voltage gain by transformers vs. amps

So I started in the modern world, where transformers are mostly considered expensive, and most circuits are transformerless. And then I learn to appreciate the sound of vintage circuits that sometimes even have interstage transformers in addition to input- and output transformers. A signal chains consisting of Preamp, equlaizers and compressors may then have 5 or even 10 transformers, all adding (hopefully) to a nice coloration.

Now I wonder, just out of curiousity: Are there any amplifiers that only use the winding ratio of their transformers for voltage gain? I mean stepup transformer -> cathode follower (or emitter follower) -> next stepup transformer -> next follower ....
Would this make any sense? Has it been done? Apart from higher cost, are there any drawbacks?

JH.

Interesting questions.

With 'colour' in mind, it sounds like a good approach: active electronics providing 'power-gain' - this since there should be something increasing, otherwise it'd perhaps not qualify as an amplifier, just as a device that shapes stuff as more current or more voltage, hey, sounds like a TX and we're round circle. Sorry for rambling, I'm curious what other posts will bring. :wink:

Transformers in that situation are used for both interstage coupling and for impedance matching.

So maybe between two tube stages you could get more gain by matching a low impedance output from one stage to a high impedance input on the next stage? Hmm... I suspect it couldn't be that simple. You'd have to design the gain stages for low output impedance, and that would probably be a tradeoff for gain in that stage anyway.

If transformers are that great, how come nobody makes a box with a dozen mic transformers in series?

There is an article in the latest (3/05) AudioXpress magazine describing a power amplifier with an input transformer, a tube, and an output transformer. Not surprisingly the enthusiastic author raves about its freedom from coloration, although I am less sanguine.

The voltage gain of the input transformer if driven from a voltage source should be about 36 based on the stated impedances. The author states that the circuit is designed to take its input from the headphone output of a CD player, and suggests that this in in the range of 50-150 ohms.

I think this may be one of the first pairings of CD players with fahnestock clips.

He runs the direct-heated triode filament off of four D cells and a variable resistor.

Rawk On!

[quote author="bcarso"]The voltage gain of the input transformer if driven from a voltage source should be about 36 based on the stated impedances.[/quote]

Sure you don't mean six? What impedances do they specify?

Also, is the tube wired as a cathode follower like JH suggests?

Peace,
Al.

Welcome Bcarso!

b

:wink:

alk, he mentions a Altec-Lansing Peerless #4722. His schematic notation is "38 Ohm" at the input and "50K Ohm" at the output (directly connected to the grid). I calculated the voltage ratio as the square root of those Z's. If anyone knows what the actual Altec specs are that would be appreciated. That is quite a step-up I agree, and maybe it is really a nominal 150 ohm to 50k ohm ratio and thus only root 333, ~18.3. That would be similar to some mic input transformers if I recall correctly.

He uses a 71-A (this is a true antiquarian is seems) as the common-cathode tube. I don't see that he states an overall voltage gain at the output of the amp, the secondary of the second transformer, a Stancor 8054, when loaded with 8 ohms.

So, this should be enough information to calculate the overall midband gain if the tube characteristics are known; the operating point could be deduced from the plate voltage of 135 and the -27V grid bias (also supplied by batterries btw).

soultek: thanks!

> may then have 5 or even 10 transformers, all adding (hopefully) to a nice coloration.

The first attempts to build big systems with many cascaded transformers sounded bad: too much response droop and particularly enough phase-shift to completely faze-out tap-dance sound. (Tap-dance also pointed out the time-delay in 2-way speakers....)

Voltage gain with transformers and tubes: hardly-any. When toobes were expensive and metal was cheap, the standard gain-stage was a tube with an amplification factor around 6 or 10 (considered "high" in the day) and a 1:2 or 1:3 step-up transformer. Total voltage gain around 10 mid-band. Plate resistances were around 10K-20K. With 1:2-1:3 step-up, that is 40K to 180K secondary impedance: very-very high for a transformer winding. 500Hz-4KHz response was considered "good".

Tubes got better: both lower plate resistances that would drive a tranny better, but also higher amplification factors. Something like a modern 12AU7 in an R-C coupled stage has a gain of 15 and bandwidth 5Hz-50KHz, without the lump of iron. Systems that did not have to drive specific impedances quickly switched to R-C coupling.

The cathode impedance is low but remember that the current available is the same as the plate. In fact the small-signal voltage gain is no better, and the large-signal output is less. So CF and high-ratio transformer is "stupid" in classic design. You only find CFs when low output impedance is vital: mostly driving power tubes into grid current. Even then the standard was a medium-Mu tube and a step-down transformer.

[quote author="PRR"]> may then have 5 or even 10 transformers, all adding (hopefully) to a nice coloration.

The first attempts to build big systems with many cascaded transformers sounded bad: too much response droop and particularly enough phase-shift to completely faze-out tap-dance sound. (Tap-dance also pointed out the time-delay in 2-way speakers....)
[/quote]

I don't know - I just thought about a recording like the following:
Neve input channel, with input and output transformer. That's 2.
Maybe a third for the mixing desk output? 3.
Make some channel compression with an early 1176 - another 2 transformers, total of 5.
Then run the final mix thru one of these expensive variable mu compressors, input, interstage, output transformer, so we're up to a total of 8 transformers in the signal chain. And that's no especially extravagant signal processing yet, is it?

So in a cheap, transformerless environment of a semi-pro studio, an (expensive, surely!) box that has 8 transformers inside won't introduce more iron than the above example.

I don't know if this is good or bad - I'm aware there are different philosophies towards iron or no iron, too - I just wonder.

Voltage gain with transformers and tubes: hardly-any. When toobes were expensive and metal was cheap, the standard gain-stage was a tube with an amplification factor around 6 or 10 (considered "high" in the day) and a 1:2 or 1:3 step-up transformer. Total voltage gain around 10 mid-band. Plate resistances were around 10K-20K. With 1:2-1:3 step-up, that is 40K to 180K secondary impedance: very-very high for a transformer winding. 500Hz-4KHz response was considered "good".

Tubes got better: both lower plate resistances that would drive a tranny better, but also higher amplification factors. Something like a modern 12AU7 in an R-C coupled stage has a gain of 15 and bandwidth 5Hz-50KHz, without the lump of iron. Systems that did not have to drive specific impedances quickly switched to R-C coupling.

The cathode impedance is low but remember that the current available is the same as the plate. In fact the small-signal voltage gain is no better, and the large-signal output is less. So CF and high-ratio transformer is "stupid" in classic design. You only find CFs when low output impedance is vital: mostly driving power tubes into grid current. Even then the standard was a medium-Mu tube and a step-down transformer.

Click to expand...

Ok, I see, as practially every tube device needs a step-down transformer at the output, the concept of voltage-gain-thru-step-up plus cathode followers doesn't make much sense.

For transistors, the situation would be different, though.

JH.