WuYit,
When I had the card mounted to the heatsink running off the full supply voltage, the heatsink didn't become very hot.
I allready have two other cards mounted to the same heatsink, and i could perhasp feel that the heatsink was slightly warmer at the end where the "problem" card was mounted. but again, no big difference, certainly nothing more than I would expect from that card running with twice the idle current as the other cards.
When I had the card mounted to the heatsink running off the full supply voltage, the heatsink didn't become very hot.
I allready have two other cards mounted to the same heatsink, and i could perhasp feel that the heatsink was slightly warmer at the end where the "problem" card was mounted. but again, no big difference, certainly nothing more than I would expect from that card running with twice the idle current as the other cards.
Elbert,
if possible, from a linearity point of view, it would be beneficial to increase the current in the output transistors. To me, the DC balance is perfectly acceptable. Sorry, but I don`t quite understand the problem.
if possible, from a linearity point of view, it would be beneficial to increase the current in the output transistors. To me, the DC balance is perfectly acceptable. Sorry, but I don`t quite understand the problem.
Well, first of all, I'd like to keep the idle current with in the 200-250 mA range specified by elektor, simply because I am not fully able to fully understand the impact of going outside this range.
I think I read somwhere about someone cranking the idle current up as high as 1 A without any other effect than warm heatsinks.
My second problem is that I get an idle current of 350-400 mA at a potmeter setting that previously (before polarity reversal and subsequent replacement of blown parts) yielded the normal 200 mA..
Is it a result of the new output transistors having a sdifferent characteristic, or is it symptomatic of a problem burried somewhere else??
If it is the output transistors, then I could allways change some resostors in the circuit arround T11 to compensate, but I'd rather not do this without understanding what's actually going on.
Fine if I had adjusted the pot for a 350 mA idle current but I haven't it is not under my controll! 🙁
I think I read somwhere about someone cranking the idle current up as high as 1 A without any other effect than warm heatsinks.
My second problem is that I get an idle current of 350-400 mA at a potmeter setting that previously (before polarity reversal and subsequent replacement of blown parts) yielded the normal 200 mA..
Is it a result of the new output transistors having a sdifferent characteristic, or is it symptomatic of a problem burried somewhere else??
If it is the output transistors, then I could allways change some resostors in the circuit arround T11 to compensate, but I'd rather not do this without understanding what's actually going on.
Fine if I had adjusted the pot for a 350 mA idle current but I haven't it is not under my controll! 🙁
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Hi E,
considering only operating temperatures, would you be happy leaving the output bias ~350mA?
If you are not sure, then tell us Ts and Tc after a thorough warm up and we can advise.
considering only operating temperatures, would you be happy leaving the output bias ~350mA?
If you are not sure, then tell us Ts and Tc after a thorough warm up and we can advise.
is this confirming you have variable resistor P1 =1k fitted and that you have not changed it's setting since the first build up using the previous set of output devices?
Andrew,
Considering temperature only, the heatsinks I'm using are not massive, but I have some fans fitted, so thermally, this should not kill the project.
Having said that, I'd like to have all amplifier channels (3 in each chassis) running at the same current, which would then mean that the fans would kick in earlier. I included the fans more as a contingency and allthough they are not terribly noisy, I'd like to avoid having them run too fast at normal listening levels.
Sorry, but what does Ts and Ts mean?
With the good cards, the temperature only rose to a comfortable 37 degrees C or so after 30 min warm-up, and my impression was that around the bad card, the temperature was only some degrees higher, no dramatic difference, I probably hadn't noticed it unless I was aware of the high idle current.
Yes, I have the 1k pot fitted. I changed the position a bit sometimes, just to see if the idle current adjustment actually worked, but allways turned it back to the full stop minimum current position.
having said that, on the other good cards, I didn't have to move the put much beyond the minimum setting in order to get a 200-210 mA idle current.
PS
Sorry for my delayed answer, I was outside shoveling the driveway!
Considering temperature only, the heatsinks I'm using are not massive, but I have some fans fitted, so thermally, this should not kill the project.
Having said that, I'd like to have all amplifier channels (3 in each chassis) running at the same current, which would then mean that the fans would kick in earlier. I included the fans more as a contingency and allthough they are not terribly noisy, I'd like to avoid having them run too fast at normal listening levels.
Sorry, but what does Ts and Ts mean?
With the good cards, the temperature only rose to a comfortable 37 degrees C or so after 30 min warm-up, and my impression was that around the bad card, the temperature was only some degrees higher, no dramatic difference, I probably hadn't noticed it unless I was aware of the high idle current.
Yes, I have the 1k pot fitted. I changed the position a bit sometimes, just to see if the idle current adjustment actually worked, but allways turned it back to the full stop minimum current position.
having said that, on the other good cards, I didn't have to move the put much beyond the minimum setting in order to get a 200-210 mA idle current.
PS
Sorry for my delayed answer, I was outside shoveling the driveway!
Hi,
the bias adjustment pot gradually changes the output voltage (Vbias) of the Vbe multiplier as the VR is changed.
BUT !!!
the output stage does not appear to respond to this gradually changing bias voltage. The output stage turns on when Vbe~400mV and Ic increases rapidly as 600mVbe is approached.
This gives the apparent effect that nothing is happening when turning the VR, then suddenly you have too much Ic and have to back off.
If you monitor Vre and Vbias using two or three voltmeters, you will see that Vbias is responding and as it approaches the critical 400mVbe (for each transistor) turn on you are ready to be more sensitive with the control adjustment.
Now back to why that sudden turn on is relevant.
A small error in positioning the pot can and will result in an big difference in Ibias. Returning to a former position by geometry (physical) position will NOT return Ibias to it's previous value.
I think it is about time you printed off an A4 copy of the schematic. Write on it as many operating voltages as you can measure. Photograph it and post the compressed but legible pic.
We can then move on from all this guessing.
the bias adjustment pot gradually changes the output voltage (Vbias) of the Vbe multiplier as the VR is changed.
BUT !!!
the output stage does not appear to respond to this gradually changing bias voltage. The output stage turns on when Vbe~400mV and Ic increases rapidly as 600mVbe is approached.
This gives the apparent effect that nothing is happening when turning the VR, then suddenly you have too much Ic and have to back off.
If you monitor Vre and Vbias using two or three voltmeters, you will see that Vbias is responding and as it approaches the critical 400mVbe (for each transistor) turn on you are ready to be more sensitive with the control adjustment.
Now back to why that sudden turn on is relevant.
A small error in positioning the pot can and will result in an big difference in Ibias. Returning to a former position by geometry (physical) position will NOT return Ibias to it's previous value.
I think it is about time you printed off an A4 copy of the schematic. Write on it as many operating voltages as you can measure. Photograph it and post the compressed but legible pic.
We can then move on from all this guessing.
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This reads like you are not at all happy with the temperatures of Ts (temperature of the sink, at the interface with the output device) nor of Tc (temperature of the device backplate, which we can't get to, to measure with any precision). Ta is the temperature of the air passing into the heatsink finning.
Andrew,
First of all, I agree that you can never hit the excact same spot twice with a pot.
The output stage does resond to changes in the pot setting, both on the good cards and the bad card, but on the bad card it is not possible to adjust any lower than about 350mA at full end stop.
Good idea about printing out a schematic with voltages annotated, I'm off now, but will do ASAP.
First of all, I agree that you can never hit the excact same spot twice with a pot.
The output stage does resond to changes in the pot setting, both on the good cards and the bad card, but on the bad card it is not possible to adjust any lower than about 350mA at full end stop.
Good idea about printing out a schematic with voltages annotated, I'm off now, but will do ASAP.
Well, I wouldn't say unhappy, but on the other hand, the heatsinks are not massively over-dimensioned as seen with some other projects arround.
D you think 37 degrees C on the heatsink next to the transistors at idle is hot?
Once again, I do lack experience for judging this..
Here's an attempt at putting some voltages down on the schematic.
I used word to add callouts with voltages to an immage, so I hope it opens with the right stuff in the right places. If not, let me know and I'll do a PDF in stead.
Hope somebody can make any sense out of this, and do let me know if there are other values I should check! 🙂
I used word to add callouts with voltages to an immage, so I hope it opens with the right stuff in the right places. If not, let me know and I'll do a PDF in stead.
Hope somebody can make any sense out of this, and do let me know if there are other values I should check! 🙂
Elbert
A similar pdf of the measured voltages for one of your working amplifiers would be good to have as a base line.
You said in an earlier post you replaced "some" of the driver transitors and that you had checked "virtually" all the transistors. Can you be more explicit, did youy in fact check "all" the transistors and which of the driver transistors did you not replace.
A similar pdf of the measured voltages for one of your working amplifiers would be good to have as a base line.
You said in an earlier post you replaced "some" of the driver transitors and that you had checked "virtually" all the transistors. Can you be more explicit, did youy in fact check "all" the transistors and which of the driver transistors did you not replace.
OldMike,
I replaced the outpit transistors, all the driver stage transistors and even the thermal compensation transistor T11 for good measures.
I allso checked the transistors in the input stage, and they were all working..
I even changed the Zeners D3 and D4 for good measures.
This is certainly more tricky than the first problem I had when i started this thread, a "bogus" T7 transistor which became obvious when unsoldered and measured.. not so easy this time.. I'm leaning more and more to the range of the idle current adjustment lacking the range to compensate from component tolerances in the new output transistors, but I'd like to be sure before I tweak that part of the circuit.
A good idea to do similar measurements on one of my working cards, but due to the chassis layout, it is not possible to measure properly on the card in situ, but as a last resort....
In one of my previous posts, I included a link to a thread where some "nominal" values from elektor were posted, that is perhaps just as good to go by?
I replaced the outpit transistors, all the driver stage transistors and even the thermal compensation transistor T11 for good measures.
I allso checked the transistors in the input stage, and they were all working..
I even changed the Zeners D3 and D4 for good measures.
This is certainly more tricky than the first problem I had when i started this thread, a "bogus" T7 transistor which became obvious when unsoldered and measured.. not so easy this time.. I'm leaning more and more to the range of the idle current adjustment lacking the range to compensate from component tolerances in the new output transistors, but I'd like to be sure before I tweak that part of the circuit.
A good idea to do similar measurements on one of my working cards, but due to the chassis layout, it is not possible to measure properly on the card in situ, but as a last resort....
In one of my previous posts, I included a link to a thread where some "nominal" values from elektor were posted, that is perhaps just as good to go by?
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Hi Elbert and others members
Measurements in the archive is an asymmetry unbalance in the value volt over the resistors R8-R9, which gives us a measurement of 0.26V , 260mV The manufacturer accepted measurements below 100mV
Τhe two amplifiers that I have done so I have managed to under 100mV with perfect match T1T2T3T4 and zener d3,d4.
Also i have made a transistor matcher that works fine.
Now I have succeeded in exiting the transistors Τ2-Τ4 12mV
Measurements in the archive is an asymmetry unbalance in the value volt over the resistors R8-R9, which gives us a measurement of 0.26V , 260mV The manufacturer accepted measurements below 100mV
Τhe two amplifiers that I have done so I have managed to under 100mV with perfect match T1T2T3T4 and zener d3,d4.
Also i have made a transistor matcher that works fine.
Now I have succeeded in exiting the transistors Τ2-Τ4 12mV
Attachments
Hi E,
remeasure the voltages at the output gates +4.1V ref audio ground for both cannot be right.
add some more voltage input pair bases.
top and bottom Vbe multiplier etc.
output offset, input offset.
Can you measure the LTP emitter resistor Vdrops with a more sensitive scale?
2000mVdc should give readings of ~940mV.
This measurement shows the emitters currents as being very close. Can you see why these 8 resistors need to be accurately matched? They tell a lot when the circuit is working or not working properly.
That raises a question about the VAS Ic and Ib leading to that gross difference in collector resistor voltages.
The near zero voltage difference on the -ve side tells us the VAS Ib is near zero. Why? is t9 or t10 damaged? The cascoded VAS, I can't read the component numbers nor values clearly.
Can you check Vbe of all transistors without blowing anything up?
Show Vdrop measurements in a different colour from ground referenced voltage measurements.
remeasure the voltages at the output gates +4.1V ref audio ground for both cannot be right.
add some more voltage input pair bases.
top and bottom Vbe multiplier etc.
output offset, input offset.
Can you measure the LTP emitter resistor Vdrops with a more sensitive scale?
2000mVdc should give readings of ~940mV.
This measurement shows the emitters currents as being very close. Can you see why these 8 resistors need to be accurately matched? They tell a lot when the circuit is working or not working properly.
That raises a question about the VAS Ic and Ib leading to that gross difference in collector resistor voltages.
The near zero voltage difference on the -ve side tells us the VAS Ib is near zero. Why? is t9 or t10 damaged? The cascoded VAS, I can't read the component numbers nor values clearly.
Can you check Vbe of all transistors without blowing anything up?
Show Vdrop measurements in a different colour from ground referenced voltage measurements.
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Andrew,
The LTP emitter reistance voltage drops were measured wit the 2 V range on my meter, giving three decimal places, i.e. 0,94V = 0,940. My mistake, I should have included all decimal places in the first place!
I agree that the measurements can make T9 and T10 suspect, yet they were replaced with new components that were measured and checked.
Hopefully, I'll be able to rig things up again this evening and do some more measurements along the lines you suggest!
Best regards,
The LTP emitter reistance voltage drops were measured wit the 2 V range on my meter, giving three decimal places, i.e. 0,94V = 0,940. My mistake, I should have included all decimal places in the first place!
I agree that the measurements can make T9 and T10 suspect, yet they were replaced with new components that were measured and checked.
Hopefully, I'll be able to rig things up again this evening and do some more measurements along the lines you suggest!
Best regards,
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