Koren Local Hero Driver Mods
The Local Hero MK-III amplifier modification by Norman Koren is a well thought out design that seeks to eliminate some of the pitfalls encountered in most amplifiers - specifically those related to global negative feedback.
There's nothing wrong with negative feedback, especially when applied in moderation to an already well designed amplifier. There is however problems that creep up like inherent instability when improperly loaded (phase shifts), reactive loads and they do a butcher job on slew rate.
Mr. Koren did a wonderful treatise on converting one of the more powerful of Dynaco amplifiers, the Mark III to using local feedback only, here: http://www.normankoren.com/Audio/FeedbackFidelity2.html
Being an authorised manufactuerer and reseller of PCB's for his projects, I had a small run of PCB's made in order to more readily share this work of his.
However, I encountered a small problem when experimenting with it in my Dynaco test bed - it was not stable.
My test amplifier had a "heartbeat" every 30 seconds or so. Having used the amplifier with full stability with many other Dynaco and aftermarket designs, I ruled out the test bed as the cause.
Upon investigation, it was determined that in some cases that the cathode feedback going into the phase inverter was the cause.
This section has a rolloff increasing with frequency, due to C34, a 680pF capacitor joining the phase inverter cathodes. But the iron itself doesn't have infinate LF response. The result is a gain stage running wide open and uncontrolled below say, 4Hz.
The "ground the 4 ohm tap and use common and 16R as cathode feedback" is not a new trick. But Mr. Koren's use of feeding the phase inverter with the cathode feedback is quite unique.
To verify this as the cause, I simply pulled R3F and R4F. The "heartbeat" went away.
Some Googling led me to realize the situation was fairly unique, as only one other person complained of this too.
So I set out to stabilize the amplifier while still keeping most of Mr. Koren's circuit (and my boards) intact.
First, I had to lose the CFB to the phase inverter while still keeping feedback there to keep the 12AU7's in check (not the most linear tube).
Experienced tech's will recognize the method of cathode feedback used on a multitap secondary as one used by the audio manufacturer, Audio Research. Audio Research also used some anode-following feedback applied to the phase inverter cathode as seen here:
However, there is *very little* feedback from the anodes of the 6550's happening, as they are fed through two 1Meg resistors shunted to ground via low value cathode resistors. That's because they use a combination of three feedback systems: 1) traditional gNFB (green line), 2) Anode follower (blue lines) and cathode feedback (red lines).
We still want to keep gNFB out of the works, so originally I tried straight anode following and got this:
(Click for full schematic)
(12AT7 driver dealt with in a moment)
First thing I did was rip out the low value capacitors. They slowed the amplifier to the point a square wave was looking more like a pyramid.
I did my tests in triode, UL and pentode positions with my test bed (ST-70 with power supply on steroids and MK-III output iron). The results were as follows:
Triode power output: 21W
Full power F3: 16Hz - 26KHz
Local NFB: 5.8dB
UL power output: 45W
Full power F3: 24Hz - 36KHz
Local NFB: 8.5dB
Pentode power output: 50W
Full power F3: 25Hz - 35KHz
Local NFB: 10.1dB
On the squarewave test in pentode, there was a bit of a leading edge spike on the top and bottom waveforms due to the transformer flyback.
Sonics were bright, mids extremely detailed and bass neutral, but the only good full power 20Hz sinewave was on the triode position. Need better damping factor. Let's add cathode feedback again.
(Click for full schematic)
The cathode feedback only adds another 1.5dB to the NFB figures, but what happen to the F3 and square responses here?
Triode power output: 21W
Full power F3: 10Hz - 35KHz
Local NFB: 7.3dB
UL power output: 40W
Full power F3: 10Hz - 43KHz
Local NFB: 10dB
Pentode power output: 40W
Full power F3: 11Hz - 46KHz
Local NFB: 11.6dB
And there was no spikes on any leading edges in pentode on the square test.
The two resistors in series (in my case, 56K @ 2W) are required because you have high AC peaks riding on an already high DC voltage. Unless you use a "barber pole" resistor, you will require the two two watters in series so they don't break down.
Pic of the (to be released by Classic Valve Design) PCB and mod:
Now about the 12AT7 where the 12AX7 should be.....
In all tests, I found the gain way too high. Even with the stock board, 200mV RMS brought me to full tilt.
I modified the values for a 12AT7 and am running it at 3mA first stage. Now it requires a more sane 500mV RMS in triode, 550mV RMS in pentode and 600mV RMS in UL for full output.
Oh and have I mentioned... I do NOT like the sonics of 12AX7's, outside of heavy NFB RIAA stages? (the Koren PAS linestage was the FIRST 12AX7 linestage I liked. It has lotsa NFB)
I began my listening session in triode mode.... that didn't last very long :-p
As with all anode feedback PP pentode methods I've listened to, the amplifier really shows its liveliness and clarity in pure pentode mode. The bass is not flabby at all. It sounds detailed (I can actually hear the bassists fingers on the strings on Madeline Peyroux "Don't Wait Too Long", which is about as treble/mid rolled-off as I've heard in a commercial release), but not tight as in SS or heavy NFB. To compare the bass.... a zero NFB MOSFET.
The mids I can only describe as "live" sounding. I had to roll the volume on the player to about 30% of an equivalent volume for my 2.5W/ch. SET for the same apparent volume on both Diana Krall's "From This Moment On" (lotsa brass) and Michael Buble's "Feelin' Good". There was complex details that were far too musical to be written off as IMD and the THD on this amp in tests is well below 5% anytime you aren't in clipping..... and I'm no where near 40W output (the wife's in bed... I'd be dead if I was ;-)
Treble in pentode mode is slightly greater than neutral.
UL mode was like the pentode, but bass sacrificed some detail for amplitude (like rolling 60 and 120Hz up by 6dB on an EQ).
Of course, sonic evaluations are subjective and if I were you, I would build and listen to it and draw my own conclusions. The ONLY opinion that matters when it comes to what something sounds like, is YOURS.