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Hagerman Bugle DC offset

post #1 of 8
11/29/10 at 8:23am
Thread Starter 

I built a Hagerman Bugle phono preamp from the pre-made PC board I bought from the Hagtech website. I used Digikey parts. I'm using two 12 SLA batteries for power.

 

I have it all put together and in the case and now when I first turned it on, I got about 6V of DC offset on one of the outputs. The rest of the inputs and outputs were <3mV DC offset. While I was investigating this, I noticed the amount of DC offset was going down steadily. After about 3 hours, it was down to 60mV. I had to stop testing at that point because I had to go to sleep.

 

Although I could have used the wrong component values, I don't see how I could have any shorts or mis-routings since the pre-made PC board is pretty idiot-proof. I also don't understand what kind of error could lead to the amount of DC offset getting better with time. Could the battery voltage between the two SLAs cause DC offset and then decrease as the batteries equalize with each other?

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post #2 of 8
11/29/10 at 9:44am
Thread Starter 

This is a schematic  (only shows one channel):

 

http://www.hagtech.com/images/bugleschem.gif

 

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post #3 of 8
11/29/10 at 1:21pm

PCB's are not 100% idiot-proof, and many a builder who cant figure out why some simple circuit wont light up has been shafted by a bum PCB. The odds are well below 1%, but its worth it to check that that there are no weird connections (adjacent pads that are bridged or short to ground planes because of construction errors are the popular ones) before you start. Just fair warning. 

 

 

Before you flip out on the PCB:

Try a different "u3" op amp if you have one handy. Its possible that one got blown up, or worse "partially" blown up. you know, the intermittent problems that are the hardest to troubleshoot and typically caused by the silliest of things.

 

If you dont have a new U3 opamp handy swap the u3 and u2 op amps and see what happens. If the problem goes away get some new op amps.

 

If that dosnt solve it check all of your solder joints on the components hooked up to u3. 

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post #4 of 8
11/29/10 at 1:44pm
Thread Starter 

Thanks. I will try swapping the opamps around.

 

I looked over the board for solder bridges and didn't find any. Hard to imagine a solder bridge causing a DC offset that decays like that though.

 

One thing I thought of, is that maybe the amp will amplify any DC offset in the inputs, so maybe I should measure the DC offset of the outputs with a shorting plug in the input RCAs before I make any conclusions about the real DC offset of the outputs.

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post #5 of 8
11/29/10 at 1:57pm

The amp will amplify DC offset, but there is a cap between u2 and u3 to stop it from getting through :)

 

Any DC offset you get should be from only the output op amp, and should be no more than a few mV at worst.

 

If you took the cap between U2 & U3 out and jumpered it put it back!


Edited by nikongod - 11/29/10 at 1:58pm
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post #6 of 8
11/29/10 at 2:32pm
Thread Starter 

No, I didn't take any caps out. I suppose one of the caps could have an internal short or something. The decaying DC offset is almost consistent with an electrolytic cap that is reforming itself, but these are not electrolytic caps.

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post #7 of 8
12/1/10 at 2:28pm
Thread Starter 

After a couple days of turning it on and checking, the DC offset decayed down to 2.5mV. The other channel was steady at 1.3mV. I swapped U3 and U2 opamps and now both channels have about 1.4mV of DC offset. How much is acceptable?

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post #8 of 8
12/1/10 at 4:08pm

Since I have an old multimeter that only goes to +/- 1 mV, I would only worry about DC offset if it got to 10 mV or so.
 

Quote:
Originally Posted by trains are bad View Post

After a couple days of turning it on and checking, the DC offset decayed down to 2.5mV. The other channel was steady at 1.3mV. I swapped U3 and U2 opamps and now both channels have about 1.4mV of DC offset. How much is acceptable?

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