Looking for feedback on Transformer Saturation Box schematic

[Sam0311]

Hey guys,

So I spent a lot of last year reading about electronics but I’ve only just recently started getting my hands dirty.

Purely out of curiosity (and because I thought it would be fairly simple to pull off) I want to see what it sounds like when you send a signal through a transformer that’s much hotter than it’s rated for. I was originally trying to keep things passive but it seems like I’m going to need to buffer the signal coming in. I’m using it with my Eurorack which has a typical output impedance of 1k ohms.

I just wanted to share the schematic on here to see if anyone can spot anything clearly wrong with my design or has any suggestions.

The idea is pretty simple, the op amps are there to buffer the signal going into the transformers. Running straight into T2 should keep things at unity gain but there is a switch to send the signal through T1 adding (I think?) +18db, the potentiometer at the end is there to tame the output level if needed. I’ll probably power the circuit with a couple of 9v batteries for now.

I’m unsure if the components in the feedback loops of the op amps are necessary since I’ll just be using them as buffers for now (I was repurposing part of a boost pedal schematic for this ). I may add a gain control on the first op amp later down the line though.

Cheers

Edit: I seem to have a habit of wiring the jacks wrong on KiCad

 

[Sam0311]

Did some more thinking and I think something like this could work? Simplifies things a bit.

 

[ruffrecords]

A TL072 will really struggle to drive 600 ohm and 150 ohm loads. There is no need to place resistive loads across their outputs; just the transformers. The easiest way to get a transformer to add more colour is to add a series resistor on its primary.

Cheers

Ian

 

[Sam0311]

Thanks for this Ian. I think the idea of the resistor was to block any DC going to the transformer, would I be safe to remove the RC filters at the op amp outputs?

The easiest way to get a transformer to add more colour is to add a series resistor on its primary.

Would you mind elaborating on this a little please? What makes this work? Is the idea that putting a resistor with significantly more resistance than the transformers primary adds colour?

Cheers

 

[warpie]

I'm not Ian but I believe he means that you don't really need the two potentiometers on the outputs of the transformers.

Also, a series resistor will add "colour" indeed but you also have to somehow compensate for the level loss.

 

[Sam0311]

For the sake of clarity by series resistor do we mean op amp output > resistor > transformer? The level loss would be down to a voltage drop across the resistor?

 

[ruffrecords]

For the sake of clarity by series resistor do we mean op amp output > resistor > transformer? The level loss would be down to a voltage drop across the resistor?

Yes. Transformer distortion depends in part on the impedance of the driving source (your TL072). The output impedance of the TL072 is very small so the transformer will add very little colour. Adding a resistor in series will add colour.

It is R4 and R7 that are unnecessarily loading the TL072 outputs. Change these to 10K - all they do is provide a dc path to ground for the output capacitors.

T1 is wired as a step up transformer. It has 16dB gain so you really need to add an output attenuator rather than an amplifier. I suggest you add an adjustable pot divider to its output before feeding to U1B.

I am not sure T2 is going to do anything other than cause the TL072 to distort.

Cheers

Ian

 

[Sam0311]

Ah okay, so I’m assuming from your reply that my understanding of what makes a transformer add harmonic distortion is maybe a bit off?

Maybe I should have provided some more information but the transformers I’m using suggest the max input level is 0dbu, I’m using this circuit with Eurorack so (If I’m not wrong) the signal going into them will be +13dbu. I thought that putting a signal through a transformer that is higher than the transformer is rated for will cause the audio to clip, much in the same way that clipping will occur if a signal goes above an op amps power rails.

I’m assuming if I change r4 and r7 that I’ll have to recalculate the value of C2 and C3 to get the desired cutoff frequency?

In the second schematic I posted, I changed the transformer to add (I think) 9db of gain which I can attenuate with the following potentiometer. The idea was that this could act as a gain control to choose how hard I was hitting the rest of the circuit.

Cheers Ian, I appreciate your input.

Sam

 

[ruffrecords]

Ah okay, so I’m assuming from your reply that my understanding of what makes a transformer add harmonic distortion is maybe a bit off?

Quite possibly.

Maybe I should have provided some more information but the transformers I’m using suggest the max input level is 0dbu, I’m using this circuit with Eurorack so (If I’m not wrong) the signal going into them will be +13dbu. I thought that putting a signal through a transformer that is higher than the transformer is rated for will cause the audio to clip, much in the same way that clipping will occur if a signal goes above an op amps power rails.

Clipping and adding some harmonic distortion are two different things. The maximum signal voltage a transformer core can take before clipping depends on the flux density. The flux density is directly proportional to the voltage applied but also inversely proportional to the frequency. A transformers maximum signal level is usually quoted at its lowest operating frequency which these days seems to be 50Hz according to most transformer manufacturers. However, because flux density is inversely proportional to frequency, the maximum level the transformer can sustain at 500Hz is 10 times as much or 20dB higher. At 5KHz it is another 20dB higher still. Bottom line is you are only going to 'clip' a transformer at very low frequencies.

In simple terms, the equivalent circuit of a transformer includes a non-linear resistor that is responsible for causing the transformer distortion. Driving the transformer from a very low source impedance like an op amp shorts out this resistor so the transformer produces little distortion. Adding a series resistor to the input counteracts this and the transformer will produce more distortion.

R
I’m assuming if I change r4 and r7 that I’ll have to recalculate the value of C2 and C3 to get the desired cutoff frequency?

In your second schematic C2 and C3 are coupling capacitors. You want them to present as low an impedance as possible across the audio spectrum. Therefore the cut off frequency with their associated load wants to be well below the audio band and preferably below 2Hz. The loads are not R2 and R3 - as I said their purpose is only to ensure there is a dc path to ground for the capacitors and in any case the TL072 is not capable of driving 600 ohm and 150 ohm loads. Change these two resistors to 10K. The loads are the transformers. The 100K pot will be reflected to the primary by the impedance ratio so it will appear at the primary as something like 9K. This will reduce a low frequencies due to the primary inductance of the transformer. To get the transformer to distort you should change R2 to be in series with the transformer primary and C2.

I am still concerned about T2. A TL072 will not reliably drive a 150:150 transformer. It inimum load is specified as 2K.

In the second schematic I posted, I changed the transformer to add (I think) 9db of gain which I can attenuate with the following potentiometer. The idea was that this could act as a gain control to choose how hard I was hitting the rest of the circuit.

Definitely a better circuit but see mu comments above

Cheers Ian, I appreciate your input.

My pleasure.

Sam

Cheers

Ian

 

[Sam0311]

Thanks Ian, very informative. I’m struggling to grasp everything you’ve said, the more I think about it the more questions arise I think a more ideal circuit could look something like this?

So if I understand correctly it’s the 10k resistor that allows the tl072 to drive the transformer? Or is it the 100k pot reflecting on to T1s primary? Is there likely not a convenient way to do the same to T2 in my second schematic and still have the output impedance of the circuit be suitably low? I'm pretty sure I can wire that transformer up to be 600:600 instead if that changes anything? It has two lots of windings.

When calculating the value of C2 should I still be treating the resistance as 600 ohms?

How would I calculate or mitigate the effects the 100k pot has on low frequencies? Should I consider changing its value?

Cheers

 

[ruffrecords]

Thanks Ian, very informative. I’m struggling to grasp everything you’ve said, the more I think about it the more questions arise I think a more ideal circuit could look something like this?

That is a lot better. 1K for RV2 is still too low for a TL072, There is no reason not to make it 10K.

It is difficult to know what value to make R2. It might be worth making it a pot so you can experiment with values. 10K is probably too high

So if I understand correctly it’s the 10k resistor that allows the tl072 to drive the transformer? Or is it the 100k pot reflecting on to T1s primary?

There are two things going on. The transformer does what it says on the tin - it transforms voltages current and most importantly impedances. So if you 'look' into the primary the ac impedance you will see is the load on the secondary winding reflected to the primary. So the 100K pot looks like 9K from the primary (it is complicated by the inductance of the the transformer but at most audio frequencies this is true).

The second thing going on is the resistance of the source feeding the primary affects the amount of distortion the transformer produces and this is what R2 does. It is in series with the impedance reflected to the primary so the total is about 10K + 9K = 19K. This is fine for the TL072 to drive. Even if R2 is made zero the TL072 still see a 9K load which it is fine with

Is there likely not a convenient way to do the same to T2 in my second schematic and still have the output impedance of the circuit be suitably low? I'm pretty sure I can wire that transformer up to be 600:600 instead if that changes anything? It has two lots of windings.

600:600 would be better but the relatively low primary inductance means there would be some loss of low frequencies and again it is too low for the TL072

When calculating the value of C2 should I still be treating the resistance as 600 ohms?

The actual load on the first TL072 varies with frequency. It will be lowest at low frequencies but you should design it so its load never fall below 2K. Then calculate C2 on that basis.

How would I calculate or mitigate the effects the 100k pot has on low frequencies? Should I consider changing its value?

The 100K pot has no effect on low fequecies. What does affect low frequencies is the inductance of the transformer primary winding.

Cheers

ian

 

[Sam0311]

Very interesting, thanks for this Ian. Lots of food for thought here and I think you’ve given me enough information that I should be able to fumble my way through this experiment. Looks like it might be a case of a bit of trial and error. Best case scenario I end up with something that I can actually use, worst case it sounds like pure but I’ve learnt some cool things along the way. Appreciate the help.

 

[ruffrecords]

Although transformers are basically simple things, in the real world there are all sorts of practical limitations that complicate matters. This means you need to design around the actual transformers you are using. Of course, manufacturers to not publish values for all the things we are interested in so experimentation is almost mandatory. Good luck.

Cheers

Ian

 

[merlin]

There was a vaguely related thread on DIYstompboxes:
https://www.diystompboxes.com/smfforum/index.php?topic=124987.0

 

[Gavin Pursinger]

Why cannot the transformer (1:1 or 1.4 ) just be hooked up in a balanced configuration off the xlr console outputs or buss outputs ? My console is a yamaha pm 3500 that has a very strong signal. I have several pairs of xformers with those ratios to try... it does not seem to me that any harm will come to the console in experimenting....thx

 

[Silveradodays]

Why cannot the transformer (1:1 or 1.4 ) just be hooked up in a balanced configuration off the xlr console outputs or buss outputs ? My console is a yamaha pm 3500 that has a very strong signal. I have several pairs of xformers with those ratios to try... it does not seem to me that any harm will come to the console in experimenting....thx

Hi,

Of course you can do that. You might still need to experiment a little with some compensation network on the secondary depending on the impedances of the equipment in the chain. Also depending on the quality of the xformer you might be deceived for it being to clean and subtle or worsening the bw and fr of your console.
hence the need to design a circuit that gives some short of control over those factors.

 

[CJ]

Here is the Greer amps soma mentioned in the stompbox link which may or may not apply to this thread ,

 

[user 37518]

Hey guys,

So I spent a lot of last year reading about electronics but I’ve only just recently started getting my hands dirty.

Purely out of curiosity (and because I thought it would be fairly simple to pull off) I want to see what it sounds like when you send a signal through a transformer that’s much hotter than it’s rated for. I was originally trying to keep things passive but it seems like I’m going to need to buffer the signal coming in. I’m using it with my Eurorack which has a typical output impedance of 1k ohms.

I just wanted to share the schematic on here to see if anyone can spot anything clearly wrong with my design or has any suggestions.

The idea is pretty simple, the op amps are there to buffer the signal going into the transformers. Running straight into T2 should keep things at unity gain but there is a switch to send the signal through T1 adding (I think?) +18db, the potentiometer at the end is there to tame the output level if needed. I’ll probably power the circuit with a couple of 9v batteries for now.

I’m unsure if the components in the feedback loops of the op amps are necessary since I’ll just be using them as buffers for now (I was repurposing part of a boost pedal schematic for this ). I may add a gain control on the first op amp later down the line though.

Cheers

Edit: I seem to have a habit of wiring the jacks wrong on KiCad

"Transformer Distortion Box" projects keep coming out every once in a while in this forum. It is a bad idea, but, if you want to go for it, get rid of the TL07x. Also, use cheap transformers, like OEP or similar, don't use a top of the line Jensen that will require a lot of voltage to fully saturate. You can also add DC to the transformer. 1:1 transformers tend to sound better than off-ratioed, so you can also use a 1:2 or 1:4 trafo. If getting nasty audio is the objective, there are a lot of things you can do. All of them sound terrible to me.

 

[mr coffee]

I agree with everybody about dropping the TL072 for this, and suggest you consider driving your transformer with something that has better output current drive like a LM386. And it will run well on 9v, too. Don't try to run it off a battery, though (LOL).

Put a variable resistor from the amplifier output capacitor to the transformer primary like Ian suggested (maybe 10 to 1K) and see if what it sounds like on a breadboard has an appeal to your ear for some usage.

Paralleling three of the windings as a primary MIGHT be helpful (be sure to get the phase right) and use the other one for the secondary (i.e., output winding), because it may increase your ability to saturate the transformer core before the amplifier runs out of headroom and reduce heating up the transformer, (but be sure it doesn't put the '386 into thermal shutdown). IIRC, the load impedance goes down once the core is in saturation, so the load on the amp gets heavier when your transformer is being saturated.

A variable resistor output load on the secondary might also be helpful in finding out if transformer saturation does some coloration you want to hear. I wouldn't go under 25 -100 ohms load and a 1K pot would be a good starting point.

Expect subtle things unless you make the '386 clip. You might want a small heatsink, too.

FWIW.

Like to hear what you find out.

Come back and tell us!

 

[Sam0311]

Hi all,

Thanks for the input, I haven’t looked at this thread for a few days so I haven’t been able to take in everything that’s been said yet but will be looking into the ideas stated here.

So I’ve been messing around with things on a bread board for a couple of days. There’s still some things I want to try out but so far I’ve been finding when A/Bing the bypassed signal and affected signal, in most configurations there is some noticeable difference in sound and sometimes I like what’s happening, sometimes not. Most of the time though the changes have been more subtle than I would like. No bother though, it’s been quite a fun learning experience so far and has got me interested in getting hold of different transformers to see what sonic characteristics they have. I think it could be cool to add some sort of soft clipping circuitry to the mix as well.

Something that’s got me a bit perplexed though is how I should be connecting up all of the grounds. To begin with I was connecting all of the grounds in the same place but the circuit wasn’t behaving as expected. I think if I’m not mistaken this is because if I send a signal through a transformers primary, on the secondary I’ll be getting the signal out on one pin and the inverted signal on the other pin. So if for example if on of the lugs on a pot was connected to the same ground as the transformer it would then be acting more like a blend between non-inverted signal and inverted signal rather than as a volume control.

I guess my question is how should I be connecting things in an unbalanced situation? How would I isolate the transformers from other parts of the circuit?