BAJA AC30TB crackle at end of sustain

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nubbin89
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Post by nubbin89 »

I've recently built BAJA's awesome AC30TB OD pedal, and after finally getting it working I've encountered something strange.

I've found that when playing at a low volume, during the end of a sustain, or when making noises with the strings the sound gets all crackly and sort of cuts in and out. Sort of sounds like a gate right on threshold, sweeping across frequencies as it closes. Reminds me of the noise my tube screamer made when the battery got super low, expect in this case my battery is good. There also seems to be a weird compression effect going on when playing loud. Other than that it seems to sounds good when playing.

The board isn't in an enclosure yet as I've only just finished troubleshooting it.

I'm running it into the front end of a Joyo Zombie.

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nubbin89
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Post by nubbin89 »

Ok, so in an incredibly annoying development I found a solder bridge between the inverting and non inverting input on TL072b (pins 5 & 6), and when I removed it, not only did it not solve the issue but now there is some sort of crazy oscillation going on. The frequency of which changed with the tone controls. I've already spent a long time looking for mistakes and cant find any!

I've also checked the bias voltages and they are all in the 4.5v region for each chip, in fact that is what led me to find the bridge as the bias was around 6.8V until I rectified the problem.

TL072 - IC1

1OUT 4.91v
1IN - 4.89V
1IN + 4.47V
VCC- 0V
VCC+ 9.44V
2OUT - 4.86V
2IN- 4.86V
2IN+ 4.74V

TL062 - IC2

1OUT 7.1V
1IN - 4.81V
1IN + 4.83V
VCC- 0V
VCC+ 9.35V
2OUT - 4.81V
2IN- 4.81V
2IN+ 4.37V

TL062 IC3

1OUT 3.43 - 3.44 (moving)
1IN - 4.68V
1IN + 0.29V
VCC- 0V
VCC+ 9.29V
2OUT - 5.42V
2IN- 4.77V
2IN+ - 4.74V

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The G
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Post by The G »

Don't make potential helpers treasure hunt for information. Help will come faster your way if you'd just point to the schematic you used.
I'll just assume you used the v2 schematic - I'd check first the non-inverting input of IC3a, it should be around VCC/2.

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nubbin89
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Post by nubbin89 »

Thanks for the reply. I think the problem lies with some confusion with the schematic and the stripboard layout.

Image
Image

On the schematic, at the bottom of the tone stack from the 1K resistor and low pot, the two connections are unlabeled, leaving me to guess where they are connected. I have tried them both connected to ground, both to v bias and one on each (both ways).

Being able to eliminate that as a potential problem would be a good start for me to properly fault find.

For now all I'm getting out of it is high pitch oscillations.

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deltafred
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Post by deltafred »

V bias.

The non inverting input (+) of IC2b is tied to v bias so the inverting (-) input also needs to be so only the AC that makes its way through the tone stack gets amplified.

The output voltage of an opamp is equal to the difference in voltage between the + and - inputs multiplied by the gain. IC2b has a gain of 12 (4700/390 the feedback resistor divided by the input resistor on the - input) so if you connect them to 0v or 9v you will drive then opamp output to a supply rail.
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The G
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Post by The G »

This ^.
Basically, in the usual pedals we are biasing the op amps inputs at the same DC bias voltage so the output is as close to the DC bias voltage as possible. The bias voltage is usually half of the voltage supply, so the output can do the largest swing without hitting the supply rails.
Some understanding of how op amps work will really be of use. A tutorial I like is this one at the Khan Academy.

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jhergonz
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Post by jhergonz »

is this issue solved?

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