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Old 26th March 2005, 12:11 AM   #1
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Default Struggling with my buffered GC preamp

I have built a second preamp from my Lm3886 GC and here is how it is set up.

1) The schematic is pretty much the same as the headphone amp application note on TIs web site for the BUF634.

2) I am using the OPA2228 as the opamp.

3) I have the gain set at 11 with 4.7K and 470R resistors.

4) Input inpedance is 100K.

5) I have a 100K stereo volume control pot from rat shack.

6) I have 220 ohm resister on the BUF634 bandwidth setting pin to V-.

Here is the problem. The thing sounds excellent at both ends of the pot within 1/4 of both ends of the pot, but anywhere near the middle I get all kinds of noise that is the worst at the middle of the pot.

Is my pot to high a value? Should I get a 10K or something?

as I noted my input inpedance is 100K. Tried with 470K and it was as bad maybe worse.

Any ideas. I have searched the forum for two hours and I am still stumped.
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Old 26th March 2005, 12:42 AM   #2
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Default Update

Ok, just connected the preamp to my headphones, and the problem does not exists when driving the headphones. It sounds great actually.

This made me think it might not like the cables to my amp or the input impedance of my amp, so I added a 120R resistor in series with the output of each buffer. Still sounds good on the headphone, but the power amp is still getting terrible problems at around mid pot. Both ends still fine.
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Old 26th March 2005, 01:10 AM   #3
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Default Update #2

OK, for kicks I tried "tuning" the noise, and got a close by AM radio station, but once again only when the preamp is connected to the GC (BrinGTs LM3886). When connected to headphone only the slightest hint of hiss right at mid pot, but otherwise silent.
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Old 26th March 2005, 08:12 AM   #4
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Yes, try a 10K pot!
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