biasing and dc offset for a sumo nine - diyAudio
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Old 24th March 2011, 09:51 PM   #1
omc is offline omc  Canada
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Default biasing and dc offset for a sumo nine

I am doing a full rebuild on one of these old amps for giggles. I don't know how to set the dc offset on this amp, or properly bias it when I am done. Can anyone shed some light or point me in the right direction? I have access to lots of equipment such as a scope and a signal generator if needed.

Thanks
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Old 25th March 2011, 12:11 AM   #2
llwhtt is offline llwhtt  United States
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These are the instructions I got in person at the Sumo factory in 1992 or so. They are for the NINE not the PLUS though they may be the same.

1) Pull one of the fuses (doesn't matter which one) on the channel you are going to adjust and replace with DMM set to amperes.

2) Turn amp on and adjust for 900ma, keep adjustment pots even. You are adjusting the pots that are near the top close together. These adjustments are for a COLD amp.

3) Set the remaining pot. (current limit) for 3 Amps at 60 Watts into 8 Ohms at 1KHz.

I've used these and they work but there has to be a better way than to "keep pots even" You might try the SAE Talk forum and see if the designer JB will give up the real deal. I've even tried to do it the Atmasphere OTL way but nothing has worked. They are both Circlotron circuitry.

Craig
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Old 25th March 2011, 12:23 AM   #3
llwhtt is offline llwhtt  United States
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As you may or may not know BOTH speaker terminals are HOT so test equipment hook up needs some thought. Anything connected to the outputs must NOT have a grounded power cord, use a cheater plug. If you need more info on this I'll find the Audio Amateur article for Ampzilla III written by JB himself back in the 80s. Ampzilla III is just like the Sumo Andromeda, fully balanced and requires the same caution using test equipment.

Craig
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Old 25th March 2011, 02:26 AM   #4
omc is offline omc  Canada
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Thank you very much, this is very helpful. If I am hooking up to the speaker outputs I will make sure I am on battery.
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Old 26th May 2011, 09:32 PM   #5
llwhtt is offline llwhtt  United States
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I finally got to meet JB last week and asked him about the NINE and it's alignment. Basically it can't be done without a distortion analyzer that has a "balanced input". All I have at the moment is a couple of H/P 334s and they are single ended. I have a Sound Technology 1701B on the way but it may be a while. JB will then tell me how to proceed. JB mumbled something about H/P gear and when I searched for the Sound Technology gear I found that they were started by ex-H/P guys, coincidence? Anyway I'll have the info but you'll need a balanced input analyzer to go with it.

Craig
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Old 15th August 2011, 02:06 AM   #6
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I'd love to hear this!
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