Fuses
I beieve it is a re hashed RA930, in which case there are fuses in line with the output of this amp. These are typically 4 amp units and near the speaker posts, leave the mains fuse be!!!!
They work well, in 12 years in the trade 75% of Rotel amps brought in to me as faulty blew the output fuse only.
.So, if you are a headbanger and take this amp to its limits, do not do this tweak.
Replace these fuses with 8 or 10 amp ones, you will not believe your ears.
Please do not cause shorts on your speaker wires or overdrive the amp as you have now virtually defeated the safety feature in the amp. This is a tweak that is safe for safe people and I accept no responibility!!!
I beieve it is a re hashed RA930, in which case there are fuses in line with the output of this amp. These are typically 4 amp units and near the speaker posts, leave the mains fuse be!!!!
They work well, in 12 years in the trade 75% of Rotel amps brought in to me as faulty blew the output fuse only.
.So, if you are a headbanger and take this amp to its limits, do not do this tweak.
Replace these fuses with 8 or 10 amp ones, you will not believe your ears.
Please do not cause shorts on your speaker wires or overdrive the amp as you have now virtually defeated the safety feature in the amp. This is a tweak that is safe for safe people and I accept no responibility!!!
Hi Drew and all,
I made two sets of four classes for year in the Politechnical School of my city about audio and I use a basic system: Yamaha CDX396, Rotel RA391 and Tannoy Mercury MXII for explain.
I dedicated the first to a introduction and, in the other three I explain many modifications that can be made for bettered the system.
If you send me a adress of snail mail, I send you photocopies of the service manual of the RA931 with all the upgrades.
Sorry for my english,
Happy days,
Raúl Couto
I made two sets of four classes for year in the Politechnical School of my city about audio and I use a basic system: Yamaha CDX396, Rotel RA391 and Tannoy Mercury MXII for explain.
I dedicated the first to a introduction and, in the other three I explain many modifications that can be made for bettered the system.
If you send me a adress of snail mail, I send you photocopies of the service manual of the RA931 with all the upgrades.
Sorry for my english,
Happy days,
Raúl Couto
The tweak pretty much will work for any amplifier with fuses in line with the output. A fuse burns out as it can not handle/gets to hot passing a required current. They thus limit / obstruct current flow, that is exactly what you do not want.
Years ago we tried different values on a RA820, somewhere around 8 or 10 amp, going higher made little audible if any difference. The std value I am sure was 4 A. Hence my estimate of going about 2.5 X the rating of the fuse.
Years ago we tried different values on a RA820, somewhere around 8 or 10 amp, going higher made little audible if any difference. The std value I am sure was 4 A. Hence my estimate of going about 2.5 X the rating of the fuse.
Hi all,
Well. Really. Are you crazy?
Drew et all: never, never, NEVER! made a modification that include eliminate the security devices.
Risk your life and your house only for have slightly best sound are totally stupid. Sorry, I don´t intended offend nothing, but are that I think.
The reason for the bad sound of the fuses are that are a variable load. The flow of the current heating the wire of the fuse change your resistence, for this, the change for other of more big amps bettered the sound: the wire change less your resistence.
Know the reason you can apply solution:
1- You can change all the fuses for the tipe called 'ceramics', are this whit a 'sand' inside. Ave less termal change.
BUT MAINTAIN THE SAME AMPS VALUE!!!
2- You can change the AC fuses for 'ceramics' AGAIN SAME AMPS!!!
and change the output fuses for 'Polyswitchs': this are a devices with the aspect of a big disc capacitor (at least for four amps) an act maintain your resistence until get your 'conmutation current', in this moment change abruptly your resistence (increase).
The other advantage are that, when dissapear the cause of the overcurrent, and leaves any time power off, they rearm automatically.
I buy in Selectronic: www.selectronic.fr, but I supposed than have other sources.
The music are part of the good side of the life but, please, maintain your common sense.
Sorry for my english.
Happy days,
Raúl Couto
P.S.- Drew, I have your photocopies, when I write the explanations of the improvements, I send you soon.
Well. Really. Are you crazy?
Drew et all: never, never, NEVER! made a modification that include eliminate the security devices.
Risk your life and your house only for have slightly best sound are totally stupid. Sorry, I don´t intended offend nothing, but are that I think.
The reason for the bad sound of the fuses are that are a variable load. The flow of the current heating the wire of the fuse change your resistence, for this, the change for other of more big amps bettered the sound: the wire change less your resistence.
Know the reason you can apply solution:
1- You can change all the fuses for the tipe called 'ceramics', are this whit a 'sand' inside. Ave less termal change.
BUT MAINTAIN THE SAME AMPS VALUE!!!
2- You can change the AC fuses for 'ceramics' AGAIN SAME AMPS!!!
and change the output fuses for 'Polyswitchs': this are a devices with the aspect of a big disc capacitor (at least for four amps) an act maintain your resistence until get your 'conmutation current', in this moment change abruptly your resistence (increase).
The other advantage are that, when dissapear the cause of the overcurrent, and leaves any time power off, they rearm automatically.
I buy in Selectronic: www.selectronic.fr, but I supposed than have other sources.
The music are part of the good side of the life but, please, maintain your common sense.
Sorry for my english.
Happy days,
Raúl Couto
P.S.- Drew, I have your photocopies, when I write the explanations of the improvements, I send you soon.
"Just installed the new T10A fuses on the ouputs of my RA-931. They must have to break in..."
Of course!
Seriously though, heed raul_77's warning. Those fuses are there for a purpose, don't replace them unless you know what you are doing. If you need more volume, buy a more powerful amp or (preferably) get more efficient speakers.
See ya,
Tim.
Of course!
Seriously though, heed raul_77's warning. Those fuses are there for a purpose, don't replace them unless you know what you are doing. If you need more volume, buy a more powerful amp or (preferably) get more efficient speakers.
See ya,
Tim.
I think you need to concentrate on your CD first.
The RA930/1 are quite well balanced designs that sound good for the bucks. If the fuses do not do it, revert back to the old ones, it is safer, you may notice adiff going back, sometimes it works like that.
What op amp is used in the pre amp stage? On 930 this was a 5532, I think on the 931 it became a op275, this si not the fullest sounding op amp. Use 2134 or 2604 or even the 5532 or NJM2114, to me these are more musical, IMHO, the op275 always had a glare in the mid I did not like.
The RA930/1 are quite well balanced designs that sound good for the bucks. If the fuses do not do it, revert back to the old ones, it is safer, you may notice adiff going back, sometimes it works like that.
What op amp is used in the pre amp stage? On 930 this was a 5532, I think on the 931 it became a op275, this si not the fullest sounding op amp. Use 2134 or 2604 or even the 5532 or NJM2114, to me these are more musical, IMHO, the op275 always had a glare in the mid I did not like.
A friend just install some new capacitors on the output stage of the amp along with the orginal ones. He also installed a couple of filters to take the edge off the top end. Noticed the improvements right away. Also adjusted the bias so that the amp operates in Class A mode for longer. Thanks buddy. Its sounding great. It is much richer sounding, deeper bass response with more impact and the sound stage is alot more open.
Hi there. Is there any chance you could pass those schematics to me too please?
I have exactly same amp and long time was thinking that there must something you can do better than the original..
Cheers! 
I know this is an old thread, but as I did, I believe more second hand of this amps are still bought around the world.
This is what I've been done to may recently acquired Rotel RA-931 MKII.
Photos can be seen at my flickr album: https://www.flickr.com/photos/jcbarros71/sets/72157644096849117/
Service manual for RA-931 (and many others) can be found at: Rotel RA-931 Manual - Stereo Integrated Amplifier - HiFi Engine
Roughly the modifications were:
Binding posts changed
All electrolytic capacitor changed (to Elna), and some ceramics (mostly to Wima).
Bridge rectifier replaced by one of 8 Amps
Output 4A fuses replaced by 4A resettable fuses
All jumper on board replaced by enameled copper wire, 20AWG (colour red) and 22AWG (colour gold) sizes used.
Some 100nF capacitors were added for filtering purposes.
Did not touch in any styroflex capacitors and transistors.
Recently I made some mods to the circuit that can be found on RA-932, RA-01 and RA-02
On the previous image, some MODs are in red, other in green.
Those pictured in green are modifications that can be found on RA-932, RA-01 and RA-02.
The red ones will be found in RA-01 and RA-02, and be advised that they affect the tone control section, lowering the volume, since the OPA2604 will be set with a gain of 6.74, when it was 19. On the previously mentioned amplifiers, this is compensated with the addition of an extra op-amp, set with a gain of 2.67.
Another mod will smooth high frequency edges, if you find some voices to sharp, for instance. This can be found on Rotel RA-840 BX3.
Four 1nF capacitors were placed in parallel with the 10uF (C501, C502, C515, C516) around the op-amp OPA2604.
The capacitors in consideration that can be seen in picture, are the green and yellow ones. They are different just because it was what I had in hand. and they will be replaced when I got new ones.
Cumps
This is what I've been done to may recently acquired Rotel RA-931 MKII.
Photos can be seen at my flickr album: https://www.flickr.com/photos/jcbarros71/sets/72157644096849117/
Service manual for RA-931 (and many others) can be found at: Rotel RA-931 Manual - Stereo Integrated Amplifier - HiFi Engine
Roughly the modifications were:
Binding posts changed
All electrolytic capacitor changed (to Elna), and some ceramics (mostly to Wima).
Bridge rectifier replaced by one of 8 Amps
Output 4A fuses replaced by 4A resettable fuses
All jumper on board replaced by enameled copper wire, 20AWG (colour red) and 22AWG (colour gold) sizes used.
Some 100nF capacitors were added for filtering purposes.
Did not touch in any styroflex capacitors and transistors.
Recently I made some mods to the circuit that can be found on RA-932, RA-01 and RA-02
On the previous image, some MODs are in red, other in green.
Those pictured in green are modifications that can be found on RA-932, RA-01 and RA-02.
The red ones will be found in RA-01 and RA-02, and be advised that they affect the tone control section, lowering the volume, since the OPA2604 will be set with a gain of 6.74, when it was 19. On the previously mentioned amplifiers, this is compensated with the addition of an extra op-amp, set with a gain of 2.67.
Another mod will smooth high frequency edges, if you find some voices to sharp, for instance. This can be found on Rotel RA-840 BX3.
Four 1nF capacitors were placed in parallel with the 10uF (C501, C502, C515, C516) around the op-amp OPA2604.
The capacitors in consideration that can be seen in picture, are the green and yellow ones. They are different just because it was what I had in hand. and they will be replaced when I got new ones.
Cumps
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would the 4a polyfuse be 30v?
RA931 has power rail voltages of +/- 37VDC and any device connected to the output should be capable of withstandiing near full rail voltages which are possible under fault conditions as no other protection is in place. With a small safety margin, >50V rating would be more realistic.
It seems the Ebay polyswitch/polyfuses are all rated 30V as are Farnell's. Digi-key in the US stocks a wider voltage range of 6 to 60V. A 30V rated part would pass audio at moderate output levels with little affect on it but then it won't survive long when tripped if the voltage across it now exceeds its rating. It'll be just as disposable as a fuse and probably affect the audio slightly.
It seems the Ebay polyswitch/polyfuses are all rated 30V as are Farnell's. Digi-key in the US stocks a wider voltage range of 6 to 60V. A 30V rated part would pass audio at moderate output levels with little affect on it but then it won't survive long when tripped if the voltage across it now exceeds its rating. It'll be just as disposable as a fuse and probably affect the audio slightly.
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