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An externally hosted image should be here but it was not working when we last tested it.
Bottom:
An externally hosted image should be here but it was not working when we last tested it.
Thanks for looking ZM.
This is the schematic I'm using - the corrected one posted by Papa:
http://www.diyaudio.com/forums/showthread.php?postid=1343785#post1343785
The schematic I used to make adjustments and match part layout is from the original volksamp A30 manual that used to be available. The part numbers match the numbering Peter used.
More pics:
ZTX550 'modified' for different pinout than IRF9610:
2SJ74BL matched and stuck face-to-face. Legs standing in different spots to make up for (again) different pinout than IRF9610:
This is the schematic I'm using - the corrected one posted by Papa:
http://www.diyaudio.com/forums/showthread.php?postid=1343785#post1343785
The schematic I used to make adjustments and match part layout is from the original volksamp A30 manual that used to be available. The part numbers match the numbering Peter used.
More pics:
ZTX550 'modified' for different pinout than IRF9610:
An externally hosted image should be here but it was not working when we last tested it.
2SJ74BL matched and stuck face-to-face. Legs standing in different spots to make up for (again) different pinout than IRF9610:
An externally hosted image should be here but it was not working when we last tested it.
replace R8 with 470R reesistor and 470Rpot , both in series
start with pot on max resistance (check with DMM)
desolder one end of R27 and put 47K pot in series ; start with pot on zero ohm (check with DMM)
set Iq and offset iteratively with both pots
later you can check AC gain of Aleph CCS - with R24 value
start with pot on max resistance (check with DMM)
desolder one end of R27 and put 47K pot in series ; start with pot on zero ohm (check with DMM)
set Iq and offset iteratively with both pots
later you can check AC gain of Aleph CCS - with R24 value
Thanks for the help ZM.
I replaced R8 with the 470+500 ohm pot and lifted one end of R27 and attached a 10K pot.
Results:
Voltage across source resistor went from 2mV to 4.3mV with R27 at 78K and dc offset went up instead of down (increasingly negative - I must have connected it wrong or read it wrong)
I put the 1K back with one leg attached via the 500ohm pot and replaced R27 with 100K + 10K pot
Voltage across source resistor went up to 183mV and I could adjust offset down to 0, but output is still very low with a pronounced buzzing/hum, so I think there's something else fundamentally wrong here
I'll retrace the jfet and ztx connections 😕
Oh, almost forgot - while it was playing, I tried to increase the volume, but after a certain point, the amp starts cutting in and out (is that oscillation?) I don't have a scope, so that's about as much as I can say. The mosfets do start to warm up however.
I replaced R8 with the 470+500 ohm pot and lifted one end of R27 and attached a 10K pot.
Results:
Voltage across source resistor went from 2mV to 4.3mV with R27 at 78K and dc offset went up instead of down (increasingly negative - I must have connected it wrong or read it wrong)
I put the 1K back with one leg attached via the 500ohm pot and replaced R27 with 100K + 10K pot
Voltage across source resistor went up to 183mV and I could adjust offset down to 0, but output is still very low with a pronounced buzzing/hum, so I think there's something else fundamentally wrong here
I'll retrace the jfet and ztx connections 😕
Oh, almost forgot - while it was playing, I tried to increase the volume, but after a certain point, the amp starts cutting in and out (is that oscillation?) I don't have a scope, so that's about as much as I can say. The mosfets do start to warm up however.
twitchie said:.....
Voltage across source resistor went up to 183mV and I could adjust offset down to 0, but output is still very low with a pronounced buzzing/hum, so I think there's something else fundamentally wrong here
.........
did you drive it with unbalanced signal , but forgetting to ground negative input?
no, input neg is jumped to ground - you can see the jumper on the right side of the board in the last picture.
I'm 99% certain all the resistors are correct. I'm going to go over the schematics again to make sure I didn't miss any jumpers
I'm 99% certain all the resistors are correct. I'm going to go over the schematics again to make sure I didn't miss any jumpers
There is something fishy about R27 in the aleph j schematics. I ended up using the placement suggested by the schematics one leg between q4 and r26 - contrary to normally between r26 and r25. You can see my quest here:
http://www.diyaudio.com/forums/showthread.php?postid=1565554#post1565554
But I think you have a different problem. How did you make that pcb? Have you plated the holes yourself or did you use some chemicals to remove the screen from peters boards?
One thing that might help you in "mysterious bug" finding is to take the schematics and write as many voltages (in reference to gnd or relative) on the nodes as you can then you might discover something or if you post it, we might be able to find the error. 😉
If you're not a black belt pdf artist you can just take a screenshot and put in the numbers in using paint.
http://www.diyaudio.com/forums/showthread.php?postid=1565554#post1565554
But I think you have a different problem. How did you make that pcb? Have you plated the holes yourself or did you use some chemicals to remove the screen from peters boards?
One thing that might help you in "mysterious bug" finding is to take the schematics and write as many voltages (in reference to gnd or relative) on the nodes as you can then you might discover something or if you post it, we might be able to find the error. 😉
If you're not a black belt pdf artist you can just take a screenshot and put in the numbers in using paint.
Thanks for the suggestion CVILLER - I didn't use anything to remove the screen from the boards as I didn't modify anything other than jumping some resistors and putting a resistor in C7 instead of a cap 😀
Here's what I measure on my board. My rail voltages are a bit high today for some reason 27.6V. Maybe demand is low today so the grids are nice and full
Here's what I measure on my board. My rail voltages are a bit high today for some reason 27.6V. Maybe demand is low today so the grids are nice and full
An externally hosted image should be here but it was not working when we last tested it.
Sure those 27 V are not due to lack of bias current?
For the front I would replace R8 with pot and try to get more bias current. Make sure that you have not inverted the ztx550 somehow. Also check that Q3 is not stealing the current.
As for the current source, it seems like you don't get it turned on properly. What are the voltages across R16 and R17? Not to be annoying, but are you sure that R15 is not connected to the unpopulated 0.47 resistor?
Try to completely remove R27.
Perhaps Q4 is acting strange. You could try to remove it (but beware you'll get a lot of current), or you could try to replace it with a ~777ohm resistor. Then you should get approx 4.3V over it, unless something else is pulling your leg....
My best guess is that you have something slightly wrong connected - there are some subtle differences in the two schematics, you might have overlooked something.
For the front I would replace R8 with pot and try to get more bias current. Make sure that you have not inverted the ztx550 somehow. Also check that Q3 is not stealing the current.
As for the current source, it seems like you don't get it turned on properly. What are the voltages across R16 and R17? Not to be annoying, but are you sure that R15 is not connected to the unpopulated 0.47 resistor?
Try to completely remove R27.
Perhaps Q4 is acting strange. You could try to remove it (but beware you'll get a lot of current), or you could try to replace it with a ~777ohm resistor. Then you should get approx 4.3V over it, unless something else is pulling your leg....
My best guess is that you have something slightly wrong connected - there are some subtle differences in the two schematics, you might have overlooked something.
My guess is that your jfets are different than Nelson's. Change R8 to a pot. It sets the current through the jfets and this current defines the voltage drop through R7. This sets the voltage bias for the output fets and they seem to be too low.
Edit..
R8 also affects the DC offset at the output. I think the correct value for R8 would give you 0 V DC offset. Playing with sim's showed me that this was also the low distortion point. Amazing that this one resistor seemed to control so much.
Jim
Edit..
R8 also affects the DC offset at the output. I think the correct value for R8 would give you 0 V DC offset. Playing with sim's showed me that this was also the low distortion point. Amazing that this one resistor seemed to control so much.
Jim
I had a look at the photos, and I think I see what potentially look like some cold solder joints. Generally speaking you have been a little light on the solder usage. You may want to go back over the board and ensure that each pad gets a nice covering of solder to ensure a proper electrical connection.
Cheers, Terry
Cheers, Terry
I replaced R8 with a 2K pot set to 1K and I put the 68.1K resistor back in R27, but now I'm getting runaway current into the lower (V-) devices with over 900mV going through the source resistors and R16 and R17 momentarily see approximately 600mV but drop to 0 immediately (powering on with variac).
I can't say this is entirely not-fun, but did I damage something?
I can't say this is entirely not-fun, but did I damage something?
Thanks Terry - I soldered on the bottom, not the top layer - I'm generally not cheap with solder and doubt there could be cold joints, but I won't rule that out.
I was measuring resistance where possible on the populated boards and everything is checking out according to the schematic (ruling out misplaced resistors), but I'm inclined to believe anything right now, including alien's messing with my brain (where did I put that tin foil cap?) ...
I was measuring resistance where possible on the populated boards and everything is checking out according to the schematic (ruling out misplaced resistors), but I'm inclined to believe anything right now, including alien's messing with my brain (where did I put that tin foil cap?) ...
I was hoping someone that knows more would pipe in, but, this suggestion may help. Lift the ends of R11 and R12 that are fed from the jfet. Tack these to the -27.6. Now the Mosfets should not be conducting. You can turn on the amp and should have time to check the jfet section for proper levels. You want to adjust R8 for the 4.3V across R7 as per Nelson's schematic. You should have a nice steady 4.3V here. If this 4.3 V DC starts cutting out, its either the power supply or the jfet portion. If not, it's on to the output section for fun and games, but at least the jfet section will now be set very close to the correct bias voltage for the output section. If your nervous about the Mosfet section, you could test the safer way and remove Q5 and Q6.
Jim
Jim
Thanks Jim. I picked up a couple of 100K pots today (and 50K too). I've had enough non-catastrophic failures that killing a couple of mosfets won't sadden me too much 
... besides, the heat is welcomed with the weather we've been having lately
Worst case, I can test on the other board which has no output devices connected.
So if I'm reading you correctly, the 4.3V through R7 is more important than the 8.3V through R8?
If I don't adjust anything else, would you expect this thing to run if I replaced the jfets and the ztx550 with irf9610 (and turn it back into a straight Aleph 3)? I haven't seen the schematic for the A3, so I don't know what the values should be (only have A30).
Thanks again
Stephen
... besides, the heat is welcomed with the weather we've been having lately Worst case, I can test on the other board which has no output devices connected.
So if I'm reading you correctly, the 4.3V through R7 is more important than the 8.3V through R8?
If I don't adjust anything else, would you expect this thing to run if I replaced the jfets and the ztx550 with irf9610 (and turn it back into a straight Aleph 3)? I haven't seen the schematic for the A3, so I don't know what the values should be (only have A30).
Thanks again
Stephen
maybe this will help ....... old and fun "article" ;
nota bene - for your conversion case ( 2 pairs of outputs , Papa's CCS )
text is more important than exact schematic
later I found that - with today's little bjts (BC546B and BC556B ) few resistor values need to be different - WR2 + series resistor , and also all WW resistors ;
this schematic was on spot with ancient Philips bjts from my drawer
nota bene - for your conversion case ( 2 pairs of outputs , Papa's CCS )
text is more important than exact schematic
later I found that - with today's little bjts (BC546B and BC556B ) few resistor values need to be different - WR2 + series resistor , and also all WW resistors ;
this schematic was on spot with ancient Philips bjts from my drawer
twitchie said:
Yes the 4.3V over R7 is the most important. The 8.3V will change somewhat depending on how accurate the 9.1 V zener is and the variation in the jfet from Nelson's.
Sorry I can't help with the Aleph3, it's the Aleph J schematic I have been studying as I have been thinking about building it.
After you finish with the jfet section you could try checking the Aleph current source up top. The last part of the circuit to check would probably be the current limiter, Q3.
Jim