Troulbe with Bias on S150 Series II

Redwingnine · 24th March 2011, 09:02 PM

I'm refreshing a Threshold S150 Series II amp (a nice sounding little gem). All was going well until I went to set the bias. The one side went without a hitch. However, the other channel is giving me some trouble. The bias level maxes out at 600ma. The bias value climbs up and down, but will not go past 600ma.

What could be the issue? Could one of the two bias transistors have a low hfe? Looking for some feedback. I've done one other S150 Series II without any issues.

Nelson Pass · 25th March 2011, 03:22 AM

Depending on the specific version, there is a resistor in
series with the potentiometer which limits the maximum
bias voltage. You can reduce the value of this resistor and
see if that does it. If it doesn't, you have to check to see
how much current is going through the bias circuit - I would
then look to see if the constant current source for this part
of the circuit is delivering.

You can use the other channel for a reference.

Redwingnine · 27th March 2011, 08:23 PM

Much Thanks, Nelson.

At least I know now where to look. I'll post what was worked out on this.

24th March 2011, 09:02 PM	#1
Redwingnine diyAudio Member Join Date: Apr 2010	Troulbe with Bias on S150 Series II I'm refreshing a Threshold S150 Series II amp (a nice sounding little gem). All was going well until I went to set the bias. The one side went without a hitch. However, the other channel is giving me some trouble. The bias level maxes out at 600ma. The bias value climbs up and down, but will not go past 600ma. What could be the issue? Could one of the two bias transistors have a low hfe? Looking for some feedback. I've done one other S150 Series II without any issues.

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25th March 2011, 03:22 AM	#2
Nelson Pass The one and only Join Date: Mar 2001	Depending on the specific version, there is a resistor in series with the potentiometer which limits the maximum bias voltage. You can reduce the value of this resistor and see if that does it. If it doesn't, you have to check to see how much current is going through the bias circuit - I would then look to see if the constant current source for this part of the circuit is delivering. You can use the other channel for a reference.

27th March 2011, 08:23 PM	#3
Redwingnine diyAudio Member Join Date: Apr 2010	Much Thanks, Nelson. At least I know now where to look. I'll post what was worked out on this.

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