Saturating a miniature audio transformer in a pedal.

[Kesh]

seeing as a good part of "tube sound" apparently comes from the output transformer getting a little saturated, i wondered if anyone had tried this on small audio transformers in a pedal at saner voltages.

is it possible?

would it sound good?

[WhenBoredomPeaks]

I have an SHO connected to a TM018 in a modular synth build of mine but i think the only thing happened is that i lost some highs.

(btw i built it as some kind of "gimme back analog" module for introducing analog artifacts in the signal but it probably were not a good idea.)

[Mike Burgundy]

Possible? Sure. Sound good? No idea.
Output transformers are very complex beasts as far as behaviour is concerned. One thing to keep in mind is that saturation will happen sooner the lower frequency you feed it, and the smaller (ie less iron) the core. It may turn out to be difficult to hit the sweet spot for pleasing saturation with a tiny transformer. This is something to experiment with. I do have a sneaky suspicion core saturation distortion in any amount sufficient to be noticable is actually quite ugly, but I might very well be wrong.

[WhenBoredomPeaks]

btw here is a relevant thread an an answer from R.G himself:

http://www.diystompboxes.com/smfforum/index.php?topic=93514.msg808971#msg808971

As a practical matter, no. Quoting myself,

You were thinking "Oh, wow! A cool new clipper mechanism!" Yes, but it only happens at a specific volt*time. You can only clip the lowest frequencies. Bummer. Not useful in effects, at least not simply.

The transformer itself does do a nice, smooth clipping that's primarily third harmonic. But it happens at a steadily increasing input voltage level as frequency goes up. Double the frequency (that is, one octave up) and the level needed to make it distort is two times as big.

One thing that might happen is that the imperfect loading might make the driving circuit distort, and the transformer would obediently transform that. But it would be the sound of the overloaded driver, and there are other and easier to use ways to get that.

[Kesh]

Thanks guys. Loads of info in that other thread.

[tca]

Have a look here: http://www.diystompboxes.com/smfforum/index.php?topic=99611.0

Cheers.

[tca]

seeing as a good part of "tube sound" apparently comes from the output transformer getting a little saturated, i wondered if anyone had tried this on small audio transformers in a pedal at saner voltages.

Hi,
   I've tested Escobedo's Ultra Class A Superdrive Power Amp



has a preamp. It sounded good! Very Fender-ish with a lot of gain.

I've also tested this version with a jfet which I dubbed TRANSFET



It has a good compression and distorts very well under overdrive (I've used a mini-booster to do it).

Here is the transformer for the TRANSFET.

Cheers.

[Gus]

CC?

A mosfet SE amp is old stuff.  here is a link
http://www.passdiy.com/pdf/ARCH%20NEMESIS.pdf

IIRC there was a 80's MOSFET design book with a SE mosfet amp.

[Kesh]

Cheers guys. 

[teemuk]

Tried it. (I actually have a small phone line transformer rigged to my current little practice amp). (Un?)fortunately it really doesn't work like that.

First of all, you'll have hard time saturating the transformer with just low amplitude signal. The voltage probably isn't going to be high enough and frequencies of interest will not be low enough. You likely have to go well below the bandwidth of typical guitar before saturation kicks in at realistic signal amplitudes.

Basically, it likely would fall down to having to use some sort of DC bias voltage to saturate the transformer and then it wouldn't be the same effect as in tube amps.

If you do get the transformer to saturate it will sound ungodly afwul. Let me ask you, have you actually ever seen audio transformer saturation on scope screen? It's not nice smooth, "musical" clipping. It's large parts of the wave basically "bitten" off, plenty - and I mean plenty - of high order harmonic distortion and mucho annoying high frequency fizz as result. In reality, audio transformer saturation is not such a usual occurence in tube amps as folks believe. Luckily.

This scope capture is taken from a push-pull tube power amp and portrays actual OT saturation (input signal is sinusoidal):


Thanks but no thanks.

There are some attributes you gain from transformers. The usual impedance mismatches introduced by the design in which small audio transformers are just more or less randomly slapped to some point of the circuit will introduce rather steep filters, which may sound nice. In my amp the interstage transformer, for example, performs a very nice low frequency cut. Pretty much like a 2nd order filter. That's about the only audible thing it does. In push-pull configuration you easily get nice soft clipping introduced by the circuit topology and in single-ended designs you may gain asymmetry usually somewhat lacking from non-transformerless SE gain stages. Of course, mostly features that have nothing to do with transformer saturation.

[chptunes]

I've also tested this version with a jfet which I dubbed TRANSFET



It has a good compression and distorts very well under overdrive (I've used a mini-booster to do it).

Here is the transformer for the TRANSFET.

Cheers.

Hey TCA.. do you drive an 8 Ohm speaker with the TransFET?  Even though the Transformer {which can also be ordered here at Mouser} has a Secondary Impedance of 1k Ohms.?..  This is an area where I still have a lot to learn.  Do you recommend any particular speaker, or types of speakers?

Thanks,

-Corey

[tca]

That circuit is not meant to drive a speaker, it is a booster/pre-amp that distorts quite well when overdriven.

[pinkjimiphoton]

using a tube amp reverb circuit, take a 47k resistor as a load instead of springs. makes a real nice, warm brown overdrive.

would sound great implemented in a pedal.

[gritz]

Not directly guitar related... but I just had to breathe fresh life into my early 70's A.M transistor radio - The on / off / volume control was really crackly and the forty-year-old electrolytics were getting a bit saggy. The audio had a constant "nasal" crossover distortion quality too. Ugh. Time to dive in with the soldering iron...

It uses the once common layout of transformer phase splitter (single ended input) feeding two identical output transistors that are wired to an output transformer in the same manner as a push/pull valve amp, so it should be easy to make it sound "warm", right?

Wrong!

Being battery powered (now via a spare 7v2 NiMH to mitigate battery cost) I had some decisions to make re. quiescent current of both tranny stages, but all I can say about the whole process is that it was very easy to make it sound pish / thin / clipped and quite hard to make it sound vaguely bearable. It's complicated! I considered bypassing the whole lot in order to drop a 386 board in there...

[Jaicen_solo]

That's why you used to be able to buy bags of NPN/PNP transistor pairs, matched for this very purpose.

[wavley]

Of course it's pretty common in the mic preamp world to put an attenuator after the output because you will, in fact, saturate the transformer in the low frequencies before the preamp starts to clip when the preamp is designed to do so.  This has been discussed and analyzed ad nauseum at places like group diy.  Some transformers do this better than others, some will fatten in the low end but then if you push them harder they will abruptly break over into a very unpleasant distortion, some just break over into an unpleasant distortion, some just sound really great when driven hard.

It's not impossible to do, it will just take a little more thought, experimentation, and listening to get the job done than some other things.  Unfortunately, I'm not sure that something like the tiny transformers we usually use in pedals around here are going to give you what you want.  I did some experimentation with actively driven small transformers inspired by R.G.'s Hum Free and found some unpleasant distortion when I tried to drive them too hard (using a scope, my active part was not clipping) switched to bigger edcor transformers and the clipping was gone, I'm sure that I could get them to saturate too, but not before hitting the rails of my active part.  Of course then purpose of mine was to be clean so I didn't really investigate further.

If I were to try and get transformer saturation in a guitar pedal on purpose I would probably look at decent mic input transformers first.  They are designed for smaller signals so it is easier to saturate, and there are quite a few of them that are designed to do so gracefully.

[R.G.]

I did some experimentation with actively driven small transformers inspired by R.G.'s Hum Free and found some unpleasant distortion when I tried to drive them too hard (using a scope, my active part was not clipping)

Hmmm.

When a transformer saturates, what happens is that the magnetic core cannot keep increasing flux beyond what it already has, and so the permeability of the material drops to that of the surrounding material, air or the equivalent. Effectively, the core flux quits changing, so the voltage on the secondaries quits changing, since it's proportion to d-phi/d-t.

This does a couple of things, one being that the secondary is now about 10,000 times less coupled to the primary (for iron cores) so the primary can do things that don't couple to the secondary. In addition, the primary incremental inductance drops by the same ratio, as the transformer becomes essentially air cored for the time the core is saturated. With a low impedance active drive, the driver can supply so much current that the beginnings of the core saturating can still be driven without the active device clipping visibly. The active device just pours in more current.

This is good for extending low end, because the paralleling of the primary inductance by reflected secondary impedance is then not what limits frequency response, it's the paralleling of the primary inductance by the active driver. As long as the driver can keep driving both the primary inductance (which is decreasing in value as frequency drops, remember) and the reflected secondary impedance, the secondary output stays up, and the response through the whole thing is still flat.

However, the core starts to saturate at some volt-time integral. As frequency drops, the time part of that product goes up, so the volt part of the product must decrease to keep the core out of saturation.This means that the voltage of the signal that can be handled without saturation decreases linearly with frequency at the low end. This is not a loss in frequency response, as the ratio of primary to secondary voltage can stay the same as long as you lower the total signal.

I'm thinking that in a situation where a low-impedance active driver is substituted for a higher-impedance source like tube plates,  when saturation is reached, the active device pours in a lot of current, and this charges up the leakage inductance to many times the normal currents in the leakages. Then when signal drops off a bit, the leakage exerts V = L* di/dt and inserts a blip of non-harmonically related voltage into the primary drive. The leakage is effectively in series between the driver and the primary, so it's free to do this. The leakage would "eat" a blip of voltage on the leading edge of saturation as drive to the primary goes up, and add a blip to the primary voltage as the primary drive goes down. This seems like it might explain some of teemuk's scope pictures.

So the impedance driving the primary might have a big effect in how this sounds.

I have seen voltage and current pictures of iron core power transformers driven into saturation. These show a nice, soft, roundover clipping on the secondary waveform. Now I wish I'd paid more attention to what was driving that setup. There is a certain amount of primarily third-harmonic distortion in all iron-core transformers from the nonlinearity of the B-H curves. RDH4 goes through this effect a bit.

[wavley]

R.G.  This subject is actually a little bit why I departed from your design a bit and started pumping up the gain to see if I could reproduce the fattening sound of pushing my mic pre's hard (Neumann V476b's with a pad on the output or a heavily modified Altec 1628)  I really liked what it was doing for my recorded guitar sound, I had a ground loop (well, mostly it was the ground loop), hey two birds with one stone.  It didn't work great with the little Triad transformers I was using (they did work great for breaking the loop, both with R.G.'s circuit and the balanced out of my 501 Space Echo) so I figured I would try the Edcor 15K:10K transformers, they sounded great, but I tried pushing so much gain to make them fatten that I was getting blocking distortion on my amps.  I didn't feel like messing with it anymore given that it solved my ground loop, turned the gain back down, and now they just live happily on my board just doing their job.

Your reasoning might explain why it's so common to see post transformer pads on so many solid state mic preamps but not so much on tube ones, I always just figured folks were already getting enough dirt from the tubes.

[tca]

Question: how much power can a typical mic transformer support/dissipate?

[Jazznoise]

That scope waveform looks vaugely sawtoothy, maybe a strong 2nd harmonic? I don't have half the knowledge on Transformers being bandied about, nothing outside the Jensen articles really and abit of the stuff in Douglas Self's work.

For those curious to try this I've seen a build where someone substituted a Neve 1073's input transformer for a cheapish one commonly availible on most electronics sites and with a little tweaking of the Zobel got very similar results. For those familiar with the 1073, this is good news. If you don't know the SoS article on the preamp tests is a good taster, it's a somewhat subtle but noticable smearing. Very flattering..for some things!

It's on my build list for next summer..and I may have aquired a suitable chassey for it. If it sounds cool, I might do a booster pedal version - though 70dB might be abit excessive!