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OTL Topologies

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The top bank would certainly needs feedback as explaint earlier. My current listen version does nothing to the bottom driver, a little NFB is fine, but excessive to the bottom will have sonic impact certainly esp. this is just one side affair, you need to test it. The SIM can tell whether symmetry conduction or not. In the sch I have with 6p12p triode has 470 ohms cathode resistor with 68k connect to cathode and output, for both drivers (the top is strapped Pentode), the SIM shows good symmetry when load is changed. I don't know yet how good it sounds compared to the current simplest version. So this is gain version (not cathode follower perhaps?) driver as the stage gain is decided by top drive gain and bottom drive follow to suit.

If you don't apply PFB then it's quite different story, it then becomes Technics version which is rare.

Btw I only reduce the cathode resistor of the top driver to increase gain. It sounds good on Pioneer CL-70.
 
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Koonw

I wonder about Sinclair-Peterson topology? his have concertina as DC coupled driver, it is characteristics the same as in on from topology from Fig.2 from my earlier post, or is something better by balancing OPS operation?

ps, maybe resistors around 6S4 tube need to be with slightly higher values, Ra=56K , Rk=41K+15K trim pot...

edit , 47uF on lower side of 6S4 cathode resistor is shown Wrong Inverse connected!
 

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I found this simplified Technics OTL design on the Web by Luca Cellai. (almost readable!) It is using separate class A drivers as Koonw suggested. Seems to be just what one would expect for the Inverted Futterman using separate drivers. Should give very close to matched tube bank gains. But there is no servo -exact- gain matching with the separate drivers. Could use a gain trim pot or some servo extension mod since the upper cathode follower tube bank will have gain slightly less than 1. (while the lower bank is servo'd to exactly one, depending on driver resistors.)

Using an LTP driver for the top tube bank (with output Fdbk to the unused grid 1) would get the top bank accurately up to gain of 1 (or matched gain with the bottom driver if they both have gain)

{and then finally, replace those tube banks with some big N channel Mosfets, they are just acting as unity gain followers}

Edit:
Two high gain feedback drivers can still mis-match by a tiny amount and produce a significant tube fight between top and bottom banks. So the gain trim has to be really closely trimmed.

If we just used a single LTP/CCS driver stage, with signal input on one grid, and the output Fdbk on the other, and the two outputs (bank gains trimmed by load resistors, or bootstrap to one load R, and a trimmed R on the other driver load R ), then the two banks would have to track exactly (no gain mis-match fighting with the single differential compare function controlling all)
 

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Looking back at that TEM3200 OTL, Broskie had suggested that the first two tubes were in cascade, followed by the cathode follower to the Mosfet.

Due to the DC coupling advertised there, I'm thinking the TEM3200 has the 1st two tubes in a dissimilar LTP/CCS with input to the 1st triode and N Fdbk to the 2nd pentode tube. The pentode side providing decent gain.

The block diagram seems to indicate that. Then the follower to the Mosfet. (which is a ridiculously large 300 Amp one, gate slewing rate?) So the same LTP/CCS driver circuit essentially as evolved in the above post! (further pre-amp gain un-needed with the extreme gm Mosfet outputs)

The Mosfets are then in a Circlotron configuration for the final output. This should be really easy to build. And cheap.

With Mosfet outputs, 60V will do fine. There are these new (Chinese) switching/linear-follower power supplies lately. Only $65 0 to 60V at 0 to 5 Amps, 300 Watt. Two of them to make a 400 Watt OTL.
 

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You need to control the top driver as well beside controlling the bottom so that they will stay matching even after output level or load change. You can build my version if you like to try :), then build another version but with asymmetry driver. You only need to change out the drive board. So far I built about 5 versions, two are still in use, the rest in the drawer.
 

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Thanks for posting your design!

6C19 triode tubes I see. 140 mA peak. Similar, somewhat bigger to the 12B4. What is the value of RL there? Oh, I see, 8, 16, 32.
How many 6C19 to work 8 Ohms?

I have a BIG box full of 21HB5 tubes, 800 mA peak, 18 watt, pentode.
Also 21LG6 tubes, 1100 mA peak, 28 Watt, pentode.
I even have a box of 13FM7 triodes, 175 mA, 10 Watt

A Mosfet OTL for summer, a Tube OTL for winter....

http://www.jogis-roehrenbude.de/Russian/6C19N/6s19.gif
http://www.wooaudio.com/docs/tube_data/6C19.pdf

http://tubedata.milbert.com/sheets/123/2/21HB5A.pdf
http://tubedata.milbert.com/sheets/123/6/6LG6.pdf
http://tubedata.milbert.com/sheets/123/6/6FM7.pdf

A couple of Xantrex/Sorensen XHR 150-7 power supplies should do. (got two for cheap years ago) The price is out of sight for two more of them now though. Guess I could use the two for both amps.
 

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Here is Sinclair Peterson sim plots and schematic for sharing.
There is a problem with trimmer for setting bias of driver, it's out of range. See attached.
 

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Oh yes I though it could drive 32 ohms at reduced level at 1.5V rms out, thd is 1%
I measured the output Z, 115 Ohms, with this load it is 0.1% at 1.5V rms out, and 2.5% at 21V rms. All depend on load and level out.

Edit: But I just check the MP 1 pair version, measure Z is only 12.5 ohms so maybe something wrong there.

Edit2: Oh ya I found it's amount of NFB is much lower with this amp compared to MP, the gain of 6au6 is much higher than 12ax7, so I guess if use same tube and same NFB level, there should be no differences.
 
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I think that one reason for high Z out is to low OLG ,and in the same time to low GNFB ,
possible solution ,that VAS stage which is 1/2 of Ecc83 need to be replaced with one 6AU6 connected and supplied similar as is by MP and Futterman OTL amp ,
second reason maybe is to low current thru 6S4 tube? , which is only 0,53mA .
 
Here`s updated Sinclair-Peterson based OTL amp, is AC coupled so no any DC protection is needed, V1 can be also EF86 or any other similar pentode, V2 can be also 6SN7 but with both triode section connected in parallel, output power tubes are 6C33C-B now, because of `stacked` PSU design both B2 & B3 voltage values can be in simple way scaled up or down just to get desired or optimum amp performance.
 

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I know this is a pretty old thread.
@banat: if you are still around, did you actually build it ? How about PS ?
My sim shows rather mediocre PSR, both B+ and B- requiring either regulator or chokes.

I just found this thread (I hear the running already ;))

I am just finalising the initial model of a 32ohm headphone OTL amp (cathode common 12BH7 driving a parallel single ended ecc99 two tubes - only three triodes used per channel).

One interesting adaptation that Broskie does is to use a cap and resistor across the rails into the CCS. Thus modulating the power supply noise into the cathode of the front end. With the P-SE the same can be done for the back end.

Originally I'd looked at PP 6AS7s, but multiplying the ecc99s drops the impedance nicely, add some NFB and it drives it well.

The next stages of this amp will be towards a LTP front end and PP backend.
 
One more prototype ....

just 1 x ECF804 (6BL8) and 2 x 4 x 6c19p per channel, not much power, rather high Z output, deliberately no additional GNFB, to test "current drive" philosophy ...

PS is tweaked ATX SMPS plus two boost converters from 14V to 125V, rectangular output into doubler to get +/-125V.
Same ATX is also used for heaters.
16x1A heaters (80A+ when cold) tripped the over current protection, so tubes are pre-heated from the 3.3V rail until a timed relay switches over to the 6V.
 

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