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OPT in or near loudspeaker?
OPT in or near loudspeaker?
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Old 1st March 2015, 09:59 PM   #1
soeren is offline soeren  Greenland
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Default OPT in or near loudspeaker?

I have some times thought that it would be better to have the OPT near the crossover a little depending on what you think about cables, may be GNFB could be delicate.

Has any of you actually tried it?

Søren
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Old 1st March 2015, 10:30 PM   #2
Miles Prower is offline Miles Prower  United States
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Having an OPT near the speeks was done all the time with PA systems. It isn't done otherwise since the audio cable would also be carrying the HV for the finals, and you'd have a safety issue there. It also isn't convenient for production amps where you never know what speeks the end user is likely to connect.
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Old 1st March 2015, 10:45 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by soeren View Post
I have some times thought that it would be better to have the OPT near the crossover
a little depending on what you think about cables, may be GNFB could be delicate.
Better to have 2 mono amps, each near the speaker, with short speaker wires.
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Old 2nd March 2015, 01:15 AM   #4
soeren is offline soeren  Greenland
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Until now the answer is no, so far we have not heard from people who have tried it.

I find that having HV DC in the cables makes things easier as you can have long cables without loss, I guess that the some times heated discussion about cables could disappear.

Safety should of course be taken care of but I do not find it more difficult than all the other HV cables in the house, it is mostly a question about choosing a reasonable plug standard.

I thought that in Europe the normal standard for PA was 600ohm symmetric.

My point is that if you are a DIY'er then you should take all the advantages of it and not try to make products which already exist on the commercial market.

I have some Tamura OPT's laying around and find that they are too good not to be used so I am thinking of some bizarre circuits in the 1-2 watt range which I have not seen before.

Søren
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Old 2nd March 2015, 01:48 AM   #5
smoking-amp is offline smoking-amp  United States
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A slightly different variant that does get mentioned sometimes (Tubecad) is to keep the OT at the amplifier, and run the global feedback back from the speaker terminals, instead of from the OT secondary terminals. That will lower the damping factor right at the speaker, and put less (effective) capacitance into the feedback with a low Zo signal.

Last edited by smoking-amp; 2nd March 2015 at 01:51 AM.
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Old 2nd March 2015, 02:05 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by smoking-amp View Post
A slightly different variant that does get mentioned sometimes (Tubecad) is to keep the OT at the amplifier,
and run the global feedback back from the speaker terminals, instead of from the OT secondary terminals.
Yes, this is called remote sensing, and should include BOTH conductors of the speaker wire. It'a a type of Kelvin connection.
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Old 2nd March 2015, 12:08 PM   #7
azazello is offline azazello  Bulgaria
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Better use OTL-OCL amp.....All things from sours to speakers will be better....
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Old 2nd March 2015, 02:16 PM   #8
mvd is offline mvd  Germany
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Quote:
Originally Posted by soeren View Post
Has any of you actually tried it?
Hello Søren

Back in the old days this was quiet normal. As an example please have a look at an radio from Telefunken, build in 1931.

Katzenkopf T 340 T340W RENS1204 Radio Telefunken Deutschland

You see, there is no speaker and no output-transformer.

One of the speakers for this radio is the Telefunken Arcodyn 505 with a field coil speaker and a build-in output transformer.

Arcodyn 505WT Speaker-P Telefunken Deutschland TFK, Gesellsc

The output transformer has taps for different tubes, from the tiny RE134, over RES164, RE304 up to the RE604, the predecessor of the AD1.

Best regards
Michael
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Old 2nd March 2015, 02:25 PM   #9
nigelwright7557 is offline nigelwright7557  United Kingdom
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I have always found two magnetic fields close to each other interfere with each other.
In the past I have had mains transformers close to valves and output transformers and picked up a lot of hum.
I would have thought the speaker voice coil will radiate and be picked up in the output transformer and then into the feedback loop ?
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Old 2nd March 2015, 03:41 PM   #10
DF96 is offline DF96  England
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Magnetic fields in air do not "interfere" with each other, as air is a linear magnetic medium. Nearby magnetic items can couple together, which is why some 1950s radios start humming before the valves have warmed up. This, though, is not "radiation" but simple local induction.
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