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anli 13th December 2004 12:33 AM

Class A for home use

I see, the question is too common, nevertheless I'll try :-)

Please, suggest a _musically_checked_ schematics for power amplifier:

- with approximately 20-30W power peak (8 Ohm load),
- balanced (bridge) symmetric topology is preferable,
- MOSFETs in output stage are preferable,
- without capacitors and transformers in signal path,
- global NFB absence is preferable.


kilowattski 13th December 2004 01:22 AM

Try the Pass Labs Forum. Maybe an Aleph or a Zen amp.

anli 13th December 2004 01:56 AM

As you can see, I have mentioned "balanced symmetrical" topology,
as one of the aim is to make an amplifier more power-effective.

Such topology is not "Nelson's path", I think :-)

AndrewT 13th December 2004 07:34 AM

could the front end of a Leach meet part of your requirement?
regards Andrew T.

kilowattski 13th December 2004 07:51 AM

Are you not looking for a class A amplifier? Efficiency and class A are not usually used in the same breath. The Aleph X is the closest I can think of.

jacco vermeulen 13th December 2004 08:52 AM

Clearly you are not interested in single ended topologies because of the low efficiency.
Meaning you want a push-pull output stage.

Secondly, you have no preference for a balanced design or amplifiers in bridged mode, as long as it has a balanced entrance.

That means practically any low power Mosfet push-pull design will do with the following constraints :
- A symmetrical circuit.
- DC coupled.
- No global feedback.

The latter is arguemental, for the last 20 years audio designers are talking local feedback.

As that still leaves a number of possible amplifier designs you may need to make a couple of choices more :
- a fully discrete circuit ?
- without a DC servo ?
- Easy to build ?
- Maximum cost ?

For example:

If you do not object to opamps in a circuit and a opamp DC servo,
with a couple of IRF630/IRF9630, a 5534 opamp and a OP07 for the servo it is possible to build a reliable amplifier fast,
the circuit components are probably less than $10 per channel.
There are a number of circuits like that around with proven results.

!!!! Just an example, of course there are way better components, i just mentioned a couple of cheap components !!!!

Other designs may call for selecting the components, or the need for a scope to measure and tweak the circuit.
For simple designs downloading a small scopefile for the pc soundcard from your fellow russian at and a simple tonegenerator will do.

Important questions may also be :
- What is in front of the power amplifier ?
- What kind of speakers does it need to power ?

anli 13th December 2004 11:13 AM


Originally posted by kilowattski
Are you not looking for a class A amplifier? Efficiency and class A are not usually used in the same breath. The Aleph X is the closest I can think of.
Saying "symmetrical balanced" I mean push-pull balanced topology, wich has
much more efficiency rather SE or something like SuperSymmetric has.

anli 13th December 2004 11:34 AM


I'll try to clarify:

- as I have said, I mean push-pull balanced topology with balanced input (I have
some ideas about discrete balanced line drive, but they are rather young :-),
- I'd prefer global NFB absence, but 5-6db feedback (to set output shifting and
output dumping factor) is not rejected at all,
- fully discrete,
- without DC servo; I'm not afraid of 100-150mV shifting :-) between balanced
- two PS are possible, for example stabilized +/-50V for all except for output stage
with single 30-40V PS,
- hard to build :-),
- cost... OK, I can say "not too expensive". At any case an amplifier sinks,
box and transformers will have the main cost, I think,
- I have a scope, a signal generator, a software to see FFT ans so on on PC,
- speakers have standard design.

Probably you can see, my English isn't perfect. Please, explain me these words
meaning: "front of the power amplifier".


PMA 13th December 2004 12:59 PM

I was thinking about similar circuit you are speaking about, but did not find time enough to realize - maybe later.

jacco vermeulen 13th December 2004 01:03 PM

Hi, Andrew.

With "in front" i mean what pre you intend to use with the power amplifier.

I think you should take a look at Mr Erno Borbely site.
He did a modular balanced symmetrical Class A Mosfet power amplifier in the '90s.

One of the versions is DC-coupled.
In Class A it does 50 watts, if you'd like it less lower the bias or the voltage.

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