Threshold Stasis 2 Class A Power Amp - bias setting? - diyAudio
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Old 26th December 2010, 01:13 AM   #1
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Default Threshold Stasis 2 Class A Power Amp - bias setting?

I have an usher amp which is a clone of the Threshold Stasis 2 Class A Power Amp. Anyone know what the bias current should be?
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Old 26th December 2010, 03:26 AM   #2
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Im going to go with 70 ma's. there's a bais pot and both individual resistors on each transister of 1 ohm as well as a board resister of ten ohms. I can set the board to .7 amps, which directly translates to 70ma's on each transister. .7 is listed for a 150 watt amp. my amps is branded as a 200wpc amp. but as others have suggested, it may actually be less. it runs pretty lke warm at .6 or 60ma's so I have no issues with using .7 or 70ma's. btw, the author of the article I clip the below from, is the designer of the amp.
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Old 26th December 2010, 04:33 AM   #3
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ok, at .7 or 70ma's the heat sinks can't be handled for 5 seconds. not hot enough to fry eggs. so I guess thats it.
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Old 26th December 2010, 04:44 AM   #4
a.wayne is offline a.wayne  United States
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If you manage to keep the smoke on the inside it would be interesting to hear how it sounds with the higher bias vs the lower ...
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Old 26th December 2010, 04:49 AM   #5
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a.wayne

thats funny. no some. the 1 ohm resister on each of the 24 output transisters are not even warm. the 10 ohm board resister is not even warm. heat sink is hot to hold but not to touch. It sounds better the higher I go. the adjust ments can take the board resister bias well past 1 amp. I think 1.2 amps puts it at max wall outlet draw. unless I can find something or somone that can rationalize going high than .7 board 70ma individual transister , going to go with .7.

thanks for all the assistance.
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Old 26th December 2010, 04:53 AM   #6
a.wayne is offline a.wayne  United States
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Hello,

What impedance are you running at , 8ohm? interesting to hear that it sounds better with that much bias. In the past i have found you do get a smoother sound, but dull and lacking life ...

what size transformer?


regards,
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Old 26th December 2010, 05:03 AM   #7
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a.wayne

you know, I was coming back to post a concern. My test speakers say that they are 8 ohms, but you know how that goes. so now I need to understand, what bias should I use at 4 ohms. if the speakers really are 8 ohms, whn I go to 4 ohms, my bias will jump pretty high. According to some white papers I read, it will double. so in theory, I might be frying eggs after all.

don't know about the tranny. there is a 12amp main fuse, and a 6amp fuse for each side of the amp for each voltage phase. 4 fuses in all. A left + fuse, a left - fuse, and the same for the right, then a master 12amp fuse.

Last edited by speakerfritz; 26th December 2010 at 05:05 AM.
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Old 26th December 2010, 10:31 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
the amplifier bias is not affected by the speaker impedance.
Set the bias to a value that minimises the output distortion.

Most ClassAB with EF output stage do this by biasing to a set Vre. The voltage across the emitter resistor of the output device. Usually <=26mVre.

ClassA bias for a push pull output stage is set to half the peak output ClassA current.
If you want 1Apk of ClassA current then total output bias is set to 500mA.
If you want 2.4Apk of ClassA current then total output bias is set to 1.2A.
Note changing the load impedance does not influence the bias setting.
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regards Andrew T.
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Old 26th December 2010, 05:11 PM   #9
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Quote:
Originally Posted by AndrewT View Post
Hi,
the amplifier bias is not affected by the speaker impedance.
.
interesting.

I got sent down this path after reading a paper by Nelson Pass about a DIY class A amp build. His check out steps goes over bias adjustments. He indicates 175ma for 8 ohms and 220ma for 4 ohms. So perhaps only in this particular project was the impedeance a factor.

I'm getting that Nelson Pass designed the Threashold line of amps, so I assumed that the basic bais concept was consistent.
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Old 26th December 2010, 05:18 PM   #10
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Quote:
Originally Posted by AndrewT View Post
Hi,

ClassA bias for a push pull output stage is set to half the peak output ClassA current.
If you want 1Apk of ClassA current then total output bias is set to 500mA.
If you want 2.4Apk of ClassA current then total output bias is set to 1.2A.
Note changing the load impedance does not influence the bias setting.
interesting info.

going to give that some though over the next hour or so.
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