# Bridging amps and SQ



## maxwerks (Jan 24, 2010)

Hello everyone, 
I am new here on this forum.. been an avid home audiophile for years, but in mobile audio, I havent experimented as much as you guys...

My question is , what is the word on degradation of sound quality, when amps are being bridged? In home audio, bridging , while always increases power, doesnt always yield the same SQ, ( pending the type of amp and the type of bridging of course ) . but mobile audio ALWAYS have the bridging feature. 

I have two old school ADS amps, 850MX & P 850.2 , eight channels each. I am looking for 8 channels dedicated per driver for an active system, what would the outcome be ( SQ not Watts ) if I was to turn each of them into a four channel amp? 

Your comments and advise are truly appreciated. Thanks


----------



## sqshoestring (Jun 19, 2007)

I just said this on another thread someplace, but I was told by a guy who designed his own amps and is a master tech...he said it will degrade the SQ however it will still be below what human hearing can pick up, for any normal amp as they all have THD well below hearing, below even while bridged. So unless its a very crappy amp or something is wrong with it, it should not matter. All else fails try it and see.


----------



## maxwerks (Jan 24, 2010)

Thank you kindly for your response... pretty much what i guessed. Cheers


----------



## SSSnake (Mar 8, 2007)

> he said it will degrade the SQ


I love subjective unsubstantiated comments.

You will likely double the level of the noise floor. Is this a problem? It depends upon the system?


----------



## MarkZ (Dec 5, 2005)

Double the noise floor level?


----------



## sqshoestring (Jun 19, 2007)

You are applying about double the voltage to the speaker, so noise would go up as much as another amp twice the size of the unbridged amp plus there could be a tiny amount of error from the two amps working together. There is also a little more THD at 2 ohms usually. But noise from the HU/etc would be amplified the same as using any larger amp that size. The amp makes more THD bridged, but I don't know all the specifics, just telling you what a guru told me. I don't have the equipment to measure distortion and have yet to find a test on it. 

In simple terms an amp varies the voltage on one speaker lead + or - and the other lead is ground. Bridged uses the 'hot' terminal from each channel, monos the two channels, then inverts the other channel. What you get is one speaker lead going + and the other - at the same time, which doubles the power to the speaker. It is similar to strapping two amps, you get some noise from two amps instead of one. But if a channel has .03% THD and now it is .6% and you can't hear it until 1%....does it matter? Now there are other factors but that is the general looks of it. For example you might get this:


> The Alpine MRP-F300 is a 4/3/2 channel power amplifier with an CEA-2006 Power Rating of
> 50 x 4 for 4 Ω @14.4V ≤1% THD+N, S/N: 80dBA, ref:1W into 4 Ω and RMS Power Ratings (Per channel) of
> 50W x 4 into 4 Ohms: (@ 14.4V ≤1%THD+N, 20Hz - 20kHz) and
> 75W x 4 into 2 Ohms (@ 14.4V ≤1%THD+N, 20Hz - 20kHz). At bridged 4 Ohms it is
> ...





> Ref475A
> 75 watts RMS x 4 channel at 4 ohms and ≤ 1% THD + N 180 watts RMS x 2 channel at 4 ohms, 14.4V supply and ≤ 1% THD + N


Most amps don't rate at 2 ohms, because the specs are not as good, but same thing if they are that low it does not matter. I don't know if some amps are worse I would assume most well known brands are respectable. Sources:
Find the Best Car Amplifiers - Reviews, Tests & Ratings
Infinity - Car Audio

If you are totally freaked about noise floors and all, then don't do it. In reality will it matter with a good amp? Very likely not. I tend to not bridge except on subs I don't care, maybe midbass, but if all I have laying here is a 4ch then I'll try it on mid/highs. Mostly why I don't is because power is cheap I just buy a big amp and use it instead of bridging, and I don't need ~200rms on mids or highs. Running 2 ohms is harder on any amp, though that does not mean it will hurt it.

BTW, you can dload a manual for it here: ADS 850MX Speaker System Owner's Manual
But it does not really say for 2 ohm, as usual they have very good specs.


----------



## MarkZ (Dec 5, 2005)

Signal is also increasing. So how is S/N going up?


----------



## sqshoestring (Jun 19, 2007)

Here is specs on an alpine, I would guess other amps are similar differences in bridged or not and 2 ohms.
Alpine MRV-T707 V12 Series (mrvt707) Discontinued Car Audio & Video Z. Discontinued Products - Sonic Electronix

"Signal increasing" I don't know what you mean, but if you put a larger amp in the noise floor from the input (HU) will be amplified more. If you bridge for double the power its the same as a larger amp; it will have more total gain. Once the music plays it too will be louder, so this would be noticed mostly at low or no output and quiet passages in music....if there is enough to hear.


----------



## MarkZ (Dec 5, 2005)

sqshoestring said:


> Here is specs on an alpine, I would guess other amps are similar differences in bridged or not and 2 ohms.
> Alpine MRV-T707 V12 Series (mrvt707) Discontinued Car Audio & Video Z. Discontinued Products - Sonic Electronix
> 
> "Signal increasing" I don't know what you mean, but if you put a larger amp in the noise floor from the input (HU) will be amplified more. If you bridge for double the power its the same as a larger amp; it will have more total gain. Once the music plays it too will be louder, so this would be noticed mostly at low or no output and quiet passages in music....if there is enough to hear.


I don't understand. Why would you need the gain to be set higher? If you want the same volume stepping, you adjust the gain so that it's the same bridged as it was unbridged. And so the noise should be the same in both cases. The benefit of bridging is that your amp doesn't run out of steam so quickly.


----------



## SSSnake (Mar 8, 2007)

The SNR should not go down unless we are considering a change in load impedance (an amp circuit in a bridged amp "sees" half of the total load impedance). This change in effective load impedance often decreases SNR (IME - not appreciably). However, the noise floor is doubled because you are differentially driving both the positive and negative leads on the speaker. So basically you get the noise from each of the two amplifier sections on one driver (hence the noise floor doubles).

This can be an issue on active systems using bridged amps on highs. Especially if you are running horns. However, most people aren't bridging amps on tweets. IME/IMO - in most applications, bridging does not decrease "SQ" enough for it to be an issue. Are there measurable differences? YES

When SQShoestring said "you have more total gain" he was NOT referring to the gain adjustments on the amp. He was referring to the increase in voltage gain from running the amp bridged (read as more ouput).



> The benefit of bridging is that your amp doesn't run out of steam so quickly.


 Well sort of... The benefit of bridging an amp is MORE OUTPUT. Bridging an amp on a speaker can often provide 4 times the output power (there are a lot of assumptions in that statement - but in a nutshell it is true). You double the output because of the second amp channel. You double it again because each amp channel is seeing half the impedance (this assumes the amp channel is stable at this reduced impedance). Again, just a rule of thumb but it is pretty accurate as long as you are within the operating parameters of the amp.


----------



## maxwerks (Jan 24, 2010)

Thanks to all .. I really enjoyed reading all your responses...

From my experience in my home audiophilia ...in a perfect world, a monoblock per driver is the ultimate way to go.. that obviously is quite hard and expensive.. and it is not the matter or wattage as much as the dedicated power supply... at any rate, while I most certainly think that the two ADS amps I have can handle being bridged into 4 ch each .. I do believe the SQ will in fact suffer.. yes I dont think that we can hear the THD... but I believe that will be heard or felt on the frequency extremes


----------



## I800C0LLECT (Jan 26, 2009)

Whatever you think will take place will be mitigated with the fact you're creating a daily driver. Correct? I don't think you'll suffer to the extent that it would play a larger role than the recordings used for playback. In the end, the added noise will not be substantial nor will it play a role. The mind is a funny thing :>

I do not have definitive numbers to back this up but the application of theory doesn't always follow through into distinctive sound characteristics.


----------



## MarkZ (Dec 5, 2005)

SSSnake said:


> The SNR should not go down unless we are considering a change in load impedance (an amp circuit in a bridged amp "sees" half of the total load impedance). This change in effective load impedance often decreases SNR (IME - not appreciably). However, the noise floor is doubled because you are differentially driving both the positive and negative leads on the speaker. So basically you get the noise from each of the two amplifier sections on one driver (hence the noise floor doubles).


But you're getting the SIGNAL from both channels too! So the S/N isn't changing! That's what I'm trying to say. It would be like saying "yeah, when I turn up the volume my noise increases". Well no kidding, so does the signal. 

So I don't see why bridging or load impedance or any of that will have an impact on the noise. It just doesn't add up for me.



> This can be an issue on active systems using bridged amps on highs. Especially if you are running horns. However, most people aren't bridging amps on tweets. IME/IMO - in most applications, bridging does not decrease "SQ" enough for it to be an issue. Are there measurable differences? YES
> 
> When SQShoestring said "you have more total gain" he was NOT referring to the gain adjustments on the amp. He was referring to the increase in voltage gain from running the amp bridged (read as more ouput).


Right, so just reduce the gain adjustment so you have the original amount of gain!



> Well sort of... The benefit of bridging an amp is MORE OUTPUT. Bridging an amp on a speaker can often provide 4 times the output power (there are a lot of assumptions in that statement - but in a nutshell it is true). You double the output because of the second amp channel. You double it again because each amp channel is seeing half the impedance (this assumes the amp channel is stable at this reduced impedance). Again, just a rule of thumb but it is pretty accurate as long as you are within the operating parameters of the amp.


Yeah. Or, an easier way to look at it... you're doubling your effective rail voltage, which translates to a 4x power (P ~ V^2).


----------



## MarkZ (Dec 5, 2005)

maxwerks said:


> Thanks to all .. I really enjoyed reading all your responses...
> 
> From my experience in my home audiophilia ...in a perfect world, a monoblock per driver is the ultimate way to go.. that obviously is quite hard and expensive.. and it is not the matter or wattage as much as the dedicated power supply... at any rate, while I most certainly think that the two ADS amps I have can handle being bridged into 4 ch each .. I do believe the SQ will in fact suffer.. yes I dont think that we can hear the THD... but I believe that will be heard or felt on the frequency extremes


Unless you turn up the volume and you clip it. In that case, the un-bridged setup will have more distortion.


----------



## SSSnake (Mar 8, 2007)

> But you're getting the SIGNAL from both channels too! So the S/N isn't changing!


I AGREE SEE COMMENTS BELOW



> The SNR should not go down


The discussion about load impedances is relevant because typically the SNR goes down with load impedance (not a linear function but there is a change). Why is there a change? I don't know but my guess would be as a result of the increased current through the devices and resultant increase in EMI noise. Now if you disagree with this statement take a look at the specs of amps at different load impedances and get back to me (if you can find the info, hard to do, what you will see is the SNR goes down with load impedance). If I remeber correctly, the maunal for my Crown CE 4000 has those measurements. If I can find a softcopy I will post a link.



> Right, so just reduce the gain adjustment so you have the original amount of gain!


No one is talking about changing gain settings on the amp. The "increase in gain" is the extra voltage/power you get from bridging.

NOISE FLOOR

The noise floor is the gain independent noise created by an amp (I'm NOT referring to input gain adjustments here). Some noise tracks the signal level. Some noise does not. Noise floor is the noise that is NOT tied to the signal level. If each amp channel has an identical noise floor then bridging the channels creates double the noise floor (Vnoise1 + Vnoise2). Notice I said nothing about the signal (because noise floor is independent of signal level).

IME - this is only an issue with tweets (where the input signal voltages are typically low and you compensate by cranking the gain knob).

Enjoy!


----------



## SSSnake (Mar 8, 2007)

NOPE not in the Crown manual. I'll keep looking.


----------



## MarkZ (Dec 5, 2005)

SSSnake said:


> The discussion about load impedances is relevant because typically the SNR goes down with load impedance (not a linear function but there is a change). Why is there a change? I don't know but my guess would be as a result of the increased current through the devices and resultant increase in EMI noise.


Interesting. I suppose that's possible.



> Now if you disagree with this statement take a look at the specs of amps at different load impedances and get back to me (if you can find the info, hard to do, what you will see is the SNR goes down with load impedance). If I remeber correctly, the maunal for my Crown CE 4000 has those measurements. If I can find a softcopy I will post a link.


Yeah, I can't find anything.



> No one is talking about changing gain settings on the amp. The "increase in gain" is the extra voltage/power you get from bridging.


Yes, I know. So if you REDUCE the gain back down to what it was before, then the problem disappears. That means it doesn't have to do with the bridging itself, it has to do with the increase in gain that accompanies bridging. 

Besides, most people ARE going to reduce the gain back down. If you have a set of midbass drivers, for example, and everything's installed and peachy. And then you decide "Hey, I'm gonna bridge the amp!", you're not gonna then live with the midbass drivers swamping the music. You're gonna reduce the gain back down to get everything balanced again.



> NOISE FLOOR
> 
> The noise floor is the gain independent noise created by an amp (I'm NOT referring to input gain adjustments here). Some noise tracks the signal level. Some noise does not. Noise floor is the noise that is NOT tied to the signal level. If each amp channel has an identical noise floor then bridging the channels creates double the noise floor (Vnoise1 + Vnoise2).


And double the signal! This is why we refer to noise in terms of signal-to-noise ratio, and not absolute noise voltages. I don't know why the noise voltage is the only thing relevant to you here.


----------



## SSSnake (Mar 8, 2007)

NOISE FLOOR AND SNR ARE DIFFERENT THINGS!

Let me break this down simply...

You know that faint hiss you hear with the volume knob at 0. That is a result of the noise floor. Guess what, the SNR at this time is 0. There is NO signal (it is all noise). 0/some number representing noise is always 0. Not much of a figure of merit. However, if you measure the voltage of the noise floor you can compare it to other amps/systems.

SNR is by definition the ratio of signal to noise. Most manufacturers take this measurement at full amplifier power (or at least they used to). I think the current standard (which most manufacturers ignore) is to measure SNR at 1W. Either way it is not a measure of noise floor.

Now if you need further clarification, re-read my posts. The information is in there.


----------



## 89grand (Nov 23, 2006)

I'm with Mark on this one. 

Look, I like sound quality as much as the next person, but I'm not going to lose sleep over a decrease (or potential decrease) in S/N when it's still below the threshold of negatively impacting the system. This stuff gets over analyzed at times looking for 1 db of noise here, 1 db there. 

Bridging most amplifiers will make no noticeable difference in sound quality.


----------



## sqshoestring (Jun 19, 2007)

You have amp noise that can change with bridging as well as load change, and if you increase the gain (double the wattage or output) that can change the noise as well as amplify input(ed) noise. I assume one bridges to increase output, though in practice you might not or increase a limited amount. All in all with a quality amp not really a concern, you may hear more noise just because you think there should be. That said I would not do it on tweeters, though I've never needed that much power on them, and would try not to on mids but would try it if I needed to. This is really why I wish there were more minimum 4 ohm load amps that delivered more power to 4 ohms. I only used bridged on subs mostly in the past and now I use mono sub amps for that. I foresee very little need to bridge a class AB anymore. About the only thing would be to power midbass with a 4ch.

I'm thinking the increase at low loads is to do with more amperage in the outputs, possibly added stress on the PS, but not entirely sure I'd have to ask someone. I'm not up on slew rates and all that, one of these days I'll get a distortion analyzer and get into it.

I've often been told to not stress an amp if you want the cleanest sound, such as running a higher load like 8 ohm speakers. But I agree the specs are so low you can't hear it anyway. Also distortion can be higher in subs, likely to a point in midbass, as you are less sensitive at low frequency.

The A/D/S have very good specs, I would not worry about a thing with bridging them.


----------



## mosconiac (Nov 12, 2009)

FWIW, my old school amps (SS MC300's) sounded worse when bridged into a "4 Ohm" speaker. I'm not one to invest in frilly interconnects or other audio gadgetry, but they most definitely sounded more "coarse". I didn't listen long enough to quantify my first impression, I just knew it wasn't for me.

My presumption is that the lower effective impedence caused higher distortion, which was audible to me. Maybe that higher level of distortion comes out in different harmonics content (odd orders maybe?)?


----------



## MarkZ (Dec 5, 2005)

SSSnake said:


> NOISE FLOOR AND SNR ARE DIFFERENT THINGS!
> 
> Let me break this down simply...
> 
> ...


Oh come on. You're arguing semantics here. In the context of the discussion ("Does bridging increase SN?"), S/N is what's important.

And you even acknowledge this in your explanation for why load impedance can potentially influence noise. You said "...my guess would be as a result of the increased current through the devices and resultant increase in EMI noise." How can you have increased current if not for increased signal?


----------



## MarkZ (Dec 5, 2005)

sqshoestring said:


> You have amp noise that can change with bridging as well as load change, and if you increase the gain (double the wattage or output) that can change the noise as well as amplify input(ed) noise.


Look. I've already addressed this. We're not talking about increasing the gain. We're talking about bridging. Like I already said, nobody is going to bridge their amp and at the same time say, "You know what? I want my midbass drivers to be 3 dB louder while I'm at it." These are two different things!

If you bridge your amp to your mids, and the gain increases by 3dB, then you reduce the gain knob by 3dB to compensate. Or you boost your tweeter and sub by 3dB to meet the new output level of the mids.

So, no, gain is not increasing unless you WANT it to.



> I'm thinking the increase at low loads is to do with more amperage in the outputs, possibly added stress on the PS, but not entirely sure I'd have to ask someone. I'm not up on slew rates and all that, one of these days I'll get a distortion analyzer and get into it.


I don't think it would be a slew rate issue. I think it would be as he said -- increased current leading to more induction noise. And yes, if the PSRR of the amplifier is poor, I could see noise being introduced that way too.

But...

If your signal is that high, ya ain't gonna hear any noise!

When we talk about noise issues, what we really care about are moderate volumes. Or no volume. And I don't see how bridging is going to affect that.


----------



## 89grand (Nov 23, 2006)

I'm not sure if people get what you're saying about bridging an amp or if they just haven't actually read it, but yeah, for nearly everyone that bridges an amp for say their midbass, they are going to be lowering the gain. 99% of the people that would do that do not want the midbass any louder to the rest of the system than before. 

The whole purpose in bridging the amp would be to limit clipping when the system was being driven hard.


----------



## namboy27 (Oct 16, 2009)

you planed on using all eight channels on one driver?


----------



## maxwerks (Jan 24, 2010)

namboy27 said:


> you planed on using all eight channels on one driver?


Oh god no, even I know better.. no I was thinking of bridging all eight channels and turn the amp into 4 channels one per driver for the front stage... although
I had my doubts and I never tried it. Now things might be different with the new Blade amp I just got.. we ll see I guess.


----------



## SSSnake (Mar 8, 2007)

> Oh come on. You're arguing semantics here. In the context of the discussion ("Does bridging increase SN?"), S/N is what's important.


WHO SAYS S/N is the only thing that's important? The context of the discussion was NOT limited to S/N. The discussion was about bridging amps and SQ. S/N is NOT the only thing to look at regarding SQ.



> And you even acknowledge this in your explanation for why load impedance can potentially influence noise. You said "...my guess would be as a result of the increased current through the devices and resultant increase in EMI noise." How can you have increased current if not for increased signal?


In the discussion about "increased EMI" I AM referring to SNR. The SNR will typically decrease as the load impedance goes down. Can you hear it *not likely*.


However, none of the above refers to noise floor. Noise floor is the minimum amount of noise for a system/component. This occurs in an audio amplifier (unless something is wrong) when there is NO INPUT SIGNAL. THAT IS NOT SEMANTICS. It IS the definition. Even when there is no input signal to the amp there is a small output. This is the noise floor. Why is it important, because every electronic component in the chain adds to this noise floor. 

AS I HAVE SAID REPEATEDLY YOU ARE NOT LIKELY TO NOTICE ANY DECREASE IN SQ OTHER THAN AN INCREASE IN NOISE FLOOR.

If you don't know what the noise floor is then that is fine. But rather than re-stating the same BS why don't consider what I am saying (1. SNR and noise floor are different 2. noise floor doubles when you bridge an amp). If I'm wrong, show me some authoritative source that says so. I am willing to admit it if I'm wrong. So far, nothing that you have stated disproves my statements.

The problem you are going to have is that I have bridged amps on tweets (and subs, and midbasses, and mids). The noise floor went up and became a problem (on the tweets). I put the bridged an unbridged amp on my Oscope and looked at the noise. GUESS WHAT! THE NOISE FLOOR doubled. Did the SNR decrease by 3 db? NOPE Why not? BECAUSE NOISE FLOOR AND SNR ARE DIFFERENT

Will this be a problem in other systems... It depends. Why did I mention it? because it CAN cause problems in installs. Will it in all installs? NO


----------



## t3sn4f2 (Jan 3, 2007)

Audio Specifications

"S/N or SNR. Signal-To-Noise Ratio
What is tested? This specification indirectly tells you how noisy a unit is. S/N is calculated by measuring a unit's output noise, *with no signal present*, and all controls set to a prescribed manner. This figure is used to calculate a ratio between it and a fixed output reference signal, with the result expressed in dB.

How is it measured? No input signal is used, however the input is not left open, or unterminated. The usual practice is to leave the unit connected to the signal generator (with its low output impedance) set for zero volts. Alternatively, a resistor equal to the expected driving impedance is connected between the inputs. The magnitude of the output noise is measured using an rms-detecting voltmeter. Noise voltage is a function of bandwidth -- wider the bandwidth, the greater the noise. This is an inescapable physical fact. Thus, a bandwidth is selected for the measuring voltmeter. If this is not done, the noise voltage measures extremely high, but does not correlate well with what is heard. The most common bandwidth seen is 22 kHz (the extra 2 kHz allows the bandwidth-limiting filter to take affect without reducing the response at 20 kHz). This is called a "flat" measurement, since all frequencies are measured equally.

Alternatively, noise filters, or weighting filters, are used when measuring noise. Most often seen is A-weighting, but a more accurate one is called the ITU-R (old CCIR) 468 filter. This filter is preferred because it shapes the measured noise in a way that relates well with what's heard.

Pro audio equipment often lists an A-weighted noise spec -- not because it correlates well with our hearing -- but because it can "hide" nasty hum components that make for bad noise specs. Always wonder if a manufacturer is hiding something when you see A-weighting specs. While noise filters are entirely appropriate and even desired when measuring other types of noise, it is an abuse to use them to disguise equipment hum problems. A-weighting rolls off the low-end, thus reducing the most annoying 2nd and 3rd line harmonics by about 20 dB and 12 dB respectively. Sometimes A-weighting can "improve" a noise spec by 10 dB.

The argument used to justify this is that the ear is not sensitive to low frequencies at low levels (à la Fletcher-Munson equal loudness curves), but that argument is false. Fletcher-Munson curves document equal loudness of single tones. Their curve tells us nothing of the ear's astonishing ability to sync in and lock onto repetitive tones -- like hum components -- even when these tones lie beneath the noise floor. This is what A-weighting can hide. For this reason most manufacturers shy from using it; instead they spec S/N figures "flat" or use the ITU-R 468 curve (which actually makes their numbers look worse, but correlate better with the real world).

However, an exception has arisen: Digital products using A/D and D/A converters regularly spec S/N and dynamic range using A-weighting. This follows the semiconductor industry's practice of spec'ing delta-sigma data converters A-weighted. They do this because they use clever noise shaping tricks to create 24-bit converters with acceptable noise behavior. All these tricks squeeze the noise out of the audio bandwidth and push it up into the higher inaudible frequencies. The noise may be inaudible, but it is still measurable and can give misleading results unless limited. When used this way, the A-weighting filter rolls off the high frequency noise better than the flat 22 kHz filter and compares better with the listening experience. The fact that the low-end also rolls off is irrelevant in this application. (See Digital Dharma of Audio A/D Converters)

Required Conditions. In order for the published figure to have any meaning, it must include the measurement bandwidth, including any weighting filters and the reference signal level. Stating that a unit has a "S/N = 90 dB" is meaningless without knowing what the signal level is, and over what bandwidth the noise was measured. For example if one product references S/N to their maximum output level of, say, +20 dBu, and another product has the same stated 90 dB S/N, but their reference level is + 4 dBu, then the second product is, in fact, 16 dB quieter. Likewise, you cannot accurately compare numbers if one unit is measured over a BW of 80 kHz and another uses 20 kHz, or if one is measured flat and the other uses A-weighting. By far however, the most common problem is not stating any conditions.

Correct: S/N = 90 dB re +4 dBu, 22 kHz BW, unity gain

Wrong: S/N = 90 dB"



















See, no signal, yet there is noise at around the *resolution limit of a 16/44 file*. Test a high resolution file and that "noise floor" drops, the "S/N ratio" goes up, and yet there is still no "signal".


----------



## MarkZ (Dec 5, 2005)

SSSnake said:


> WHO SAYS S/N is the only thing that's important? The context of the discussion was NOT limited to S/N. The discussion was about bridging amps and SQ. S/N is NOT the only thing to look at regarding SQ.
> 
> In the discussion about "increased EMI" I AM referring to SNR. The SNR will typically decrease as the load impedance goes down. Can you hear it *not likely*.
> 
> ...


Ok, then please explain why the noise floor would go up if you bridge your amp. You had an earlier explanation (the EMI thing) that obviously doesn't apply when there's no input signal. So, why do you believe that this noise floor will increase?



> If you don't know what the noise floor is then that is fine.


There's no need for that.



> But rather than re-stating the same BS why don't consider what I am saying (1. SNR and noise floor are different 2. noise floor doubles when you bridge an amp). If I'm wrong, show me some authoritative source that says so. I am willing to admit it if I'm wrong. So far, nothing that you have stated disproves my statements.


And nothing that you've stated proves your statements. All I'm asking for is an explanation. You keep insisting that noise floor will double. You cited your crown amp manual or something, I'm not sure. Mechanistically, you don't have an explanation. Mechanistically, I *have* provided an explanation for why it wouldn't increase. So I don't know why you're throwing all this on my shoulders.


----------



## SSSnake (Mar 8, 2007)

t3sn4f2,

Good authoritative source. From the same source...

S/N or SNR (signal-to-noise ratio) An audio measurement of the residual noise of a unit, stated as the ratio of signal level (or power) to noise level (or power), normally expressed in decibels. * The "signal" reference level must be stated. Typically this is either the expected nominal operating level, say, +4 dBu for professional audio, or the maximum output level, usually around +20 dBu. *The noise is measured using a true rms type voltmeter over a specified bandwidth, and sometimes using weighting filters. All these thing must be stated for a S/N spec to have meaning. Simply saying a unit has a SNR of 90 dB means nothing, without giving the reference level, measurement bandwidth, and any weighting filers. A system's maximum S/N is called the dynamic range. See the RaneNote Audio Specifications. 

In car audio prior to CEA-517 (which references CEA-490-A) SNR was typically specified at maximum output level (as per one of the cases above). I believe that this is still the case with several manufacturers. I believe that CEA-517 called for this measurement to be performed at 1W (I can't confirm this because I can't find my copy).

The problem with the reference you provided is that they don't specify the calculation. It is my belief that the formula for SNR is as follows:

20 log Vs/Vn or 10 log Ws/Wn

Where Vs is signal voltage 
Vn is noise voltage
Ws is signal power
Wn is noise power

With these definitions at 0 input voltage SNR is always 0. Is there noise present? Yes. It is what I am referring to as the noise floor. 

In your screen shots you show this noise and it is indexed as db. My question is at what "signal reference level" (see above) was the measurement taken or referenced. My guess is that it is truly dbu rather than db (I can't say that definitively but that would be my assumption since dbu is a fairly standard measure in pro audio).

MarkZ,

Your latest post asks me "mechanistically" prove my point. Here is my attempt. 

Can we agree that at Vin = 0V we have some Vout that is greater than zero for each amp channel in a non-bridged configuration? If so, then when we bridge amps we are connecting one amp channel with a phase inverted amp channel. In this case both amp channels provide voltage to the load. If those amp channels are identical then the voltage delivered to the amp is now Vout (from amp channel 1) + Vout (from amp channel 2) or 2*Vout (with Vin still equal 0V). This doubles the noise floor. You can't turn down the input gain to get rid of this noise (the input signal is already at 0V - _in practice you can turn down the gain to reduce the output resulting from the amplification of the noise floor of previous components - but that just complicates the issue_). 

I believe that this "mechanistically" proves

At this point you have 0V signal and 2*Vout noise deliverd to the load or as I said double the noise floor. 



You can say that this isn't important because when you listen to music Vin is greater than 0V. This is very true but as I referenced in earlier posts this is "that faint hiss you hear with the volume knob at 0". The problem is sometimes that hiss ain't so faint. 

SNR as defined by t3sn4f2 above "means nothing, without giving the reference level". With this I am very much in agreement. The confusion seems to arise from the measurement technique they employ (shunting the input and measuring the output voltage). This ignores the potential increases in noise over the output range of the amp. IMO - this is not always a good assumption (especially in a car amplifier). Also, we have ignored the measurement bandwidth but in this discussion I believe that is warranted.

Now, if someone can prove to me that the NOISE FLOOR does not double when you bridge an amp I will be glad to admit I was wrong but as I have maintained several times:
1.) SNR and noise floor are different things
2.) The noise floor doubles when you bridge an amp

You can prove this "mechanistically" or maybe just logically . So far no one has touched on noise floor. The discussion has centered on SNR. Again, they are two different things and I agree that SNR is not likely to change (given a doubling of load impedance).


----------



## aphexacid (Oct 24, 2009)

89grand said:


> I'm not sure if people get what you're saying about bridging an amp or if they just haven't actually read it, but yeah, for nearly everyone that bridges an amp for say their midbass, they are going to be lowering the gain. 99% of the people that would do that do not want the midbass any louder to the rest of the system than before.
> 
> The whole purpose in bridging the amp would be to limit clipping when the system was being driven hard.



This.


----------



## SSSnake (Mar 8, 2007)

MarkZ,

I forgot to mention that I was not "throwing this all on your shoulders". I stated what I believe to be fact supported by both logical and empirical evidence (agreed I didn't get as detailed on the logical portion earlier and you did have to trust me on the empirical evidence). But you're arguments have consistently addressed only SNR. Which again is not the same as noise floor. This is why I asked you to produce an authoritative source or some other firm evidence. If I had discredited one of your statements I would provide the same.

T3 provided some good info on SNR. However, it again fails to address noise floor.


----------



## MarkZ (Dec 5, 2005)

SSSnake said:


> MarkZ,
> 
> Your latest post asks me "mechanistically" prove my point. Here is my attempt.
> 
> ...


That all depends. Where is the noise coming from?

Some noise comes from the source. Some comes from ground loops. Some comes from induction through the cabling. Some is internally generated. It appears to me that you're only focusing on the last one and ignoring the others. I'm sure you'd agree that the reduction in the gain mitigates the first three I listed. Yes? I think we've all experienced the increase in noise that usually accompanies turning that gain knob up.

Assuming we're in agreement there, let's talk about the internally generated noise (ie. generated AFTER the gain circuit)...

This noise is going to be present in both channels whether you bridge or not. If, for example, you don't bridge your amp, and drive one DVC coil per channel, you'll ADD the two noises in the subwoofer. That is, one noise will be applied to one coil, the other noise will be applied to the other coil, and it will add appropriately -- if the noises are correlated, it will double; if the noises are uncorrelated, it will increase by a factor of sqrt(2).

If, instead, you bridge the amp, the noise present in one channel and in the other channel will add going into the single series-wired coil. It adds in the same way as with the unbridged configuration -- if it's correlated, it will double the voltage; if it's uncorrelated, it will increase by a factor of sqrt(2).

What I was addressing in my last post was where I thought you were going with it. I thought you were saying there's an additional noise source, perhaps coming from the increase in current through the output devices or the power supply switching or something like that. I acknowledged that that's very possible (MZ: "Interesting. I suppose that's possible."). But my point was that it doesn't translate into anything audible because the tunes are cranking. I think you agree with this.


----------



## SSSnake (Mar 8, 2007)

We're talking about Amplifier SQ as a result of bridging, so all of the noise comes from........ the amp.

So yes turning the gain knob mitigates all of the noise issues we were NOT talking about earlier....

We are definitely talking about the internally generated noise. However, the amp is the gain circuit (plus the power supply) so I'm not sure why you are saying after the gain circuit.... Do you mean the gain control circuit (pre amp stage)?



> This noise is going to be present in both channels whether you bridge or not.


ABSOLUTELY CORRECT. However, before we bridged the circuit only one of the amp channels was connected to this load. Changing the discussion to a deal with a dual coil sub changes the situation completely. Because in this new case you created we have noise from two channels being evaluated. You are now scuplting the scenario to fit your logic. Additionally, I believe that I mentioned a specific case where this was a potential issue. TWEETS. Not too many dual coil tweets out there.

Your point about uncorrelated and correlated noise is completely valid. My statement about the amp channels being "identical" (BTW - I mean theoretically identical - I realize physical circuits are never truly identical) forced my discussion to the correlated case. Plus it was much simpler and communication seems to have been an issue.

I'm not sure why you keep going back to my EMI statement. In every post I have agreed it has a minor effect on SNR. Here is the actual post:



> The SNR should not go down unless we are considering a change in load impedance (an amp circuit in a bridged amp "sees" half of the total load impedance). This change in effective load impedance often decreases SNR (IME - not appreciably).


Now, I don't know how much clearer I can be. In this statement I said SNR twice, never said noise floor, and said "not appreciably" when discussing audibility of the SNR change.

I do have a couple of questions... 
Do you believe that the noise floor doubles when bridging? It appears that you do from your last post. However in previous posts you did not.



> When we talk about noise issues, what we really care about are moderate volumes. *Or no volume. And I don't see how bridging is going to affect that.*


I do believe that we have proved the noise will be higher in the "no volume" case.

Do you believe that a doubling of the noise floor can be audible? Particularly in the case of bridging amps onto tweets.



I know that I am being a royal pain here but to be quite honest you were earlier. My posts were twisted in an effort to disprove my stattements. In the end I stand by the two facts that I repeated over and over.

1.) SNR and noise floor are different things
2.) The noise floor doubles when you bridge an amp


----------



## MarkZ (Dec 5, 2005)

Got it.


----------



## sqshoestring (Jun 19, 2007)

One issue with an amp, is that transistors react differently to voltage changes compared to amperage changes. This is why home audio runs high voltage amps they work nice...but car audio went 4 ohm and higher current/less voltage because a big PS was harder to do on 12v. I don't know the specifics of how exactly this affects distortion levels, only that high voltage is typically preferred for quality and less heat in the outputs. Given electronic components are better today and this is less an issue than in the past, so I don't know how much an issue it really is likely not much because amps are so clean now. 

More gain...well maybe you don't run more gain but the OP didn't say that. If gain is the same then you eliminate input noise increases right?

If I'm not doing a show car I don't even care about noise floor long as I don't ever hear it. Car is such a poor noisy environment. But I will say I have heard distortion in cheap amps bridged. They were old amps I doubt even cheaper new amps have that bad of spec today....but I still have the practice of trying to only bridge in the bass region where it is not much of an issue. I tend to value actual experience more than anything, if I know the technical issues or not. I have not been into electronics that long like some of you, but installing I have.

Gain of an amp I would describe as the increase of voltage from input to output. If you have a 4v max HU and the amp clips at 60v, that is multiple of 15 for example. The gain control is only to match the gain properly with other equipment such as the HU input level and load placed on the amp, as well as setting the output level you need (though actually in db).

Another issue is amp design, though design has become fairly advanced and standard in many cases making most amps operate similarly.


----------



## thesolution23 (Feb 1, 2010)

I never run an amp below 4 ohms per channel in my car. I have done the tests and there is an audible difference in sound when you bridge an amp or run it at 2 ohms per channel. The main difference is in overall spaciousness and dynamics. The sound just isn't as "sweet".


----------



## lycan (Dec 20, 2009)

Let's talk about distortion ... namely, harmonic distortion ... instead of noise 

One argument says that distortion might _increase_ in the bridged configuration, since each amp channel is driving half the load impedance. HOWEVER ... certain distortion components (namely, the _even_ ones  ) will tend to _cancel_ in the bridged configuration.

Bottom line for me has always been : if the amp can handle the effective reduced impedance load (without excessive distortion), bridging can't possible degrade "sq". There's simply no mechanism to cause it ... if each channel in the bridge is fine with half the load impedance.

Where do these rumors get started?


----------



## lycan (Dec 20, 2009)

only one point to offer on the noise issue ... yes, the noise floor increases. But the noise _voltage_ at the output doesn't double, the noise _power_ doubles. Was this already covered? If yes, me=sorry.

Even with identically designed and built amp channels, even with "matched" components, the thermal noise (and flicker noise, and shot noise) from the electronic elements in the amp channels will be uncorrelated. This means the RMS noise voltage will increase by a factor of sqrt(2) ... it's the output noise _power_ that will increase by a factor of 2.

Bottom line (again, for me), is that noise increase will be a non-issue. If you're using VERY sensitive drivers that don't need much _signal_, the noise increase may be objectionable ... but you wouldn't be bridging an amp on these drivers anyway.

So ... if the _distortion_ from each channel with half the load impedance is not objectionable, bridge-away 

Yes, i'm also ignoring damping factor ... as we all should


----------



## MarkZ (Dec 5, 2005)

lycan said:


> only one point to offer on the noise issue ... yes, the noise floor increases. But the noise _voltage_ at the output doesn't double, the noise _power_ doubles. Was this already covered? If yes, me=sorry.


Yes. Several posts ago I said if it's uncorrelated noise it will increase by a factor of sqrt(2).


----------



## lycan (Dec 20, 2009)

MarkZ said:


> Yes. Several posts ago I said if it's uncorrelated noise it will increase by a factor of sqrt(2).


sorry, again


----------



## MarkZ (Dec 5, 2005)

Nah, that's cool. You explained it more clearly than I did.


----------



## SSSnake (Mar 8, 2007)

NOT THIS THREAD AGAIN!!!! :laugh:



> But the noise voltage at the output doesn't double, the noise power doubles. Was this already covered? If yes, me=sorry.


Your typical amps are voltage gain blocks. As long as the power supply can deliver enough juice the amp will act as a constant gain block independent of load. The point being if the gain is 10:1 then 1V in equals 10V out until the power supply poops out.

When bridging, if you double the load impedance (so that each channel see the pre-bridge load), then the output voltage and power should double. Vbridge = Vout1 + Vout2 = 2*Vout1 and Rbridge = Rload1 + Rload2 = 2*Rload1. Therefore, Pbridge = Vbridge^2/Rbridge = 2*Vout1^2/Rload1 or 
Double the output voltage and double the output power (true for noise as well).


If the load impedance is left the same (so that each channel sees half of the pre-bridge load), then the output votage doubles and the output power quadruples. Vbridge =Vout1 + Vout2 = 2*Vout1 and Rbridge = Rload1. Therefore, Pbridge = Vbridge^2/Rbridge = 4*Vout1^2/Rload1. Again quadruple the output power and double the output voltage.

Am I missing something?


----------



## MarkZ (Dec 5, 2005)

SSSnake said:


> Vbridge = Vout1 + Vout2 = 2*Vout1
> 
> Am I missing something?


That equation only holds true if Vout1=Vout2. The point is that if they're uncorrelated, their "powers" add, not their voltages.

Think of it in terms of adding guassian white noise. It's described by its sigma (standard deviation) or sigma-squared (variance). Sigma-squared corresponds to power, sigma corresponds to voltage. If you have two completely random signals (ie. uncorrelated noise) and you add them, the resulting sigma = sqrt(sigma1^2 + sigma2^2). It's basically the pythagorean theorem applied to orthogonal vectors.


----------



## SSSnake (Mar 8, 2007)

OK. We're back to the correlated and uncorrelated discussion. Yes as stated previously, in the uncorrelated case (which most closely matches the real world) the noise floor voltage will increase by a factor of SQRT(2).

Sorry, I read this 


> But the noise voltage at the output doesn't double, the noise power doubles


as an increase in power without a corresponding increase in voltage (I should have known Jeff would not suggest this). 

Thanks Mark!


----------

