Class A for home use

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A

anli

Member
Joined 2004
gaydenko.com
Hi,

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.

Andrew
 
jacco vermeulen

jacco vermeulen

Member
Joined 2004
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 edu.ru 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 ?
 
A

anli

Member
Joined 2004
gaydenko.com
Jacco,

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
output,
- 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".

br,
Andrew
 
jacco vermeulen

jacco vermeulen

Member
Joined 2004
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.
 
jacco vermeulen

jacco vermeulen

Member
Joined 2004
Another suggestion :

Norman E. Thagard's design published in Januari/Februari 1995.

Also a modular design, with dual powerlines for the front and the output stage, with bipolar/Mosfet cascode output stages
(MJ15024-MJ15025-IRF511-IRF9521)

Intended for 100 watt Class A, but also with separate voltages for the voltage and current amplifier. The voltage and bias of the output stage can be lowered to the desired need.
This amplifier can drive just about anything, the output has a maximum dissipation of 3500 watts.
In musical terms it does not rank highest on the list of amps i built.

I built a number of Class A push pull Mosfet amplifiers with dual powersupplies.
In my view the obvious choice, the total cost for the powersupply is lower, dual powersupplies have a natural separation and add to stability.
 
A

anli

Member
Joined 2004
gaydenko.com
zinsula

zinsula

Member
Joined 2004
Check out this one:

http://www.diyaudio.com/forums/showthread.php?postid=334963#post334963

(the zip file at the end of the post)

balanced, symmetrical, no feedback, MOSFET output, etc.

Maybe one wants to use other output devices, but replacing them doesn't seem to difficult. And you may bias them up to the level you want. If it has too much power, you may reduce the voltage of output stage rails....:cool:

Ciao, Tino
 
A

anli

Member
Joined 2004
gaydenko.com
zinsula

zinsula

Member
Joined 2004
anli said:
Tino, thanks! It is interesting for me, as the schematics resembles me my
own (deprecated by me: my view to audio design is shifted now slightly) project:
Click to expand...

You're welcome. Why did you change your view of audio design, and in what direction exactly? At least, your project was symmetrical, balanced, had mosfets....and feedback.

Ciao, Tino
 
A

anli

Member
Joined 2004
gaydenko.com
zinsula said:


You're welcome. Why did you change your view of audio design, and in what direction exactly? At least, your project was symmetrical, balanced, had mosfets....and feedback.

Ciao, Tino
Click to expand...

Tino (well, is "Tino" your name? :),

After reading plenty of (often disputable) articles, now I think:

1. each stage (input-gain-driver-output in common topology) _must_ have
own clean spctrum. Saying "clean spectrum" I mean quick lowering of
harmonics (something like 40db between 3d and 5th, for example).
Here I can accept the case when first even and first odd harmonics may be
even about -(40-60)db ;

2. global NFB commonly brings more difficulties rather advantages (due to needed
frequency correction each active part is not used as well as it can; global NFB
lag; as a reslt - harmonic spectrum is far from state-1); more advanced
designers may add plenty of info about other global NFB surprises, but see
this post end words :)

3. global NFB introduces own distrotions, which are not commonly measured, but
our ears listen them well. As the aim is an amplifier for home use, I don't
want an amplifier wich is excellent for market, I want an amplifier to be excellent
for my and my friends music emotions.

There are many threads here concerning listed states. I'd like to emphasize,
I'll be happy this one will be about _concrete_ schematics designs rather
unending theoretical disputes :)

Let's try!
Andrew
 
M

Marc Vi

Member
Joined 2004
suplifier.nl
I’ am designing a fully discrete balanced symmetrical DC coupled class A amplifier with no global feedback using transistors (but Mosfets are also possible). A DC servo (with opamp) is possible but not necessary. The circuit will not be complex and will be rather easy to build. The costs will be fairly low, depending on the parts you use. The power will be about 35 W at 8 ohm.

I will publish the circuit in Christmas holidays.

Marc.
 
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