Acurus A200 schematic here

[hags]

I haven't posted in quite awhile. I thought maybe someone besides myself could use this.



Maybe someone can tell me why these amps are notorious for ear shrieking treble gain.

[mlloyd1]

don't know if i can answer your question, seeing as i don't recall ever listening to one of these amps. however, i'm puzzled by something i see on the schematic: regarding the diff amp input parts, (2SA798 and 2SC1583), i thought those were 5 pin devices that did not offer access to the individual emitter connections, so i'm wondering how/if those 10 ohm emitter degeneration resistors are really connected.

mlloyd1

[bobodioulasso]

What are Q5 and Q12 for?

[jaycee]

Q5/Q12 would protect the VAS from current overload if a VI limiter triggered - there isn't one shown on the schematic though.

[Chamberman]

I think most of the reason for the treble gain you speak of is the relatively low amounts of bias the Acurus products are set for. I would think that upping the bias voltage beyond the factory specs, as long as the heatsinks can handle it, would help to alleviate this. The addition of some CRC filtering in the power supply couldn't hurt either.

Here is the Mondial bias voltage setting for their products. Interestingly enough the Aragon 004 series original bias setup procedure says to set them for 10 mv and not the 6-8mv that this procedure calls for.

[hags]

Originally Posted by Chamberman

I think most of the reason for the treble gain you speak of is the relatively low amounts of bias the Acurus products are set for. I would think that upping the bias voltage beyond the factory specs, as long as the heatsinks can handle it, would help to alleviate this. The addition of some CRC filtering in the power supply couldn't hurt either.

Here is the Mondial bias voltage setting for their products. Interestingly enough the Aragon 004 series original bias setup procedure says to set them for 10 mv and not the 6-8mv that this procedure calls for.

I have not heard this with regard to other amps with similiar problems.
I was not aware that an increase in bias current will affect frequency issues.

I am open to trying this. Where am I measuring the 6mv at or across???

[Chamberman]

Increases in bias voltage have many times been attributed to a warmer sound, not to mention it warms the heatsinks up and subsequently your room.

The voltage readings are taken across the emitter resistors (.5 ohm/2 watters). If you do not have a way to measure the heatsink temps, if you can hold your hand on the thicker baseplate area of the heatsink you are probably okay.

[hags]

Ahhh, ok,
After a long warm up, the amp has been on all day. I read 2.3mV on one channel and 3.1mV on the other. Probably not good.

I have adjusted the bias as advised and now have 6mV on both channels now.
I will periodically check the bias over the next 24 hours and then screw the top back on.

Chamberman, thank you, again, for the bias info!

[Chamberman]

Wow that is low. Definitely keep an eye on the temps. Its possible your meter reads a little low too! The heatsink temp will tell you if your getting too high on the bias. You'll have to report back on how it sounds after the change.

[Chamberman]

Hags, I am interested in something. I have a schematic of the A250 and it is a clone of the one you posted. Even the rails are the same +/- 85 vdc. Did you ever measure the rails on that thing. I'm interested in whether they are in fact +/- 85 vdc. I'm thinking that the A200/A250 amps are the same with the only difference being the rail voltage. I'm guessing the rails are probably really +/- 70 vdc on the A200, this would match up with the Aragon 8000 series which put out 200 watts per channel and used very similar circuitry.