Koren Local Hero and Mods

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Koren Local Hero and Mods

Postby Geek » Sat Apr 23, 2011 4:05 am

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:

Image

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:

Image

(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.

Image

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.

Sonics test is forthcoming and I'll have a report on this, ASAP :-)

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.

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)

Cheers!
-= Gregg =-
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Postby Geek » Sun Apr 24, 2011 3:09 am

Pic of the (to be released) PCB and mod:

Image

Cheers!
-= Gregg =-
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Postby dcgillespie » Mon Apr 25, 2011 7:36 am

Superb work as always Gregg!

Another little known fact about applying cathode feedback within a push-pull stage is that distortion -- normally reduced in direct proportion by the amount of the gain reduction -- is actually reduced by the square of the feedback factor. Therefore, the connection is extremely effective, although not all transformers remain stable with such configurations.

Achieving high orders of stability in FB amplifiers is something I have championed for many, many years, always believing that how effectively and to what stability level NFB is applied has more significance on reproduced sound than any other design element. While I have been able to achieve the same levels of stability Mr. Koren describes while still using traditional global feedback designs and levels, there is no doubt that the results of his work is significant in helping to explain the sonic differences between the various approaches to amplifier design.

Thanks for sharing your work.

Dave
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Postby Geek » Mon Apr 25, 2011 3:21 pm

Hi Dave,

No problem! I like sharing studies in odd configurations.

I've grown rather fond of these cathode feedback methods since I started using them in SE amps (cathode resistor bypassed to the transformer secondary with a big cap). The result is a better DF (tighter bass) and a "lesser" transformer can be made to sound like a grade or two higher ;)

Taking local feedback off the anodes directly I'm finding to be a rather effective (and stable) method. Because the load variations are reflected so well due to the step-up, you have a more stable amplifier and I've successfully run these in test beds with no load and not had the power oscillator effect. Fullrange speaker ready! :))

I've found a thread (Audiokarma) where the Local Hero was definately picky to output transformers. Some, as with me, it did not work. others, no problem.

I have four MK-III OPT's.... the two matched ones are in this test bed. Then I have two that have a wicked (100+ ohm) DCR difference and sound different. Consistency back then didn't seem a standard.

Cheers!
-= Gregg =-
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Postby dhuebert » Mon Apr 25, 2011 7:34 pm

Thanks, Gregg! I'm always ready to read about approaches I haven't considered! Keep it coming.

Don
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Postby Geek » Tue Apr 26, 2011 12:16 am

It feels good to know these little articles have a reader or two :))

I'm now listening to this on my main system.

I began my listening session in triode mode.... that didn't last very long (sick)

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)

There's my $0.02 [:)

Cheers!
-= Gregg =-
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Postby DeathRex » Tue Apr 26, 2011 12:54 pm

So you could, almost without modifying anything, drop in a 12AU7, or with a bit of rewiring, a 6DJ8, 6BZ8, or 6N1P? And what if you take the feedback from the EL34's cathode (lowering the 2 56K resistors) instead of the anode?
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Postby Geek » Tue Apr 26, 2011 4:01 pm

Hi,

DeathRex wrote:So you could, almost without modifying anything, drop in a 12AU7, or with a bit of rewiring, a 6DJ8, 6BZ8, or 6N1P?


The first stage and cathodyne is really straightforward and out of the FB loop, so yeah, you could with just changing resistor values, add pretty much any dual triode with the same pinout.

Actually, 6N1P is a really nice sounding tube in power amp front ends, IMO and I might just try that :-)

Just watch your B+ on that stage then.... I've had Philips 6DJ8's and some 6N1P arc-over with the full B+ before the heaters warm.

My test bed has a standby, so I'm not concerned, but all stock and most DIY Dynaco's do not.


And what if you take the feedback from the EL34's cathode (lowering the 2 56K resistors) instead of the anode?


Actually, that's similar to the original Koren diagram.

We could apply the FB at a different point... maybe in the grid-leaks due to the phase. But this is good, since the voltage developed across the 10R cathodes are low enough you could go straight DC coupling to the grid leaks of the 12AU7.

Cheers!
-= Gregg =-
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EL34 PtC FB

Postby msmpe » Mon May 02, 2011 10:33 pm

Hi Gregg,

Thanks, very interesting. The amp I'm working on at the moment also uses plate to cathode local feedback. The amp came out of a Zenith console using the 7695 SE'd with a 12AX7 VA. The origianl Zenith circuit was full of all kinds of goofy stuff! But, I found a very simple Westinghouse circuit that used the PtC FB, so I'm adapting it to this amp. If it sounds at all decent [using original OPT and 2-way 6x9 speakers], I'll publish the circuit because it appears to be an ideal LFB solution for small SE amps.

Can you give us more information on the PCBs for the EL34PP?
8>) Mike

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Postby Geek » Tue May 03, 2011 1:21 am

My test bed uses EL34 in PP (I don't like KT88's). Working great and am resisting my partner's suggestions to pull it out to test another board (lol)

The board can be used with anything, really. Though it's made to fit a MK-III chassis, I just made painted MDF adapter plates (my test bed has a hole for PC-3's). I daresay you can use it on PP EL84's, just use a low gain tube up front... 6922 or something.

I have a bunch on my desk if anyone wants boards (PM me).

Cheers!
-= Gregg =-
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Postby jgf » Fri May 06, 2011 10:41 am

What's the effect of OPT cathode feedback on output impedance? IOW will an 8 ohm speaker work well across com & 16 with 4 ohm grounded? I suppose increased damping factor helps?
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Postby Geek » Fri May 06, 2011 4:22 pm

Hi,

You put an 8 ohm speaker across com and 8 ohms.

The ratio does not change. You hook it up as if everything was "normal". 4 ohm goes to com and the grounded 4-ohm tap and the phase is even the same with the speaker's "+" terminal grounded by the 4 ohm tap.

I have not measured the difference in CFB output impedance.

Cheers!
-= Gregg =-
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Postby jgf » Sat May 07, 2011 8:25 am

Duh... simple enough! for some reason I thought it had to be balanced about the ground connection.

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Re: Koren Local Hero and Mods

Postby PeterG » Fri Feb 07, 2014 12:26 pm

Hi Geek, found this when searching on Norman Koren's Local Hero amp design. Read through your comments but am a bit mystified by your comments about the problems you had with the heartbeat instability and your pointing out the cathode feedback to the phase splitter being the problem and pulling the R3F and R4F resistors to make the problem go away. Are you sure these triode sections are the phase splitter? In looking at Mr. Koren's schematic, the phase splitter is actually TU2, with TU3 and TU4 acting as simple drivers between the phase splitter and the TU5 and TU6 power output tubes. Providing the cathode feedback to TU3 and TU4 drivers as well as the output tubes should have enhance the stability of the amp, but with your amp it caused the instability. Just not sure why.
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Re: Koren Local Hero and Mods

Postby Geek » Fri Feb 07, 2014 5:30 pm

Hi,

Yes, not the PI, the amplification after the PI. My mistake.

Happened a couple of times with the design (myself and a client), the mod tweaked it and it's fine (y)

Cheers!
-= Gregg =-
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