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Old 4th May 2009, 09:27 PM   #1
diyAudio Member
Join Date: Nov 2004
Location: Rome
Default rotel ra 820

Hi to all, I'm putting my hands on a Rotel ra 820 for an upgrade, especially passive components. I'd like to transform the unnecessary (for me) "mono" switch in a direct-tone defeat control.

It seems that the schematics of this early model is quite hard-to find. I've found the Ra-820b one that is sufficient for the power amp (very similar, apart from the doubled power transistors), but for the line path I'd need an help.

Anyone there??

p.s.: someone tried already to bypass the 0,22 Ohm output series resistor? How does the amp works? It remains stable?
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Old 11th May 2009, 12:05 PM   #2
diyAudio Member
Join Date: Nov 2004
Location: Rome
no replies, no information googling... it seems this amplifier has a very low popularity...

Well, I decided to go on by myself, and this is what I found: it seems the line path is formed by a discrete three-transistors signal amplifier and a passive tone control circuit.

There are a couple of question:
I'm not familiar with electronic theory, so I'm not able to calculate the amount of gain of the line stage (I want to subsitute it with a good op-amp). It isn't really a problem, I can try different solutions until I find the right one, but maybe the schematics can be useful for other friends, just let me know and I'žll draw and post it.

The second question regards the tone control circuit: it seems a classic Baxandall net, but the treble control is a little bit different, instead of the classic two capacitor that carry the signal to the potentiomenter, it show a completely linear net (a simple r-pot-r ciruit) with only one cpacitor conncted to the central pin, where the signal is taken (I hope I've explained it well...)

Might I have an help to better understand this kind of circuitry?

Hello to all

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