• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Output Audio Transformer Impedance

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Administrator
Joined 2004
Paid Member
Interesting topic. Getting back to the "how big if you don't need bass?" question that SY posed.. I asked Bud this same questions some months ago, hoping that a transformer used above 500Hz could be smaller, cheaper and better. This seemed to make sense to me, as bass response seems to be a the major problem. (see 60Hz power transfos vs 400Hz or higher).

However, Bud didn't think that going small was a great idea even for a >500Hz transformer. I'm still not clear on why.
 
Interesting. Will this still apply in an amp without loop NFB? I haven't built any PP OT amps with the 6AS7 yet, and will keep this in mind. (Tho to be honest I find it hard to believe this is much of a problem, since I've not heard of such problems from other similar projects)

It could be a problem if PP amp has global NFB, and 99% have it except really odd designs. This problem was described in details in one of the books but I could not recall what its name (sorry, I have several Gigs of old books).
 
Last edited:
However, Bud didn't think that going small was a great idea even for a >500Hz transformer. I'm still not clear on why.

All hinges upon frequency response vs core response. If you are using an amorphous core, the core will support the FR above 500 Hz, no capacative coupling needed or allowed, due to FR peaks of astounding degree. Means that half of the information content is thrown away, but the remainder is usually good enough for most folks.

For core that does not support signal above 500 Hz, capacitive coupling, moderated by the antenna event rise time that is forced by the permittivity of the core material forming the Fe bounding box, is all you have. This means that the greater the coupling area, across the dielectric barrier the more information you will retain above 500 Hz. This is true for all materials other than amorphous core, though to a greater degree as you head toward the M series of electrical grade steels.

The reason you can use smaller core at signal levels is because you can actually use the nickle core grades that do extend their FR support, to 3.5 kHz for 48% and 10kHz for 80 %. There are drawbacks to both of these materials, but a good designer knows about them and works to subdue their effects. One drawback for E/I core is that the nickle laminations are not stamped in sizes that will allow power transform above about 5 watts down to 20 Hz and 15 watts for a steeply limited 500 Hz. C core is often a better choice here, though there are drawbacks to C core also, one's a good designer can work around.

The reason for the enormous amount of retained information you have commented on, from my designs, is that I do not throw away the capacitive coupling. I also know how to passively demagnetize the E/I core I utilize, and I know how to cause a dielectric circuit to be formed in the coil, that mitigates the distributed capacitance out in the depth of windings and enhances that of the capacitive coupling, across the dielectric barrier, between primary and secondary antenna events. To make this work optimally I need wide coupling surfaces and minimum winding depths, hence a larger core structure, to get the winding length up, is needed.

Other materials require other techniques and the few of us who do work in the outer edges of the audio transformer performance envelope, all have our own peculiar take on what is best. At these levels of performance, your choice amongst us is just a matter of personal taste.

Bud
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.