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Old 30th April 2003, 08:03 PM   #1
EC8010 is offline EC8010  United Kingdom
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Default OTL discussion is pertinent...

I will soon need a small treble amplifier to drive ESg-1 ribbon tweeters. Six quality watts should be sufficient. PP 2A3 with a really nice output transformer seems the obvious choice. I did (briefly) consider OTL, but I don't see that a small OTL is possible, it seems to me that they become less impractical the bigger they get (10 off 6528 would work rather nicely).

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Old 30th April 2003, 08:23 PM   #2
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A single 6528 or 6336 transformer coupled would do splendidly. IIRC, you can get about 10-15 watts Class A with 375V B+ and an ip of 75 ma per section, running with a 4000 ohm p-p transformer. A pair of 6BX7/6BL7 will also do well, I'd expect.
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Old 30th April 2003, 08:47 PM   #3
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Default When I said "quality", I meant "unsurpassable"...

I'm well aware that 6528 can be really quite good (either SE or PP). 6BX7 are rather variable, and not necessarily distortion matched, although the few 6BL7 I've seen seem better. None of the previous valves were designed for audio, so they have coarse grid windings. Conversely, the 2A3 was designed for audio and low distortion.

I'm probably going to use PP 2A3, but I'd be delighted to be persuaded about 6W OTL into 8 ohm (resistive).
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Old 30th April 2003, 09:03 PM   #4
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To run a tweeter? Nah, you don't need OTL.
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Old 30th April 2003, 09:12 PM   #5
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Default FURTHER THOUGHTS...

Hi,

Quote:
To run a tweeter? Nah, you don't need OTL.
I'm not so sure about that, I'd be tempted though, especially so with such a fast driver as a ribbon.

Then again, much will depend on the x-over frequency.
I don't know the specs for this particular one but I vaguely remember it can go quite low.

If it's a three way design I'd biamp with two sets of OTL, both using the same topology but of different powerratings.
That should make for a seamless integration, in theory that is.

Actually my SD3s sport a ribbon for the tweeter section as well but I use the passive x-over with the OTL running full-range.
It really works very well in this combo.

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Old 30th April 2003, 09:20 PM   #6
SY is offline SY  United States
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Yes, but I bet it doesn't go so low as to require a big xfrmer.
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Old 30th April 2003, 09:26 PM   #7
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Default You lost me...

Hi,

Quote:
Yes, but I bet it doesn't go so low as to require a big xfrmer.
??.

Xformer? Where? What?

I must have missed something...

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Old 30th April 2003, 09:35 PM   #8
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I've not decided on crossover frequency yet. The ribbon has to integrate with a Jordan JX92 aluminium-coned very nearly full-range driver. Allegedly, the ribbon can be used down to 1500Hz, but I'm not quite set up to measure this yet (bought an old computer today to solve this problem). I want to keep the crossover away from the critical midband, but then, a ribbon is a far better transducer than a cone.

Either way, the lowest frequency that the tweeter amplifier has to be able to handle with negligable distortion is 200Hz, rather than 20Hz. This greatly eases output transformer design, but I'm interested by OTL thoughts. I do not have a vested interest here, but I have heard PP 2A3 amplifiers producing nice treble through electrostatics.
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Old 30th April 2003, 09:49 PM   #9
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Default REMINDS ME OF...

Hi,

Quote:
I've not decided on crossover frequency yet. The ribbon has to integrate with a Jordan JX92 aluminium-coned very nearly full-range driver.
Reminds me a lot of a Max Townsend design from the mid-eighties, he ran the Jordan full range and used the ribbon ( can't remember which one) only to extend the Jordans' bandwidth.
A lovely speaker that was.

Quote:
Either way, the lowest frequency that the tweeter amplifier has to be able to handle with negligable distortion is 200Hz, rather than 20Hz. This greatly eases output transformer design
True, but than you'll need it custom wound I presume?

Quote:
but I'm interested by OTL thoughts.
I recall SY had a Futterman 12B4A amp and that tube had crossed my mind for this service.
One thing to keep in mind is that the OTL should not be oscillating at all or the ribbons will be fried within seconds...
But you knew that already, didn't you?

Quote:
I do not have a vested interest here, but I have heard PP 2A3 amplifiers producing nice treble through electrostatics.
The 2A3 and the 6080 are historically related and both are favs of mine...for a tweeter you could just as well opt for SE and forsake the phase spitter in the process.
It's just my opinion but I feel this is the only place I'd consider SE.

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Old 30th April 2003, 10:08 PM   #10
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I'm proposing to roll the Jordan off (hopefully only at 6dB/octave) to tame its cone breakup. And yes, the ribbon is intended to extend bandwidth (and to see if >20kHz really matters).

Obviously, any output transformers will need to be custom-wound, but transformer manufacture is a cottage industry anyway, so this isn't a problem.

Crikey! Tendencies towards oscillation will not be tolerated at all. Perfect square wave performance into the actual load is required.

OK. I never thought I'd hear it from Mr OTL (PP), but 6080 SE could be considered (I've gots lots of 6080). Were you still thinking of OTL?
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