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Shrad

Sat Nov 30 2013, 10:56AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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I have some nice toroids with 220V primary, 30-0-30 + 10-0-10 + 5 secondaries, which I kept for this usage and I haven't been able to test yet due to the lack of a good LCR meter, but which would at least be an alternative...

Proud Mary

Sat Nov 30 2013, 12:27PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

I was thinking that you 'may' be able to use a 240-12 volt step-down transformer, with a turns ratio ~20:1. Some have thinner laminations than others (required for reducing eddy currents at the higher frequencies).

I have a few, but none with particularly thin laminates. I've not done any measuring yet, though.

I'm absolutely no authority on valve audio output transformers, but I do know that the art lies in impedance matching with an adequate bandwidth.

Too little primary inductance and you lose your bass response. A primary inductance large enough to provide good gain at low frequencies may then have too much inter-winding capacitance, through which the high frequencies are lost like bees out of a string bag. So the transformer will always be a compromise - often a financial compromise - with bass and treble controls there to compensate for the system's weaknesses.

10H was considered an adequate inductance for ordinary domestic AF applications - the wireless and the TV - with 25H - 40H for 'hi-fi'.

tobias

Sat Nov 30 2013, 12:37PM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Is stripping down not so old silicon amplifiers for the transformers a good idea? Sometimes I see old Onkyos for $5...

Ash Small

Sat Nov 30 2013, 01:15PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

tobias wrote ...

Is stripping down not so old silicon amplifiers for the transformers a good idea? Sometimes I see old Onkyos for $5...

From what little reading I've done on the subject so far, I think the ouput impedance of the power valve/tube is much higher than the output impedance from a power transistor circuit, at least that was my interpretation of what I read.

If you can find an old valve-tube radio or tv, or something, otherwise, from what I've read, it's do what PM tried, or look for a suitable mains step-down transformer.

There are some other 'tricks' that people use, for example, you can get away with a 1:10 turns ratio in some circumstances, and some say they even have some advantages.

Of course, you could always consider 'winding your own'

Proud Mary

Sat Nov 30 2013, 01:21PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

tobias wrote ...

Is stripping down not so old silicon amplifiers for the transformers a good idea? Sometimes I see old Onkyos for $5...

I don't know on that one, Tobias, but as valves are high voltage high impedance devices, I would guess that impedance matching transformers found in old silicon (BJT) amplifiers would not be suitable for valve use. Ask someone who knows more about semiconductor audio power amplifiers.

tobias

Sat Nov 30 2013, 01:31PM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

Tap 4 should be named Jethro Tull then! :)

Proud Mary

Sat Nov 30 2013, 02:03PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

tobias wrote ...

Tap 4 should be named Jethro Tull then! :)

Shrad

Sun Dec 01 2013, 09:36AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

I read somewhere that sawing through an old power transformer to add an air gap would greatly improve frequency response

toroids are also a good bet, as you can easily find one with multitaps and their frequency response is wider already with less capacitive losses than EI cores

Proud Mary

Sun Dec 01 2013, 02:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Shrad wrote ...

I read somewhere that sawing through an old power transformer to add an air gap would greatly improve frequency response

toroids are also a good bet, as you can easily find one with multitaps and their frequency response is wider already with less capacitive losses than EI cores

I've seen bits and pieces scattered about the web, but have yet to see a finished design where someone has successfully converted a toroidal mains transformer into a good quality AF output transformer, though they may exist somewhere.

I suspect it's cheaper and less hassle in the long run to cough up and buy over-priced vintage AF O/P transformers, or their modern copies. No amount of care with the rest of an audio design can ever compensate for the weakest link of a bad O/P transformer.

Shrad

Sun Dec 01 2013, 08:17PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

I'll still practice using these...

I have a 500VA rated double C laminated core which I salvaged from an old 3 phase power transformer, and I'd be willing to wind my own once I'm used to those tube designs

I have some literature, and I've read online about it for about 6 months so I guess I'll have to try one day or another, and I just started to tidy the room I use to practice our art ^^

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