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Old 6th February 2003, 12:38 PM   #1
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Default Technics variation of Futterman SEPP.

I just came across the Technics version of Julius Futterman's SEPP cct for OTL amps. This one is supposed to make both upper and lower tube act as a cathode follower rather than the upper one a follower and the lower one a common cathode, or both of them a common cathode as in JF's cct.. I couldn't find hardly anything about it on the web, and Andrea Ciufolli's site seems to be down at the moment. Could anyone tell me how it works, and/or post a few links?

/Circlotron - he can't follow someone else's cct. Now the shoe is on the other foot.
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Old 6th February 2003, 12:50 PM   #2
EC8010 is offline EC8010  United Kingdom
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It looks like a White cathode follower preceded by a cencertina phase splitter. with parallel derived, series applied negative feedback. It's a "principle of operation" circuit (the DC bias for the output valves is missing), but you knew that. I suspect that the feedback loop is to enforce correct behaviour of the lower valve when the output valves are operated in Class B.

What do others think?
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Old 6th February 2003, 01:52 PM   #3
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The return point of the split load catode resistor should be your clue- it's the Futterman circuit.
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Old 6th February 2003, 02:11 PM   #4
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Default OTL

Hi,

The circuit you show hasn't got anything to do with the Technics 20A at all.

BTW,don't copy the Technics 20A if you value your speakers.

With regard to the circuit you show:

The phase splitter is crossed to compensate for the inherent difference in amplification/Zo of the output stage.
In essence the output stage now operates as a true pair of voltage followers providing for lower Zo than the original JF circuit.

Rp/2+2mu.

Andrea Ciuffolis' OTLs adapt the same technology for his OTL amps.

The circuit was posted here:

OTL

Cheers,
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Old 6th February 2003, 08:01 PM   #5
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Speaking of Futterman, here's my 6AS7G adaptation, yet to be built:

Click the image to open in full size.

Hmm, come to think of it, that -100V should really be more like -120V. And the supplies should be higher, more like +/-100V instead. Likewise, the 6SN7 will need more supply voltage to push the higher voltage swing...

Tim
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Old 6th February 2003, 09:46 PM   #6
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Default OTL

Hi,

Tim,

Your amp would work better with higer voltage rails.

I can't wait to see your PSU and adjustments on this idea...as most of the other members know I'm a big OTL fan.

Good luck,
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Old 6th February 2003, 09:59 PM   #7
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Oh boy oh boy oh boy

I was thinking of using a 48V transformer feeding a doubler for the 60V rails, a 24V 2.5A tranny for the heaters (if I had four 6082s I wouldn't need that ), and something else for the +300V.

Tim
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Old 6th February 2003, 10:11 PM   #8
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Default OTL.

Hi Tim,

While I'm aware that you know that the bias arrangement won't work...you'll need -120V at least to bias the bottom output,I find your circuit interesting.

OTL amps benefit from stiff,well regulated supplies.

Don't rely on the cross-coupled phase splitter to lower Zo.
The theory claiming that it will turn this into a double CF at the output stage is simply wrong.

The cross-coupled splitter may balance it more but it sure is no cure all.

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Old 7th February 2003, 12:00 AM   #9
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1. I drew this a while ago.
2. As such, I assumed the topology would work, based on the text.

I've never sat down and really tried modeling operation...

OBTW, I once read a post where a guy calculated the gain of each section. The plate follower gain was very close to the CF's gain. Only difference would be the distortion, which would be very high (indeed, nearly the Gm curve) for the plate follower, and much lower for the CF (having local NFB).

So should I slather on some global NFB?

Tim
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Old 7th February 2003, 12:16 AM   #10
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Tim

Regarding your circuit diagram, I mailed you about this earlier, the bias of the upper output tubes must be referenced to the cathode otherwise the bias and the drive voltage will change with the output envelope, think about what the grid to cathode voltage will look like and how this will affect current in the tube. Using the bias as in your drawing will give a lot of distorsion.

Regarding the way this circuit works, the 100% feedback from the output to the cathode of the phase splitter reduce the gain in the lower output tube making it the almost the same as in the upper one. To say that both tubes works as CF is not correct but as the gain of the lower tubes is reduced to the same value as in the higher tube the circuit will have an output impedance the same as 2 CF in parallel, Rp/(2*(1+u)) which for 2 6C33C tubes will be ~80/2*(1+2.7) = 10.8 ohm which is very close to what I have measured in my own amp.
This is also a factor 2 lower than in a circlotron and almost 4 times lower than in a original futterman circuit.

Regards Hans
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