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Audio Research SP6a DC on output
Audio Research SP6a DC on output
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Old 30th September 2001, 11:26 AM   #1
Luke is offline Luke  New Zealand
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HI I have recently bought an Audio Research SP6a preamp and very happy with it generally.
There is however a problem with it when powering it on and off. It get a large amount of DC as the woofers make a very unnatural noise and the cones go forward for quite a few seconds and then gradually go back to their resting position.

I know the amp has had quite a bit of mod work done on it and upgraded with hovlands in the signal caps etc, new cv4004 tubes and a couple of electolytics have been replaced, apparently due to hum on the output.

What I dont know as I have no schematic, is whether it has had a time delay muting relay disconnected at some stage or whether there is a problem with filter caps or something.
I have no idea if the filter caps are original or been relaced, think they may be 20 years old and could be the problem? If so where can i get replacements, they are quite high capacitance, i think 2000uf @ 400V and there are lots of them. They are stainless steel can Mallory electrolytics I think.

Is anyone out there familliar with this pre that can offer me some assistance in diagnosing this fault?

Thanks Luke

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Old 30th September 2001, 04:07 PM   #2
GRollins is offline GRollins  United States
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Sounds as though the muting relay is not working. It should keep the output shorted to ground for perhaps a minute after turning the unit on, and should short to ground immediately on turn-off. The DC transients are completely normal, in and of themselves, but it is definitely the duty of the preamp to keep from passing DC through to the output.
Filter caps in the power supply have nothing to do with this problem. (Mallory cans are made of aluminum.) If you want to replace the cans, by all means do so, but don't expect it to cure your turn-on/off thump problem.
I would recommend not using the preamp until the relay is working. The amount of DC passed under these circumstances can destroy speakers.

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Old 30th September 2001, 04:31 PM   #3
Mark A. Gulbrandsen is offline Mark A. Gulbrandsen
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Join Date: Sep 2001
Location: Salt Lake City
Contact Audio Research as they are very cooperative with supplying manuals and parts. I have had trouble however getting parts for some of their older units but this is usually a cabinet item request sort of thing. I had an SP-6 years ago and I seem to remember that it had a muting relay in it. So be sure to check this out. As Grey said, it can and will eventually ruin your speakers. If you have to run the preamp before you repair it, turn it on first and wait a minuite or so and then turn on the power amp. If there is still a large turn on trainsent then it may be that there is DC on the output of the preamp all the time and the amp has a servo circuit that is trying to compensate for this. If all seems normal powering up this way and there is no transient when switching the amp on it is probaby safe to use it. To power down do the opposite and turn the amp off first, wait a minuite or so and then switch the preamp off.
If my burn't out memory serves me, my SP-6 sounded alot better when left on all the time. But you will have to decide if this is the right thing to do in your circumstance.
Mark Gulbrandsen
Salt LAke City, UT

[Edited by Mark A. Gulbrandsen on 09-30-2001 at 11:34 AM]
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Old 2nd October 2001, 10:22 AM   #4
Luke is offline Luke  New Zealand
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Smile thanks guys


Thanks Grey and Mark for your help.
I cant see anything that looks like a relay, tis pretty hard to work on these things, plenty crammed into small spaces:-)

Seems its the relays. Think next step is to get a service manual and maybe to talk to someone re parts for this thing at ARC.

regards Luke
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Old 19th April 2002, 02:44 AM   #5
Morello is offline Morello  Sweden
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Location: Sweden
Talking Relay

There is probably the muting relay at the output that is not working. However, the SP3 didn't have such a relay.

Best Reagrds\Joe Morello
Audio design engineer. Commerical amplifiers: O&G Engineering RS2000 and Holographic Audio AMP-27 among others.
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Old 19th April 2002, 03:30 AM   #6
Mohan Varkey is offline Mohan Varkey  Australia
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I owned an SP6A 1980-1987. I have the manual that came with it. I can mail or fax a copy to you if I am able to locate it.

From memory, the mute switch simply shorted the output to ground. This switch should be on until the warm up period is complete. The manual says that your woofers will dance otherwise.

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