Rotel problems

[johnandchris]

HELLPPPPP!!!!

I have recently purchased a Rotel RA-971 mk 2 which gets very good reviews. I have it hooked up to a marantz cd5400 and tdl nucleus 2 bookshelf speakers. It is connected using pure silver speaker cable and solid silver phono interconnects (own make and design, i know these are fine).

the amp plays fine at low volumes, however when turned up to anywhere near halfway, distortion sets in and it becomes unlistenable past half way. It almost sounds like it spikes (heavy bass is distorted). I had it hooked to a seperate power amp (rb-970bx mk2) and was bi-amping. I removed the power amp thinking that this was the problem, however it appears it was not.

What could be the problem. Is the amp doing something it shouldnt (i have had the top off, checked fuses etc, nothing appears out of the ordinary). Are my speakers struggling (they say on the back for use with an amp from 20 to 100w, and the rotel is rated at 60w) or could it be something else.

Any help would be greatly appreciated

Thanks
JOhn

[johnandchris]

Is isnt the speakers. I recently bought a brand new (discontinued) Wharfedale Pacific evo 20 floorstanders and the problem persists.
The problem is odd. The very low bass punches out with purpose and is absulutely fine. Similarly the high frequencies are fine.
It appears to be in the middle that things go a touch fuzzy. I am beginning to think it is just me going mad.
ANy ideas??

Dont suppose anyone lives near westbury in wilts who could pop in and have a listen look?

Regards
John

[homemodder]

Hi john

I have one of these amps, they were great at their time (1992 i think) your problem sounds very strange to me, somehow i think you are overdriving your amp. I still have mine works beautifully, although i have made a lot of tweaks it. This amp can sound awesome with some changes. Does your amp use ad827 as the preamp. If you have the manual try to adjust the output bias accordingly, set it slighty higher and this amp becomes so much more sweeter. The quality of the preset used on these amps are rather poor so i even suggest replacing this.

Alex

[johnandchris]

Thanks for the reply alex.
I dont know the specifics at the moment. What do i have to alter, i know in the amp there are a couple of blue plastic 'screws' in boxes. Is this what you are talking about?.
Also you said that yours was modified, could you give me detailed information on what you modified, what you replaced etc, i am very interested in doing the same.
I dont think i am overdriving the amp, because the problem occurs at low volumes as well as loud (never taken past half way due to the wife!!

I really dont want to change the amp as they are a superb bit of kit, high end performance for a budget price, although modifications are a difinite.

John

Ps. I ahve a copy of the technical manual on pdf, so can follow simple instruction. I am reasonably handy with a soldering iron and know very basically how to operate a multimeter.

[jaycee]

No - no no no, do not fiddle with the bias. This will not cure this problem, and if done incorrectly you will blow the output stage of the amp and possibly your speakers.

By the sound of it, you have a fault in the preamp section. Try with the tone controls bypassed.

[johnandchris]

Hi. Thanks for the reply. I never use the tone controls, they are turned off. When you say a problem in the pre-amp part, what could it be, a faulty component?

How would i test this.

ANY HELP appreciated.

regards
John

[jaycee]

It's a shame the amp doesnt have a "pre out"... thatd make it easy to test!

Is this amp brand new, second hand ? If second hand, do you know if the previous owners twiddled with it?

Does the volume control make any "scratchy" noise through the speakers?

[johnandchris]

Hi.

I bought the amp second hand but it does not appear to have been messed about with. It does have a pre-out, and i have managed to lay my hands on a rotel 971 mk11 power amp. Should i hook this up and try the ra-971mk11 integrated as a pre-amp. The volume pot appears to be fine, no scratching.
Regards
John

[cabbagerat]

Distortion at high volumes is often caused by the power supply rails collapsing under load. Perhaps you should test the rails with a multimeter, and replace the PSU caps (often a problem with older amps).

[homemodder]

Hi John

A little secret about these Rotel amps, they are very modestly rated, and will give you much more then 60w they rated at. They have 2 pairs of 2sd1047/2sb817 output transistors and can deliver huge amounts of current to power even the most difficult of loads.
Your idea of testing the preamp is good way to verify if preamp is good, cabbaqerat made a good point although these amps use slit foil high quality electrolytics which still should be okay.
The usual problems with these amps are the powersupply for the preamp section, which what i can recall is simple regulater with 2sd600/2sb631 and zener, verify if you have correct voltages at the opamps; +- 12v. Also have a look at the zobel network at the output. Some of these amps even when new tended to oscillate a bit causing the failure of sometimes the resistor other times the capacitor. A third problem i have come across is the emitter resistors on the output transistors sometime fail and even seen changing of values for no apparent reason, they are made by mpc i recall; check that they all measure 0,22 ohms I will try to dig up the schematics somewhere here on my harddrive and let you know the exact part numbers to look out for.
Later i can explain about the biasing and some really easy tweaks that can be made to these amps, one of them is to cure the slight oscilation problem some of these amps have.

Alex