An inexpensive audio transformer with DIY projects

[johnk_10]

I can tell you that it looks basically identical to the sine at the input with the dual gang drive pot all the way up.

 

[boomertech]

I can tell you that it looks basically identical to the sine at the input with the dual gan drive pot all the way up.

At the output of the circuit or at U2a pin 1?

-Frank

 

[johnk_10]

at U2A pin 1. when I turn the drive control al the way down, there's a TON (like 10X!) of boost with the sine wave rounding off.

 

[boomertech]

at U2A pin 1. when I turn the drive control al the way down, there's a TON (like 10X!) of boost with the sine wave rounding off.

Cool! The gain looks right, but the pot VR1a is wired backwards.

-Frank

 

[boomertech]

The two segments of VR1 are gain and anti-gain. So if VR1 is wired correctly, the output of U1b should remain fairly constant throughout the range of VR1, but there may be some clipping seen that comes from U2a when the drive is increased.

-Frank

 

[johnk_10]

okay. i just reversed them, only VR1a and, unfortunately it still sounds terrible. (fizzy with a gated decay)

 

[boomertech]

We need to do some checks... prototype pains.

With a test tone (100Hz) and the clip control at minimum, set the drive control to have a 6Vp-p signal at U2a pin 1. Then check U2b pin 7 to have the same 6Vp-p signal (only inverted).

Then as you turn up the clip control, view that the clipping level is decreasing (decreasing clipping level is heavier distortion). Depending on the taper for VR3 used, you may only start to see clipping near the end of its rotation with this level of signal.

-Frank

 

[boomertech]

We may also need to apply HF filtering later in the circuit to tame the fizzyness that may be present. But that's the fun voicing part that comes after the basic circuit is working 100%.

-Frank

 

[boomertech]

We need to do some checks... prototype pains.

With a test tone (100Hz) and the clip control at minimum, set the drive control to have a 6Vp-p signal at U2a pin 1. Then check U2b pin 7 to have the same 6Vp-p signal (only inverted).

Then as you turn up the clip control, view that the clipping level is decreasing (decreasing clipping level is heavier distortion). Depending on the taper for VR3 used, you may only start to see clipping near the end of its rotation with this level of signal.

-Frank

This is what you should see at U2A pin 1 with the clipping control:

-Frank

 

[boomertech]

John, on your tranny PCB pin 1 should be directly across from pin 8.

-Frank

 

[johnk_10]

yep, that's what I see alright.
when I adjust the drive to get 6.6V Vpp on pin1 i have 15.5V(!) on pin 7

 

[boomertech]

Also on your main PCB the Q1 and Q2 transistors look backwards. D1 and D2 should be connected to the collectors and your layout shows them connected to the emitters.

-Frank

 

[boomertech]

yep, that's what I see alright.
when I adjust the drive to get 6.6V Vpp on pin1 i have 15.5V(!) on pin 7

Then R8 or R9 is the wrong value.

-Frank

 

[boomertech]

I am not seeing R9 on your PCB. There should be a 10K from C11 to U2 pin 6.

-Frank

 

[johnk_10]

John, on your tranny PCB pin 1 should be directly across from pin 8.

-Frank

00ps. you're right.

 

[johnk_10]

good eye! i'll fix that now.

 

[johnk_10]

dang all these little errors. I cut a trace and tacked a 10K in there for R9 so pin 1 and pin 7 match now.
it still sounds awful though.

 

[boomertech]

Cool! we are getting there.

Measure the signal at the junction of R10/R11. It should be 1/2 of the signal at U2 pin 1.

-Frank

 

[johnk_10]

yep. it is 1/2 of its Vpp.

 

[boomertech]

Great!

Now check to see if the clipping changes like the simulation image above at U2a pin 1.

Also check that output of the circuit has undistorted signal with the drive turned down and clip control turned down.

-Frank