DC compensated SE output transformer

[fscarpa58]

hi all

I recently read US patent 2895019 by M.Farber
about compensating opt SE trafos to cancel the constant
flux as in PP transformer.

This is achieved by the use of a choke or
a capacitor and, if true it is a great advance in
SE tube amp design.

I carefully read the patent but I am still very doubtful.

opinions?

thanks
bye

Federico Scarpa

[ThorstenL]

Konnichiwa,

Originally posted by fscarpa58
I carefully read the patent but I am still very doubtful.

Especially figure 3 (see attached) seesm very workable.



I have recently started working on "electronic chokes", that is mosfet (or BJT) structures that behave like chokes on top of being ripple killers like classic gyrators.

If you take a traditional PP Output Transformer and feed one from a traditional gyrator and the other from a bootstrapped gyrator (electronic choke) and hang your SE valve of the original centertap you have an SE Output at 1/4 of the primary impedance of the original transformer.

Pretty neat way to build cheap SE Amp's with 6AS7 or 2A3 and the like. You could probably get away using certain mains torriods even.... The "PhrugalPhiles" SE Valve Amp in essence.

I like it.

Sayonara

[Yvesm]

Yes, yes, appealling, but ....

You loose that kind of "magnetic bias" from DC current in the tranny that help to stay in the flatter portion of the iron Mu curves


Nothing for nothing, this could produce a SE amp sounding like a PP one

Yves

[ThorstenL]

Konnichiwa,

Originally posted by Yvesm
You loose that kind of "magnetic bias" from DC current in the tranny that help to stay in the flatter portion of the iron Mu curves


Nothing for nothing, this could produce a SE amp sounding like a PP one

One of my tricks to make my Push-Pull amplifiers sound a lot more "SE" is to allow or rather to deliberatly introduce a modest degree of DC imbalance, enough to shift us a bit up the BH curve and to move the problem region from "minimal signal" levels into the 0.1W++ region. The same can be done for the compensated SE.... ;-)

Sayonara

[hermanv]

I have used this very simple circuit several times. It works amazingly well considering its simplicity (depends mostly on the choice of FET).

DC Voltage drop is more or less equal to Gate Threshold (typically 5-10VDC) think of an L in series with a zener.

I have not developed an equation for the equivalent value of L, it is dependent on transconductance and operating point. Start with a R of 50K or so and a C of 10uF this will get you low in the audio band.

Power switching FETs are a poor choice, too high a gate charge. Higher transconductance is better of course.

[Yvesm]

Originally posted by Kuei Yang Wang
Konnichiwa,



One of my tricks to make my Push-Pull amplifiers sound a lot more "SE" is to allow or rather to deliberatly introduce a modest degree of DC imbalance, enough to shift us a bit up the BH curve and to move the problem region from "minimal signal" levels into the 0.1W++ region. The same can be done for the compensated SE.... ;-)

Sayonara

Agreed for unbalance deliberatly introduced in a "compensated" SE or even in a PP, however I don't know how the iron will react when passing by zero "bias" at some points in the cycle

In a "classic" SE design, the OPT may be estabished so that DC induction is higher than peak AC in order it never reach zero.

Did you experienced that ? Do you use a (small) gap in this situation ?

I bet "Konnishiwa" is something like "Hi there", isn't it ?

Yves.

[fscarpa58]

hi,

Especially figure 3 (see attached) seesm very workable.

I agree that circuit of fig.3 works.

but I feel cir of fig.2 will never work.

bye

Federico

[Sch3mat1c]

Originally posted by fscarpa58
but I feel cir of fig.2 will never work.

The amp-turns cancel, and the cap prevents voltage appearing on the top winding; unforunately, it also prevents volts from appearing on the bottom winding, through transformer action.

Tim

[silurato]

Hi there

This is my doubt for fig.2: doesn't the "bypassed" half of the primary appears like a "parasitic load"?

I mean, the tube is loaded with 1/4 of the load reflected from the secondary, it's ok, but what about the bypassed half?

I can't focus this point...

[Sch3mat1c]

Exactly. Now, how that got past the PTO...

Tim