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Quick measurements of Pass Aleph 1.2 monoblock amplifier

restorer-john

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I suggest the 60 Hz may be old main bank caps. Big old cans life span is not that long. Just re-did my old amp. Audible hum to just detectable at the noise floor. Can't hear a thing at the speaker or feel with my finger. It is only a 60W amp and I used 4 x 6800 on each rail, so not excessive.

Not denying you may have had some poor condition capacitors in this case, but to state 'big old cans life span is not that long' is just misinformation. I have plenty of 30-50 year old amps well within original spec- with original caps.

4x6800uF per rail is an awful lot (27,200uF+27200uF) for any amplifier, letalone a "60W" amplifier.

I have some big 200+200W/350+350W amplifiers with 22,000uF to 44,000uF per rail and that is overkill.
 

tvrgeek

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I wonder, it seems my testing correlates with the work from Hiraga and Wigman on the amplifier end, but leaves of what I found on the effect on drivers. This is something that might be extrapolated from the classic static SNAID testing.

Hiraga's conclusion may have more to do with the speakers available at the time than the actual profile of the harmonics. The better the speaker with respect to harmonics exciting resonances, maybe the effect is less. For those who insist no one can hear any difference in amplifiers, maybe if we had a "perfect" speaker, we would have achieved that goal.

I am no genius. I was led down this path by John Curl. In his usual help, he provides no answers, but points you to the issues and lets you rediscover for yourself. Credit belongs to him. A good teacher as you remember what you discover better than remember what you are told.

I believe the key to audible differences in amps both dynamic performance of the amp as well as interaction with the load, on the amp but more importantly to the load. These do not show up in any static fixed load test. As we have to talk about speakers, the levels of distortion are well above where the peer reviewed papers have identified our level of detection is.
 
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pkane

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I suggest the 60 Hz may be old main bank caps. Big old cans life span is not that long. Just re-did my old amp. Audible hum to just detectable at the noise floor. Can't hear a thing at the speaker or feel with my finger. It is only a 60W amp and I used 4 x 6800 on each rail, so not excessive.

Maybe, but there’s no hum even with my ear to the speakers (88dB/W/m) and you can see the level is way down in the noise residual. Disconnecting the DAC also lowers the 60hz component.

There is a tiny hiss from the tweeter that I can only hear when my ear is right next to it, but not from a few inches away.
 

tvrgeek

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Not denying you may have had some poor condition capacitors in this case, but to state 'big old cans life span is not that long' is just misinformation. I have plenty of 30-50 year old amps well within original spec- with original caps.

4x6800uF per rail is an awful lot (27,200uF+27200uF) for any amplifier, letalone a "60W" amplifier.

I have some big 200+200W/350+350W amplifiers with 22,000uF to 44,000uF per rail and that is overkill.
I disagree with all you have said. Sorry.
I spent many years in failure analysis so I understand the life cycle of wet electrolytic caps pretty well. It seems the bigger the can, the more susceptible they are to drying out. Many very old amps the caps are dry paper and do not age. The industry standard for large electrolytic is about 7 to 10 years. Heat is the critical factor. The higher the ripple current, the warmer. Where stored can make a difference. Six months on the top of a high-bay in Colorado, they are toast. FWIW, the 4200's in my 20 year old preamp were not to spec, but the 5200's in my little integrated are just fine. If an amp is never used, and sits in AC. it will last far longer.

On overkill. Absolutely not. 4 x 6800, not 68,000. So 27,000uF total per rail. I am not talking about ripple, but in maintaining the rails during transients. This makes very big audible differences. It seems Rotel shares your view and this may explain why they are totally lifeless to listen to even though their architecture is very good.

I offer this possibility: The bank size not related to the amp power, but to the dynamic current needed. If I only need 60W without clipping, it does not matter if the amp can produce 300, the dynamic current is the same. So, how big a rail supply is has more to do with the power required to not droop the rails, not power available. I have a suspicion this is the primary reason so many find big amps better sounding than small. It is the dynamic current. If you look at the best of the smaller amps, you will find proportionally larger banks. ( There are a few other things too) Also remember, for the same uF, if you double the voltage, you double the coulombs stored.

Besides, in my amp, the ripple to the output side is more than 110 dB down. I use a pi-filter to the VAS and IPS so I get another 10 dB there. Good enough.
 
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pkane

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I disagree with all you have said. Sorry.
I spent many years in failure analysis so I understand the life cycle of wet electrolytic caps pretty well. It seems the bigger the can, the more susceptible they are to drying out. Many very old amps the caps are dry paper and do not age. The industry standard for large electrolytic is about 7 to 10 years. Heat is the critical factor. The higher the ripple current, the warmer. Where stored can make a difference. Six months on the top of a high-bay in Colorado, they are toast. FWIW, the 4200's in my 20 year old preamp were not to spec, but the 5200's in my little integrated are just fine. If an amp is never used, and sits in AC. it will last far longer.

On overkill. Absolutely not. 4 x 6800, not 68,000. So 27,000uF total per rail. I am not talking about ripple, but in maintaining the rails during transients. This makes very big audible differences. It seems Rotel shares your view and this may explain why they are totally lifeless to listen to even though their architecture is very good.

I offer this possibility: The bank size not related to the amp power, but to the dynamic current needed. If I only need 60W without clipping, it does not matter if the amp can produce 300, the dynamic current is the same. So, how big a rail supply is has more to do with the power required to not droop the rails, not power available. I have a suspicion this is the primary reason so many find big amps better sounding than small. It is the dynamic current. If you look at the best of the smaller amps, you will find proportionally larger banks. ( There are a few other things too) Also remember, for the same uF, if you double the voltage, you double the coulombs stored.

Besides, in my amp, the ripple to the output side is more than 110 dB down. I use a pi-filter to the VAS and IPS so I get another 10 dB there. Good enough.

There's no indication is due to caps, and appears to be a simple (but hard to isolate) ground loop. The level of 60Hz wouldn't go down 15-20dB when I disconnect the DAC if the caps are leaking, but it did. When I measured residual noise, the level went down to around 1 micro-volts, wouldn't happen if caps were the problem.

Pass Aleph 1.2 has about 0.1F total capacitance, by the way.
 
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pkane

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@pkane

Do you know what this is? ~16.5kHz?

View attachment 179932

I get one at 16kHz here. Is it a USB packet 'harmonic' or something?
Looks to be a bit above 16kHz, so not really sure, but this wouldn't be USB packet noise then. I also noticed it, but didn't try to chase it down since it was at such a low level.
 

pma

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So 27,000uF total per rail. I am not talking about ripple, but in maintaining the rails during transients. This makes very big audible differences.

... Hmm, like 1dB usually. I suppose you have some proof of your claims.

A250_burst_1k.png


50Vp/4ohm with burst, 312.5W equivalent sine power into 4ohm.

With continuous sine, clipping is +/- 46Vp. So we get about 1dB.

Capacitance overkill results mainly in higher mains related noise, due to electromagnetic field radiated during charging current peaks into the capacitor bank. Look at JC5 measurements and you will see.
 
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pkane

pkane

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I decided to model this by mixing together 500Hz and 1kHz (reduced by 40dB) signals in Audition. If I arrange the two waves so that the positive and negative peaks of the 500Hz signal align with the negative peak of the 1kHz that seems to match his description of 'negative phase':
View attachment 179914

Feeding this into REW's RTA gives a 2nd harmonic distortion phase of 82 degrees (I probably didn't match the peaks perfectly). Doing the same but with the 500Hz peaks aligned with the positive peak of the 1kHz signal gives a distortion phase of -90 degrees.

If I align the waves so that the down->up zero-crossing of the 1kHz signal aligns with the peaks of the 500Hz signal like so:
View attachment 179919
then I get a distortion phase of 173 degrees (a less than perfect alignment again), similar to your result here. Aligning the peaks of the fundamental with the up->down zero crossing gives a distortion phase of 0 degrees.

Of course, the little diagrams he uses to define the term shows waveforms whose alignment falls somewhere in between these two options, but it does seem closer to my initial guess with the coincident peaks. So maybe he hadn't come up with the 'negative phase' idea when he designed this amp or the distortion phase changes significantly across the spectrum (perhaps more likely given the 33 degree change you saw from 1 to 5kHz).

Don't know if Pass was still trying to discover his "negative phase" rule-of-thumb with this amp, but I do have to admit (fully subjectively speaking, of course) that imaging with this amp + speakers + room + EQ is unbelievably good :)

I also wrote software that can generate tunable distortion (with phase), so at some point I'll try it on the little, squeaky-clean PA5 to see if I can hear the difference.
 
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charleski

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I found the differences in the general "slope" of distortion between Dominant Pole and Miller compensation to be the audible difference between two modified Rotel amps. As regaining the same level of stability required other small changes that effected the speed, they may also effect the phase of both fundamentals and harmonics.

Is there a reference where Mr. Pass talks about this?
Apart from the Positive Feedback article I linked there are a couple of other resources where Pass talks about his general approach:
Old Stereophile interview from 1991
PassDIY article from 2008
Neither of these older articles mentions the ‘negative phase’ idea, so it may be fairly new.
 

pma

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Conjugate in the complex plane shows negative phase angle. Nothing in common with audio, however. Negative angle refers to opposite direction of rotation of the vector.
 

restorer-john

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On overkill. Absolutely not. 4 x 6800, not 68,000. So 27,000uF total per rail. I am not talking about ripple, but in maintaining the rails during transients. This makes very big audible differences. It seems Rotel shares your view and this may explain why they are totally lifeless to listen to even though their architecture is very good.

27,200uF per rail is excessive for a 60wpc amplifier.

Not sure why you want to bring up Rotel, but here we are. So, let's talk about them. Their designs are solid and always have been. Their ratings are always conservative, especially in the all important power output and particularly into low impedances. They have quality transformers, more than adequate filter capacitance and also ensure there is plenty of current capability in paralleled silicon- something a lot of other manufacturers cheap out on. Doesn't matter how much energy you have potentially if you can't get it to the load without excessive losses.

They offer good value for money, are extremely reliable and the company makes HiFi and nothing else. Privately owned and successful.

Rotel do however make a habit of bandwidth limiting in my experience, especially at the low end (LF). That may be why you think they sound lifeless? I've got a few Rotels here, medium powered integrateds and small pre/power ones I picked up over the years. Lovely things. One gets almost daily use for TV/Movie sound, along with a standalone Rotel D/A converter taking SPDIF (mixdown to PCM 2.0) from the TV. It's running a pair of Mordaunt Short floorstanders and I have not felt the need or desire to mess with that system for nearly 10 years. (Amazing for me). ;)
 
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