High DC offset voltage at preamp output

[mmerig]

My preamp has high DC offset voltage (9.7 and 10.4 VDC, left and right ) at the output terminals. It is a Hafler DH110. I have not found anything on whether this is a DC coupled preamp. The schematic shows what looks like a coupling film capacitor at C19 (470 nfd, 250V) , but it seems unlikley that two film capacitors would fail. I pulled one and it was not shorted and metered at 490 nfd. I checked/replaced all three electrolytics in the line amp section for one channel, and this did not help.

I noticed that the sound was not as clear as it normally is. Another problem is a big transient at turn-on and turn-off that can blow speaker fuses. These were the motivations for checking.

The muting circuit seems to be working in that it takes about 5 sec before voltage comes up to about 10 at the outputs. However, just before it does, there is a sharp spike to about 20 volts. Could a faulty muting circuit cause high offset?

The power amp has a 2 mfd coupling cap at the input. Would this filter all of the DC from the preamp? Even so, I would still like to reduce the DC offset as much as possible.

Perhaps another possibility is unbalanced transistors, but it seems unlikley that both channels would have this problem.

A friend put it on his scope, and noticed a 10 V peak to peak ripple current at the output for both channels. Probes into the power supply indicated that it was okay (low ripple, voltage at specification). The problem seems to be in the line amp section, but an almost equal voltage for each channel suggests that it happens before the line amp. Volume and balance controls have no effect.

Any suggestions?

[Jan Dupont]

Do you have the schematic ???

My first thought was that one supply rail was down, but since you have meassured the supply with good results, my next idea is that something is wrong at the input circuit... Malfunctioning opamp, fet or trannie....

[mmerig]

Thanks for the reply. I have the original manual with the schematic, and Hafler provides pdf's of the manual that includes the schematic. Should I send you a copy? The file is 950K.

[Jan Dupont]

Please do that....
Maybe I can post it on one of my sites so others can participate

[anatech]

Hi mmerig,
Your preamp has capacitively coupled outputs. The muting section (Q15) could cause your DC offset. The easy way to check this is to remove the transistors (FET's) and turn the unit back on. Measure the output voltage.
If that's not the cause, check the voltage on each side of the output caps. Also check that your ground reference is sitting at ground potential.
-Chris

[mmerig]

Anatech, your advice on the muting circuit helped, thank you. I pulled Q15 from the left channel, and DC offset is now 0.00 mV on the left. It is slightly higher (11.2 volts versus 10.4) on the right channel.

My ohmeter check of Q15 shows that it is bad. I'm going to order and replace both Q15 and Q115, (both are J112 FET), but will order two extra just in case the IC3 is faulty and had broken both j112's.

My original hunch was that IC3 was bad, and I ordered one a week ago. But when I saw the delayed voltage rise at the output after turn-on, I figured the IC3 was okay. But after seeing no delay in the voltage rise at the power supply, it increased my suspicion about the muting circuit. Pulling Q15 helped confirm things.

Luckily, these parts are only about 50 cents apiece.

[anatech]

Hi mmerig,
I love a cheap fix, don't you?

-Chris

[mmerig]

Replacing Q15 and Q115 fixed the problem. I did not have to replace IC3. It has worked fine for a few days so far. Thanks for the suggestion.

I can only guess why both of these transistors failed, and I think it may have been static electricity. I live in a cold, dry climate, heat with wood, and usually wear wool. Unless I ground myself on the wood stove before I touch anythingg else metal, I usually get a small shock.

[Giaime]

You lucky man! How's that preamp sounding? I never tried one of that brand... Is it a phono pre?

[anatech]

Hi mmerig,
Those FET's aren't very happy with the high reverse bias voltage. You should check it as it may need clamping. Better yet. Yank out those parts and install a relay to short the signals to ground during mute.
Failure of these parts is reasonably common in many brands. Just think, there are affecting the sound before they actually "blow".

Hi Giaime,
Yup, it has a phone stage.