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24.20.10.236
In Reply to: RE: Futterman H3 amp 'upgrades' posted by rogerh113 on July 31, 2019 at 10:31:43
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I've used OTL amplifiers exclusively, for the past nearly 50 years. From the mid-70s and up to around the mid-1990s, I owned several different pairs of Futterman amplifiers and used them to drive either one or two pairs of KLH 9s, depending upon my living space at any point in time. With two pairs of 9s run in parallel, the bass was thunderous. With one pair, not so much. But it was common knowledge that a single pair of 9s would not make much bass, in part because of phase cancellation and in part because of the limited radiating area. Nevertheless, I am kind of puzzled as to why JGH got poor results in low bass performance with the Futtermans driving 9s, compared to the Marantz 9s, especially since most ESLs tend to exhibit impedance maxima in the very low bass that I would have thought would stress the Marantz. I have made impedance vs frequency curves with my full range Sound Lab ESLs, and the impedance below 100 Hz goes sky high, 50 to 100 ohms at 20Hz. I assume the impedance of a KLH9 below 100Hz would be much higher than its stated nominal impedance of 16 ohms. The Futtermans should have been able to drive the 9s at low frequencies at least as well and maybe better than any of the two transformer-coupled tube amplifiers that were compared. I can only think that the review is from 1970. By the time I got hold of Futterman amplifiers, Julius had dramatically increased the coupling capacitance to at least 1600uF, IIRC. Maybe that was the problem. Maybe also the low frequency load presented by the KLH 9s to a Marantz 9 or Dyna Stereo 70 was sufficient to generate euphonic harmonic distortions in those amplifiers. (Grasping at straws here.)
Your thinking the Nines impedance below 100cps was greater than 16 ohms is correct it went upwards and higher than 30 ohms.
Into higher impedances a larger coupling cap isn't helping.
1600Uf into a 16 ohm load the low frequency -3dB cutoff is about 6Hz. If the impedance is 50 ohms its about 2Hz.
But we can see from this that if the output coupling cap was less, there would have been phase shift well into the upper bass; the ear perceives that as a loss of impact.
I'm sorry, but aren't you saying that 1600uF DOES help, compared to 800uF, to convey low bass frequencies? True, if the impedance at low frequencies is still 16 ohms, then you'd want even more than 1600uF to get all the low frequency music into the speaker.
Yes on all counts :)
Thanks. Very strange review and manufacturer reply. Sounded like the unit submitted was pre or early production, with the manufacturer working out the bugs.
I did spot some posts of people planning to renovate an H3, and contacted them to see how it turned out. No replies as yet. Perhaps more than a couple of un-started projects out there.....
...a case of don't wish for anything too hard as you might get it.
The review is very strange, but from the lens of 1967 when transistors ruled the earth. I didn't find Mr. Holt's opinions to be consonant with mine, but it is one more data point for you.
Good luck and when you find your project I hope that you will see clear to photographically chronicle it here, or in DIY. It seems most intriguing.
One thing about this hobby is that personal taste and experience come into play heavily. When I acquired my obscure SET parts, some said I was nuts to pursue it. The eyes of my long suffering wife rolled back in her head, but survive she did (and enjoys the amp immensely now). I won't even mention what happened when I rolled some massive, gutted Altec Magnificent cabinets (with the requisite patina) into the garage.
I think the project would be very revealing. A design so far ahead of its time, which has spawned a small industry and cult following of OTL amplifiers, many current versions being highly sought after. Just what would that Futterman amp sound like if brought up to current standards using high quality components.
Just the impact of an IEC and high quality power cord might be significant. Good wire, quality resistors and capacitors - very interesting. Things that Mr. Futterman did not have access to.
I have already considered building the improvements somewhat in stages, both to see if each works in the amp, and to gauge what the impact is. Perhaps a bit more work, but it might also be educational and fun.
Key is to find the appropriate patient (Futterman H3) to perform the operation on. I also would like to have the final amp appearance relatively unchanged from the original.
I think the project would be very revealing. A design so far ahead of its time, which has spawned a small industry and cult following of OTL amplifiers, many current versions being highly sought after.
As a bit of an historical note, when I first invented my take on OTLs I had no idea who Futterman was or that OTLs were a thing. I was barely out of high school...
Nt
And studiously sidestepped a lot of the pitfalls in his design. That's why our OTL is unconditionally stable.
....there is an ad in it for an OTL amp with a matching very high impedance loudspeaker. I'm coming off the top of my head here but around 100 ohms. Have you heard of that Ralph? I should dig it out and post it here.
For the Beveridge models 2 and 2SW a direct-coupled amplifier is situated in the base of the gigantic speaker(s). No coupling capacitors OR transformer, driving a totally unique take on ESL, not really an ESL as we know it because the diaphragm does not carry a bias voltage; it's at ground reference. Which is maybe why after nearly 40s years my 2SWs still sound divine. (Knocking on wood.) I've owned the 2SWs for about 6-7 years now, but the amplifiers are dated inside, "1979".
Other later models of Beveridge speakers (models 3 and up) are much more like conventional ESL speakers and require an outboard amplifier.
Nt
I think I just wanted a chance to talk about Beveridge.
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Nt
Stephens and Philips Norelco offered high impedance speakers with matching OTL amplifiers.I believe the high impedance Stephens TruSonic speakers were 500 ohms; while Philips Norelco had both 400 ohm and 800 ohm offerings.
As I recall, each of their OTL amplifiers were cap-coupled SRPP designs.
Edits: 08/13/19 08/13/19
Nt
Lots of interesting material here - scroll down for info on their OTL amplifier.
...I wouldn't mind a pair of those corner horns, and that mid-century modern house to listen to them in.
This combo (OTL + high impedance speaker) sounds interesting. I hope you can find it.
My Stax F83 speakers have a variable but overall very high impedance, around 160 ohms in the midrange. The NYAL Stax combination works very well. (But I now have a subwoofer to take
care fo the low frequency, where the speaker's impedance strangey goes back down to 4 ohms).
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...in the Magnificents. I am quit jealous.
A friend of mine has a couple of Futterermans that he deploys with ESL57s and smaller Altecs. They were both rebuilt by local Portland OR tech Mat Kamna. I wish that I could give my impressions, but my friend has so much gear, and it changes so quickly, that I have a hard time nailing anything down.
I hope that you will share your project when it rolls around.
I actually converted them to 9cu ft Altec 604 enclosures (so a wee bit smaller, but by no means petite).If and when I get going, I will put something up and see if there is interest. Likely to need some conceptual help with the project as well, as I am no expert...
Edits: 08/10/19
Nt
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