Simple upgrade for Rotel RA-01 amp.. - diyAudio
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Old 14th December 2004, 03:52 AM   #1
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Default Simple upgrade for Rotel RA-01 amp..

A friend left a Rotel RA-01 with me . I opened it up and discovered that it looks very similar to a Rotel RA970BX .
There is a preamp with an OPA2604 with a gain of about 4 or 5 folllowed by the power amp. The volume control is right at the input of the opamp. I found that he opamp had an elco at the input and output and the power amp had one at its input.
Both suspect locations.
So I took out the power amp input cap and replaced it with a box type 2.2uF PP film cap (250v). I did not have a lower voltage unit.
This did make a difference. So then I shorted out the elcos on the preamp . I did check to see what the dc voltages were at the input and output of the opamp. The input was lower than my DMM scale and the output was -1.6mV (L ch ) and -4.4mV ( R ch).
This actually also goes to the preamp output at the back of the amp.

The new input cap probably needs some burning in but the initial impression is much improved bass , better sounding transients and much better voice reproduction. Treble has also improved a lot - it is much more musical than earlier. Bass extension has also improved considerably.

If anyone is interested I could post pics of the mod though this might not be required to do this simple mod.

Incidentally it is easy to convert this amp to a power amp ( no input gain stage ) but with volume control still active. Or one can configure it to work as a power amp with inputs at the "preamp out" sockets and everything else out of the way. Input impedance will be about 47K ohms.
A great amp to play around with - easy access to all parts.

Might be possible to tweak the power amp a bit. Must look at that later . No time right now.

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Old 14th December 2004, 12:09 PM   #2
Hennie is offline Hennie  South Africa
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Join Date: Nov 2004
Location: Pretoria
I am also into tweaking Rotel equipment. I am not familiar with the RA-01. Were those electrolytics Black Gates?

You may still find that the pre-amp uses an electrolytic in the series feedback path. If so you can probably short out that one too provided that the equipment you connect has no or small DC offset.

The electrolytic in the power amp series feedback circuit is more difficult to get rid of. You can look at the suggestion I made in the following thread:

Eliminating capacitor in amplifier series feedback loop

I used that technique with other amps including Rotel, but with some there is a shunt compensation cap in the feedback network so be careful.

How about upgrading the OPA2604?
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Old 22nd June 2015, 08:11 PM   #3
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I know this is an old thread but do you still have the pictures of the changes to the rotel amp? I own this amp but feel it would benefit from a bit more at the bottom end.
It would be my first go at this sort of thing so any advice would be much appreciated.

Thank you.
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