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Old 12th February 2006, 08:18 AM   #1
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Default Rotel rb-991 in class A

I'm considering building a rotel rb991, but with decreased supply voltage(~+-35v) Then runnung it into class A mode. I will have to build 4 channels for my bi-amped system.

Any ideas on this?

Schematics are available on
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Old 12th February 2006, 11:03 AM   #2
lineup is offline lineup  Sweden
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The Power amplifier RB-991 is a very simple and standard circuit.
This is from looking in the Service Manual Schematics.

Should not be very difficult to modify.
I think you can do it.
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Old 12th February 2006, 05:31 PM   #3
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Default Re: Rotel rb-991 in class A

Originally posted by appelkoos
I'm considering building a rotel rb991, but with decreased supply voltage(~+-35v) Then runnung it into class A mode.
I see no problem ,as the LTP are feed by CCS , the operation points will not change significantly , with the lower voltage rails...

Go ahead...
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Old 13th February 2006, 02:44 AM   #4
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Not worth the effort. RB991 looks like a very good amp on paper but its sound quality is very mediocre.

Inner resolution is pathetic and the stock amp sound quite hollow. It seemingly presents a very wide (larger than life) sound stage, but the depth and height of the stage is hardly resolved.

I have modified the power supply of an RB991 considerably, using MUR1560 rectifiers, CRC filtering, capacitor snubber, mods to the feedback section and change to silver-plated teflon-insulated cables. The amp has become a lot cleaner but low level signal resolution is still lacking to a large extent.

Try a mini-aleph or a real aleph and you will be a lot more pleased. Perhaps, an Aleph-X would be even better.
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Old 13th February 2006, 04:02 AM   #5
fab is offline fab  Canada
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Default Question on schematics

What is the purpose of C627 and R665 between + and - of amp on the schematics?
(I have seen that also on the Denon schematics)
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Old 13th February 2006, 06:17 AM   #6
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Thanks Sam, That was actually spot on the question I wanted answered. Due to lack of availability, I will have to use IRFP250's for the aleph
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Old 24th May 2006, 09:30 PM   #7
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I know this is a late response to this thread but this is something I was considering doing myself.

I wish to disagree with the negative review of the RB-991. I have tested it with quite a few different amps the two most expensive being a pair of 1.7k tag maclaren monoblocs and an 8k Krell Class A.

The Rotel was in a different league to the TAGs, beat them in every department, not quite as good as the krell though.

I guess you just paired the amp with the wrong gear.

What appelkos was suggesting sounds exactly like what I wanted to do, What would you reccomend Sam?
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Old 25th May 2006, 02:31 PM   #8
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I did not consider running the RB991 in Class A, but I did consider lowering the voltage and increasing the bias. However, the heatsinks of the amp will certainly not allow for Class A operation.

Secondly, you will find that the amp PCB has been designed for L/R channel symmetry and there are too many flaws in the layout, due to symmetry being a priority.

I also think that Rotel's engineers have not applied some basics. For examply, the 10 ohm resistor between Input and Power ground is rated at 1/4 watt. That will go up in smoke in case of any small malfunction. The amp that I worked on had this problem with one channel. It developed this problem all of a sudden. I have replaced this with 2 watt type on both boards. There is no room on the PCB for anti-parallel diodes in shunt with this resistor.

This amp also tends to oscillate and the T6.3A onboard fuses tend to blow. I have changed the feedback resistor value but am not happy.

Micro-dynamics and inner details are either lost all together or quickly fade away. The sound spectrum is not dense. The sound stage is very wide, but lacks good resolution; the stage is not deep and stage height is missing all together.

Try to use this amp in a dedicated subwoofer application and you will realise its weakness even in the bass region.

By the way, this amp is being used with Rotel Preamp (I think the model is 1070 or 1080) and Klipsch RF-3 speakers.

If someone does take the pains to make this amp sound very good, it will be a commendable job.
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Old 25th May 2006, 09:32 PM   #9
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Hi Sam,

Thanks for getting back to me.
I run a RC-991 through the RB-991 into a pair of Acoustic Energy 309s.

The unit is undeniably flawed, but what it does right I really like.

I agree about the fuses, I have a pack sitting next to the amp at all times =-). It was worse when i lived in the country, I actually ran the amp with the case off to save time becuase I had to swop them so often. This problem was removed when i fitted an in-line mains filter but that damaged the sound.

I must admit my knowledge of electronics is very basic, but if you could offer any help with the amps flaws, or reccomendations on how to improve it, I would greatly appreciate it.

What do you mean by the amp oscilates?
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