What is Transformer Saturation?

[leadfootdriver]

I assume this applies to the Output Transformer. How does this affect the sound of an amp?

I have an attenuator that I've experimented with. The low level un attenuated tones sound crusty and honky. But when you turn up the volume, the mids scoop out, the bass and treble are emphasized, and the feel is juicier. This is a general statement as it affects some of my amps more than others.

I can hear the difference between power amp clip and preamp, but there must be more going on in an amp than just waveform cliping. Anyone have more insight on this?

[Roberto]

You can read this: Non-Linear Transformer Behavior
and this: Elecraft K3 Receive Audio

You will find why saturated cores give mostly odd harmonics.

[pdf64]

Saturation of the OT is very unusual; as it would cause the primary inductance to collapse, I think designers strive to avoid it.
It would only probably occur at the very lowest frequencies at max power.

[R.G.]

What pdf said. In general, guitar amp output transformers don't saturate. If they do, it's only when driven with the lowest possible bass frequencies. Possible, but not often. It is also possible that one could cleverly design a guitar amp output transformer to saturate (or get it from especially bad - or good, I guess - luck) but it's really uncommon.

This is because to get saturation, you have to drive a transformer with a certain amount of voltage*time. You can do this by putting a large voltage on it and letting it ramp up in current for a short time or a lower voltage for a longer time. The voltage that an output stage can put on an OT is limited by the power supply, so the way you saturate it is by putting the voltage on it for a longer time per half cycle - a lower frequency.

In general, the voltage needed to saturate an OT primary will be twice as big at twice the frequency, so it's not possible in ordinary guitar amps to saturate them with higher frequencies - only the lowest.

You cannot saturate a transformer from the secondary side, so loading has nothing to do with it.

You *can* cause saturation to happen by picking an OT designed for a lower power supply voltage (that is, a lower power output stage, with a lower power supply to do this) and subbing it into a higher power supply voltage amplifier, but again, this is only at the lowest frequencies, as "resistance" to saturation drops linearly with frequency.

You **might** get saturation by driving a guitar amp designed only for guitar with a bass, but even there, the preamp would have to actually pass the fundamentals of the bass signal, and many amplifiers simply don't pass that much bass. Some bass amplifiers only pass the second harmonic in their preamps. So the true fundamental may not even get to the output stage to try to saturate the OT.

So - how does saturation affect the sound of an amp? It's unlikely that any of us have ever heard that. It's very, very unlikely that what you're chasing is saturation.

[leadfootdriver]

Thanks for the replies.

So am I hearing the coloration of the attenuator?

[pdf64]

Without user and bench experience of your rig, and ideally being in the room to have 1st hand experience of the settings / sounds, it's not really possible to say for sure.
You may be confirming the general findings that stage gear doesn't sound its best at very low volumes, and that tube amps are best pushed to high signal levels to reach their sweet spot.

[Gregg]

My experience with testing mostly guitar OTs is you can saturate them only with frequencies in the 30-40Hz range but that is at full power only. The transformer will start to buzz and the sine wave will start to "break" which is very unpleasant to hear as the odd harmonics content will be predominant.
In real life that's almost impossible because a guitar OT is very unlikely to be fed such frequency content as in most guitar amps the low frequency content could be decreased several times in the preamp until it reaches the power amp. Even bass amps' OTs are not designed to work with such frequencies.

[J M Fahey]

Yes, there's 2 terms loosely used in the Guitar World which aren't really accurate (being kind) : transformer saturation and speaker saturation, specially as applied to alnico speakers.

The real fact is that some amps are great forn tasty hand controlled playing, nice crunching, etc. ; such amps do exist, theres a lot of them, and are loved by many players.
Generally old and low powered amps.
So player looks at them and *see* small transformers, wimpy speakers, and attributes that sound to them.
Which is "confirmed" in his mind because modern amps have big iron and heavy sensitive ceramic speakers and do not have "that" sound.

Modern in Guitar World spacetime, meaning late 60's early 70's on , of course

But what you are actually saturating is power tubes, grids rectify and amps become crude 40's technology compressors.

Transformers and speakers have little to do with that.

[teemuk]

se-output-trans-calc-3 contains examples of transformer saturation captured on oscilloscope, top waveform is voltage, bottom current:

@100 Hz input, not much visible distortion.

I'm pretty sure this is about lowest frequency outputs from typical guitar preamps. If it's "high gain" design it may introduce distinct hi-pass filtering already around 1 kHz.

@32 Hz


@25 Hz


I would imagine the waveforms contain plenty of odd and even harmonic distortion, probably such that has very high amplitude of higher order harmonics. All in all, it doesn't look like distortion you could classify musically pleasing.

[printer2]

So - how does saturation affect the sound of an amp? It's unlikely that any of us have ever heard that. It's very, very unlikely that what you're chasing is saturation.

Haven't heard it? Well then you must not be using the right transformers, or the wrong ones if you actually think of it. I dislike the sound of saturation. My latest amp with two 12AQ5's and a 70V line transformer has a problem with the low notes on my E string.

[J M Fahey]

No doubt, line transformers are not gapped and saturate with the slightest DC through them.
To boot, since typical background music service is not critical at all, often driving a ton of cheesy 3"t o 6"speakers they are even less suitable to pass low frequencies .... low meaning anything below 200 Hz.

[R.G.]

Haven't heard it? Well then you must not be using the right transformers, or the wrong ones if you actually think of it. I dislike the sound of saturation. My latest amp with two 12AQ5's and a 70V line transformer has a problem with the low notes on my E string.

Notice that I didn't say it couldn't happen. I said it's unlikely. I can easily concoct situations that would show up transformer saturation.

In fact I have. I used to design switching power supplies for a living, and the stock in trade there is to design magnetic components right up to the edge of saturation. I've missed the edge a few times and so I am pretty familiar with how to get to the edge.

But yes, you can get there. It's just very unlikely that any of us have heard it.

[Jazz P Bass]

I just had in a Dynaco St70 amp.

What a beut.

In testing it, the output went clear down to 35 Hz!

Below that it got ugly.

What I find of interest is amps like the Fender Bassman.
They don't do well below 50Hz.
Which isn't even the low E.

[pdf64]

the Fender Bassman...don't do well below 50Hz.

I seem to remember reading that was part of the spec (at least in regard of the 5F6A), in order to avoid damaging speakers, and so warranty repairs.

[Jazz P Bass]

That makes sense, as speakers where not at all robust back then.
(think 25 watt speakers in the am/fm/ phono consoles)

(I seem to recall Hartley Peavey becoming so disgusted with the available speakers, that he desided to manufacture his own.)