MAC Autoformers?

C1Ferrari 9-6-2018
Very cool thread ;-)

 +1.  Great thread.  And thanks to Ralph and Roger for providing their informative if somewhat different perspectives.

Roger, regarding...

The Futterman amplifiers have over 60 dB of feedback and a good reputation.

I've seen that stated before, but I'm wondering how an amp can be designed with 60 db of feedback.  Wouldn't its open loop gain have to be enormous to support a closed loop gain that is reasonable?

Best regards,
-- Al

P.S:  Bruce (Bifwynne), great to see you posting again recently. 

@ramtubes 

So Roger, …. bottom line, do you concur that using the 4 ohms taps on my Ref 150SE should yield a flatter FR, less distortion and longer tube life?? I gather you are saying yes. 

P.S. the ARC engineers obviously were trying to balance a lot of design ideas and techniques when putting the Ref 150SE together.  Just enough negative feedback to get the output impedance low enough to handle a host of speakers designed to be driven my low impedance SS amps and a robust power supply (1040 joules) to handle the tough spots that require a lot of current (i.e., low impedance and highly reactive phase angles in the bass region).

P.S.S. -  I have about 1800 hours on my KT-150s.  Can you help me source some matched tubes that will bias well my amp?  I have 8 fresh KT-150s sourced from another vendor sitting in a box.  They don't bias well.  Could you match them for me?  

@almarg - been distracted.  Glad to be back.  Al, if you get a chance, check out Roger's amp design presentation here:

https://www.youtube.com/watch?v=aO1Q281FQxo

The guy knows his stuff.

BIF 

The link Roger posted did not seem to work so here it is again:

https://www.stereophile.com/content/devore-fidelity-orangutan-o96-loudspeaker-measurements

@almarg 

  I've seen that stated before, but I'm wondering how an amp can be designed with 60 db of feedback. Wouldn't its open loop gain have to be enormous to support a closed loop gain that is reasonable?

The open loop gain is enormous and obtained entirely in the first tube. That tube drives a split load phase inverter which provides EQUAL drives to the output tubes. Many think the top output tube is a follower and not equally driven but it indeed is equally driven.

What is so clever about the circuit is that it is able to employ that high level of feedback without oscillation. This because of the very wide bandwidth of the circuit up to many hundered KHz. Conventional tube amps are limited in feedback by the output transformer.

While the original circuit is quite simple, published in AES Journal, it takes some time to fully appreciate what Futterman created.