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tmblack 5th March 2005 07:25 PM

What do uncompensated amps behave?
My power amp does not oscillate but if sounds overly crisp and detailed
in a bad way.
I used a current mirror instead of resistors in a LTP.

What tests can be applied to test the compensation of an amp?


anatech 5th March 2005 07:49 PM

Feed the amp a 1KHz square wave and 'scope the vas stage. Actually, you can look at the waveforms throughout the amp. In a marginally stable amp, you may see ringing or low oscillation on the waveform. You should have a load on the amp and you can use a low level. Watch you don't burn out the zobel network if so equipted.


tmblack 6th March 2005 06:13 AM

Scoping the 1kHz signal at the output with a load looks fine with no
There is no inductors at the outputs, perhaps this is needed.


Graham Maynard 6th March 2005 07:35 AM

Hi Tom,

Do you use a small value 'C' in parallel with the NFB sensing resistor from the output; typically about 22pF for 10k.

This can reduce 'crispness' but the value should really be determined on an individual amp. circuit basis, either trying a handful of different small value components, or by using an insulated air spaced SW variable and then externally measuring it for your chosen preference.

Cheers ......... Graham.

anatech 6th March 2005 03:51 PM

Hi Tom,
Go back, back into the amp .... 'Scope the vas and other stages to see what's going on inside. You can then tell if you tend to overdrive the vas if you turn it up some into a dummy load. I'm sure you don't feel like listening to tones up high. That & you'd take out mids and tweeters doing that.

GRollins 6th March 2005 09:12 PM

There are numerous possibilities here, but your comment that you used a current mirror for the differential is interesting. I avoid using active loads whenever possible because they tend to produce an etched sound. Those who think measurements are the be-all, end-all love current mirrors and other, similar circuits. Those who actually listen to their systems tend to view them with a great deal of suspicion.
The usual reason people use an active load, whether current sources or their cousins, current mirrors, is to get as much gain as possible out of a stage. That's frequently because people intend to turn around and burn off that gain as negative feedback. High rates of negative feedback are also associated with poor sound quality.
Inevitably, someone will pop up at this point and start screaming about how it lowers distortion, etc. Your proper response to such a post is to ask them how often they listen to live, unamplified music. Amplified music doesn't count--it has to be unamplified, elseways you're listening to the PA system, and PAs make lousy stereos. Technical perfection that doesn't sound like real music isn't perfection, is it?
If you can't find evidence of actual misbehavior in the circuit, you might try using a resistive load and a lower rate of feedback. It might, possibly just cure the problem.
I'll back out now and let the measurements uber alles crowd fill up the next fifteen pages.


AKSA 6th March 2005 10:59 PM


I categorically and unequivocally agree with all you wrote......

My experience too.....



tmblack 6th March 2005 11:12 PM

I scoped at the VAS collector and it looks just great but turning off the power to the output stage did make it spike a bit.

I decided to add some CC across the collector and base of the VAS.
It now sounds appropriately balanced.

Greg, I'm really glad you posted about current mirrors.
How about using cascode transistor loads like in the Leach?

Where do you find live music without PA's these days?
Even acoustic instruments are amplified at concert halls.


lumanauw 7th March 2005 12:17 AM

2 Attachment(s)
Hi, Grey,

This is a never ending story, between listening argument and measurement argument:D
Have you ever look to Electrocompaniet schematic, like AW120? It's quite complex, using mirrors, but its something. The RE degeneration are big everywhere, quite strange, why one use current mirrors but put big degeneration at the same time?

andy_c 7th March 2005 12:37 AM

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