# I too have a PDX4.150 now, my thoughts



## CBRworm (Sep 1, 2006)

First let me say that I have been into car and home audio for over 20 years. That being said I am pre-biased against the idea of a full-range class D amp, but the form factor and power savings are driving me to give this thing as un-biased an evaluation as I possibly can. Really it is just the form factor, I don't care about power saving right now.

First, as I have to say the form factor is absolutely perfect. Mounting this under my seat will be a breeze.


Thoughts:

I am surprised how high the idle current is, it is as high as any A/B amp I have tested - right over 2.25 amps at 14.4volts. Who cares though - I was just expecting it to be so efficient that there would be no idle current draw.

Current draw under load - wow - this thing uses no current at all. For my initial testing I am using a 10 amp bench top power supply with analog volt and amp meters. Normally I can only use this supply for fairly low volumes as the highest setting for the current limiter is 10 amps. 

With this amp, I can drive my speakers as loud as I can stand it in the house (with a music source) without reaching the 10 amp mark on the meter (at 14.4 volts). Normally I would start checking an amp out on this PS and then move to the Iota for higher volume testing. 

The speakers I am testing it with currently consist of the following: Peerless HDS tweeter, DLS IR3 midrange, and Seas CA21RNX woofer all using a passive crossover. It is an easy load for any car amp.

Driving 1 speaker with the PDX and one with a Mmats SQ2150, I can not tell which is which. They don't sound exactly the same, but going back and forth I can't pick out by ear a 'better' sound. And the input signal is stereo at this point, so they may not be recieving the exact same source. The speakers are side by side about 12 feet from me. I do not notice anything missing or added. The Mmats amp I am using is not exactly a flat reference, but it is a sound I am very familiar with, and one I like.

With the inputs disconnected and the gains set in the middle at the 'norm' detent there is more hiss than I would like, and a little bit of a ringing noise. It sounds like switching noise, but it could be sneaking through from the power supply. I will reevaluate this when it is in the car driving the SEAS directly.

Driving both speakers with the PDX, it sounds great - nothing lacking that I can hear. And it plays fairly loudly without hitting the 2.5 amp mark on my power supply. With my A/B amps (any of them) the amp meter acts like a VU meter even at low volumes. Playing at around the 5 amp mark for an hour the amp got surprisingly warm (I could smell it), but not really hot. With the efficiency ratings and input voltage I figure this should be around 20 watts x 2. In my car it will rarely need to put out that much.

Looking at the output waveform compared to the input waveform it looks very close except for what I can only describe as a slow return to zero after a hard hit. I saw this on the PDX1.1000 I tested (and cooked) also. It may be due to my scope, or methods. I can't hear it and I don't see it on the A/B amps I have tested. The source I am using today is not my normal M-Audio 1010, but an onboard sound chip from a different PC. It has a lot of noise so I am not going to base too much on this.



This is good - if it had not passed this test, it would not move into the car test.


Normally my next step would be to hook it up to the IOTA supply and a deep cycle battery and use it to drive (in this case) 2 DIYMA subs bridged and see how much power it can put out before clipping, and also how much current draw, how hot after an hour or so of almost max output (with miami bass), etc.

I am going to skip the high power testing in the house because in the car it will only be driving seas neo tweets and DLS IR3 mids active. Essentially this amp will never see a load if it works well enough for this application. Besides, my neighbors (who live about 15 feet away) always complain when I have duelling DIYMA's.

So my initial impression is: Pleasantly surprised.

Stay tuned for the in-car testing where I plan to hook it up (using relays on the outputs) to be able to switch it instantly between the XXK4150 and the PDX4.150. After being level matched of course. I may even not label the switch so I don't know which is which. 

If anyone knows of any damage I am likely to do by disconnecting the speakers from an amps outputs mid-music, let me know. There are no capacitors on the outputs, only speakers.

If this works I will be able to recover a good amount of trunk space (small car) and move my CG lower and forward a little bit.


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## CBRworm (Sep 1, 2006)

After running for a few hours with a light load it gets pretty darn hot. But since most of the heat is being generated by the power supply (I think), maybe it won't get any hotter under load. I am sure it is designed to tolerate high heat. I know the old V12 alpines used to get really hot as well.

I am getting used to the XXK's which get really really hot, but only when you play them really really loud.


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## ErinH (Feb 14, 2007)

Waiting to hear thoughts on XXK comparison, since everyone here seems to tout that as one of the better amps out.


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## Abaddon (Aug 28, 2007)

With the heat you are describing, would you be cautious to stack this on top of another 4.150 under the seat in your car?


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## CBRworm (Sep 1, 2006)

No, They are designed to be stacked so I am sure it would be fine.

I might be hesitant to stack them in a trunk with no airflow and temps well over 100f - but in the passenger cabin there will be airflow at the very least and potentially air conditioning to keep them cool.


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## CBRworm (Sep 1, 2006)

I too am waiting to hear my thoughts VS. the XXK. 

I don't doubt that the XXK is a more accurate amplifier (at least on paper), I am really interested to see if I can tell a difference in my car. I am hoping that I can't. I figure that the PDX should certainly not be the weak link in my system unless it does something audibly bad - but the specs are better than my ears.

Hopefully I'll hear something in the next day or so.


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## GlasSman (Nov 14, 2006)

Anxiously awaiting.....


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## Fellippe (Sep 15, 2006)

I've used the PDX for rear fill and center channel use for about 2 months now, and having heard it in my car and another, I only have nice things to say about them being a former Alpine V12 & JL 300/4 owner. 

This amp is clearly superior to those lines and is a great value. 

Combined with cool running operation, tiny footprint, and stackable capability, there's a lot to love about the PDX series.


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## CBRworm (Sep 1, 2006)

Ok, I have had it in my car for almost a day now. 

I was really impressed with the overall sound quality - I did not get the levels matched exactly with the XXK, but it sounded really good. . . If the sound was lacking anywhere at all it would be in the mid-range (around 1k). The highs where I expected it to be soft sounded crystal clear. The speakers it was connected to only do from 450hz up, so I didn't get to test mid-bass - but my testing in the house showed plenty of bass.

Again, let me say that I am pre-biased against full-range class D amps, so take this with a grain of salt - but I was really hoping that this would work - but it wont.

I can hear the amps power supply through the tweeters loud enough that I can hear it while driving slowly with the sound turned all the way down (at the deck). 

This fatal flaw in my eyes is brought on by a couple things.

First I am driving a pair of Seas NEO metal domes directly from the amplifier. They are fairly sensitive - pointed at my head, and they are playing the amplifiers power supply very clearly. It sounds like a turbo spinning up and down with load. So if I have a load on the back channels, the whistle in the front channels (tweeters) speed up. I suspect this is exacerbated by the fact that my gain settings are whacky. I listen to my music loud and clear. I have the gains set so the normal threshold of almost too loud is right around 20 on the W200/H701, but it will play without distortion up to right around 30 - but more than a couple of minutes and your ears will be ringing. 

On the PDX that works out to the Seas gain being just north of dead center, the the DLS IR3 mids gain maxed out. (On the XXK this works out to be just left of center and 3/4 - not that it matters in any way) So I suspect that because I have the gains set fairly high connected to a tweeter I am amplifying a lot of background noise. I am also picking up noise from my air conditioner fan. My car has a PWM fan controller and I can hear it through the tweeters as well. When I tested the amp in the house I heard the PS noise, but I thought it might be coming from my bench top power supply.

If it weren't for the high noise floor, I would use this amp. I suspect that running through a passive crossover the noise would be almost unnoticeable.

I may try it running my midbasses bridged. Right now they are getting 320 watts per channel from a bridged XXK4150 and I have a pretty good idea of how they sound, if the pdx does anywhere near that it would be sufficient.

I would not hesitate to use this amp somewhere else, just not in my car on my tweeters.

Once I get my scope back from my dad I will look at the noise and see what the frequency is and maybe I can make something to filter it out on the output, but for now it's going to be on the bench for a while.

Once again, I must tell you that as much as I wanted to like this amp, I was pre-biased against it before I even bought it. It probably would have had to be better than the best A/B amp out there for me to like it.


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## t3sn4f2 (Jan 3, 2007)

Hey CBR, this might be a dumb question but do you keep your tweeter attenuator at "0" on the H701 and do you think if adjusting all amp gains for the main volume listening range you have of 20-30 to 25-35 be enough to bring the hiss from the tweeter down to a point where it is not noticeable anymore?


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## MidnightCE (Mar 5, 2007)

CBRworm said:


> Ok, I have had it in my car for almost a day now.
> 
> I was really impressed with the overall sound quality - I did not get the levels matched exactly with the XXK, but it sounded really good. . . If the sound was lacking anywhere at all it would be in the mid-range (around 1k). The highs where I expected it to be soft sounded crystal clear. The speakers it was connected to only do from 450hz up, so I didn't get to test mid-bass - but my testing in the house showed plenty of bass.
> 
> ...


are you running an outboard processor? My PDX4.150 made the same nasty noise when i had an eq hooked up. I now have ARC KAR amps.


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## CBRworm (Sep 1, 2006)

I was running an H701 with it. I am planning on getting the 6 channel KS. I think that will solve all my problems including space.


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## CBRworm (Sep 1, 2006)

t3sn4f2 said:


> Hey CBR, this might be a dumb question but do you keep your tweeter attenuator at "0" on the H701 and do you think if adjusting all amp gains for the main volume listening range you have of 20-30 to 25-35 be enough to bring the hiss from the tweeter down to a point where it is not noticeable anymore?


The tweeter level is usually around -4 which gives me a little bit of room to bring it up on dull recordings. Moving the range up to 25-35 might get rid of some of it, but not enough. Besides, I do sometimes make it up to 35 now, just not for very long. If it were just hiss, I might be able to deal with it, but it is more of a variable pitch whine - even without the engine running.


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## ErinH (Feb 14, 2007)

I have the 4.100 on my tweets. Giving probably (if I had to guess) 50w to them. I don't notice any hissing.

The other 2 channels are on my midranges which are getting about 30w rms or so.


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## Guest (Oct 22, 2007)

couple random thoughts ...

- the idle current is no surprise ... the efficiency improvement to be expected with Class D is with _hot_ signals, not with _low_ signals  Given some basic housekeeping and processing functions, all amps will draw _some_ current with _no_ signal. In other words, the efficiency of _any_ amp at zero signal will be identically zero 

- at first, i thought that maybe the observed "slow return to zero" was simply the transient response of the LC low-pass filter on the Class D output stage. It wouldn't matter, because even if visible on a 'scope, the low-pass cutoff must be beyond 20kHz on a full-range Class D ... meaning, it wouldn't be audible. But then it occurred to me that this filter is in cascade with the _very_ sharp anti-imaging filters following the D/A conversion ... in which case, the transient response of an LPF _higher_ in frequency wouldn't be visible, with any singals recorded on a CD. In other words ... no signals recorded on a CD, and processed by the DAC anti-imaging filters, are "sharp" enough to excite any subsequent low-pass transient response. So i'm not sure what that might be ...


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## BTA (Nov 5, 2005)

I dunno man, you must really like stuff loud.

I don't think I've ever set gains higher than half on any amp I've ever owned or installed.


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## chadillac3 (Feb 3, 2006)

werewolf said:


> couple random thoughts ...
> 
> - the idle current is no surprise ... the efficiency improvement to be expected with Class D is with _hot_ signals, not with _low_ signals  Given some basic housekeeping and processing functions, all amps will draw _some_ current with _no_ signal. In other words, the efficiency of _any_ amp at zero signal will be identically zero
> 
> - at first, i thought that maybe the observed "slow return to zero" was simply the transient response of the LC low-pass filter on the Class D output stage. It wouldn't matter, because even if visible on a 'scope, the low-pass cutoff must be beyond 20kHz on a full-range Class D ... meaning, it wouldn't be audible. But then it occurred to me that this filter is in cascade with the _very_ sharp anti-imaging filters following the D/A conversion ... in which case, the transient response of an LPF _higher_ in frequency wouldn't be visible, with any singals recorded on a CD. In other words ... no signals recorded on a CD, and processed by the DAC anti-imaging filters, are "sharp" enough to excite any subsequent low-pass transient response. So i'm not sure what that might be ...


What's interesting is Leon didn't mention anything about such a slow decline and he was using a Fluke portable scope-meter.


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## CBRworm (Sep 1, 2006)

I just typed a page long reply . . . the server was too busy and it's gone. . .

Anyway. to sum it up I said that the effect isn't audible, but if there is a rythmic beat playing into two amps with two DIYMA's side by side. The PDX one looks like I am playing a record without the subsonic filter on. The speaker is tight during the beat, then just kind of floats. The other amp, the speaker is always tight. I saw this with both the PDX1.1000 and the 4.150 

Maybe when there is no input the outputs either short (causing the speaker to return to center slower) or the outputs float down - which is what it looks like. 

I tried to capture it on video, but it is not very easy to see. Maybe if I could figure out how to host the video's I would post them somewhere (other than youtube)

My other thought was that maybe the AB amps I have do in effect have a subsonic filter at 5hz or so and don't reproduce DC signals - whatever it is could be coming from my source and not reproduced by the AB amps.

I don't know - it doesn't really matter - they work fine for their intended purpose, it is just different than what I am used to.

My scope is a Tektronix TDS2002 (which is currently at my dad's), my last scope was a Tek TDS220. Both of them could see the 'float' but I had the scopes hooked up at the terminals on the speaker, which could very easily be reading voltage produced by the speaker when it is 'floating' back to center if the amps outputs are essentially open. I did verify that with a DIYMA hooked up to a scope I can see the waveforms for other music playing in the room.


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## Guest (Oct 23, 2007)

No question ... most audio amps do not have a flat bandwidth all the way down to DC. Any coupling capacitor in the signal path will create a high-pass filter. So maybe what you are observing, as you have suggested, is a high-pass transient artifact ... about how long does the signal take to return to zero? Longer time indicates lower frequency, so this may very well be it ...


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## bdubs767 (Apr 4, 2006)

So then what you guys are saying the pdx amps may have natural roll off on the top end?


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## CBRworm (Sep 1, 2006)

I didn't notice any high frequency roll off. It sounded good to me up there, but I didn't look at it on an analyzer.


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## Guest (Oct 24, 2007)

bdubs767 said:


> So then what you guys are saying the pdx amps may have natural roll off on the top end?


only if Alpine is lying about their specs :

http://www.alpine-usa.com/US-en/products/product.php?model=PDX-4.150&lang=en&tab=F


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## npdang (Jul 29, 2005)

I've measured the response, it's flat 20hz to 20khz. I believe it was the Pioneer ICE amps that had a rolloff up top.


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## bdubs767 (Apr 4, 2006)

npdang said:


> I've measured the response, it's flat 20hz to 20khz. I believe it was the Pioneer ICE amps that had a rolloff up top.


any comment on the noise that has been related to the power supply dang?


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## mazzasec (Oct 25, 2007)

good review!


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## DearS (May 14, 2005)

So is the PDX on par with the ARC?


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## CBRworm (Sep 1, 2006)

I was very impressed overall with the PDX, but in my case the high frequency whistle (possibly related to my processor?) the PDX did not turn out to be a suitable replacement for the XXK. 

It is a very impressive little amp though.


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## oneiztoomany (Oct 1, 2007)

i am not nearly as technical as many of the posters on this board. however i do have both a PDX 4.100 AND a PDX 1.1000 and do not notice any unwanted noise like you are mentioning.

i hear every little thing and am obsessed with getting rid of unwanted noise so trust me i would notice it.


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## DearS (May 14, 2005)

CBRworm said:


> I was very impressed overall with the PDX, but in my case the high frequency whistle (possibly related to my processor?) the PDX did not turn out to be a suitable replacement for the XXK.
> 
> It is a very impressive little amp though.


Thank you for sharing your opinion.


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## Suneet (Jun 19, 2007)

I'm having noise issues with my tweeters using my PDX 4.150 and DCX-730. Previously, I had the tweets powered passively with an x-over and didn't have any noise issues.

Could the PDX be contributing to the noise?


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