Zen v.4 Bias Sensitivity Difficulty

Kashmire · 15th February 2004, 02:16 AM

I built a Penultimate Zen with the recommended component values for biasing Q1 (a 25K trimpot and 47.5K fixed resistor for R4 and R5). Starting with the trimpot fully CCW, the bias voltage at the drain of Q1 is low. As the trimpot is advanced, the bias voltage doesn’t change much. When R4 is almost full CW, the bias voltage suddenly changes from about 8V to 40V. The sensitivity is incredible – the bias voltage sweeps the entire range with only a few degrees of rotation on the trimpot. It’s impossible to set, because it is so sensitive. Both channels operate the same way.

My conclusion is R5 should be decreased, so that R4 is better matched. Is my thinking correct?

BDP · 15th February 2004, 03:08 AM

Quote:

The sensitivity is incredible – the bias voltage sweeps the entire range with only a few degrees of rotation on the trimpot. It’s impossible to set, because it is so sensitive. Both channels operate the same way.

The pot adjustment with the supplied values works just great, no problems setting the bias. I would look some where else for your problem. If you could give more info, voltage reading, maybe someone could help you out.

BDP

Nelson Pass · 15th February 2004, 05:06 PM

I would use an ohmmeter to check the actual values of the
resistor and trim pot out of circuit. Also, you could simply
subsitute in a resistor value, say 10K or 15K instead of the
trim pot and see what you get.

pro · 26th February 2004, 11:43 AM

Are you shure that the trimpot is connected in the right way?
I think only two legs mast be connected and one left floating.

15th February 2004, 02:16 AM	#1
Kashmire diyAudio Member Join Date: Dec 2003 Location: Midwest	Zen v.4 Bias Sensitivity Difficulty I built a Penultimate Zen with the recommended component values for biasing Q1 (a 25K trimpot and 47.5K fixed resistor for R4 and R5). Starting with the trimpot fully CCW, the bias voltage at the drain of Q1 is low. As the trimpot is advanced, the bias voltage doesn’t change much. When R4 is almost full CW, the bias voltage suddenly changes from about 8V to 40V. The sensitivity is incredible – the bias voltage sweeps the entire range with only a few degrees of rotation on the trimpot. It’s impossible to set, because it is so sensitive. Both channels operate the same way. My conclusion is R5 should be decreased, so that R4 is better matched. Is my thinking correct?

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15th February 2004, 05:06 PM	#3
Nelson Pass The one and only Join Date: Mar 2001	I would use an ohmmeter to check the actual values of the resistor and trim pot out of circuit. Also, you could simply subsitute in a resistor value, say 10K or 15K instead of the trim pot and see what you get.

26th February 2004, 11:43 AM	#4
pro diyAudio Member Join Date: Feb 2004 Location: Milano	Are you shure that the trimpot is connected in the right way? I think only two legs mast be connected and one left floating.

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