Tweeter Amp for OB or Horns

[IslandPink]

Phew !

Power supply thoughts :
Been looking for mains transformers, I had it in my head ( and on PSUD ) that a simple 240V isolation transformer around 60-70VA would be enough, with a full bridge this should give around 300V after smoothing. I went looking and found nothing of this spec so far. All the 240V secondary transformers I've found are 1kVA and above. Please advise, anyone !
Space-wise I'm thinking there probably isn't enough space for damper-diode rectifiers and a 2xOD3 shunt, but enough for one or the other - ie. a solid-state bridge with some smoothing and a CCS and a shunt, or a bridge using 2x 6AX4 or whatever but no shunt on the end - What do you think, Nick ? - which would you choose ?

[Nick]

For the transformer I would look through the Hammond catalog.

I would use consider Cree Silicon Carbide Schottky Diodes for the bridge and the CCS/Shunt

[simon]

Have you tried SiC diodes Nick? I intend to try them when I eventually rebuild GM70, but I'm interested in what you found if you have.

[Nick]

Yep, I used them in the last two 300b's I built (Garry on AOS has one). Cant say if they were better, but certainly not worst than UF4007's

[IslandPink]

Thanks, the plan is coming into focus. I will check out Hammond then, and get Gerry H at Crowthorne to find me some nicer OD3's, all the ones I have are taped-up between the base and bulb. Will ask about 7N7 or 14N7 too, I like loctals and have a scheme for soft-mounting them, as on the current 4P1L drivers.
Can you think if any repercussions of the 55H chokes 'unloading' at low frequencies ? The driver stage on this will most likely see the full FR, as the high-pass will be done in the coupling cap to the 45's .

[IslandPink]

This looks interesting. 50VA should be enough
http://www.hammondmfg.com/pdf2/1182E240.pdf
So is 271.5 the typical unloaded voltage out ?
I need to sim in PSUD to make sure I can get something like 330V to feed the CCS before the 300V shunt.
I need to allow for 245V mains here too.
I think Mike explained how to properly set up the mains in PSUD a while back, I'm a bit rusty but will check back on the shunt-reg power supply thread.

[Ali Tait]

How about an OTL?

Reason I mention it is Ray’s OTL at the last Owston is the best treble I’ve ever heard I think.

[IslandPink]

I know , I've trying to avoid thinking about that - but you are correct.
It does seem they need quite a lot of valves and running power just to get a couple of watts out - which I'm trying to avoid - plus they often need feedback, which tends to decrease soundstage width. Maybe this wouldn't necessarily be the case to just drive eg. a ribbon. However I think in the future I'll look into it once the speaker questions are resolved.

[izzy wizzy]

I often wonder whether an OTL is a "technical detour" to solve a perceived problem that doesn't exist in real life as opposed to the test bench. Much like cap coupling vs transformer coupling. I guess it depends what aural matrix you are trying to create. From what you have, wouldn't a simple SET be the go?

I don't think the circlotron OTLs have feedback but that's also a lot of power supply.

cheers,

Stephen

[IslandPink]

I would say that the best ones I've heard ( eg. Ray's from last time I went to Owston ) are very good indeed and it's not one you have to think about too hard to judge vs. any other amp - the texture of violins etc in the HF is really realistic.
However I do have a special set of OPT's from Bud that use smaller cores and litz-wire secondaries , and it would be fun to build and see how these compare to an OTL - I haven't used them in anger yet.

[IslandPink]

ps. Would anyone like to hazard a guess about whether I can find my (3) 45's anywhere at the moment ?

[simon]

Ha! I know that feeling - I'm wondering where my 5751s are.

[IslandPink]

Exactly. I know I had them for the 3-stage 300B amp, and I wouldn't have sold them.

Transformer - I'm remembering the method in PSUD now. In the data sheet the off-load voltage is given ( 271.5 ) for 234V across the series'd primaries. I need to adjust this for my mains ( ~245 ) and then put this larger number in the V rms box for the PSUD model. Then the DCR for the 'red-blu' secondary at 64R is put in the other box. Then as long as I calculate approx. the right circuit load ( R or I ) after the PS, everything should come down to the correct voltages.
On that basis I can easily get about 330V out from bridge + 6uF + 9H + 440uF. Might be good to get a slightly higher value choke and get ripple down a bit , though I'm not sure how much effect 30mV at 100H will have above 1kHz.
Anybody got anything over 10H at 100mA or a bit more ?

[IslandPink]

I would use consider Cree Silicon Carbide Schottky Diodes for the bridge and the CCS/Shunt

So is this the sort of thing I need ?
https://uk.rs-online.com/web/p/rectifie ... s/7027440/

[Nick]

Yep.