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Transformer as a voltage gain element

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#21
Borat said:
A while ago ago i had an idea - an amplifier that uses solid state devices for current gain and transformers for voltage gain.

there would be ZERO voltage gain from any active device. all the voltage gain would come only from transformers.

theoretically such an amp could be fairly linear without any feedback - no ?

what i was told then ( at partsexpress.com ) is that there is no need for this because you can simply use an op-amp to get all the clean gain you want. i am hoping on this forum i will get a more intelligent answer 🙂

there would be no input or output transformers by the way. all the transformers would be internally used for gain.
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Here you go:

Wavebourn said:
The PCB is not ready, but while waiting for it I've got a crazy idea: to use similar source followers both for input for and output stages, with a step-up transformer between them. The result will be a "No-GNFB" amp, but it's for a SS forum.
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http://www.diyaudio.com/forums/showpost.php?p=1889875&postcount=39
 
#22
ChrisA said:
I think what you are talking about is coupling two amp stage with a transformer, a "transformer coupled amp". These have been build but the problem is that there is a cheaper way to get better quality.

Always you have to think about budget. It doesn't matter if you have $10,000 or $10 to spend, you always want the best that your budget can buy. I think if you were to cost out the parts yo'd find the transformer used to couple the stages costs 10X or more then capastor coupling

Also you have to remember that you would have to use a "real" transformer and not the theoretical transformer. The real transformer will have resistance and capacitance and some kind of frequency response. You could make all of this work very well but the question is still if you couldn't get better sound for the same cost using a different design

IMO the best use for transformer coupling is in radios because (1) you can take advantage of the limited frequency response and use the transformer as a filter and (2) the transformers designed for those frequencies are small and much lower cost. In radios you see transformer use for voltage gain all the time. It's very common there but not cost effective for audio.
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2 points:

1 - since when does a capacitor step up voltage ?

2 - it WOULD in fact be for a narrow bandwidth amplifier. i have already put passive speakers behind me. i am 100% active now and dont plan to go back.

tentatively 4-way active with crossover points @ 60 hz, 400 hz and 3 khz or so.

the bottom two amps would be mainstream prosound amplifiers. the top two i want to be audiophile units and that's where this could come in.

one amp - one transducer. a transducer is a bandpass device so it is OK if so is the amp.
 
#23
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#24
Borat, Wavebourn,

What step up ratio are you looking at? If you can bot provide me with a source impedance step up ratio and expected target capacitance I can give you a pretty good idea of performance vs cost.

Bud
 
#25
Steve Eddy said:
I guess it depends on how one defines "quality."

Also depends on how one defines "best."
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No, what I wrote does not depend of anyone's definition of "quality" or "best". It applies to all ddefinitions.

In every case, even if you hapen to like plastic "boom boxes" with exagerated bass for playing rap. Most reasonable people will want what they think is the best for a given price.

I think this opinion, that transformer coupling is rarely used because it is not cost effective has been widely held by many people. I first read it in Radiotron 4Th ed. (published in the 50's) I think it's in the section on audio coupling transformers.

Yes you are correct that it is used when it can be cost effective. Mic preamps might be one of those places because the signal level is so very, low in a mic pre. Transformers cost depends strongly on physical size and size depends strongly on curent flowing through the transformer. I said radio was another use because at higher frequecies the size can be greatly reduced.

I think there are also some audiophile grade transformer coupled power amps too. But again it comes to economics: If you are going to build the "best" $200 amp or the "best" $2000 amp, most people come up with the about the same thing.

I don't think you have to actually build a transformers coupled amp to know if it would work. I think it is pretty clear that it could work It is just a matter of "bang per buck"
 
#29
ChrisA said:
I think this opinion, that transformer coupling is rarely used because it is not cost effective
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toobz arent cost effective either and yet they're used.

i think the reason is different.

i think at first toobz were used becuz there was nothing else.

then transistor amps were made because they were smaller and cheaper.

at no point was there any effort to develop a new technology to make it SOUND BETTER.

it was just starting from whats available and trying to make it cheaper.

and now we're in a situation where people pay $100,000 for an amp that uses technology that was supposed to be cheap meanhile a $100 transformer is ruled too expensive

LOL ?

maybe transoformers arent used because they don't sound like tubes and audiophiles wouldn't buy them ? we have class AB amps like Bryston for "objectivists" and SE tube amps for "subjectivists" and there just doesn't seem to be any room for an amp that sounds good but doesn't color music at the same time ?
 
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#30
Borat said:
a single linear gain stage should certainly beat multiple nonlinear gain stages that all intermodulate with each other like crazy.
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Why do you think it will be more linear?
But, I have to admit, designing such an amp the designer would at least think of power amplification. And it may sound much better than when they think of voltage amplification without non-linear currents in mind. 😉
 
#33
Borat,

Ok, 1:100. That's going to take what ever capacitance is in the downstream circuit and multiply it by 10,000, so you will have to look very carefully at what you intend here. I think you are more likely to get what you want with two or even three driven stages. As far as I know the capacitance will be from grid of the SS device to ground. This is a SE circuit, correct?

But, to continue, what is the drive side voltage going to be, and is your number peak to peak or rms. Also, will there be any DC current flowing in the primary?

Bud
 
#34
Unless you're looking to create some 800 watt monster, you don't need 40dB of voltage gain.

Use a bridged approach and a pair of 1:10s per channel would be more than sufficient.

CineMag's CMMI-10C is ideal for this. They're 1:10 and will take a 2 volt RMS input signal. Wire the primaries parallel and series the secondaries.

Drive 'em with a source impedance of a couple hundred ohms or less, load each secondary at around 200k and keep the load capacitance low.

se
 
#35
Unless you're looking to create some 800 watt monster, you don't need 40dB of voltage gain.

Use a bridged approach and a pair of 1:10s per channel would be more than sufficient.

CineMag's CMMI-10C is ideal for this. They're 1:10 and will take a 2 volt RMS input signal. Wire the primaries parallel and series the secondaries.

Drive 'em with a source impedance of a couple hundred ohms or less, load each secondary at around 200k and keep the load capacitance low.
Click to expand...

Yup. Good company and quality products.

Bud
 
#37
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#39
jerluwoo said:
Transformer gain stages have been used in PA amps for a long time. Heres a very simple one. http://www.epanorama.net/schematics...s-linear/Amplifier-Pa_-_Pa-Mobile-15W_Rms.pdf
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Q2 provides most of the voltage gain in the bullhorn circuit above. T1 might be
1:2 overall as a phase splitter. T2 could be up to 4:3 step-down or approaching
1:1 but not likely step-up.

Interesting circuit, with the SE transistor + transformer phase splitter. It can
be simple and economical for limited frequency response!

But it's not the same at all as using only step-up transformers for voltage
gain plus unity gain buffers to drive the primaries and maybe the output.

I thought of building a step-up iron-only microphone preamp, but that has
morphed into something better ;-)

The cinemag transformer referenced above is speced at +24.5dbu, which
would be about 11V RMS giving about 30 watts into 8 ohms with about
1.1V RMS input. Very interesting indeed. The distortion spec is .004% at
1KHz at normal operating levels; how is that not a low-distortion amplifier? 😎
 
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