# Tweeter 'Hiss' after Mod



## storm

Would like to seek expertise advice on my current issue. I had my 6-channel amp's opamp and power supply upgraded to better quality parts / grades. The SQ turned out fantastic after some burn-in. Resolution was greatly improved with excellent transparency.

However I can hear this disturbing 'hiss' sound from the tweeters. The hiss sound was never present before the upgrade. Amplitude of the hiss varies with different recordings. Sounded similar to those hiss you often hear on vinyl playback. Threw the Zero Bit track for checks but could not hear the hiss sound in the track. 

I'm no technical expert. However, will be great if anyone with previous experience or knowledge can advise what are the likely problems. Possible that the problem was introduced during the upgrade process? If yes, specifically which area? I'd like to think mean time that the amp's resolution is revealing the recording quality during playback


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## ChrisB

It sounds like your amplifier wasn't biased properly for the opamp swap.


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## Luke352

So you have background hiss that changes in amplitude dependent on which tracks you play, BUT if you play a Zero Bit track you have no hiss. Think about that for a minute, if you have no hiss playing the 0 bit track then it is plainly obvious that you have no audible system noise and that what you are hearing is background hiss on your music CD's most have background noise of some sort which will vary depending on the quality of the recording. 

So why are you hearing it now? Well probably because you weren't really listening for it before, it's like when somethign breaks on your car and after you fix it you hear lots of noises that weren't there before, they were you just weren't listening. Or possibly the background noise on CD's was being covered or lost in your systems normal noise and now that you've made improvements you've dropped the noise floor enough to reveal the background hiss on your music CD's.


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## Luke352

storm said:


> However I can hear this disturbing 'hiss' sound from the tweeters. The hiss sound was never present before the upgrade. Amplitude of the hiss varies with different recordings. Sounded similar to those hiss you often hear on vinyl playback. *Threw the Zero Bit track for checks but could not hear the hiss sound in the track.*
> 
> I'd like to think mean time that the amp's resolution is revealing the recording quality during playback


See my post above but the bolded part is the key giveaway.


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## MarkZ

ChrisB said:


> It sounds like your amplifier wasn't biased properly for the opamp swap.


op amp bias?


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## thehatedguy

Sounds to me you might have increased the gain in the preamp.


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## MarkZ

thehatedguy said:


> Sounds to me you might have increased the gain in the preamp.


Good call. The gain adjustment for many amps is nothing more than a pot on the front end of an op amp. Bet that op amp is one that got changed. Bet the gain changed along with it.


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## Luke352

thehatedguy said:


> Sounds to me you might have increased the gain in the preamp.





MarkZ said:


> Good call. The gain adjustment for many amps is nothing more than a pot on the front end of an op amp. Bet that op amp is one that got changed. Bet the gain changed along with it.


Did you guys read his post? If what he says about having NO HISS when playing a 0 bit track is correct then it's simple there is NO problem with the amp. All he is hearing is typical background hiss as found on most Music CD's. I'm not saying that there hasn't been some gain increase there could've been, BUT if there is no HISS when playing a 0 bit track then anything he hears must be coming from his other CD's.


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## thehatedguy

Which would come from a higher gain structure.

If there isn't anything to amplify like a zero bit track, what can you hear?


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## Luke352

thehatedguy said:


> Which would come from a higher gain structure.
> 
> If there isn't anything to amplify like a zero bit track, what can you hear?


Agreed, but his concern seems to be that there may have been a fault in the work. I think we can agree that there isn't and that what he is hearing is either as you have said a result of higher gain amplifying the CD's noise floor or the amps noise floor which previously hid the CD noise floor has been dropped enough to now reveal the CD noise floor.


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## trigg007

and so the weak link is the tweeter (maybe)...

increased *** (insert adjective) typically yields more detail


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## ChrisB

MarkZ said:


> op amp bias?


It was a joke, albeit badly written.:laugh:

I take it the OP was serious about SQ burn-in, enhanced resolution, and clarity from an OPAMP swap for something installed IN A MOTORIZED vehicle. I guess the channel separation went up, crosstalk went down, and there was a warmer, richer, more vibrant sound too. 

Almost reminds me of a review I once wrote after having something modified where all I did was take various audiophile quotes and slam them together in one email. I fell for the psychoacoustic factor of being told it would sound better, spending money for it to sound better, and thereby believing it sounded better. The reality of it is I can't remember jack when it comes to sound. Heck, I couldn't even tell you what I ate for supper last Friday.:laugh:

Sorry for the poorly placed joke and threadjack OP.


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## storm

Thanks for all the valuable comments. Probably summarised by Luke352 that the hiss was "a result of higher gain amplifying the CD's noise floor or the amps noise floor which previously hid the CD noise floor has been dropped enough to now reveal the CD noise floor."

If the aforementioned is true, is it possible to 'lower' the preamp gain or match the opamp to the default preamp gain? While the increased resolution and details were great, the hiss sound does has an impact on the overall music appreciation experience.


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## MarkZ

Don't you have a gain knob?


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## storm

Yes, and it's set all the way down now


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## MarkZ

Yikes. Then I don't know what to tell you. Buy a lower noise head unit or have the amp modded again.


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## thehatedguy

Going to sound dumb, but lower efficiency tweeters would unamplify the hiss.


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## storm

I loved the L1 Pro SE!!! I will not trade it out unless for a Dyn Esotar 2 

If the problem started cos of the amp mod, then I'd like tackle from there. Short of replacing the opamps with originals (cos I really loved the increased SQ now), is there any other way? Is opamp matching required to begin with?


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## MarkZ

It only involves replacing a single resistor if you want to cut the gain down.


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## envisionelec

I read the post to mean that the hiss *in the track* could not be heard separate from the hiss _*from the amp*_.

OpAmp swaps don't change the gain - it's impossible.

What can and will happen with some of these esoteric opamps is that they break into oscillation due to instability. Many types are not "unity-gain" stable or they cannot tolerate reactive loads of any type. The result is ultrasonic oscillation that excites resonances in the amplifier causing subharmonic distortion. You hear this as hiss.

Put 'em back....


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## MarkZ

envisionelec said:


> I read the post to mean that the hiss *in the track* could not be heard separate from the hiss _*from the amp*_.
> 
> OpAmp swaps don't change the gain - it's impossible.


Wait a second here. We don't know what else was modded. It's possible the modder changed the feedback resistors as well for stability concerns. For reasons you touched on in the rest of your post.

It's also possible that he changed the feedback resistors because it's part of his scam.


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## envisionelec

MarkZ said:


> Wait a second here. We don't know what else was modded. It's possible the modder changed the feedback resistors as well for stability concerns. For reasons you touched on in the rest of your post.
> 
> It's also possible that he changed the feedback resistors because it's part of his scam.


_Maybe._ Modders usually can't read resistor values, though.


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## envisionelec

cajunner said:


> I've been doing some light reading on this oscillation issue with op amp swaps, and it seems to me that "rolling your own" can induce some unwanted nastiness even with supposedly superior specifications from the new replacements.
> 
> if it's "sub-harmonic resonances" or just distortion, I wouldn't have an accurate way to tell, unless somebody can approximate what the two signals sound like for us in the descriptive.
> 
> if you had a TL07x series, the recommended replacement/upgrade from Texas Instruments is TLE207x, but whether or not the intended result in better sound is acquired through replacement has yet to be explained...
> 
> I believe that the move would be a lateral one? Perhaps sonically indistinguishable due to the circuit having compatible characteristics?
> 
> If putting in one of these 12 dollar op amps is causing something like tweeter hiss, and you don't want to necessarily change the sonics of the amp, should one just put in what would be an upgrade in the material specs and technology, (TLE207X) would this be a good way to go?


 Driving a mostly capacitive load is difficult for many "high grade" op amps, and these loads are often found in high-pass filter networks. An opamp is unstable when it cannot error correct - just like a power amplifier. Instability manifests itself in many ways: oscillation, distortion or even destruction.

Opamp selection should be done on a case-by-case basis. You have to know what the circuit is doing before dropping in a replacement.


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## storm

The amp is going to the bench again to rectify the nasty hiss. I have a few queries though:

1. Mine's a 6-channel amp driving a 3-way active front; the opamp replacement was done for ALL in the circuit board. I intend to switch the tweeter channels with the midrange channels. If the hiss is still present, then probably the root of the problem is the opamp swop right?

2. Other than replacing the opamp with originals, are there any other options? Is there a way to find out what are the 'matching' opamps available to prevent the possible oscillation? e.g from specs?

3. Will changing the resistor values to lower the preamp gain help if the hiss is indeed caused by the opamp's oscillation?

Appreciate valuable inputs. I'm a noob in technical stuff


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## envisionelec

storm said:


> The amp is going to the bench again to rectify the nasty hiss. I have a few queries though:
> 
> 1. Mine's a 6-channel amp driving a 3-way active front; the opamp replacement was done for ALL in the circuit board. I intend to switch the tweeter channels with the midrange channels. If the hiss is still present, then probably the root of the problem is the opamp swop right?
> 
> 2. Other than replacing the opamp with originals, are there any other options? Is there a way to find out what are the 'matching' opamps available to prevent the possible oscillation? e.g from specs?
> 
> 3. Will changing the resistor values to lower the preamp gain help if the hiss is indeed caused by the opamp's oscillation?
> 
> Appreciate valuable inputs. I'm a noob in technical stuff


You can do a lot of research on the subject yourself. Anything on opamps by Walt Jung is a good read. Also, Douglas Self had some good information on opamp stability. Now that he's published, it looks like he closed his page. Here are some excerpts from his book.

One thing to remember is that not all Opamps are unity-gain stable; also known as a gain of 1. Simply put, 1V input will give you 1V output or 1V/V. A gain of 2 will give 2V output with 1V input (2V/V). Lowering gain below unity is not possible and must be performed as attenuation using resistors and it will not do anything for stability. Gain of -1 is an inverted output (180°) of equal magnitude (-1V/V). Changing the gain of the circuit is not recommended unless you have (or can trace) a schematic and know exactly what to expect. 

A lot of these opamps have great specs - if you're using them as instrumentation front-ends. They're _very picky_ and require an above average understanding of them. Many also do not have zero-offset output. Some cannot drive low impedance loads - even a 1k resistive ohm load is too much! Others don't have the correct output configuration to drive a the next circuit stage and will "rail stick" at the slightest input imbalance. Still others won't drive a modestly capacitive load without going straight into oscillation. 

Then there is sensitivity to parasitic oscillations. A marginally stable opamp _for your particular configuration_ will ring or oscillate only under certain circuit conditions. [As an aside - this is a big reason I do not buy low-cost car amplifiers with built-in filtering. The "designer" is using the cheapest part and has not done stability analysis on the design. A simple frequency sweep that they do in magazine reviews don't pick up stability margin problems with the filter design. But, I digress - it's a topic for another day.]

Here's a good one for you. I designed an amplifier about seven years ago that used a fairly high quality, low offset op amp in a DC servo loop. I built the whole thing up and flipped it on - POP, BANG. Fuses blew. Outputs blew. After several rounds of checking, I pulled the output devices and found that my preciously chosen op-amp was oscillating, driving the output madly to both rails at around 500khz (fully clipped!). Note that I did hear a tremendous HISSS from my tweeters even though the fundamental oscillation occurred far above human hearing! I found that I had missed a key specification in the choice of the op amp - it's ability to drive a capacitve load (fairly high current) was limited. The opamp wasn't driving a capacitor, directly, but the feedback pole in the amplifier certainly is. Being a servo compensation loop, the op amp's sole job was to keep the DC operating point close to zero, automatically, reducing the requirement for a the large series capacitor that adds a pole to the amplifier's LF response. I replaced the $3 Burr Brown opamp with a 75 cent variety and it worked perfectly, holding offset to below 500uV. 

I own several really good (yet expensive) books on the topic. One is Walt Jung's (pronounced 'young') OP Amp Applications Handbook. Well worth the $80 investment (looks like it's only $51 now). 

A book that was just recently published by my good friend Bob Cordell is Designing Audio Power Amplifiers. Bob is an amplifier guy - but he's plain spoken and his book is only $38. Plus, he's approachable via email and over at DIYAudio.com




*
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## MarkZ

Don't waste your time. Just put the LF351 back in there and call it a day. Then move on to things that will actually have an audible impact.


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## Oliver

Since you really like the sound

have you tried to EQ out the offending frequencies?

hiss might be 2K- 7K


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## storm

cajunner said:


> I'd agree on this, just put the op amp it came with back in on the one that controls the input gain.
> 
> you don't have to change any of the other ones, your amp's improved resolution will still be there but the gain structure won't be out of whack and you will have the increased sound quality of no hiss, or an ultra quiet background.



I didn't know that there's a single op amp that controls the input gain!!! This sounds like a plan mate! I think I shall try this out to see whether it works to rid the hiss. Fingers cross...


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## storm

just a quick update that the issue was resolved. we swopped in another BB opamp at the input stage; 'hiss' was not due to system noise. the noise was due to CD recording, though now not glaring.


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