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Old 26th May 2003, 05:38 PM   #31
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Hi,

Quote:
also if it was just because it share's the B+ with the previous stage, then the lower buffer would require the same treatment wouldn't it?
Not necessarily since they operate differently.
If you look at the bottom buffer on the diagram you'll notice two fundamental differences:

The bottom one runs at full B+ and it has the cathode directly referenced to ground.
The top one however is far more complicated to figure out and the easiest way to understand (for me at least) is to look at the current flow as explained in the patent application.

I tell you upfront that I'm not quite sure what it is those zener do in that particular position...
I can only assume they're necessary because of the global NFB loop.

Quote:
Granted, as you said previously there are no complimentary pairs in tubes, you will always have this imbalance with the upper and lower tubes, I think that both the Futterman and the B.R. design's deal with that so well, you can say it doesn't exist.
I get the feeling you are looking at the amps as a whole whereas I only meant to point to the fact that the outputstage, by it's very nature, is unbalanced.
Futtermans' solution to the problem is problematic although an amazingly simple idea at first sight.
B.R. solution is by far the better one since the bottom halve of the outputstage is not affected.

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Old 26th May 2003, 11:48 PM   #32
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Quote:
I get the feeling you are looking at the amps as a whole whereas I only meant to point to the fact that the outputstage, by it's very nature, is unbalanced. Futtermans' solution to the problem is problematic although an amazingly simple idea at first sight. B.R. solution is by far the better one since the bottom halve of the outputstage is not affected.
Yes! Finally I think we are in total agreement.

Here's basically what I think is going on. The zener's provide a 450V envelope for the upper buffer to operate in. With the cathode referenced to the audio output, and the zener's tying the plate to the audio output as well, the setup provides a floating ground for the buffer. So as the audio output rises, all voltages associated with the upper buffer rise with it. Thus the output on the cathode with respect to ground is the input on the grid plus the audio output. The degenerative feedback is completely compensated for. The inspiration is Futterman, but the implementation is completely different. The Futterman concept relys on the fact that by tying the cathode to the output, the plate of the phase splitter goes up corresponding to the audio output, but the cathode is unaffected because it is a cathode follower and cannot give you more than .9 of it's input. The degenerative feedback is compensated for but by doing so a bunch of forward voltage gain get's eaten up by the cathode follower action of the phase splitter.

Whew! You have no idea how long it took me to get my head around this. And I'm still not sure I've got what's going on in the Futterman 100% right.

The nifty thing is, now you have two cathode followers driving the upper and lower tubes, with identical input and output impedances, everything is completely balanced. There's still one thing that's bugging me though. I think there is still a bit of voltage gain imbalance because the plate of the phase splitter is providing the DC bias for the upper buffer and it is not floating with the output. So in this respect, the B.R. design relys on the cathode follower action to compensate the way it does in the Futterman.

The only solution I can see for this is to provide separate bias' for the buffers and then AC couple it. Now, however you have added chances of instability, not to mention another capacitor in the signal path, and I don't know that the problem is all that bad anyway. Or for that matter if it even is a problem.

So that's my take, what do ya think?

Cheers,

Chris
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Old 27th May 2003, 12:42 AM   #33
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Thumbs up CONGRATS.

Hi,

Very well...so my assumption of you being a smart person has paid off.

Sorry, couldn't resist.

Quote:
Yes! Finally I think we are in total agreement.
We have been for a while already...guess my ego was standing in the way.

I think I'll have to print out some of the diagrams to study...and read the last couple of pages the A.R. book.

The actual effect of the NFB loop is notoriously hard to follow for we have to think in both D.C. and A.C. terms... not always simple.

Quote:
Whew! You have no idea how long it took me to get my head around this. And I'm still not sure I've got what's going on in the Futterman 100% right.
Your're good, very good even.

Quote:
The nifty thing is, now you have two cathode followers driving the upper and lower tubes, with identical input and output impedances, everything is completely balanced.
When we look at it at the output (LS) yes....but there's a but.
Although B.R. attributes a snubber role in case of a fault for the shunt series R/C network at the output, this is actually only partly true.
It also performs the role of evening out the impedance of the L.S. as seen by the amp.
Not perfectly so, but stil better than nothing at all...
BTW. , once you have your ESLs you can leave this out or optimize for the xformer of the ELS if needed.

This is extremely important in view of the fact that hardly ( if any at all) speaker has a constant impedance with regard to frequency.
Usually measured as 8 Ohm at 1KHz for instance, this impedance will change with frequency.(Just stating the obvious.)

Do you see what that would do to our frequency response if we left that network out?
And how the global NFB would affected too?

Quote:
The only solution I can see for this is to provide separate bias' for the buffers and then AC couple it. Now, however you have added chances of instability, not to mention another capacitor in the signal path, and I don't know that the problem is all that bad anyway. Or for that matter if it even is a problem.
It is a problem in theory, in practice however I feel the FB loop will act very fast to compensate for this...better than having an other cap in the signal path and create all kinds of other probs.

Now, if your interested in other ways of tackling this SEPP/OTL imbalance, I'm happy to go through it together with you...or for instance discuss the corners cut in the B.R. design etc.
Be my guest, but be forewarned that I believe in the influence of so called passive components, wires and a raft of other smaller details...such as PSUs.

Cheers and enjoy your amp,
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Old 27th May 2003, 02:11 AM   #34
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Default Hi guys

Hate to but in, but is there anyway, I could look at those schematics? A website to check out and maybe print out?

Thanks, and glad all is ok.
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Old 27th May 2003, 10:30 AM   #35
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Default OTL.

Hi,

Quote:
A website to check out and maybe print out?
Hope this helps:

THE B.R. PATENT.

Cheers,
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Old 27th May 2003, 05:37 PM   #36
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Default Thanks.

Thanks FD. Interesting reading.
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Old 27th May 2003, 10:14 PM   #37
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Quote:
Although B.R. attributes a snubber role in case of a fault for the shunt series R/C network at the output, this is actually only partly true. It also performs the role of evening out the impedance of the L.S. as seen by the amp.
I think he says that in the overview somewhere but I'd have to go back and check. I do remember him saying something about keeping the amp stable into high inductance loads. I've just started reading up on compensation and stablization so it will be a while before I could state anything for certain on how that affects anything in the amp let alone the global NFB, but I will. Right now I've relaxed my reading a bit, I almost blew out a cerebral cortex figuring out the Futterman vs Rozenblit deal.

As far as taking it out altogether if driving an ESL, I thought about that when you mentioned it way back, I may install a switch for dynamic/ESL, but that won't be for a while. My audio budget is a long way from ESL's right now.

Which reminds me, Positron (if that is in fact your real name[/quote], careful if your trying to figure out the amp by what's on those patent drawings. Figure 1 is prior art, and is neither the Futterman nor B.R. In the body of the text of the patent he uses it to explain his advantages over previous art. I'm not sure why he chose it and not the Futterman. Do you know who's design that one is Frank? I'd like to see a real schematic of it. Perhaps it's on the same page further up, I'll check later, I'm wondering if that was the way it was done prior to Futterman.

Right now, I have to go defend my honor on the DBLT thread.

Cheers

Chris
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Old 27th May 2003, 10:45 PM   #38
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Hi Chris,

Since the sun was shining this afternoon I decided to give it a little well deserved rest, took the A.R. copies to the park an read it through.

I must say B.R. is even more down to earth than I'd imagined.
He seems to very well aware as to the where and how things matter most.

Not a criticism, just an observation.
Were it for my own use, I'd approach the idea differently, but that's just my personality talking.

That "Prior Art" mention in his patent application had me fooled for a while too.
Reading that, I commented to you " no not B.R. but Futterman has this patented, if you recall that unfortunate passage.
Really confusing, I wish they'd left that out or tagged it differently.

Anyway, in the book the whole idea is explained from A-Z in his blunt style ( reminds me of how I come across at times ) and I think you see the picture already so I don't feel any need to dwell on that.

More interestingly though, at least to yours truely, is that the idea can be easily adapted to non-OTL SEPP amps that are OPT coupled, especially when a PP OPT is used, but...I divert.

Quote:
As far as taking it out altogether if driving an ESL, I thought about that when you mentioned it way back,
Yep, you could.
If you look at older OTL designs that were developped for 16 Ohm loads and so on you'd often encounter different compensation networks for different loads.

In my own system none is present, since every driver, bar my ribbon tweeter is impedance corrected right at the driver terminals.

By pure pot luck I pushed the WWW button on Positrons' post earlier today and found an old faithful from the days of TAA, SP and GA... more importantly it provides a link that actually shows you how these networks influence the speaker behaviour in a grapical view.
Ineresting stuff, thanks Positron.

I hope that answers your concern.

Quote:
Right now, I have to go defend my honor on the DBLT thread.
There's little honour to be found in that battlefield, I'm afraid.

Cheers,
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Old 28th May 2003, 03:03 AM   #39
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Default WWW button?

You may have me confused with someone else, as I could't find any www on my posts. I think another should get the credit, not me.

I don't think I was functioning correctly earlier, as I have a schematic for one of Futterman's designs, I believe an OTL3.

Sometime, I shall have to look at it again.

Thanks Guys. Have a great week.

Steve
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Old 28th May 2003, 03:10 AM   #40
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Default Www.

Hi,

The button I meant is the one in your profile, visible to us in the left hand corner and in your case pointing to SASlabs.

Whether or not you have anything to do with that company is up to you, I don't care either way.

Sorry if I inadvertantly mistook identity...

P.S. Location seems to fit, hence my assumption.
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