SAE A205 amp, mosFET?

[SirPaulGerman]

I was offered a Sae A205 amp, there is not much info out there, plenty of power, looks like a Acurus or Adcom mosfet amp.
but I am not sure.

Is this a Mosfet amp ?
what are the bias number without a service manual ?
Is the design any good ?

100wpc Class A/200wpc Class AB1.
4x Toshiba 2SA1302, 4 x 2SC3281 each side

thanks for your help

 

[adason]

Not a single mosfet in it.

 

[SirPaulGerman]

Is a very simplistic amp with a lot of power

 

[adason]

Definitely very powerful classAB amp, but never 100 watts in classA. Thats nonsense. Maybe a watt or two.

 

[SirPaulGerman]

What are the numbers to adjust the bias ?

 

[planet10]

From my SAE experience in the old days, i would look at it as a parts supply for a better sounding amplifier.

dave

 

[SirPaulGerman]

What are you suggestions for new boards ?

 

[planet10]

Depends on the power supply and heat sinks available.

dave

 

[SirPaulGerman]

I did some research about the amp, according to guy from audiokarma, here are pseudo service manual by Motorola

http://www.cieri.net/Documenti/Altr...eband-Low Feedback Design (AN1308 - 1992).pdf

see fig 26 and and 35

I not very tech savvy, what is the roll of Motorola on the design of the amp ?

 

[Extreme_Boky]

You need to read the whole article. It's actually very good.

They designed two amplifiers around their (famous) BJTs...

 

[SirPaulGerman]

Did he designed the A205 with the same philosophy ?
are fig 26 and 35 the schematic pretty much ?
Not much info out there
What do you make of the design ?

 

[adason]

Is a very simplistic amp with a lot of power

where did you get that from?

 

[SirPaulGerman]

The way it looks and some comments from audiokarma

 

[adason]

well, so far you were wrong about mosfet...
you were wrong about 100 watts classA...

 

[SirPaulGerman]

Your welcome to review the amp ?
Not a very tech person, that's all

 

[ostripper]

Yet another "leach" amp ( fully complimentary).

Strengths =
- Very good PSRR , they cancel supply ripple natively.
-simple and easy to understand with simple compensation - very stable.
Weaknesses = 3rd and 5th distortion products and will always require offset adjust
and-or P/N matching.
Below is the simple version of this topology , used in most A/B subwoofer plate amps (80% of them). Still .01% THD.
Dayton plate amps are this circuit.

 

[SirPaulGerman]

thanks for the review of the amp, got it for $75 plus shipping
How do you adjust the bias ?
There is a switch is the back for 100 watts and 200 watts, what is the difference ?

Some pic of the amp


http://i462.photobucket.com/albums/qq350/62sunbeam/67b7a04c.jpg

http://i462.photobucket.com/albums/qq350/62sunbeam/4e5162ae.jpg

http://i462.photobucket.com/albums/qq350/62sunbeam/f342b23d.jpg


http://i462.photobucket.com/albums/qq350/62sunbeam/de983236.jpg

http://i462.photobucket.com/albums/qq350/62sunbeam/6fe2405a.jpg

 

[ostripper]

Good deal , the case and the amp for 75$ ... I'd buy it. Trafo alone goes for 100$+

Bias , first test one of the emitter resistors (big grey ones). 20mV would give you @40mA per device.
I don't see any trimmers for either offset or bias on any google picture of this amp.
What were they thinking ??
Board layout looks different from either schematic.
Zener biased LTP's , no ripple eaters , no adjustment trimmers offset by top quality silver mica caps and a beefy
power supply ???? strange.

OS

 

[SirPaulGerman]

No bias adjustment ?
what about the switch in the back ?

 

[ostripper]

Looking closely at the pictures , 8 trannies in the middle are the LTP's and cascodes (+/-). can see the 2 current sources (2 trannies all alone at the top).
No large driver transistors ... it might be the circuit I posted driving LATFET outputs.