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Old 5th October 2004, 04:19 AM   #1
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Default Threshold Stasis S500 Series II

Hello,
Does anybody know how to check the correct bias for this amp? There are two driver boards with single pots each and I guess that is the one to adjust. For those who know, what will be the voltage reading across the pot (rheostat connected as a potentiometer) for correct bias adjustment?

reason being is that when I power ON the amp, a loud "thump" can be heard on the speakers (both channels) but not during power OFF.
I was thinking that it could be because the bias is to high(low?)so a small signal from turning on the power switch is being amplified greatly. During operation, the amp is very quiet even if the preamp volume knob being use is at close to maximum. I tried a Variac as power source and even at 5VAC input power, the "thump" is still loud.

Any comments will be greatly appreciated. I love the amp and want to keep it, that problem is my only concern to its operation. BTW, I already replaced all the big filter caps at the output.

If somebody knows or can provide me an schematic to troubleshoot, please kindly do so.

I believe "the man" himself (Mr. Nelson Pass) readily participates in this forum when not busy and I hope he can read my inquiry.

Thank you very much. Any suggestions will be greatly appreciated.

Abe
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Old 5th October 2004, 09:35 PM   #2
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I've got two of these beasts. They're still pretty good low frequency amps, even twenty years on.
The thump is part of the game. Changing the bias will not make it go away. You've got two options: Modify the circuit or grin and bear it. My advice is to leave well enough alone.
Yes, the pot is there to set the bias, but you don't set the bias by reading the voltage across the pot. Initially, you set the bias by reading the voltage across one of the emitter resistors, then close the thing up and let it cook for a while. Final bias is determined by temperature; you stick a thermometer in the rear-most screw hole into the top of the back heat sink. I don't have all the numbers commited to memory (I'm at work), but can get them once I get home if you need them. That said, the bias pot is hard to get to and twitchy as hell. I tried to come up with a good way to get a multi-turn pot down in there, but it's not easy, given the orientation of the part and lack of easy access.
If you still want to mess with the bias, I'd recommend wrapping your screwdriver with tape. There are a lot of potential short circuits between you and that pot.

Grey
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Old 5th October 2004, 10:09 PM   #3
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The best measurement comes around 40 deg C. on the
base of the heat sinks, best sound at 50 degrees.
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Old 6th October 2004, 03:32 AM   #4
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Grey,

Thanks a lot! What would be the reading in volts on the emitter resistor during initial biasing? Please do post or email me any info/numbers when you have time to do so.

Mr. Pass,

Thanks for your input also. I really appreciate it!



regards both,

Abe
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Old 7th October 2004, 04:19 PM   #5
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First, check the temperature of your heatsinks. You may not need to adjust the bias.
If you still think you want to mess with the bias, start with 90mV across one of the emitter resistors with the amp cold. There's not really any point in reading all the emitter resistors. They will read somewhat differently depending on the characteristics of the individual output device. All you're looking for is a starting point.
Put the top back on and monitor the temperature by inserting a thermometer into the screw hole at the back of the amp. You'll need to pull the top to get to the bias pot, obviously, but be sure to put it back on every time, as it is part of the thermal environment. Plus or minus a degree or two between the channels is okay. Let the amp idle for an hour or more in order to get everything stable.
As I said above, the bias pot is fairly twitchy. Patience is the key. Adjust in small increments. Given the possibility of thermal runaway, it's better to run the amp too cool than too hot, although it will sound better if you run it warmer.

Grey
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Old 8th October 2004, 03:32 AM   #6
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Thanks Grey!

I will do just that.

Abe
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Old 21st November 2009, 04:08 AM   #7
a.wayne is offline a.wayne  United States
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Default S500 Bias

Hello Gents ,
Digging up and oldie ... A question regarding S500 1 vs series 2 opti Bias ..

How difficult is it to update a series 1 and what is the advantages of the opti-bias in the series 2 ?

regards,
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Old 21st November 2009, 02:46 PM   #8
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It is mostly replacement of the front end as I recall.

The optical bias is improved in that it is more thermally
stable, that is to say the bias value drifts less with temperature.

If you're fooling around with all that, you probably want
the front end that never made it to production, which I
did before I left in '91.

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Old 21st November 2009, 03:29 PM   #9
a.wayne is offline a.wayne  United States
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Hello Mr Pass,

Thanks for the response and your offer . i wanted to review my options to see if i should upgrade or go looking for a series 2 ..

Also , My S500 has no turn on thiump as the others have described and it does take a long time to come up to temps , my next step was to see what it was biased at , as the sound is much better and different after a couple of Hrs .

Now how would one obtain that special Pass front end ?
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Old 21st November 2009, 05:39 PM   #10
a.wayne is offline a.wayne  United States
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almost forgot , apart from the series 2 badging , how do you tell a series1 from a 2 ...
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