Rerurn to Romy the Cat's Site


In the Forum: Audio Discussions
In the Thread: An amplifier for Tweeters
Post Subject: Re: An amplifier for TweetersPosted by cv on: 9/4/2006
Hi Romy,

I think you may be struggling to get 10-12db of gain out of a spud amp (the term is a play on "one tuber").... May just about get it with 6C45.... remember you will be stepping down something like 12 to 18:1 on the OPT.

best case: 10db required (gain = 3.15), transformer is 3k:16, n=13.69, so gain of the tube stage needs to be 43. This needs a mu of about 57 from the tube assuming an RI of about 1k... Need to find high mu examples of the 6C45. With a 2k5 load, I get a mu of about 55.

The thing with an air-core is that you need as low a driving impedance as possible. 6C45 at high current may well be ok, but the best tube in that class is the E55L triode connected, which has an Ri of about 600 ohms IIRC. This means that you could get away with 50mH of primary inductance. But it doesn't *quite*  have the gain you need.

I built some air cores to drive a ribbon project which is on long term hold. I haven't listened to them but they measured reasonably well, something like -1dB at 19kHz when driven from 600 ohms. The primary inductance was about 250 mH and the turns ratio was huge as they were designed to be 3k:0.6 ohms.

I don't have the design to hand but can obtain it in a couple of weeks when I head back home; it was something like a 3" diameter cylinder, with 20 or so grooves machined into it. Each groove was a 3 mm wide and a few mm deep, and the primary.secondary sections were alternately wound P-S-P-S etc. There was also an axial trough machined into it to enable one to get to all the terminations easily.

However, I suspect that the bobbin from a standard SE transformer, sans core, may come close to meeting the specs you require, in which case, just purchase a pair without the iron and give it a go. Maybe give Bud a call?

The only issue I can see might be signal radiating from the cores.

Alternatively, for a fast core - a whlle back I had Sowter wind some 100% mumetal outputs, good for 2kHz up (135mA, 3k:8).  They sounded absolutely *magical* on my tweeters. I hope to get them rewound for use with the S2s and a different OP valve at some point... but the point is that a 100% mumetal core in your application could be tiny, and very, very "fast", whatever that means...

It's a tradeoff between:
high interwinding capacitances and no-core (air) or
 magnetic effects and low turns count, low capacitance (mumetal).

I can't guess which would be preferable, especially with the 60kHz cutoff on the tweeter x/o!
If you can persuade a winder to lend you an existing bobbin or have some you can use, that would  be a quick test of the concept.

Brainwave - the 7788/E810F has a higher gain than the 6S45.... should just about do it!

Hope this helps,
cv

Rerurn to Romy the Cat's Site