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Old 28th November 2006, 05:35 PM   #1
poynton is offline poynton  United Kingdom
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Default OTL SE - Help Wanted with Idea

Hi.

One of the 'classic' designs for a simple OTL amplifier is shown below :-

http://www.audiodesignguide.com/otl/otlse.jpg

However, this is not true OTL as it uses chokes as cathode loads.
The chokes could be replaced with resistors but this would not be an efficient idea.

My idea would be to replace them with a current source. (As I have seen used as plate loads etc.) See link :-

Click the image to open in full size.

Anyone care to comment whether this would work or even if it has already been tried in this manner ??

Many thanks.

Andy
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Old 28th November 2006, 06:46 PM   #2
JPPP is offline JPPP  Finland
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Hi, I would like to know too if somebody has tried this. There is a webpage containing this schematic with a note that it has actually been tested with excellent result. But no other info.

http://www.tubebuilder.com/schematic3.html

I just build open baffle speakers with Goodmans Axiom 201's (16ohm, 98db/m) and I do happen to have 8 pieces of 6as7 tubes, so I guess a low power OTL would be a nice project at this stage.

yours, Jussi
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Old 28th November 2006, 06:46 PM   #3
JPPP is offline JPPP  Finland
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oops, of course I was talking about the original schematic, not the mosfet part, but still I am very interested to hear any comments.
Sorry.
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Old 28th November 2006, 08:23 PM   #4
rdf is offline rdf  Canada
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Doesn't replacing the choke with a CCS cause you to lose half the available voltage swing? I don't know for sure but I think it's analagous to replacing a plate choke and losing the ability to swing plate voltage over B+. If so a simple solution would be to use a positive and negative supply for the output stage. A clever rabbit might additionally be able to make use of the servo philosphy behind SY's Heritcal Line Stage and eliminate the output cap.
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Old 28th November 2006, 08:37 PM   #5
Fuling is offline Fuling  Sweden
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Swing is not the issue here, current is.
One idea that I´ve been thinking about but never tested is to load the cathode followers with a dynamic (Aleph-type) current source. This would roughly double the output power from a given number of output tubes, though I can feel that it would be so much of an hybrid amp that it would be better to replace the tubes with another Mosfet


Anyway, I have a box of 6C19 low Rp triodes and a pair of suitable cathode chokes so one day I´ll build myself an SE OTL similar to Rosenblitz´s design.
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Old 29th November 2006, 06:19 AM   #6
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Note that the max allowed continous anode current for each section of a 6080 is 100mA, here the current is 250mA! Also the power dissipation of each section is 165V * 0.25 = 41W which is way over the allowed 13W for each half. It is a recipe for disaster where the anodes will be glowing and the tubes will burn out in a very short time.

There is no shortcut to a class A OTL design, either you will get very low power or you will burn out the tubes.

Regards Hans
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Old 29th November 2006, 06:44 AM   #7
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Hi Poynton,

One thing you have to be careful of with CCS-loaded cathode followers like your posted schematic is they like to turn into power oscillators at the drop of a pin unless the load is a fair stable resistance.
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Old 29th November 2006, 09:12 AM   #8
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by Geek
Hi Poynton,

One thing you have to be careful of with CCS-loaded cathode followers like your posted schematic is they like to turn into power oscillators at the drop of a pin unless the load is a fair stable resistance.


HI.

Thanks for the input.

Do you know if this type of arrangement has been tried before as an output stage or as intermediate stages ??

Andy
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Old 29th November 2006, 09:26 AM   #9
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by tubetvr
Note that the max allowed continous anode current for each section of a 6080 is 100mA, here the current is 250mA! Also the power dissipation of each section is 165V * 0.25 = 41W which is way over the allowed 13W for each half. It is a recipe for disaster where the anodes will be glowing and the tubes will burn out in a very short time.

There is no shortcut to a class A OTL design, either you will get very low power or you will burn out the tubes.

Regards Hans
Hi.

Thanks for the answer.

I am aware of the numbers on the diagram which seem to possibly go against the published data for the 6080.

However, the circuit was developed in 1952 and, as noted in the post from JPPP, does work !! ( presumably without self-destructing ).

The idea obviously requires work.

Andy
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Old 29th November 2006, 11:50 AM   #10
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Quote:
The idea obviously requires work.
Don't get dissapointed if you will find out in the end that it will selfdestruct. It is one thing to exceed dissipation and current parameters in a class AB or B OTL as it doesn't happen very often, (only at music peaks) but in this case the circuit draw constant high current all the time.

If it was safe to run any tube continously at 2.5 rated current or 3.5 times rated anode dissipation I believe more would do so gladly.

That the circuit was developed in 1952 doesn't say anything about its performance or reliability, that OTL's haven't been built comercially according to the schematic says more. You can find quite many of similar overoptimistic circuits on the net. That someone unknown on the net claims to have been testing the circuit doesn't give much of credit either.

My own interest in this is not to promote class A OTLs but I want others to realise that it is possible to build a very reliable OTL with reasonable output power but then it must be working in class AB, real class A or single ended OTL is a moot idea except for anyone accepting very low output power.

Regards Hans
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