I typically see a rise in output Z at the higher end of the band. But you are talking very small impedances in this graph- not really an issue in the grand scheme of things.
The loop at the end?
You can look at several of my amplifier/receiver videos and see the plots of output impedance- the videos are chapter marked which should make it easy to find the output impedance graph.
I am looking forward to going back to your videos now that i have a more clearly understand the test procedures.
My comparison at this time is Bob’s graph in the test procedure. In his example i don’t see anything that resembles that loop back.
Thank you
V2 will be out very shortly. It has taken a little longer than I thought 
I’ll try my best to release it while we are still in 2024. When it comes out, I’ll provide it to Matt and also post it on my web page at cordellaudio.com. V2 will cover most of the automated amplifier tests, but also contains new material and improvements in the section that has already been released as V1.
Cheers,
Bob
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There is an “upgrade” kit for Hafler DH-220 and DH-500 to replace everything except the power supply and output transistors:
https://audioxpress.com/article/you-can-diy-the-dh-220c-mosfet-power-amplifier-part-1-the-circuit
I rebuilt my 1989 vintage DH-500 and it’s a superb power amp. The DC coupling (via servo) alone made a huge difference in the level and dynamics of bass from a pair of Magnepan MG-IIIa. The “kit” is a set of raw PCBs and a bill of materials at Mouser. It took a few days to get it together, well worth the time and certainly worth the money!
What does that have to do with the Bob Cordell tutorials?
It just has to do with the DH220C design that Rick Savas I did and which is available with PCBs and was a construction project in AudioExpress. It is a full redesign of the Hafler DH220 that is also adaptable to the DH500.
Did you ever look at the David Tillbrook Hitachi MOSFET designs in ETI back in 1981? ETI-477.
They were some of the very first proper amplifier builds I did as a teenager after the Leo Simpson designed Playmaster 40/40 (1976/7).
It was soon after I got into Perreaux and their 2150B. Pretty sure Peter Perreaux was one the absolute first to bring a Hitachi MOSFET application-note based integrated to market and then of course the big power amp lineup all the way to the 5550 into the mid 80s.
I still pickup pro power amps I come across cheap or broken from that era as they often plastered “Mosfet” on the front panels and they are chock full of those lovely Hitachi TO3s. I’ve still got several blank fibreglass unpopulated 477 boards and a few 477 populated boards picked up for a few dollars at a garage sale.
Version 2 of the QA403 Tutorial is finally ready, and I have posted it on my website at cordellaudio.com under the Audio Instrumentation tab. I have also provided a copy to Matt.
Version 2 includes with it a ZIP file that has all of the Measurement Settings in it that were used for the measurements and plots in the tutorial. Each measurement settings file has a prefix associated with it that corresponds to the figure in the tutorial for which it was used.
Happy New Year and cheers,
Bob
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