An inexpensive audio transformer with DIY projects

[johnk_10]

here's the OPA2134 with 220R's and 1K's for R17/R18 @ -10db:

 

[boomertech]

here's the OPA2134 with 220R's and 1K's for R17/R18 @ -10db:

If you put that 220R in for R24 with R17/R18 at 220R the blue trace should be very close to the violet trace. But since decreasing the resistor values didn't have a positive outcome I would say keep the 1K's for R17/R18 and 2K2 for R24. Those 4.7uF capacitors for C5/C11 should improve the low end response a little bit.

When the LED's were across the primary, did it have a crunchier sound? I know the output was low because of the lower voltage across the primary, but how was the tone?

-Frank

 

[johnk_10]

If you put that 220R in for R24 with R17/R18 at 220R the blue trace should be very close to the violet trace. But since decreasing the resistor values didn't have a positive outcome I would say keep the 1K's for R17/R18 and 2K2 for R24. Those 4.7uF capacitors for C5/C11 should improve the low end response a little bit.

When the LED's were across the primary, did it have a crunchier sound? I know the output was low because of the lower voltage across the primary, but how was the tone?

-Frank

the LEDs didn't work at all. the output was so low (i.e. just barely audible) I couldn't determine what it's tone was like. IMO, it doesn't need them.

the 4u7's brought a tiny bit of low end back.
I actually prefer it with the 220R's since it drives the tranny harder (=more dirt if you want it).

the blue line is with the 4u7's and the violet line is with the 1u's, so not much of a difference at all.

 

[boomertech]

Let's keep the 4.7uF's because it brought the response at 20Hz up by approx 5dB.

I'll make the changes to the schematic with the 220R's, no LED clipping and an optional note for the emitter 1K's on Q1/Q2.

Am I forgetting anything?

-Frank

 

[johnk_10]

Let's keep the 4.7uF's because it brought the response at 20Hz up by approx 5dB.

I'll make the changes to the schematic with the 220R's, no LED clipping and an optional note for the emitter 1K's on Q1/Q2.

Am I forgetting anything?

-Frank

yeah, i'd change C1 to a 1u. look at it now with the two 4u7's and a 1u for C1:

BTW, it sounds great and I prefer 220R's for R17 & R18. it can get pretty dirty that way and the freq response is the same as with the 1K's.

IMO, it's DONE!!!!

 

[boomertech]

yeah, i'd change C1 to a 1u. look at it now with the two 4u7's and a 1u for C1:

BTW, it sounds great and I prefer 220R's for R17 & R18. it can get pretty dirty that way and the freq response is the same as with the 1K's.

IMO, it's DONE!!!!

Since you have your T-RTA all fired up, let's try one more experiment. I was playing around with LF resonance in the simulator that might be just what this circuit needs. Put a series cap of 22uF to the tranny primary pin 2... I'm showing a 3dB bump at 22Hz.

-Frank

 

[johnk_10]

are you saying that it goes between R17 and Pin 2 of the tranny?

 

[boomertech]

Yes.

 

[johnk_10]

hmmmmm. just tried it with the 22u and there's absolutely no difference.

 

[boomertech]

hmmmmm. just tried it with the 22u and there's absolutely no difference.

My simulations have been off from the actual circuit. Try a 10uF, you should see a resonance bump in the lows, if nothing else is messing with the LF response.

-Frank

 

[johnk_10]

IMO, I don't think that any coupling cap will make a difference there since, with a value at 10u or above it's basically acting like a straight wire, unless i'm missing something(?).

 

[johnk_10]

My simulations have been off from the actual circuit.

whether your simulations are off or not, the circuit sounds great just the way that it is right now.

 

[johnk_10]

I just tried a 10u electrolytic for C8 and here's the result. it boosted the lows by quite a bit (Blue line) verses C8 being a 1uf (violet line). it's only 1dB down @ 30Hz and about 2dB at 20Hz now so i'd go with a 10u there.

 

[boomertech]

IMO, I don't think that any coupling cap will make a difference there since, with a value at 10u or above it's basically acting like a straight wire, unless i'm missing something(?).

The tranny primary coil and a series capacitor make a tank circuit that has a resonance frequency... that's the principle behind how they make wah pedals. If the capacitor in series with the coil is large enough, there won't be a resonant bump in the audio range and it acts more like a coupling capacitor that you would see with a resistive load. But if the cap is sized just right you can bump up the lowest frequencies in the audio range. The series resistance with the tank circuit limits how large the resonant bump can be. I just thought it would be an interesting experiment and if it worked out...

COOL!!!! Your post came in as I was typing.

-Frank

 

[johnk_10]

y

The tranny primary coil and a series capacitor make a tank circuit that has a resonance frequency... that's the principle behind how they make wah pedals. If the capacitor in series with the coil is large enough, there won't be a resonant bump in the audio range and it acts more like a coupling capacitor that you would see with a resistive load. But if the cap is sized just right you can bump up the lowest frequencies in the audio range. The series resistance with the tank circuit limits how large the resonant bump can be. I just thought it would be an interesting experiment and if it worked out....-Frank

okay, I get it now.

with the 10u on the output, if it had any more low end than that, it would start sounding tubby.

 

[boomertech]

FINAL!!!!

Please check it over.

-Frank

 

[johnk_10]

that's it! Great job Frank!
it's a really nice sounding (and original!) circuit thanks for going the distance with me. drool

 

[johnk_10]

soundclips coming tomorrow.

 

[boomertech]

that's it, except I have a 2K2 for R24 on mine. I also have an OPA2134 for U2 in mine right now, but IMO, it doesn't really make any difference if you use an RC4558 (it draws 10ma less with the RC4558).
the whole circuit draws 20ma (with two 4558's, and 30ma with a 4558 and an OPA2134.

FIXED!

Thanks again for playing!!!

-Frank

 

[johnk_10]

right back at ya!