Using power transformers as output transformers

[Learner]

Hi JH,
I donno if you are aware of the more economical Edcor transformer alternative

http://www.edcorusa.com/transformers/audio/matching.htm

I guess the down side of this transformer is that they are only 1:1 ratio which does not add any gain to the signal comparing to the original UTC? I have been looking at adding an op amp gain stage after the transformer and making the input switchable for either low impedance 600ohm input from the transformer or high impedance input direct into the op amp. Or replace the 600:10k transformer with a current to voltage amplifier.......

Likewise I would like to do that to the output stage also, adding an op amp at the output stage :shock: :? with a buffer for low impedance output , straight out of the opamp with 10k neg feedback for hi impedance output.

Personally, I don't like the idea or the thought of adding op amps into tube circuits. However, my engineering sense tells me that if the op amps are running within its linear range it should not make ANY difference.......same principle would apply to all active components.

Any thoughts?

 

[CJ]

Torroid for LA2 output:

Well, the BH7 has a Rp of about 5k, and you pushing about 10 ma.
Triodes generate less distortion than pentodes due to their lower plate resistance, so this is working in your favor.
The 10 ma is not working in your favor.
But wait, isnt there a cap feeding the pri in the LA2!
One thing working in your favor is that most power transformers use M6, which is better for low end.
LEts see, 1.3 watts/2=0.65 watts at 50hz/1.41 (root 2) =35 hz,
.65/2=0.325 watts at 35/1.41=24 hz, 162 milliwatts at 24/1.41=17 hz.
8 db at 17 hz. not too good. add to that the dc problem, and well....

Torroids can take dc in a push pull circuit, but you have to balance it carefully.
The struture of a torroid output is working in your favor, as they have a much more symetrical winding struture as far as winding resistance and coil balance, which also means better leakage inductance and capacitance. This is why they can get away with a smaller core and still get high inductance.

Torroids need less magnatizing current.
Less magnatizing current means lower impedance windings, (larger gauge wire), which could work against you as far as pri. inductance, which is essential for low end distortion.
Now, here is where it gets interesting. Lets say you do generate more harmonic distortion at the low end. The ear will take the second and third harmonics and convert them to the fundamental at low frequencies. They did tests where the the fundamental was removed and only the harmonics were present, and the listeners still heard the fundamental even though it wasn't present!
A transformer that does not generate many harmonics at the low end sound soft, not as musical as one that does. So there is such a thing as too much pri inductance, which explains why I hat Dumble guitar amps. (huge cores)\

The LA2 uses neg feedback in the ouput stage. This is in your favor. The higher dampening factor will mean less high end rolloff,

What the heck, give it a try. Specs on transformers can't compare to actually doingt an experiment.

If you don't load down the secondary to much, you might get away with it!

:guinness:

 

[jhaible]

I just ordered a Lundahl output transformer.

They have them on stock at Jac Music in Germany, so I'll have
it next week. I also considered Sowter, but they have two weeks
delivery time.)

The cute little power toroid will ba saved for less critical
projects.

JH.

 

[tony dB]

Edcor is sounding very nice in my lineleveltogtroutputconvertor (TM)
and very cheap compared to Jensen, Lundahl, ...

 

[clintrubber]

Edcor is sounding very nice in my lineleveltogtroutputconvertor (TM)
and very cheap compared to Jensen, Lundahl, ...

Hi Tony,

Just wondering, where did you buy those Edcors ? Could you find them in Hellgium ?

Groeten & proost,

Peter

 

[jhaible]

[quote author="cjenrick"]The struture of a torroid output is working in your favor, as they have a much more symetrical winding struture as far as winding resistance and coil balance, which also means better leakage inductance and capacitance. This is why they can get away with a smaller core and still get high inductance.

Torroids need less magnatizing current.
Less magnatizing current means lower impedance windings, (larger gauge wire), which could work against you as far as pri. inductance, which is essential for low end distortion.
Now, here is where it gets interesting. Lets say you do generate more harmonic distortion at the low end. The ear will take the second and third harmonics and convert them to the fundamental at low frequencies. They did tests where the the fundamental was removed and only the harmonics were present, and the listeners still heard the fundamental even though it wasn't present!
A transformer that does not generate many harmonics at the low end sound soft, not as musical as one that does. So there is such a thing as too much pri inductance,

Very interesting!

I should take this little toroid to work and connect it to the impedance analyzer. I should measure different transformers to get a "feeling" for typical values.

I wholeheartedly agree to what you say about low frequency distortion. I really love the sound of that quite undersized O.E.P. transformer in my 1176 clone. The output stage probably hasn't much in common with the original, but I set the levels and loading just so that I get hefty waveform distortion at low frequencies just *before* the amplifier clips. As I have that limiter in the stereo sum, just before it goes to the Quantec Reverb (which doesn't have very much dynamics by today's standards), the output level is quite fixed at that point where the transformer is, so I'm running the transformer always around its "sweet spot".

JH.

 

[rmburrow]

Let's see, a 120 to 12 volt woulfd have a turns ratio of 10:1, so yes, you could probably get some audio out of it.
It is not interleaved, so the freq response is going to be questionable.
Try it.
cj

Tried that,  audio quality poor...

 

[gyraf]

Yes, it's a game of chance if the transformer is not designed for the purpose.

Which also means that even though one 12V power transformer (yours) sounds crappy, this does'nt indicate that all 12V transformers are bad for audio.

I've had decent results with some after experimenting. Don't recall exact type at the moment.

BTW: Necroposting, thread is 15 years old (!)

Jakob E.

 

[merlin]

Just send me your address for a copy

No idea if you're still on this site, but I'd be interested to see a copy too.