PMA
I have tried whitout signal and nothing, I have also grounded chassis.
MikeB
Voltage across r14 is 270mV!!! (with virable resistor turnet at 0 volume)
Heatsinks are 45C with bias 200mA.
Maybe I schould put mpsa18 as input tr. Suplly is around 40V.
I have tried whitout signal and nothing, I have also grounded chassis.
MikeB
Voltage across r14 is 270mV!!! (with virable resistor turnet at 0 volume)
Heatsinks are 45C with bias 200mA.
Maybe I schould put mpsa18 as input tr. Suplly is around 40V.
borys, as long as voltage does not get above 40v, the mpsa18 should be fine, it has max rating of 45v.
270mv gives an effective hfe of 122. It's possible that the turn on thump is caused by that. The mpsa18 has at least 10times the gain. And it is ~4 times faster...
About your grounded chassis, is it only connected to power-gnd ? You should keep signal-gnd isolated from the chassis.
Mike
270mv gives an effective hfe of 122. It's possible that the turn on thump is caused by that. The mpsa18 has at least 10times the gain. And it is ~4 times faster...
About your grounded chassis, is it only connected to power-gnd ? You should keep signal-gnd isolated from the chassis.
Mike
With mpsa18 is the same situation - on thumb.
Across R14 I have 25mV now - its better.
The chassis is connected only to supply GND. Input gnd is isolated.
Across R14 I have 25mV now - its better.
The chassis is connected only to supply GND. Input gnd is isolated.
Okay, one cause excluded... Is the feedback DC-cap really 470uf ?
One of the output devices or drivers might be defect, do you have this symptom on both channels ?
Mike
One of the output devices or drivers might be defect, do you have this symptom on both channels ?
Mike
Feedback dc cap is 470uF as in 5,3 project. This thumb is in both channels, devices IMHO are good (same series and voltage acros emiter resistors are the same).
I have no idea what could make this on thumb.
I have no idea what could make this on thumb.
Hi dytln_02,
Thanks for responding. My audio issue is with Windows XP only. This is the third sound card in this computer since an update killed it.
Hi borys,
Could you please measure the DC offset on each side of the differential pair bases in mV? A turn on thump suggests either a problem with both 470 uF caps (maybe the type is bad for this), or a mismatch between the gains that takes time to be servo'd out by the diff pair set.
-Chris
Thanks for responding. My audio issue is with Windows XP only. This is the third sound card in this computer since an update killed it.
Hi borys,
Could you please measure the DC offset on each side of the differential pair bases in mV? A turn on thump suggests either a problem with both 470 uF caps (maybe the type is bad for this), or a mismatch between the gains that takes time to be servo'd out by the diff pair set.
-Chris
anatech
I have bridged DC cap (470uF) and on thumb is all the time -2,5V, the cap i low impedance samwha WL. DC offset on each side of the differential pair bases is aprox 950mA - very high.
I have changed a diff pair couple of times with not matched (658/625, 658/670) mpsa18 and out offset voltage is 182mV all the time (it was 2,5mV).
I have bridged DC cap (470uF) and on thumb is all the time -2,5V, the cap i low impedance samwha WL. DC offset on each side of the differential pair bases is aprox 950mA - very high.
I have changed a diff pair couple of times with not matched (658/625, 658/670) mpsa18 and out offset voltage is 182mV all the time (it was 2,5mV).
I have manage with this thumb.
I have hanged diff pairs in both channels, not in one only - this was prapobly the reason of this offset.
With both channels with mpsa42 offset was 2,5mV.
With mpsa42 and mpsa18 in secon channel offset was 180mV all the time.
Thumb is but very very smal.
THANKS VERY MUCH
Now I know almost everything about symasym 🙂
I have hanged diff pairs in both channels, not in one only - this was prapobly the reason of this offset.
With both channels with mpsa42 offset was 2,5mV.
With mpsa42 and mpsa18 in secon channel offset was 180mV all the time.
Thumb is but very very smal.
THANKS VERY MUCH
Now I know almost everything about symasym 🙂
Hi borys, with the dc-blocking cap shorted it is normal to have that big DC-offset.
Okay, i did not understand "hanged diff pairs", what was the problem ?
Has the DC-offset now returned to 2.5mv ? Everything above 5mv is abnormal for symasym.
A slight turn on thump is normal, but should be too small to be audible/visible.
With a 658/670 match in input and the DC-cap, the DC-offset still should be very low (far below 180mv).
Mike
Okay, i did not understand "hanged diff pairs", what was the problem ?
Has the DC-offset now returned to 2.5mv ? Everything above 5mv is abnormal for symasym.
A slight turn on thump is normal, but should be too small to be audible/visible.
With a 658/670 match in input and the DC-cap, the DC-offset still should be very low (far below 180mv).
Mike
Hi Mike.
Now I have offset aprox 32mV (it is higv I know), diff pair is 670/670. After change diff pairs in both channels offset decreased to 32mV from 180mV. Changing hfe in diff amp don't change the offset. With (658/625, 658/670) offset is still 32mV.
(sory for mistakes - my English isn't good)
Now I have offset aprox 32mV (it is higv I know), diff pair is 670/670. After change diff pairs in both channels offset decreased to 32mV from 180mV. Changing hfe in diff amp don't change the offset. With (658/625, 658/670) offset is still 32mV.
(sory for mistakes - my English isn't good)
Hi borys, there is definitely a fault somewhere... Is maybe the DC-Cap (c19) still shorted ?
If cap is not shorted, could you read voltages base to gnd Q1/2 and voltages across r5/6 ?
Mike
If cap is not shorted, could you read voltages base to gnd Q1/2 and voltages across r5/6 ?
Mike
From GND to bases q1/q1 is aprox 58mV, acros r5/r6 is 967mV. When I disconnect one of channels offset decrease to 12mV(16mV secon channel), when both are connected to supply offset is 32mV.
Hi borys, you have identical base voltages at q1/2, but outputoffset of 12mv ?
And if you connect 2nd channel to same voltagesupply, offsetvoltage changes ? OMG ! That will be funny to debug...
Have you verified that r19 and r14 are identical ? (22k)
Can you measure voltages across r2 ?
Mike
And if you connect 2nd channel to same voltagesupply, offsetvoltage changes ? OMG ! That will be funny to debug...
Have you verified that r19 and r14 are identical ? (22k)
Can you measure voltages across r2 ?
Mike
r19 and r14 I have checkd first and they are identical. Acros r2 is 33mV.
Yes offset increase to connect both channels - maybe this is power supply fault. Between GND and bases diff tr is precisley 61mV on each transistor (mpsa18).
Yes offset increase to connect both channels - maybe this is power supply fault. Between GND and bases diff tr is precisley 61mV on each transistor (mpsa18).
You definitely have a grounding problem, it's very likely that this does not only create DC-offset, it will also create heavy distortions.
Is the offset on r2 still there if you completely disconnect input ?
Mike
Is the offset on r2 still there if you completely disconnect input ?
Mike
If I disconnect input there is the same offset on r2. I supposed that is a grouding problem becouse I had EMI distorsions (even heard), when I connect my CD I can hear distortions from high voltage network (~240V) at a higher level, with another amp everythink is ok. Input is very sensitive.
borys said:
Most probably signal wiring problem. Also, try to diconnect PSU 0V from chassis (you wrote it was earthed).
Whitout signal cables (PCB) and grounding chassis is still 32mV. Maybe there is something wrong with the vas ? I'm wonder why changeing the diff pairs doesn't change the offset voltage. When I heat up one of mpsa18 by the soldering iron offset normaly change with the hfe of mpsa18.
22k virable resistor decrease distortions from my CD (I have one 22k only).
22k virable resistor decrease distortions from my CD (I have one 22k only).