An inexpensive audio transformer with DIY projects

[boomertech]

I do have a distortion analyzer (an HP8903A) but it's been a while since I've used it so it would take me quite a while to set it up.

I won't be much help with the analyzer, because it has been over 20 years since I have used one. But I do remember that there is a button on the HP analyzers that switches it from remote HPIB control to panel control. So there is a good start for you...

-Frank

 

[boomertech]

I found youtube demo's of that analyzer. Here is one that should get you close to what you need for making the saturation distortion measurements.



-Frank

 

[blindrabbit]

in order to get the circuit to reach some dirt, the clip has to be full up and the drive past about 60-70%, otherwise it's a super high output clean boost.

Nothing wrong with that...in fact, describing it that way makes it sound pretty appealing to me! I'd love to have a combo clean boost/warm mild OD pedal.

 

[johnk_10]

I found youtube demo's of that analyzer. Here is one that should get you close to what you need for making the saturation distortion measurements.



-Frank

thanks! i'll see if I can set that up.

 

[boomertech]

Nothing wrong with that...in fact, describing it that way makes it sound pretty appealing to me! I'd love to have a combo clean boost/warm mild OD pedal.

The changes to the circuit won't get rid of the operations that John described... we will just make it so it has the ability to go to eleven.

-Frank

 

[johnk_10]

after running the TrueRTA tests, it seems that a 2k2 is working best for R26 to me.
now I think we should remove/bypass VR1b and see what it does.

 

[boomertech]

after running the TrueRTA tests, it seems that a 2k2 is working best for R26 to me.
now I think we should remove/bypass VR1b and see what it does.

Did the T-RTA calibration work as I described or did you go by ear?

Crank the largest signal that you can with the drive at max and clip at min... then give me the max peak-to-peak voltage at the junction of R10/R11. When I have that, then I can scale the gain for the output amp without VR1b in the circuit.

Please take the output DC offsets from pins 1 & 7 on both U1 and U2.

Thanx!
-Frank

 

[johnk_10]

Did the T-RTA calibration work as I described or did you go by ear?

I calibrated the best that I could using your method.

Crank the largest signal that you can with the drive at max and clip at min... then give me the max peak-to-peak voltage at the junction of R10/R11. When I have that, then I can scale the gain for the output amp without VR1b in the circuit.

Please take the output DC offsets from pins 1 & 7 on both U1 and U2.

Thanx!
-Frank

okay, i'll do that next. should I still use a 30Hz signal? and does it matter where the output level is set?

 

[boomertech]

I calibrated the best that I could using your method.



okay, i'll do that next. should I still use a 30Hz signal? and does it matter where the output level is set?

You can use any freq that you like... I'm just looking for the max transformer output at R10/R11, so the output level doesn't matter.

-Frank

 

[johnk_10]

and what should the input signal Vpp be? i'm just using my scope and a fluke 87 MK V right now.

 

[boomertech]

and what should the input signal Vpp be? i'm just using my scope and a fluke 87 MK V right now.

Any input that drives the U2 primary drivers to clipping with Q1 and Q2 in circuit. Just set the clipping control to minimum so that the max signal hits the primary coil.

-Frank

 

[johnk_10]

with a 500mv Vpp input signal, the clip at minimum and drive at max, the signal is clean up to 10.3Vpp at pin1 of U2.
at the junction of R10/R11 it's clean and the voltage at that junction is 4.4Vpp.
pin 1 of U1 is .046V and pin 7 is.315V
pin 1 of U2 is .005V and pin 7 is .000V

 

[boomertech]

with a 500mv Vpp input signal, the clip at minimum and drive at max, the signal is clean up to 10.3Vpp at pin1 of U2.
at the junction of R10/R11 it's clean and the voltage at that junction is 4.4Vpp.
pin 1 of U1 is .046V and pin 7 is.315V
pin 1 of U2 is .005V and pin 7 is .000V

Thanx!

Be back in a few with the changes.

-Frank

 

[boomertech]

*** IMAGE REMOVED - UPDATED IMAGE LATER IN THREAD***


Changed VR1 to A1M
Changed R11 to 100K
Removed VR1b

Please check to see if I have all of your resistor values correct.
Please check that the range of the output level control works well for all drive settings. We can adjust it further if necessary.
The drive control will now add large amounts of gain and the level control will need to be able to bring the output back down to less than unity.

Thanks for all of the work!
-Frank

 

[johnk_10]

it looks like you have them all correct.
the only thing that I see is that U1a is called U1b (i.e there's two U1b's).
I make the changes later tonight and test it. i gotta spend some quality time with the wife right now.

 

[boomertech]

it looks like you have them all correct.
the only thing that I see is that U1a is called U1b (i.e there's two U1b's).
I make the changes later tonight and test it. i gotta spend some quality time with the wife right now.

U1a Fixed

-Frank

 

[johnk_10]

made the changes and it's the best it's ever been. YAY!
it's only issue now is that the level control doesn't attenuate the enough (not even to bypassed unity level) when the drive control is turned up.
IMO, I would be best if it went to zero output.
it's taper is also mostly at the end clockwise, so an anti-log pot might work best there, but IMO, the circuit needs to be reworked so the level control can get to zero output like a normal master volume control.

 

[johnk_10]

well, I changed R17 back to a 1K (like it was in the first schematic) and now the level control can go lower than bypassed level with the drive control dimed, and it still has more than enough boosted output available.

 

[boomertech]

made the changes and it's the best it's ever been. YAY!
it's only issue now is that the level control doesn't attenuate the enough (not even to bypassed unity level) when the drive control is turned up.
IMO, I would be best if it went to zero output.
it's taper is also mostly at the end clockwise, so an anti-log pot might work best there, but IMO, the circuit needs to be reworked so the level control can get to zero output like a normal master volume control.

well, I changed R17 back to a 1K (like it was in the first schematic) and now the level control can go lower than bypassed level with the drive control dimed, and it still has more than enough boosted output available.

Try this for the output level circuit:
Change R12 to zero ohms and put R17 back to 10K
VR2 may have a better feel with a B or C taper pot.

-Frank

 

[johnk_10]

tried that and now it doesn't get past bypassed unity level