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morricab wrote:

You are also right that to get everything you want from an output transformer requires a VERY expensive construction and top materials.  However; even an expensive transformer is no guarantee of success.

Sorry to accent it again but the DSET is. A LF transformer of DSET dos not care about capacitance and HF phase shifts - is just pumps inductance with core mass or with turns. The MF-HF transformer of DSET do not care about inductance of power saturation as LF are filtered out from the channel. What might be more simplistic and elegant?

morricab wrote:

Overall SET amps like Lamm and KR audio sound cleaner and more correct AND they also measure cleaner and more correct...with little or no corrective feedback (Lamm uses a bit I believe and KR audio uses none).

It would be interesting to learn why Lamm wean for feedback in ML2. Obviously he wanted to keep the amp as much as possible accommodated for wider margin of perspective speakers. Also he had no good driver in input stage as 12AX7 can not handle the line-level input (too low bias). So, he jacked up the 12AX7’s bias with feedback in order do not drive the input stage into overload.

morricab wrote:

I think that in fact measurements DO mean something when the amp is not using negative feedback, which seems to mask the true behavior of the amplifier (ie. open loop linearity).  The more linear an amp is open loop (ie. without negative feedback) I think overall the better it will sound.  This includes errors from the power supply, circuit layout (eg. hum and noise), OPT, thermal issues etc.  All of these things will have a profound affect on the open loop linearity and many designers cover up these issues (but do not cure them) with negative feedback. I mentioned the dynamic bias issue with PP amps.  It is more a problem with OTL but it is also a problem with transformer coupled PP amps and is partially why they don't achieve the overall purity of a SET.  When you bias an amp statically, you can match them pretty closely.  However; put a demand on the tubes and watch them diverge!  I know of only one amp on the market that addresses this issue with a kind of dynamic biasing scheme (not disclosed) and that comes from VacuumState.  It is an 18 watt 300B PP amp (the designer refers to it as differential as it has no phase splitter and is fully differential) that is the first I have heard to get the low level resolution and tone color of a SET.  Also bass is quite controlled and natural.  It is better than most SETs I have heard and as it uses a very good OPT, my guess is that it has minimal distortion in the bass as well in addition to being zero negative feedback.  www.vacuumstate.com

What you say sounds logical to me but I have no further PERSONAL experiences with feedback in power amps. I heard many of then with different feedbacks, including positive feedback but was about all that I did with feedbacks….

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