A-75 Bias voltage

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I am finishing up my A-75 (after many years of fits and starts). My present problem is in the measurement of DC voltage ascross the +DRV and -DRV points in the circuit. The construction article says this should not exceed 8 vdc or there may be problems (drawing excessive current) when the output stage is connected and fired up. (By the way, I interpret the wording in the article to mean that neither +DRV nor -DRV should see more than 8v difference from ground. )

When I first made this measurement I got excessive values. Following the article's advice, I reduced R80 from 3300 ohms to 2700 ohms. The situation is improved, but I still get 8.5 to 8.75 vdc difference (and about 17.5 volts between +DRV and -DRV).

I am using IRF 231 and 9231 TO3 devices I bought years ago and don't have a lot to spare, so I don't want to endanger the output stage AT ALL. Not to mention the enormous time I spent building them with sockets, 10 gauge bus wire, and point to point resistor connections.

What should I do. Lower R80 again? If I connect the output stages will that lower the voltage somewhat in and of itself? Do I have any margin of safety here?

By the way, when I adjust P1 and P2, I can't exceed about .850vac across R7 and R8 without the waveform starting to go from finely traced to a sort of smeared or fuzzy line (always symmetrical though). My cleanest wafeforms occur with a level around .775vac. Wassup with that fuzzyness? Is that the dreaded high freq. oscillation I read about? How do you check for that?

I have kept all parts substitutions to the list of equivalent devices in the article, and used 1% MF R's throughout.

Any help would be greatly appreciated.
Joined 2001
Paid Member
First, be sure that P3 is set for maximum resistance (5K ohms).

Next, measure the voltage across DRV+ and DRV-. To do this, set your DVM to DC Volts and put its red test probe on DRV+ and the black probe on DRV- and note the reading. You are looking for 7V or less so you will be able to dial the output stage bias current down to zero for initial testing.

Don't connect the output stage until you can get this voltage to 7V, or you could lose the output stage. You could reduce R80 further, but if you had to go much below 2700 ohms I would suspect another problem.
Congrats on making it this far with an ambitious project. Mine took a while also mainly due to the price of the parts and a couple of sidetracks with my heatsinks. As for your questions:

Your dc voltage across r7/r8 is normal. Mine barely made it to .850 and the fuzziness is in all likelyhood due to oscillations. These can be eliminated by installing a 150 mf electrolytic bypass on the incoming rectified power to the driver card. I screwed mine into the socket strips on the card, one for positive and one for negative on both sides. Watch your polarity, maintaining the negative side of the cap to the most negative point. For the negative rail this means cap positive will be to ground and cap negative will be to the negative rail. This should make the fuzzyness go away.

Your +/- drive output voltage question I cannot answer as it has been too long a period. There is however a lot of A75 material in here if you do a search on it. You may find what you are looking for. If not, leave an email at pass diy. It seems to me that you are alright with what you have but I am not sure enough to tell you to go ahead with it. You are wise to find an answer first as this is not a cheap project and you could not easily replace that style of output device. I used the 9240/240 to3p devices and custom built cards for them. I know you put a lot of work into yours and hopefully soon you'll be rewarded. It really is unbelievable to hear. Good Luck:)

Many thanks to PassFan and Mr. Berry for their responses. I am now quite worried however, that I have a problem, given that Mr. Berry indicates that bias voltage measured from +DRV to -DRV should be 7 volts and, as I indicated in my post, I have more than 8 vdc from EITHER to ground, and over 17 vdc between the two DRV points!! To quote Mr. Pass: "Double-plus ungood".

Can anyone suggest where I might have gone wrong to get a bias voltage that is 10 vdc above where it should be with P3 at full CCW? My regulated voltage locked in at 47.5 volts as I ramped up the Variac, so I don't see how there could be a problem there.

The only other curious thing I see is that when first adjusting P1 and P2 (when they are set pretty far CCW) I almost immediately see a clipped waveform as I begin to turn up the Variac from zero AC, then the signal flattens out and disappears at about one quarter or so up the Variac scale, leaving a flat trace line and almost no DC offset from the center grid line on my Tek. Turning up the P1 and P2 pots from this point brings up the trace on the scope again, which looks symmetrically clipped until I reach around .700 vac. (then, as I mentioned, above .850 it gets a little furry).

Should I call for an Exorcist? Or just advertise the unit on e-bay as a boat anchor?
Joined 2001
Paid Member
Suggest you locate bias MOSFET Q11 (IRF610 or equiv.) on the PC board and measure the voltage across its D and S terminals with the circuit under power and P3 at 5k ohms. This should be the same as the voltage across +DRV and -DRV. If you still get 17V or so there, my suspicion is that Q11 is installed backward or is bad.
Don't give up on it. Mine drove me nuts for about 2 weeks but I finally got it. Send Nelson an e-mail and ask him. He's at passdiyaudio. Don't write him a dissertation just ask him your question about the +/- drive and if it's wrong what could cause it. If you ask too much he will prod you to seek on your own. I always learn more this way anyhow, but you will get it and you will be glad you did.:)
Q11 Vgs multiplier should be adjusted to produce about 2 times the Vgs of the output mosfets thus the amount of bias current for the output.
Measure the voltage across P3 which would be Vgs of Q11, then measure the voltage across R80. Lets say for example that the Vgs is at 4 volts and your pot P3 is set at 2.5K then 4 volts divided by 2.5K = 1.6ma, this same current will flow through R80, 1.6ma times 3.32K = 5.3 volts, so the total drive voltage would be 4volts plus 5.3volts = 9.3 volts. If this voltage is to high then you would need to increase P3, so lets say you were to increase P3 to 5K then you would have 4 volts divided by 5K = .8ma, this current times R80, 3.32K, = 2.6 volts. Total DRV voltage would be 4 volts plus 2.6 volts = 6.6 volts. If your Q11 Vgs is very high then you may need to adjust R80 down to reduce the DRV voltage. With the numbers you are getting I would doubt that is the case.

Hope this helps to solve your problem.
A-75 bias voltage: continuing efforts

Mr. Berry: thank you for your suggestion to check Q11. The orientation of the device is correct in both channels. When I tried to measure the Vds however, I discovered there is NO dc potential between these points on either channel's device. :confused:

I e-mailed Mr. Pass about this last week and got his suggestion to check resistor values in the neighborhood of Q11. These appear to be OK from resistor codes, but I have not yet tried to pull them out of the circuit and test them individually.

If there is no Vds on either channel's Q11, would you conclude that both devices are blown?

Thank you for your help.
Well. I have checked resistors in the neighborhood of the Bias Mosfet, Q11, and can't find anything amiss on either channel. Ditto for orientation of devices. However, with Vdc ( I stated this wrong in an earlier post, describing it as Vac--I'm learning slowly, anyway!) across R7 and R8 at about .800 each, and a nice waveform trace on the output, and with my too-high +DRV to -DRV voltage of 17+ Vdc, I get NO Vgs on Q11, and No Vds on it either. Same for both channels. I supposed I should conclude that both Q11's are toasted? I guess I will have to pull them out of the circuit and check. I built this thing stock with OCSL boards and made no omissions except R81 and the lag comp Cap. Oh yeah, no fan or Q for it either on power supply board.

This has been a real puzzle. I have built a few things since my first dynaco kit in '76, and my Leach amp has been putting along (however boringly) since '78 without incident. But I can't seem to get over this hump. I must have killed too many braincells along the way.
You are aware of some misprints on the cards from OCSL as well as the layout in the original article. I vagely remember something about this and it was addressed in the questions section an a later issue. I'm out of town working right now and don't have access to my old notes. If you caught that then nevermind but if not you may have some resistors loaded wrong.:rolleyes:
Thank you PassFan for the additional thought. Yes. When I bought the OCSL boards there was a memo included in their shipment; the memo from Mr. Pass listed a few errors that had crept into the board layout drawing. I thought I accounted for them all, but it is worth a comparison to the schematic one more time, and I will do that.

Thanks again for the help from everyone at DIYaudio. This forum helps keep the OMG! (panic) level down.

Larry Wright,

Well I still don't see where I got a problem with R's on both channels. And yesterday I unsoldered one of the Q11 bias mosfets and checked it in the original testing circuit described in the article. At 15 vdc I get a Vgs measurement (for an IRC 510) of 3.49 volts--right in the range for all the other 510s I measured.

Before this, I tried to check junctions on the device with the diode test function on my trusty old Beckman MM. (Forces 5ma of current through). Got weird results: expected voltage drop in one direction, and OL in the other, then after switching leads back and forth, a very low reading in place of the OL reading (which would be bad--shorted innards or something). Switching leads again, however, I get first the expected voltage drop in one direction, then back to OL. Almost like the device was 'latching' in one state or another. Maybe you can't test a mosfet in this manner??? In any case, back into the published test setup, and the device measures normally every time (and right in line with two other 510's that have never been in the circuit)... Got my copy of The Art of Electronics Friday and gonna ed-u-cate mahself, but still don't got no background music to read by!!

Larry Wright
I'm struggling with the same problem. Have checked Q11..VGS 4V and replaced R80 even though the one I pulled out measured spot on. I'm still measuring 18V across +/-DRV. The amp has been in sevrice for 15 years but recently went out on me. At this point I'm suspecting it may be a bad zener (either Z3 or 4). Don't have any spares in the box so before I go that route thought I check here.
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