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

Cheers,
Ashok.
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Old 14th December 2004, 12:09 PM   #2
Hennie is offline Hennie  South Africa
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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:

http://www.diyaudio.com/forums/showt...threadid=47197

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
sjmunday is offline sjmunday
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Hi.
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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Old 9th June 2017, 06:10 AM   #4
ashok is offline ashok
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Simple upgrade for Rotel RA-01 amp..
If you are still interested.....
I saw these two replies only today !

I'll put up some more info next week and possibly some pictures. Might re-look at it again. I did think it was a bit bass light !
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Old 23rd June 2017, 10:10 PM   #5
misfits73 is offline misfits73  United Kingdom
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Quote:
Originally Posted by ashok View Post
If you are still interested.....
I saw these two replies only today !

I'll put up some more info next week and possibly some pictures. Might re-look at it again. I did think it was a bit bass light !
Yes please, upload pics and any other info

Cheers
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Old 24th June 2017, 05:27 AM   #6
ashok is offline ashok
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Simple upgrade for Rotel RA-01 amp..
Just found the amp ! Cleaned it up and will take some pictures.
I vaguely remember using bypass film caps on it but I don't find them there now ! All opamp have sockets now but really not sure which types were the original. I'll pull out the service manual and check. The film cap at the input of the power amp is 2.2uF/250 V polyester type. Probably only had that at the time. Not a very good brand. I'll change it to Solen or something else and see what happens. Maybe even try it direct coupled if the dc offset isn't too bad. Need to look at the circuitry also to see if anything is worth tweaking.
Very tiny power amp section !
Pictures coming up soon.

Service manual at
http://www.acousticpsychos.com/Files...A01-int-sm.pdf

Owners manual at
http://www.rotel.com/sites/default/f...s/RA-01-OM.pdf
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Last edited by ashok; 24th June 2017 at 05:32 AM.
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Old 24th June 2017, 09:39 AM   #7
ashok is offline ashok
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Simple upgrade for Rotel RA-01 amp..
Default RA-01 pictures

Three pictures.
The full amp innards. The power amp section showing the film caps at the input ( grey). Note that the two black caps taped to the board are the original input capacitors. The third picture is the phono stage with conventional NE5532 with NFB equalisation. All opamps are the original ones.I had removed them and put in DIP sockets. But while I might have tried other opamps I don't recall any results ! I would have put back the original ones so that they don't get lost.( NE5532 for Phono, OPA2604 for line stage and NJM 2114 for tone stage ).

From the circuit diagram I can see some places where possibly one could remove some electrolytic capacitors. Need to look at it carefully. The power amp could be improved slightly. I'll have to do all this to one channel only to compare with the stock circuit. Can't do it quickly as there isn't so much time.

To start with , I am going to test it out in normal condition for a couple of days. Then make some changes to see what it does to the sound ! Luckily the tone control can be switched out.
Attached Images
File Type: jpg RA-01.jpg (492.6 KB, 297 views)
File Type: jpg RA-01-phono.jpg (645.8 KB, 283 views)
File Type: jpg RA-01-power.jpg (938.4 KB, 277 views)
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Old 25th June 2017, 08:46 AM   #8
AngelP is offline AngelP  United Kingdom
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Hi Ashok,
Good work on improving the Rotel. From I can see in the schematics the amp section is exactly the same as the RA-931, so you could possibly apply the mods I have described in
Improve a Rotel amp THD by 20dB!

now that I have the current mirror and EF-VAS modules.

I used Wima 3.3uF caps to replace the electrolytics. It is a PE type, but it is a rolled (not stacked as in the grey Multicomp types) film, and it is a direct drop-in which makes things look nicer. The improvement over the e-lytes is clear, but I simply haven't been able to hear any difference between the red Wima PE and PP types.

If you choose to go DC coupled be aware that the Tone/Direct switch could be a break-before-make type (it is in the RA-931) and you could get a nasty sharp thump when switching the tone control in and out.

Happy modding!
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Old 25th June 2017, 11:40 AM   #9
misfits73 is offline misfits73  United Kingdom
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When playing some test tones, 1Khz in particular there is a high pitch noise like distortion coming from both tweeters for 2 seconds and then gets normal.It occurs when switching between the test tone and HD tracks encoded above 44Khz but also when i switch on the amp and play directly the test tone.
I can't blame my DAC as it's not happening with other amps i have tried.
Any ideas?
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Old 26th June 2017, 07:21 AM   #10
ashok is offline ashok
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Simple upgrade for Rotel RA-01 amp..
AngelP I saw your thread earlier. Didn't realise the that the circuits were identical. I'll have a look.
I've been playing the RA01 for the past two days. It sounds much better than it "used to ".
But things changed. The speakers are the same but the DAC is different . Input cap to the power amp is now a film cap ( 2.2uF 250V)
LF is pretty good and HF is decent but I think mids and HF aren't as smooth as they should be. That sounds better on my Behringer A500 or Creek 5350. Could be one of the coupling caps(?). I certainly found direct coupling sounding better on many amps. Even with a larger dc offset it's worth it if the offset is below 0.1 V or so. Ouput stage MUST have DC protection ! They usually kick in at over +/- 0.6 V dc offset.

Looking at the simulation of the power amp circuit I find the power supply rejection is mediocre. Improving this by cleaning up the power supply might help improve the sound. Besides there are a lot of electrolytic capacitors in the signal path.Need to look at them carefully and see where they can be removed in spite of a 'small bang' when operating the tone switch ( which I never use in any case !)
Then they have a cap across the BC of the bottom driver which kills the -ve supply rail PSRR. But need to see how it affects stability at HF. Need to use the scope for that. If I'm not careful I'll blow some devices ! That would be a catastrophe !
Misfit73 , can't you get hold of a scope to see what's on the output when you get those strange sounds ? Is the switching causing a burst of oscillation ? This amp doesn't seem to have the usual miller cap on the VAS. The cap goes from Collector to ground along with a large resistor. I don't have the Spice data for these transistors. Using the BC series and MJ series I get a large peak at around 1 Mhz in the frequency response. Can be tamed by adding a cap for stability , though this might be VERY different with the actual transistors used. But one could always add a small cap to see if the noise goes away. Want to try ?
On many occasions I have noticed that mid and HF performance improves dramatically with a change in capacitor type. Some areas benefit more than others. Before taking off parts I'll have to re-look at everything. But I think , things might improve at least a bit.
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Last edited by ashok; 26th June 2017 at 07:37 AM.
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