check the voltage across R4. It should be passing zero current and thus should have zero Vdrop.
You state 30mV indicating that the capacitor is passing/leaking if the Vreadings are correct.
The voltage across Vr1 seems wrong.
This should be passing current and so should have a Vdrop.
Check your readings.
You state 30mV indicating that the capacitor is passing/leaking if the Vreadings are correct.
The voltage across Vr1 seems wrong.
This should be passing current and so should have a Vdrop.
Check your readings.
IIRC, the voltage at R4 was fluctuating. That may explain the Vdrop. I will measure across it to be sure.
Vr1 has no effect on the bias right now though I did check to make sure it is working. It may have been set to lowest resistance at the time of the readings. I will have to check that. I have been bringing the amp up very slowly with the variac through light bulb and cap multiplier, and with an ammeter connected to one of the rails to make sure I don't have the bias too high. It's Thanksgivings Day today so not sure if I will get much done on it today. If I do I will report back.
Blessings, Terry
OK I got a chance to change out Q8. That has me closer but I still can't raise the bias. Bias is .013mA at the rail. VR2 has to be all the way up to get the offset down to 63mV.
I have posted a new schematic with the voltages I have now.
Thanks, Terry
I have posted a new schematic with the voltages I have now.
Thanks, Terry
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Great news, that was it. I just replaced Q10 and now all is well. Now I just need to look back through and see what I had the bias set to. Thanks to all of you for letting me work this out here. Now I can finish my case and get this thing melded into my system.
Blessings and Happy Thanksgiving to all.
Terry
Blessings and Happy Thanksgiving to all.
Terry
Member
Joined 2009
Paid Member
Hi Guys,
I had a strange, (to me) thing happen when I hooked up everything in the case today. When a single channel is hooked up to the power supply, and the bias set to 100mA all is well. but when both channel are hooked up together, the bias goes off the chart and blows the fuses. I thought something was hurt so I pulled it all apart and went to testing each board. Thanks goodness they both seemed just fine once the fuses were replaced. I reinstalled them in the case and when powering up through my light bulb tester the light goes bright again so I shut it down and backed the bias all the way down and then brought them up slowly until I had 100mA per rail on each side. I did notice that the adjustment is very touchy but will hold. Any idea why the bias would be so different when connected to the same PSU?
Thanks, Terry
I had a strange, (to me) thing happen when I hooked up everything in the case today. When a single channel is hooked up to the power supply, and the bias set to 100mA all is well. but when both channel are hooked up together, the bias goes off the chart and blows the fuses. I thought something was hurt so I pulled it all apart and went to testing each board. Thanks goodness they both seemed just fine once the fuses were replaced. I reinstalled them in the case and when powering up through my light bulb tester the light goes bright again so I shut it down and backed the bias all the way down and then brought them up slowly until I had 100mA per rail on each side. I did notice that the adjustment is very touchy but will hold. Any idea why the bias would be so different when connected to the same PSU?
Thanks, Terry
Member
Joined 2009
Paid Member
It was delicious!!!
As for the bias, I was able to set it to 150mA per rail and let it play for a couple hours without issue. Hopefully it is fine. It was just really weird that the bias changed so much when a second channel was connected to the PSU. Both channels reacted the same way. At 150mA it is running just slightly warm to the touch. Before I had it in a case I was running it at 225mA per rail and it ran pretty warm. It sounds fine at the 150mA so I will leave it there for a while.
Thanks, Terry
As for the bias, I was able to set it to 150mA per rail and let it play for a couple hours without issue. Hopefully it is fine. It was just really weird that the bias changed so much when a second channel was connected to the PSU. Both channels reacted the same way. At 150mA it is running just slightly warm to the touch. Before I had it in a case I was running it at 225mA per rail and it ran pretty warm. It sounds fine at the 150mA so I will leave it there for a while.
Thanks, Terry
Something weird is still going on. It played "OK" for a while with a stand alone CD player but when I plugged in a preamp and turned it up the power supply fuses blew. I may be that I am creating some type of electric field or something. I replaced the fuses turned down the volume and gave it another try. After playing for a few seconds it will start making a sort of sizzling sound. If I pull the power cables from either one of the channels it plays fine. It also played fine when I had the heatsinks laid flat on the work bench with the power supply in front of them. Tomorrow, I will run some longer wires so I can route them around the end of the PSU and see if that cures it. I have included a pic of how it is presently wired.
Blessings, Terry
Blessings, Terry
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Try to decouple supply wires by 220nF capacitors. Because of MOSFET sources coupled to rails the two channels interfere.
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I'll see if I have some 220nf caps. I wonder why it worked perfectly on the bench? Seems one channel is more sensitive to bias adjustment than the other. Thanks for the suggestion. I will try it out.
Blessings, Terry
Member
Joined 2009
Paid Member
Look carefully at the earthing, this is one bit of wiring that probably changed when you moved it all into the chasis. I can't see from the photo what exactly is connected to what and to where. But any speaker currents that end up on the signal ground will be bad news. And earthing also includes your mains house wiring so you will want to have an earth loop break somewhere. The fact that the mains house wiring is part of the earthing can influence stability when you plug in and out the source into the RCA inputs. If you can post a clear diagram of your wiring scheme the clever chaps here will likely spot anything out of sorts.
You may have something there. When on the bench there was no earth ground at all. I used only a two wire cord which hooked to the primary of the transformer. The only thing earthed right now is the case and the shield on the input cable but the shield is only connected to the case. In the picture I posted, the green wires are the the circuit ground.
I do believe that the preamp leads are earthed at the preamp so that may be the issue. I will have to rethink this.
Thanks, Terry
I do believe that the preamp leads are earthed at the preamp so that may be the issue. I will have to rethink this.
Thanks, Terry