Thank you Chris! So in the high bias channel I've got to decrease the resistance, right? So I should be able just to turn the pot down... I will try...
For the 3 diodes string, I'll try to see how it works... but what about that vbe multiplier? I'll search the forum and will post what I get.
For the 3 diodes string, I'll try to see how it works... but what about that vbe multiplier? I'll search the forum and will post what I get.
Hi Giaime,
Yes, decrease the resistance to lower the bias current. There are many examples of a vbe multiplier. Don't get fancy. The circuit really only needs two fixed resistors, a potentiometer and a transistor. A resistor is in series with the pot to prevent someone from turning the control so far that the outputs are instantly destroyed. The pot and resistor go between the base and emitter, another resistor between the base and collector. The transistor normally mounts on the heatsink.
-Chris
Yes, decrease the resistance to lower the bias current. There are many examples of a vbe multiplier. Don't get fancy. The circuit really only needs two fixed resistors, a potentiometer and a transistor. A resistor is in series with the pot to prevent someone from turning the control so far that the outputs are instantly destroyed. The pot and resistor go between the base and emitter, another resistor between the base and collector. The transistor normally mounts on the heatsink.
-Chris
Hello! Chris, please, not to offend you, but... Are you sure that H714 is a normal diode and not a Zener? And are you sure I can replace it with a 1N4001? I think that's the problem because everything else seems to be working well...
The two channels now are IDENTICAL: the only difference is that diode. On the other channesl (that one that biases at 25mV) that diode (H713) is still original, on the cut-off channel (where voltage across that resistor goes down to 0mV) I've replaced it.
I remember a friend of mine searched in his transistor replacement books and founds that S3016, or better, 1S3016 is a 16V Zener diode...
Are you sure it's a normal diode like you said in a previous post?
The two channels now are IDENTICAL: the only difference is that diode. On the other channesl (that one that biases at 25mV) that diode (H713) is still original, on the cut-off channel (where voltage across that resistor goes down to 0mV) I've replaced it.
I remember a friend of mine searched in his transistor replacement books and founds that S3016, or better, 1S3016 is a 16V Zener diode...
Are you sure it's a normal diode like you said in a previous post?
Hi Giaime,
This diode should never be reversed biased to begin with. I can't find the original spec on this part although we know 1S numbers are diodes. Anyhow, in this circuit position, we want a low drop. At 16V everything has gone south. What is the measured voltage drops across both these diodes?
-Chris
This diode should never be reversed biased to begin with. I can't find the original spec on this part although we know 1S numbers are diodes. Anyhow, in this circuit position, we want a low drop. At 16V everything has gone south. What is the measured voltage drops across both these diodes?
-Chris
Ehm... I can't measure the voltage drop across both of the diodes, one just broke into little pieces... On the one that's still alive, I measure the same of a tipical 1N4001 (don't remember the value). I hoped that was the problem... a simple fix. I promise you I will work onto the amp tomorrow morning. I will post...
Edit: you say low drop... this makes me think. Should be that the original diode has developed some partial open circuit (and so it measures too high)? I replaced the other one reading this one, and I replaced with a 1N4001. But if in origin the Vd was (i.e.) 200mV, would that make trouble? Maybe I can replace them with Ge or Schotty diodes...
Edit: you say low drop... this makes me think. Should be that the original diode has developed some partial open circuit (and so it measures too high)? I replaced the other one reading this one, and I replaced with a 1N4001. But if in origin the Vd was (i.e.) 200mV, would that make trouble? Maybe I can replace them with Ge or Schotty diodes...
Hi Giaime,
No, the voltage drop is what I expect. The bias pot is in parallel with the diode. That's why there is a low drop. The diode will only conduct if the current through the pot reaches the drop of the diode (0.45~0.5 VDC). That is why a zener does not make sense here. This is also why I would condem the pot if the diode was burnt out across it. The minimum resistance of the pot is therefore very important, as is the voltage drop across the bias diode.
-Chris
No, the voltage drop is what I expect. The bias pot is in parallel with the diode. That's why there is a low drop. The diode will only conduct if the current through the pot reaches the drop of the diode (0.45~0.5 VDC). That is why a zener does not make sense here. This is also why I would condem the pot if the diode was burnt out across it. The minimum resistance of the pot is therefore very important, as is the voltage drop across the bias diode.
-Chris
Yeah! I have pretty much fixed the amp in question.
The high bias on right channel seemed to be a pot problem. I turned it around a couple of times. Then switched the amp on, got 3mV. I turned for 7.5mV, and after a while that value drifted to 13mV. I re-turned down to 7.5mV and now it stays there.
On the low bias channel, it clearly a problem of the thermal compensation diode that broke its leg and I fixed it. It should be the soldering iron's heat that made it a little leaky or something. With the pot on fully counterclockwise, it stays at about 3mV. Too little you'll say; I agree but the amp sounds good that way (yes, it would sound better on 7.5mV that means 30mA per device, instead of 12mA, but I don't care too much. I will verify on the scope if it does too much crossover distortion).
The happy part is that the amp is playing techno music for about an hour at full volume, on 4 ohm and bass all the way up and it barely gets just little over room temperature. Good!
What do you think, Chris? Shall we put the "end" word to this infinite thread?
The high bias on right channel seemed to be a pot problem. I turned it around a couple of times. Then switched the amp on, got 3mV. I turned for 7.5mV, and after a while that value drifted to 13mV. I re-turned down to 7.5mV and now it stays there.
On the low bias channel, it clearly a problem of the thermal compensation diode that broke its leg and I fixed it. It should be the soldering iron's heat that made it a little leaky or something. With the pot on fully counterclockwise, it stays at about 3mV. Too little you'll say; I agree but the amp sounds good that way (yes, it would sound better on 7.5mV that means 30mA per device, instead of 12mA, but I don't care too much. I will verify on the scope if it does too much crossover distortion).
The happy part is that the amp is playing techno music for about an hour at full volume, on 4 ohm and bass all the way up and it barely gets just little over room temperature. Good!
What do you think, Chris? Shall we put the "end" word to this infinite thread?
News! News! News!
Oh my God I now understand why the amp wasn't working in some ways...
http://www.diyaudio.com/forums/showthread.php?s=&threadid=55630
Chris, did you thought about that? I not...
Oh my God I now understand why the amp wasn't working in some ways...
http://www.diyaudio.com/forums/showthread.php?s=&threadid=55630
Chris, did you thought about that? I not...
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