Mosfet OT saturator!! Yarrrr!!!

[jrfrond]

I think I posted this somewhere else on the forum, but I experimented with something like this a long time ago using a miniature audio OT and JFET's. I got it to work and it sounded decent, but I was able to get much the same result out of linearly-biased CMOS buffers e.g. CD4049. You can find that in the Sunn Beta series of amps.

Univox released their "Mobile-Ohm" series (models U130L/B and U200L/B) of SS amps in the mid-70's that utilized bipolar power transistors with a push-pull transformer-coupled output and selectable speaker impedance (hence the moniker). When you turned these amps up, you could get the transformer to saturate nicely and they sounded quite good! Remember, a problem with bipolar transistor clipping is odd-harmonic distortion. Odd-harmonics tend to cancel in transformer push-pull designs. As to WHY you would want the saturation/compression effect in a SS, well, THAT is a question for the ages, isn't it?

[teemuk]

Univox released their "Mobile-Ohm" series (models U130L/B and U200L/B) of SS amps in the mid-70's that utilized bipolar power transistors with a push-pull transformer-coupled output and selectable speaker impedance (hence the moniker).

The transformers in those were actually just plain autotransformers used for
impedance matching. Similar stuff that had been a mainstay in solid-state PA,
paging and 70V Audio system amps for about a decade before Unicord even
adobted the idea.

Interestingly, not much is talked about transformer-induced sonic qualitites
of the aforementioned. It's only when that stuff winds up to guitar amps all the
talking suddenly begins.

For example, this Marshall amp - also from Unicord, used the very same idea
as Mobile Ohm series. No push-pull transformer coupling, just plain autotransformer
as an interface scaling the output signal voltage to match the load. You could
naturally replace it with an ordinary transformer with separate primary and secondary
but that would be more bulky and expensive.

[lowell]

Ok so I'm pretty new to this autotransformer talk. I get that there's no primary/secondary and that it's just basically a primary w/ a tap or multiple taps, or a sliding mechanism as in a Variac... so could the primary of a center tapped output transformer be used as an autotransformer?
Finally I'm lost in the world of impedances w/ an autotransformer, AND solid-state impedance matching for that matter too. How does one choose a transformer for coupling drains or sources to a speaker? What I mean is what is the output impedance of your average MOSFET or silicon power transistor? Is it the RdsON in the data sheet? FYI I get how OTs work in tube power amps for impedance matching to the speaker with the winding ratio and that transformers have a VA (wattage) rating not be exceeded.

[J M Fahey]

Hi Lowell. No, it's not a RDSon or any other *device* parameter per se, but an optimum load impedance that the active device must drive, which in moderns OTL amps is the (speaker) load impedance itself.
When designing the (modern)amp, it's taken account of and the project is sized as to to drive it properly.
When an important and unavoidable mismatch exists, as in tube amps, we include a transformer which tries to match "driver to driven".
Besides that, it introduces some quirks in the sound that we find tasty, I guess that is what you intend to get.
Read the Peavey and Pritchard patents, you will find enough food for thought there as to get belly ache, and then some.

Hi Teemu, did you notice that the speaker fuse placement is wrong? It should be between the transistors output proper (which always see the same load impedance) and the output autoformer.

Hi Wilder: no, not protons (which are far heavier), the answer lies in:

Hi Steve:

There were never any "PNP" vacuum tubes, to make one you would need an "anti-cathode" that emitted positrons.

No Sir, wrong answer !! Most of Isaac Asimov's illustrious writing career was based, precisely, on the existence of the Positronic Brain, the basis of human-like Robots, the first of which was none other than R Daneel Olivaw, the famous detective.
Although Asimov never mentioned anticathodes, those Positive Electrons must have come from somewhere.
Don't tell me that it was only SF !!!!!!! :