Massive amp modification - basic questions - Rotel RB991 donor

inteificio · 29th November 2010, 12:50 PM

Hi,

I have a Rotel RB-991 which is not only a poor match for my speakers, but gets heavily slated on this site.

This was the original problem, now considering a more drastic solution.

Rotel RB991 vs Acoustic Energy 309 - fail

For a summary, speakers are very sensitive, amp only sounds good when being driven more than 20%, which is way to loud for my room.

Now I do not want to stick the RB-991 in the bin and start again, and I cannot sell due to some basic mods which have ruined the look of the case (speakon connectors being the main one.)

I am thinking that the transformer/power supply/main caps are worth keeping.

Is their a good DIY board that I could slot in where the current ones go, taking advantage of the good heatsinks, supply, caps?

Any suggestions?

I believe the rail voltages on this are 70v, might be wrong, not even sure if it is relevant.

Are these coke can sized caps just used to maintain the supply or are they used for something else?

A quick example of how sensitive these speakers are, would be what I am currently driving them with. A modded SonicT can happily drive these speakers to a level that is uncomfortable. I should add this room is large, 5mx15m.

I have no ideas where to start or what to do, so this is an early stage research.

Any suggestions/ideas?

No budget at the moment (as no job), so quite open on that (no more than a few hundred though, hopefully a lot less)

inteificio · 29th November 2010, 12:55 PM

As the heat sinks on this amp are huge, thinking about a lower power class A?

jaycee · 29th November 2010, 04:14 PM

I doubt there is any drop in board.

The large cans are indeed the main power supply capacitors. The rail voltages are about 75v unloaded (depending on mains voltage).

Now I remember that I commented on your other thread - did you try increasing the bias, and did that help?

inteificio · 29th November 2010, 06:31 PM

Hi Jayce,

I increased the bias, but didn't notice any difference =-(

Can't remember what I set it to, will check again when the spare parts arrive (one of the power smoothing caps leaked).

My knowledge of amps is small, but this is how I see the situation.

I have a good quality 75v power supply and balanced inputs. All I think I need is an amplification board that runs off 75v and I should just be able to swap the cables over and get a new amp.

Is my view logical, or so over simplified that it won't work?

jaycee · 29th November 2010, 07:29 PM

If one of the smoothing caps has leaked, this is a sign of trouble in itself. Yes, sorry to say it is a very simplistic view. It's like saying you can take a Ford Escort, put a Ferarri engine in it and get an Enzo. I hope im not sounding too harsh there

Thing is, the Rotel shouldn't sound that bad at all. Now you mention a smoothing capacitor has leaked, it tells me theres a problem with it.

wahab · 29th November 2010, 07:34 PM

Can we see a schematic of this amp ?..

Rotel had quite a few good amps, unfortunately there are
some design choices that can make a basicaly good amp
sounding mediocre.

Particularly, they rely on massive shunt compensations that
litteraly kill the amp s qualities if implemented as heavily..

aparatusonitus · 29th November 2010, 08:03 PM

[QUOTE=wahab;2381311]Can we see a schematic of this amp ?..[QUOTE]

Yes ;-) http://bwgroup-support.com/techmanuals.html

inteificio · 29th November 2010, 08:50 PM

The cap that leaked was C803.
Not certain what that board does, but as the protection and power LEDs are linked to it my guess is the protection circuit, Rotel tech said it was a power smoothing cap.

jaycee · 30th November 2010, 10:38 AM

It is indeed the protection board - Strange for the cap to leak, but it wouldn't cause poor sound issues.

The RB991 would certainly make a great chassis for a different amp, but it would be strictly DIY, nothing drop-in.

wahab · 30th November 2010, 11:38 AM

[QUOTE=aparatusonitus;2381353][QUOTE=wahab;2381311]Can we see a schematic of this amp ?..

Quote:

Yes ;-) Service Manuals

Thank you, Aparatusonus

I once had a gif schematic and it s exactly this one , safe
that your link gives the full version.

A very fine amp and i once made some sims to see its behaviour.
Seems to me that compensation is heavy and doesn t help the
dynamic behaviour.
Also, the input differentials have no degeneration.
All in all , it should be slightly modded to give its best ,
since the topology is quite good.

29th November 2010, 12:50 PM	#1
inteificio diyAudio Member Join Date: Dec 2004 Location: York	Massive amp modification - basic questions - Rotel RB991 donor Hi, I have a Rotel RB-991 which is not only a poor match for my speakers, but gets heavily slated on this site. This was the original problem, now considering a more drastic solution. Rotel RB991 vs Acoustic Energy 309 - fail For a summary, speakers are very sensitive, amp only sounds good when being driven more than 20%, which is way to loud for my room. Now I do not want to stick the RB-991 in the bin and start again, and I cannot sell due to some basic mods which have ruined the look of the case (speakon connectors being the main one.) I am thinking that the transformer/power supply/main caps are worth keeping. Is their a good DIY board that I could slot in where the current ones go, taking advantage of the good heatsinks, supply, caps? Any suggestions? I believe the rail voltages on this are 70v, might be wrong, not even sure if it is relevant. Are these coke can sized caps just used to maintain the supply or are they used for something else? A quick example of how sensitive these speakers are, would be what I am currently driving them with. A modded SonicT can happily drive these speakers to a level that is uncomfortable. I should add this room is large, 5mx15m. I have no ideas where to start or what to do, so this is an early stage research. Any suggestions/ideas? No budget at the moment (as no job), so quite open on that (no more than a few hundred though, hopefully a lot less)

29th November 2010, 08:03 PM	#7
aparatusonitus diyAudio Member Join Date: Jan 2004 Location: Split, Croatia	[QUOTE=wahab;2381311]Can we see a schematic of this amp ?..[QUOTE] Yes ;-) http://bwgroup-support.com/techmanuals.html __________________ Non é mai abbastanza...

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29th November 2010, 12:55 PM	#2
inteificio diyAudio Member Join Date: Dec 2004 Location: York	As the heat sinks on this amp are huge, thinking about a lower power class A?

29th November 2010, 04:14 PM	#3
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	I doubt there is any drop in board. The large cans are indeed the main power supply capacitors. The rail voltages are about 75v unloaded (depending on mains voltage). Now I remember that I commented on your other thread - did you try increasing the bias, and did that help?

29th November 2010, 06:31 PM	#4
inteificio diyAudio Member Join Date: Dec 2004 Location: York	Hi Jayce, I increased the bias, but didn't notice any difference =-( Can't remember what I set it to, will check again when the spare parts arrive (one of the power smoothing caps leaked). My knowledge of amps is small, but this is how I see the situation. I have a good quality 75v power supply and balanced inputs. All I think I need is an amplification board that runs off 75v and I should just be able to swap the cables over and get a new amp. Is my view logical, or so over simplified that it won't work?

29th November 2010, 07:29 PM	#5
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	If one of the smoothing caps has leaked, this is a sign of trouble in itself. Yes, sorry to say it is a very simplistic view. It's like saying you can take a Ford Escort, put a Ferarri engine in it and get an Enzo. I hope im not sounding too harsh there Thing is, the Rotel shouldn't sound that bad at all. Now you mention a smoothing capacitor has leaked, it tells me theres a problem with it.

29th November 2010, 07:34 PM	#6
wahab diyAudio Member Join Date: Nov 2009 Location: algeria/france	Can we see a schematic of this amp ?.. Rotel had quite a few good amps, unfortunately there are some design choices that can make a basicaly good amp sounding mediocre. Particularly, they rely on massive shunt compensations that litteraly kill the amp s qualities if implemented as heavily..

29th November 2010, 08:50 PM	#8
inteificio diyAudio Member Join Date: Dec 2004 Location: York	The cap that leaked was C803. Not certain what that board does, but as the protection and power LEDs are linked to it my guess is the protection circuit, Rotel tech said it was a power smoothing cap.

30th November 2010, 10:38 AM	#9
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	It is indeed the protection board - Strange for the cap to leak, but it wouldn't cause poor sound issues. The RB991 would certainly make a great chassis for a different amp, but it would be strictly DIY, nothing drop-in.

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