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OPT at frequency extremes

If I understand correctly the difficulty in designing wide bandwidth OPTs is getting both high primary inductance and low shunt capacitance making it difficult to do both ends well.

In multi-amped systems where each amplifier will be limited to three or four octave range would it be theoretically possible to wind OPTs that reach the extremes of frequency seperately. That is to say would it be possible to come up with a transformer that goes down to say 10Hz as long as it only has to get up to 200-300Hz range and one that can go up to 30kHz as long as it only has to go down to about 2kHz?



diyAudio Moderator Emeritus
2003-01-18 7:57 am
Near London. UK
And if you look at the recent Motorola console thread, you'll see it was done commercially. The problem with a transformer is not getting one or other extreme, it's getting both simultaneously. It's easy to make a transformer with only a few octaves of bandwidth, but increasing that number of octaves is much harder. Where you place those octaves is a function of primary inductance. Lots of older amplifiers were distinctly stingy on core material and primary inductance, but their HF response is satisfactory, so they make great tweeter amplifiers. Am I allowed to suggest solid state for bass?
Oh yes you can suggest SS for bass. You are not the first and there is a good possibility that it will start that way and even end that way. :) It just occured to me that getting the proper iron might be much easier in the multi-amp situation (I will eventually have tri-amp on each side plus sub).

The sub will handle 16-70Hz or so. Therefore the midbass amp will only have to handle down to 60Hz or so. It seems any respectable SE transformer should be able to handle 60-12K or so. Thus the first amp I build will probably be the one for the midbass. I could initially use it as a computer amp since my sound card can't do more than about 50-10k or so anyway.

The main h.f. problem is capacitance between primary and secondary; not so much primary interwinding, i.e. you can get a goodly h.f. response despite transformer size. Suitable C-cores will keep windings small (I assume here that you have a choice or can wind your own).

As a typical illustration (no fancy stuff involved) I am at present using a 130W output transformer with the following specs: Primary inductance = 120 H; leakage inductance = 6 mH (measured in the usual way); equivalent interwinding capacitance approximatelt 2 nF. Overall size using C-core = 10 x 11 x 12 cm. This will give in my application a l.f. (-3dB) point of 7 Hz and an h.f. (-3dB) point of 35 KHz, the latter mainly as a result of the pri/sec. capacitance. This is just the transformer, before application of negative feedback. I use 3 secondary sections which was probably overkill: the h.f. -3dB point because of leakage inductance alone would have been 400 KHz! (For the record the output tubes are 2 x 2 6L6GC in p.p./parallel.)

Bi-amping is of course advantageous for other reasons: Eliminating of loudspeaker cross-overs.