Tried with trimpots for bias
OK. I have replaced 1K R8 resistor with 470 fixed + 500 ohms trimpot. DC offset could be adjusted to zero volts. As the output transistors get heated up over a period of time, the offset drifts upto 40mv and requires readjustment of the trimpot.
The amp is a lot hotter than F5 after 45 mins or so. Clearly requires bigger heatsinks than F5.
This is the exact behavior of both the channels. Once the boards get cooled, I will measure the resistance across the trimpot and see if I need to replace with a fixed one.
Planning to connect some old speaker and check audio.
Another point, I want some advice on is: once the power is turned off to the power supply, the DC offset goes and crosses over 1V and then slowly dies down to zero. Does it signify a turnoff thump? I hope this does not kill my speaker.
TIA.
OK. I have replaced 1K R8 resistor with 470 fixed + 500 ohms trimpot. DC offset could be adjusted to zero volts. As the output transistors get heated up over a period of time, the offset drifts upto 40mv and requires readjustment of the trimpot.
The amp is a lot hotter than F5 after 45 mins or so. Clearly requires bigger heatsinks than F5.
This is the exact behavior of both the channels. Once the boards get cooled, I will measure the resistance across the trimpot and see if I need to replace with a fixed one.
Planning to connect some old speaker and check audio.
Another point, I want some advice on is: once the power is turned off to the power supply, the DC offset goes and crosses over 1V and then slowly dies down to zero. Does it signify a turnoff thump? I hope this does not kill my speaker.
TIA.
Thanks. Connected old speakers and checked. The turn-off thump is quite mild and I do not think is a problem.
As far as how it sounded: The gain seems to be less. Required much high level of input signal than a F5. Highs were quite nice, but missing mids/lows and a high level of distortion. After all this, I am forced to come to the conclusion that the JFETs may be fake ones and do not exhibit the properties of 2SJ74BLs.
I have ordered a set of matched JFETs from Blues and should be with me shortly. Will check with them and see where it goes.
Cheers.
Success on 1 Channel!!
Changed the JFETs to some old stock I had. Learnt to measure IDSS and got a pair at 10mA. Put them in place of the fake JFETs in one channel. Offset adjusted to 2mv, after outputs reaching operating temperature (HOT). Played music.
Working absolutely wonderful. Looks like working to full spec of Mr. Pass's Aleph J. Outstanding! Cannot yet get the other channel to work, as I do not have one more set of real FETs. Waiting for my shipment to arrive.
Thanks for all the help. I still need to get the other channel to work. Will post the developments.
Cheers.
Changed the JFETs to some old stock I had. Learnt to measure IDSS and got a pair at 10mA. Put them in place of the fake JFETs in one channel. Offset adjusted to 2mv, after outputs reaching operating temperature (HOT). Played music.
Working absolutely wonderful. Looks like working to full spec of Mr. Pass's Aleph J. Outstanding! Cannot yet get the other channel to work, as I do not have one more set of real FETs. Waiting for my shipment to arrive.
Thanks for all the help. I still need to get the other channel to work. Will post the developments.
Cheers.
2nd Channel having problems
OK. Took the good JFETs out of the working channel and put them in the 2nd channel and tested.
The board is not working right. There is low gain and distortion. What could be the issue. JFETs are not the culprits. The outputs are matched pairs.
Can the suspect be one of the Q2, Q3, Q4 ZTX devices? I will start measuring the voltage points and see where this goes.
Any troubleshooting tips?
TIA.
OK. Took the good JFETs out of the working channel and put them in the 2nd channel and tested.
The board is not working right. There is low gain and distortion. What could be the issue. JFETs are not the culprits. The outputs are matched pairs.
Can the suspect be one of the Q2, Q3, Q4 ZTX devices? I will start measuring the voltage points and see where this goes.
Any troubleshooting tips?
TIA.
I am using unbalanced input. Grounded the -input. What data points should i measure and post? Please let me know. Earlier you posted someone else's document giving voltage points on aleph 30. I will measure similar points on aleph j and post.
Thanks.
Measurements
Left (Working) Right (Problem)
R8 .. 8.73 8.65
R7 .. 5.38 5.38
Z1 ... 9.4 9.5
R18 .. 0.518 0.521
R19 .. 0.514 0.526
R16 .. 0.525 0.525
R17 .. 0.489 0.500
DC Offset .. 22mv 18mv (Both channels reduce with increase in temperature).
Q4 (C-E).. 5.3 5.3
Q4 (B-E).. 0.32 0.32
Q4 (B-C).. 4.4 4.5
Q3 (C-E).. 5.25 5.23
Q3 (B-E).. 0.323 0.324
Q3 (B-C).. 4.9 4.9
I am completely lost. They seem to be performing identically in terms of electrical characteristics. Left channel sounds perfect, right channel has poor gain and distortion when input level is increased.
Thanks.
Left (Working) Right (Problem)
R8 .. 8.73 8.65
R7 .. 5.38 5.38
Z1 ... 9.4 9.5
R18 .. 0.518 0.521
R19 .. 0.514 0.526
R16 .. 0.525 0.525
R17 .. 0.489 0.500
DC Offset .. 22mv 18mv (Both channels reduce with increase in temperature).
Q4 (C-E).. 5.3 5.3
Q4 (B-E).. 0.32 0.32
Q4 (B-C).. 4.4 4.5
Q3 (C-E).. 5.25 5.23
Q3 (B-E).. 0.323 0.324
Q3 (B-C).. 4.9 4.9
I am completely lost. They seem to be performing identically in terms of electrical characteristics. Left channel sounds perfect, right channel has poor gain and distortion when input level is increased.
Thanks.
Problem persists, nothing identified
Removed the board from the heatsinks, checked each and every solder and polarity and values of passive components with a magnifying glass. Cannot find anything. Cleaned small solder blobs here and there and scrapped cleared with a knife the gaps between each terminal of jfet.
First connected the faulty amp to one channel of the preamp and one of my regular Fostex speaker. Played music. Low gain. Preamp volume control goes all the way upto 1/2 of the dial. Pronounced emphasis on treble and audible distortion, predominantly in the highs. DC offset around 22mv to begin with.
Disconnected the preamp and connected the good amp board. Same preamp feed and same speaker used, to eliminate variations. Played music. Full bodied superb and well balanced sound. Volume control around 1/4 of the dial.
Some component that alters the frequency response and gain of the amp seems to be the cause. To me JFETs are ruled out as bad, as I swapped between the good and bad amp. Can they be one of the caps - C1, C2, C3 or one of the Q2, Q3, Q4 ZTXs? No clue.
TIA.
Removed the board from the heatsinks, checked each and every solder and polarity and values of passive components with a magnifying glass. Cannot find anything. Cleaned small solder blobs here and there and scrapped cleared with a knife the gaps between each terminal of jfet.
First connected the faulty amp to one channel of the preamp and one of my regular Fostex speaker. Played music. Low gain. Preamp volume control goes all the way upto 1/2 of the dial. Pronounced emphasis on treble and audible distortion, predominantly in the highs. DC offset around 22mv to begin with.
Disconnected the preamp and connected the good amp board. Same preamp feed and same speaker used, to eliminate variations. Played music. Full bodied superb and well balanced sound. Volume control around 1/4 of the dial.
Some component that alters the frequency response and gain of the amp seems to be the cause. To me JFETs are ruled out as bad, as I swapped between the good and bad amp. Can they be one of the caps - C1, C2, C3 or one of the Q2, Q3, Q4 ZTXs? No clue.
TIA.