adding an output transformer stage to a Son of Screamer type circuit

[mordechai]

I would like to add a little thickness/heft to a Tubescreamer-type OD by adding an output transformer...something like the output transformer stage we find in the Hudson Broadcast.  The schematic is here:

https://www.pedalpcb.com/docs/Simulcast.pdf

If, following the output capacitor in the SOS circuit, I add the Hudson Broadcast transformer section from R14/R15 section (the 15K/33K resistors) onward, would that do the trick, or would it introduce problems?  Would it affect the level/volume? 

[Mark Hammer]

Hi Mark. 
I'd be curious to know what the role of the transformer is in this application.  Is it contributing to the tone, or simply providing some sort of impedance-matching function.
Note as well that the charge pump is delivering a substantially greater supply voltage than you might be using in a TUbescreamer-like circuit.

[Marcos - Munky]

The transformer used is a 10K ct : 10K ct. A 1:1 transformer.

That's all the info I have on it, I don't know what it does. But maybe that info is useful.

[FiveseveN]

I don't know what it does.

Makes the pedal more expensive.

[mordechai]

Hi Mark. 
I'd be curious to know what the role of the transformer is in this application.  Is it contributing to the tone, or simply providing some sort of impedance-matching function.
Note as well that the charge pump is delivering a substantially greater supply voltage than you might be using in a TUbescreamer-like circuit.

Hi Mark,

The reason I ask about this is because before the pandemic hit, a guitarist for whom I've made some fuzzes was doing a show in Philly and asked me about a pedal he was using that had an output transformer.  He liked the thickness it imparted to the tone when switched in, and asked if I could add something like that to a Tubescreamer-type device. 

Five months in and I'm now turning back to pedal building to try to cope with some of the losses I've experienced from the Coronavirus, which has claimed the lives of two family members and three childhood friends.  So I wanted to give this a shot because I am feeling depressed and live in a country where half the population is ensuring that as many innocent people as possible die and that this crisis remains with us for very, very long time. 

I've used a transformer in an octave fuzz before but I don't know enough about them to determine how they would work as a simple output stage in an overdrive.  In a recording studio I know, the transformers in the different analog pre-amps and compressors make a notable difference on the tone and add more low/low-mid harmonic saturation in a way that is different from a simple EQ boost in that region of the signal.

The Hudson device is obviously quite a different device than a TS-style overdrive but I thought it would be worth exploring and asking. 

[John Lyons]

Sounds like an interesting idea.
Sorry to hear about the ones you've lost.
Be strong.
John

[merlinb]

If, following the output capacitor in the SOS circuit, I add the Hudson Broadcast transformer section from R14/R15 section (the 15K/33K resistors) onward, would that do the trick, or would it introduce problems?  Would it affect the level/volume?

It will work fine, but you will probably want to reduce R14 to something in the range of 100 ohms to 1k, otherwise you will lose volume.

[FiveseveN]

Well now I feel bad for being cynical.
Do you know which pedal your friend was using? Was it the Broadcast? Or something that offers the option of bypassing the transformer? Or does "when switched in" refer to the entire pedal? Because that doesn't tell us how much "thickness" the transformer is responsible for and how much the rest of the circuit or the circuit as a whole (even what comes after if the transformer isn't followed by a buffer).

[Mark Hammer]

First, saddened to hear about the personal impact of the pandemic on you and those close to you.

That transformers can saturate and contribute to the texture of sound is well-established.  The nagging question is what the conditions are that facilitate this, and whether what you have in mind will meet those conditions.

That aside, a little relief.  And maybe a bit of hope.

[PRR]

....you will probably want to reduce R14 to something in the range of 100 ohms to 1k, otherwise you will lose volume.

Yeahbut.... if the source impedance is low it absorbs much of the transformer's flaws (flavor).

[mordechai]

Well now I feel bad for being cynical.
Do you know which pedal your friend was using? Was it the Broadcast? Or something that offers the option of bypassing the transformer? Or does "when switched in" refer to the entire pedal? Because that doesn't tell us how much "thickness" the transformer is responsible for and how much the rest of the circuit or the circuit as a whole (even what comes after if the transformer isn't followed by a buffer).

It was a Greer Amps "Soma" he was using.  Looking at the face of the pedal now on the Greer Amps page, it doesn't look like the thing has a switch for the transformer so it looks like it's just always in the signal path.  I'd love to get a look at the schematic for the whole circuit, maybe there's something fancy going on with how they incorporated the transformer. 

It did have a really nice heavy saturated tone (not hi-gain or super crunchy, just thick and present), and he had said that somehow getting a TS-style overdrive to have some of that additional heft would be of interest to him.  Months ago I tried futzing just with the output cap and volume control of a Son-of-Screamer circuit to see if it could do the trick but it just wasn't the same and just sounded like a woofier version of the TS-style drive and not in a pleasant way.  So figuring out how to integrate the transformer could be a nice way to get that sound, which I guess is sort of Neve-like (I'm thinking of the beef you get from a 1073 pre-amp)...

[merlinb]

Yeahbut.... if the source impedance is low it absorbs much of the transformer's flaws (flavor).

IME when you throw a transformer into the circuit, people hear whatever they want to hear, whether it's audible or not!

Also, I noticed the transformer secondary is grounded. There's no need to do that,; if you leave it floating you get the benefit of ground loop breaking.

[mordechai]

Yeahbut.... if the source impedance is low it absorbs much of the transformer's flaws (flavor).

IME when you throw a transformer into the circuit, people hear whatever they want to hear, whether it's audible or not!

Also, I noticed the transformer secondary is grounded. There's no need to do that,; if you leave it floating you get the benefit of ground loop breaking.

So is there a purpose to the ground loop there, or is it a design flaw/redundancy?  That is, does having the secondary grounded do something that leaving it open wouldn't do?

[amptramp]

A lot depends on whether you have the transformer operated at zero DC current or you operate it with DC current like the output of a single-ended amplifier.

In the example shown by PRR, the capacitor blocks DC so any effects from saturation would be symmetrical about the zero signal level and this would favour odd-order harmonics.  If you have DC current through it like a single-transistor driver stage, it will operate somewhere between zero current in one direction and maximum current in the other, so saturation effects would be in one polarity only and would favour even-order harmonics.

I have one old radio (a 1938 Minerva made by the Universal Radio Company) that uses a 6C5 driving push-pull 6F6's via a phase splitter transformer that is DC isolated via a capacitor to the transformer and the neat thing about it is it has a quarter inch jack for audio input so youi can plug a guitar directly into it.  My daughter, SpawnofRon, is the guitarist in the family and she was quite skeptical about how good a guitar would sound through it, so I set it up for her to play.  The dinner-plate eyes said it all - now she knows why tube amplifiers are jealously guarded by guitarists.  The sound was superb and plenty of volume as well - tube watts seem to be mightier than solid-state watts.  Push-pull 6F6's are good for about 11 watts but they sound louder.

[merlinb]

So is there a purpose to the ground loop there, or is it a design flaw/redundancy? 

I suspect it is a design accident or the result of adding the transformer to an existing circuit where the output jack was already grounded and couldn't easily be disconnected.

[mordechai]

A lot depends on whether you have the transformer operated at zero DC current or you operate it with DC current like the output of a single-ended amplifier.

In the example shown by PRR, the capacitor blocks DC so any effects from saturation would be symmetrical about the zero signal level and this would favour odd-order harmonics.  If you have DC current through it like a single-transistor driver stage, it will operate somewhere between zero current in one direction and maximum current in the other, so saturation effects would be in one polarity only and would favour even-order harmonics.

I have one old radio (a 1938 Minerva made by the Universal Radio Company) that uses a 6C5 driving push-pull 6F6's via a phase splitter transformer that is DC isolated via a capacitor to the transformer and the neat thing about it is it has a quarter inch jack for audio input so youi can plug a guitar directly into it.  My daughter, SpawnofRon, is the guitarist in the family and she was quite skeptical about how good a guitar would sound through it, so I set it up for her to play.  The dinner-plate eyes said it all - now she knows why tube amplifiers are jealously guarded by guitarists.  The sound was superb and plenty of volume as well - tube watts seem to be mightier than solid-state watts.  Push-pull 6F6's are good for about 11 watts but they sound louder.

Thanks for this comment, it sheds a little more light on the situation.  If I understand correctly, then, the 100uF cap in PRR's image would lead the transformer to operate without the DC current, and thus it would not generate the even-order harmonics that would lead to a pleasing type of harmonic saturation.  Do I have that right?  If so...would it be more advisable, then, the remove that cap, have the signal go right to R14, and put the output cap between transformer and the volume pot?  Would that create a scenario where the transformer generates even-order harmonics?

[merlinb]

If I understand correctly, then, the 100uF cap in PRR's image would lead the transformer to operate without the DC current, and thus it would not generate the even-order harmonics that would lead to a pleasing type of harmonic saturation.  Do I have that right?

All transformers introduce distortion, of various flavours, whether there is DC or not. Some more than others, of course, so it depends what transformer you use. A cheap little transformer is likely to add plenty of flavour even without saturation.

If so...would it be more advisable, then, the remove that cap, have the signal go right to R14, and put the output cap between transformer and the volume pot?  Would that create a scenario where the transformer generates even-order harmonics?

That would saturate the transformer so hard you would get no output. The best option is to build the circuit as PRR/me have already shown. Then listen.
Then add a resistor in *parallel* with the cap. This will allow some DC to leak past the cap and through the transformer primary, which will push it towards saturation. You can play with the parallel resistor value as much as you like, heck make it a 100k variable resistor and call it a saturation knob if you like.

[PRR]

Transformer flaws are some function of Level.

I don't understand putting an arbitrary transformer at an arbitrary point and expecting "magic".

[mordechai]

PRR,

I don't "expect magic".

I asked a simple question about a simple subject, something I'd like to know more about.  That shouldn't evoke a snide remark carrying an implicit, derisive insult.

I'm a university professor in my day job.  My field is ancient Israelite religion and archaeology.  If a student asked me an honest question about the meaning of a passage in the Bible, I wouldn't try to humiliate them be rubbing their face in what they don't know about ancient languages. 

So don't do that to me.   

I'm not expecting anything.  I'm just trying to learn, and to do so as a way of coping with deaths in my family from this pandemic. 

I get that you know more than I do about stompboxes.  Really.  You know way, way more.  But unless your vast knowledge actually helps in developing a vaccine that can save lives or can bring back my relatives -- do me a favor and keep your hubris to yourself.

[mordechai]

If I understand correctly, then, the 100uF cap in PRR's image would lead the transformer to operate without the DC current, and thus it would not generate the even-order harmonics that would lead to a pleasing type of harmonic saturation.  Do I have that right?

All transformers introduce distortion, of various flavours, whether there is DC or not. Some more than others, of course, so it depends what transformer you use. A cheap little transformer is likely to add plenty of flavour even without saturation.

If so...would it be more advisable, then, the remove that cap, have the signal go right to R14, and put the output cap between transformer and the volume pot?  Would that create a scenario where the transformer generates even-order harmonics?

That would saturate the transformer so hard you would get no output. The best option is to build the circuit as PRR/me have already shown. Then listen.
Then add a resistor in *parallel* with the cap. This will allow some DC to leak past the cap and through the transformer primary, which will push it towards saturation. You can play with the parallel resistor value as much as you like, heck make it a 100k variable resistor and call it a saturation knob if you like.

That variable resistor as a saturation control is very interesting.  I will give it a shot on the breadboard when I try to build this thing up next week.  Question...would the resistance from the variable resistor obviate the need for R14, or would it be advisable to have R14 at a nominal value so that if the variable resistance was completely removed, there would still be a safety net of sorts before the transformer stage?