Unwanted Preamp Distortion

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EricssonB
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Joined: 07 Jun 2012, 04:53

Post by EricssonB »

So I'm nearly complete with the first of a few preamps that I'm building for me and my friend. Just about got this one done when I noticed that it seems to distort upon the power being applied, and then accumulate until you switch it off. My hunch is that it's in the power section.

Schematic that I came across on someone else's build:
Image

My layout with a different power section added:
Image

Soundcloud link if you need to hear it:


Audio clip with all pots on full. Downloads enabled in case you want the subtleties not provided by the audio stream.

Switched on at about 7 seconds. Just beyond the mid-point time warp that skips a minute you can hear how gated and distorted it gets. Not sure what that's about yet. Switched off for a bit then switched back on -- everything's okay.

Any takers?

Side note: this thing is pretty hot. May need to throttle it down a bit.

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blackbunny
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Post by blackbunny »

3 things bother me in this circuit.
Firstly, the circuitry around the input seems unnecessarily complicated, and would likely result in a severely bandpassed frequency response with a prominent midrange hump.
What are you using this circuit to amplify? Single coil guitar pickups? Humbuckers? Passive bass pickups? The input could easily be redesigned to suit the source a lot better.

Secondly, I would change the power supply voltage dividing resistors from 2M2 to 100k to allow more bias current.

Thirdly, running a preamp and active tone controls - with about 15db of cut and boost available - from a 9v supply means that it will run out of headroom very easily when boosting the EQ unless you are amplifying very small input signals, such as a piezo pickup or very low output pickup. The TL07x and TL06x will latch up to the supply rail when the signal voltage swing exceeds +- 4.5v , causing serious distortion and oscillation when the tone controls are fully boosted.

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