OTL Asylum OTL, Output Transformerless Amplifier User Group.

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ OTL Asylum ]

---

[ Alert Moderator ]

---

Hi, Tyu

Posted by Lew (A) on December 18, 2013 at 07:44:01

In Reply to: RE: Roger Modjeski OTL posted by airheadair on December 14, 2013 at 18:19:56:

Happy holidays.
I agree with Tyu re the Counterpoint SA4. I believe the major issue with it was that it was a direct-coupled Futterman-type design, sort of a contradiction in terms. Julius Futterman always used output coupling caps in series with the speaker to block DC. His totem pole topology inevitably presents DC at the output, along with the AC signal. This required a bank of high value electrolytics, in order to pass bass frequencies. If there is any sonic weakness to the Futterman circuit, that's it. One could ameliorate the negative sonic effect of capacitor-coupling by bypassing the electrolytics with high quality film caps, a trick I only learned to do when I was on my last pair of Futterman type amplifiers, a set of Prodigy OTLs. Julius himself ended up using photoflash caps, which were the fastest electrolytics of his day. He stopped making amplifiers only when he died, in either 1979 or the early 80s. I consider him a great man of audio. Prior to owning the Prodigys, I had a pair of "H3aa" monoblocks, which Julius built for me in 1979. Now back to the Counterpoint SA4: The innovation of the SA4 was to eschew the use of coupling capacitors by virtue of some sort of servo mechanism that balanced out the DC at the output. It was unreliable and often led to mini-disasters. The SA4 also used a 12AX7 at its input, rather than the 6EJ7 and 6FQ7, stipulated in the Futterman design. There may have been issues associated with that. There was at least one other design from the midwest or the West Coast, the KSS, which also used such a method to eliminate DC from the output. Those were even worse, in terms of reliability, IIRC. Those two products did more to damage the reputation of "Futterman" amplifiers and of OTLs in general than anything else that actually applies to a true Futterman design.

What I noticed in RM's post is that it seems he is going to use a servo mechanism to block DC in his new product, as well. I guess we have better ICs and other parts available these days with which to do that reliably.

Altho I revere Julius, I have to say that my first pair of Atma-sphere amplifiers blew away the Prodigy amplifiers, and the Prodigy amps were very very good classical Futterman designs, in and of themselves.
In that, I also agree with Tyu.

Tyu, I am playing with a pair of Beveridge 2SWs. Direct-drive is amazing.

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---