I want to explore the effect of transmitting a guitar signal through a small saturated magnetic core. But I don't want to do this at high power levels, using saturation of the output audio transformer (in tube amps), as is usually done. I'm aware that some Wah pedals do exhibit some distortion using a small inductor, but I want to consider other approaches.
I want to do is to do this in the "small", at a signal power level. Something like passing a signal through a "core memory donut", or iron washer etc. The idea is to find something more readily available than a core memory ring.
My fear is also that a washer will not be effective enough, may also require higher power levels than desired.
The link below describes how to avoid saturation of toroid cores:
The example he cits suggests a max current around 2.4 Amps for the example he cited. So this rules out toroids in my mind, and probably even smaller ones.
One thought I had was to use some steel wire, bend it into a very small ring and crazy glue the ends. Then try to put some windings on it to turn it into a saturable signal transformer. I'd have to guess at the windings in this case since I'd have no clue on the magnetic properties of such.
I'm looking for suggestions or better ideas. Or is it simply better to purchase a small inductor? If so, what inductor range is suitable for op-amp signal levels?
Ferrite EMI suppression rings are generally high permeability and saturate very easily, just a few amp-turns. You could get away with an inductor of perhaps 1000 turns to distort signal levels on the order of mA.
I think you'll find that 1., 1000 turns will give a hell of a lot of inductance, and 2., "small signal" properties are, well, small; distortion is inherently low under this condition.
Uncommon materials are less linear. Square permalloy is wonderful stuff. You can find cores at Nebraska Surplus Sales, namely 50B12-1D at:
Beware, permalloy is to magnetic cores what transistors are to vacuum tubes -- it switches a lot faster, so you'll see a "harsher" cutoff when it saturates.
Deep Friar: a very philosophical monk.
George Herold expounded in news:5b1906be-5b90-4d1f-add1- email@example.com:
Diodes are used in everything from "Overdrive" (soft distortion), to "Distortion" (harsher) to Metal distortion (extreme). Those are trivial to reproduce.
What I'm looking to do is to emulate the best attributes of magnetic distortion that happens in certain brands of tube amps by saturating the audio output transformer. This leads to (or supplements) a more pleasing distortion sound.
FETs have interesting characteristics as well, but it is the magnetic attributes that I am after.
Tim Williams expounded in news:ic1a6l$tkk$ firstname.lastname@example.org:
I could approach 100 mA with a 2n2222 perhaps. Alternatively, I could do more with a LM386. But more than that would be more than I want to get into at this point in time.
Interesting... Harsher is ok if it "sounds good", so some experimentation might be in order. But I want to explore the "normal" first, as this is probably what I'm after. I liked your SS/tube analogy. That hits home. ;-)
At the risk of displaying my ignorance yet again. ("What, Can't you keep that thing under wraps Herold?") What are the 'attributes of magnetic distortion' that you are trying to emulate? I figured that as the transfomer saturated the signal amplitude would level off. (Less gain at large amplitudes) But perhaps there is more to it? Does it also depend on the frequency?
Saturation in tube amp output transformers is a low frequency, high amplitude phenomenon. The effect is a sort of farting sound, where the whole amplifier gain kind of gets shunted to zero once the transformer saturates. It stays "off" until the signal reverses and the transformer moves out of saturation.
The frequency depends on amplitude, so if say 10V saturates at 50Hz, only
2V will saturate at 20Hz, or you'll need a whopping 200V to even get close in midband ~1kHz. Of course a typical 10W, 8 ohm tube amp won't make more than 9Vrms, or maybe 13V peak, square wave / in clipping, so there is a limited range of frequencies which will saturate. (Guitar amps are probably wound with dinky transformers that saturate even easier...)
The suddenness of saturation will control harmonic production. You can imagine the physics of farting: expanding gas opens a path through a restriction; restriction opens, pressure drops, restriction closes. It's a relaxation oscillator, where gas comes out in bursts, hence the pulsating sound. If the pulses are very sharp, it will have a popping, clicking or buzzing quality to it; if they are more rounded (cheeks together..), you get a deeper, more resonating sound. In the same way, the sharp saturation of permalloy will have a reedy sound, while the softer saturation of iron or ferrite will result in less harmonics and, I suppose, for obvious reasons, a more "tubey" sound.
For the circuit, obviously you'll want the inductor in parallel with the signal, i.e. from signal to ground, same as the original transformer does. You'll need a source with moderate current capacity, and some series resistance (so saturation doesn't overload it), which an LM386 with 10 ohm series resistor would be good at. It must be AC coupled, which means you won't see saturation below some frequency, because that frequency is too attenuated to cause saturation; for guitar, 20Hz is probably fine. You want the system overdamped, so check that R, L and C come out okay. Or maybe you do want it a little underdamped, so there's an LF resonance that adds boominess.
For cheapasses, the "expensive" inductor could be replaced with an integrator and clamp circuit. Integration counts volt-seconds, just like an inductor does; when the count gets too high (or too negative), the signal gets clamped. The actual signal path is not integrated, so it still has the ordinary frequency response, but it does get shunted when "saturation" occurs. In this case, harmonic content would be controlled by comparator gain -- if it crosses the threshold and instantly the signal goes to zero, you'll have loads of harmonics. If it goes up and lazily starts tapering off a little at a time, you'll get a rounded curve. This has the advantages of being able to control all aspects -- inductance, saturation flux and sharpness.
Deep Friar: a very philosophical monk.
George Herold expounded in news:a578d6b2-37bb-4c31-b841-64c4cb5b4695 @o14g2000yqe.googlegroups.com:
In short, the technical term is the "tone". ;-)
Since there is an amplifier involved, there are aspects like class A (and hence a DC bias) etc. that factor into this (for some smaller amps). For other amps, there is the class B or AB amps (like Vox AC30 is contraversially reported to be when driven hard).
In my google travels (forget where) I saw that adding DC bias eventually makes the 3rd harmonic higher, and drops the fundamental in the process. With even more bias you can see even more of that and the 5th rising up. This is one controllable aspect that I want to experiment with. I have seen this kind of behavior from diode circuits too (for one diode only), but I suspect that the magnetic process is better (cleaner) sounding. It is this tough to measure quality that I want to explore.
I also suspect that the BH curve has other side effects that are difficult to reproduce in other ways. Finally there may be interactions with the amplifier which give it its characteristic sound. For example, the final sees more resistance when the core saturates, which must change the response of the amp. There might even be some signal folding going on, depending on amp etc. So I need to investigate some of that.
So really, I am just looking for a jump off point for some magnetic saturation experiments. Whether they lead to something useful or not, is another matter.
Tim Williams expounded in news:ic1if7$q9a$ email@example.com:
I wonder about some amplifier designs in this regard. I have read (or perhaps imagined) that in some finals there could be signal folding going on. Once the transfomer saturates, any extra signal will only see resistance. If this changes the nature of the load seen, I could see the possibility of it's gain actually decreasing even if only slightly, for the isntance during saturation. This too, would introduce new harmonic content. But I need to be careful to evaluate things from the speaker end and not the primary. ;-)
What I do know for sure, is that preamp induced distortion alone is not nearly as satisfying tone wise as combining it with final stage distortion.
So there is some "tone" aspect to final amplifier and/or magnetics. Companies like Murcury Magnetics seem to be successful at enhancing otherwise tame little amps like the Valve Junior.
Intentional tube amp overloading is not normally an effect of core saturation, it's one of tube GM shift at reduced plate voltage - a form of amplitude compression. There are other forms of overloading in the tube element with more objectionable effects, but these are rarely induced intentionally; they don't sound nice and the tube screen elements can be damaged in an uncontrolled manner.
Thanks for the explanation Tim. Perhaps one could just use a smaller transformer (made out of the same material as the output tranny) in the signal chain. If you can get the same current-turns-area as the output tranny then it should saturate in the same way. I can imagine a 'distortion knob' that puts more current through the small transformer and then throws the extra signal away before the output. (That way you'd have more distortion without having to rail things out.) If you wound your own transformer you could add a third set of turns for DC bias.
Why not get yourself some little audio transformers - Farnel list quite a few, some with power ratings as low as 10mW which should be easy to overdrive. Some of the nicer tiny ones are a bit pricey so you need to look yourself and pick some.
You could even make a tiny guitar amp using fets and transformers.
George Herold expounded in news: firstname.lastname@example.org:
I'm not against winding my own. In fact I can get some surplus cores locally, which would seem ideal for that. The difficulty is determining in advance what it will take to saturate them.
I'll have to review their stock to see what sizes there are to choose from. Perhaps I can get a crude test done with a signal generator, if I can get it "small" enough. Otherwise, I'll have to breadboard a LM386 for the purpose.
Well, I should start by saying that I know very little about magnetic materials. But if you can find out what the core is made of, and can get a B-H curve of the material. (I'm not sure this is called a B-H curve.) Then as long as you don't let all the different magnetic 'units' confuse you it should be fairly easy to get an idea of how many turns at what current (given the Area) you will need to saturate it. I'm sure there are many others here who can help with that.
Hi Warren, I was scribbling down some numbers last night as I watched my daughter's dress rehersal for "A Christmas Carol", she's the ghost of Xmas past.
I think iron tends to saturate at a field of about 1 Tesla (10^4 Gauss). I don't know the permability of your core, but a WAG is maybe
100 to 1000. Which means you need an 'air core' field of 10 to 100 Gauss. Now that is a pretty easy field make if you've got and amp or so of current. Heck, I made a pair of 8 inch diameter Helmholz coils that give 100 Gauss at 3 amps.
Have you taken any physics? Do you know how to calculate the B field for a simple solinoid?