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An inexpensive audio transformer with DIY projects

#41
johnk_10 said:
Hi Frank,
I haven't started etching this one yet. do you think it's ready for 'real-world' testing now? if so, i'll start drawing up a PCB for it.
Click to expand...

I just removed C12, because I didn't like AC coupling that node to the -V rail. I don't think there will be that much of a DC offset at the output of U1b and the resistors in the feedback path around U1b are small enough that background noise shouldn't be an issue with a little DC running thru them.

It's not on the schematic, but put a resistor position on your PCB layout across pins 2-4 on the tranny... just in-case we need to do something there.

Also, before mounting the tranny to the PCB, place a jumper from T1 pin 2 to pin 8 for testing the circuit drive control and clipping circuits. After those circuits check out then mount the tranny (remove 2-8 jumper). We will have to adjust the current thru the primary coil, so please socket R18 and R19.

-Frank
 
#43
*** IMAGE REMOVED - UPDATED IMAGE LATER IN THREAD***

John, I did some shower door math again and placed R26 across the primary coil in the schematic. With the tranny having the ability to easily saturate I changed R18 and R19 to 1K's to eliminate loading effects to the OPA2134's during transformer saturation. This will make it easier to calibrate the value for R26 for maximum saturation.

If you have put that (R26) resistor position across the primary on your PCB then the only thing that has changed are the R18/R19 values. We will need your o-scope and signal generator to calibrate the maximum tranny saturation with R26. I will run thru the procedure when the other circuits check out.

-Frank
 
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#47
i finally got the pcb drawn up for it. i'm going to triple check it again in the morning, but i'm 99% sure that it's correct.
once i'm 100% sure, i'll etch it, build it and test it. i'm socketing R18, R19 & R26 on it.
i'm also using a small daughter board for the transformer which will allow me to fit it in the smallest enclosure possible.
 
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#48
johnk_10 said:
i finally got the pcb drawn up for it. i'm going to triple check it again in the morning, but i'm 99% sure that it's correct.
once i'm 100% sure, i'll etch it, build it and test it. i'm socketing R18, R19 & R26 on it.
i'm also using a small daughter board for the transformer which will allow me to fit it in the smallest enclosure possible.

[Invalid or Expired Link Removed]
Click to expand...

Nice job John!

Please socket Q1 and Q2 on your prototype, because that will make calibrating the maximum transformer saturation a bit easier. After you have the values for R18, R19 and R26, you won't need to socket anything for future builds.

I have been trying to find sources that quantify amp distortions into approximate tube/circuit vs tranny ratios... So far, no luck. There may be no tranny distortions at all in some amplifiers, so I guess looking for general data is a waste of time. But here is a nice graph from MP amps:

sagspectrum.gif


I am thinking that 5% tranny distortion at 30Hz with a very large primary signal will be a good place to start, since the soft clipping in this circuit will allow for much higher distortion percentages. We need to keep the low frequency tranny saturation under control or you will have the same issues with HF loss that you heard with the GK amp simulator.

I am really enjoying these power amp dirt simulators we have been working on.:) I was just looking at the new Bogner boost/compress pedal and thought that strapping on a version of the Demeter Compulator with this circuit may also make a really nice pedal.

-Frank
 
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#51
johnk_10 said:
the PCBs are in the etching tank now.
BTW, I have a socket for R26. what value should I use for it (for starters)?
Click to expand...

For R26 we need to find the value. Temporarily take a 10K pot and run a wires from the wiper and an outside lug and use it as an adjustable resistor.
When we start calibrating the max saturation, this pot will make the procedure so much easier to find the value for R26.

Try the circuit first with the 2-8 jumper on the tranny pads before installing the TY250. This way you will know for sure that everything else is good before we work with the tranny.

-Frank
 
#53
johnk_10 said:
when calibrating I guess that i'll also have to keep in mind that the highest value that of a dual gang pot is an A250K?
Click to expand...

Nope, a 250K may work perfectly fine for the drive (gain/anti-gain) control. The tranny calibration will not change with the gain abilities of the circuit. We just need to be able to get a 12Vp-p (6Vp) signal from U2a to set saturation... and that shouldn't be a problem with a 250K pot.

-Frank
 
#54
well, I got it built up, but something's definitely not right since i'm getting ZERO output out of it.
i'm thinking that I must have an error in my layout somewhere, but I've looked it over a LOT of times and I sure can't find one.
right now, it's wired without the TY250 with lugs 2&8 jumped together and a 10K trimpot for R26.
i'll have to take some voltages and put it on the scope tomorrow morning to see where the signal is disappearing.
 
#55
johnk_10 said:
well, I got it built up, but something's definitely not right since i'm getting ZERO output out of it.
i'm thinking that I must have an error in my layout somewhere, but I've looked it over a LOT of times and I sure can't find one.
right now, it's wired without the TY250 with lugs 2&8 jumped together and a 10K trimpot for R26.
i'll have to take some voltages and put it on the scope tomorrow morning to see where the signal is disappearing.
Click to expand...

Remove R26 until the tranny is installed.

-Frank
 
#56
okay, I found a solder bridge on my PCB that was causing the signal to short out.
now I want to make sure that my pot lugs are hooked up correctly.
everyone seems to draw them differently. I've numbered them in this diagram, but i'm not sure if it's correct? (the picture of the pot on the left is viewed from the bottom).

TransBoostPots.jpg
 
#57
johnk_10 said:
okay, I found a solder bridge on my PCB that was causing the signal to short out.
now I want to make sure that my pot lugs are hooked up correctly.
everyone seems to draw them differently. I've numbered them in this diagram, but i'm not sure if it's correct? (the picture of the pot on the left is viewed from the bottom).

TransBoostPots.jpg
Click to expand...

I draw the wiper direction arrow for clockwise rotation (turning up). 3 = cw, so that's good.

-Frank
 

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