Are all amps being built wrong?

This statement is false. No Futterman amp ever had anything like 60dB of feedback!! The most any had was more like 20dB, and because the amp had very wide bandwidth, oscillation (caused by its phase margin being exceeded by the feedback) was sometimes an issue. So to your closing comment here- they are well-known to **not** be stable into complex loads.

@atmasphere Roger Modjeski based his OTLs (he built several, mostly as custom projects, I have two of them) on the Futterman circuit and in his last design prior to his death he was specifically referencing the H3 circuit. In an effort to solve the oscillation issue in his first prototype and which you noted, one thing Roger found is Futterman himself omitted some things from the H3 schematic, specifically the lack of notation for the ferrite beads which Roger eventually discovered because he had me source an H3 for him to study (the NYAL versions of this circuit certainly had their issues, perhaps due to the schematic omissions as well). One look underneath and there they all were. Now when Roger completed his design I asked him how much feedback he was using. The response was 60 dB which in addition to adding the beads to the circuit (although far fewer than Futterman used) solved the oscillation issue in the second prototype of his design.

Now I don't remember why he came up with 60 dB, but since jasonbourne52 also mentioned it, I'm inclined to think it was what Futterman used. Of course this could all be a coincidence and Roger could have come up with that amount of feedback on his own through testing.

In an effort to solve the oscillation issue in his first prototype and which you noted, one thing Roger found is Futterman himself omitted some things from the H3 schematic, specifically the lack of notation for the ferrite beads

@clio09 

IMO this was intentional. I have a lot of anecdotal evidence from customers that say that the Futterman amps held together while amps made by others using the Futterman circuit didn't. However Harry Pearson recounted an incident where Futterman brought one of his amps to Sea Cliff for audition but before it could be entirely set up, one of the amps went into oscillation and failed (this was in response to a letter to the editor from Harvey Rosenburg in the late 1990s). But all you have to do is leave out one bead by accident and you're sunk. Futterman made most of his amps himself to my understanding.


60dB of feedback in a tube amplifier is an impressive feat! Normally you have such prodigious issues with not exceeding the phase margin of the amp (OTL or not) that most would not attempt such a thing. If 60dB is correct I'm quite impressed (and stand corrected)! The ones I've seen did not have any such value- I doubt that they even had 60dB of loop gain (that's the gain of the amp plus the amount of feedback).  But some of the Futterman amps have impressively low output impedance figures, such that they would have easily behaved as a voltage source, even though they made more power into higher impedances.


Kron-Hite made laboratory amplifiers in the 1960s. In their manual for the amp (which used KT88s) they claimed 80dB(!) of feedback. Its hard to imagine how they pulled that off- that amp was quite stable. I had a pair of them for a while in the late 1970s and they compared very favorably to an ARC D-75 that a friend of mine had.

Julius used hollow ferrite rods on the wiring of his amps - at least the one's I've seen. 

I seem to remember Modejeski discussing that Futterman 60dB feedback figure in one of the threads on the Music Reference AudioCircle Forum (though dormant, all posts are still viewable), and maybe in one of his three seminars at The Burning Amp Festival, which I encourage everyone to watch on YouTube. All contain a wealth of free information and wisdom, as does the MR AC Forum.

@jasonbourne52 - It's been a couple years so I would have to look at that Harvard H3 again. I seem to recall 1" long beads, but they could have been rods. They were all around the power tube sockets.

@bdp24 - I'll see if I can dig something up to verify. I keep forgetting those AC circles are still accessible although AC banned Roger.

@atmasphere - I have about 3 hours (cassette tape) of Roger interviewing Julius Futterman (also interviews with Saul Marantz, Sid Smith, and Brian DePalma) that he recorded when visiting Julius in NY years ago. Upon opening the door to his shop Julius asked Roger if he could come back in an hour as he was about to get started eating his lunch (which he brought to work everyday in a brown paper sack). BTW - if you are interested in dissecting an H3 I have two of them. I'll send one off to you if you like. Perhaps we can find out if he was able to pull off that 60 dB of feedback.