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Old 22nd October 2002, 09:09 AM   #11
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Hi Bas,

Not to worry then.
Just an audio misunderstanding.Happens.

Cheers,

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Old 22nd October 2002, 09:32 AM   #12
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What amps have you got? If you don't mind me asking?

Cheers,
Bas
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Old 22nd October 2002, 09:48 AM   #13
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Hey Bas,

Me, I am using my Croft OTL MKIII powerhouses.
These are two monoblock designs using 10 6080 for the output,
they can take the more powerful 6336A's as well but they sound too sweet to my taste then (probably a tad too much Miller effect.
Everything is hardwired with silver solid core and all coupling caps are MIT polystyrene and tin foil.
Critical resistors are replaced with Holco H2 and a Vishay bulkfoil in the feedback loop.
A pair of 12BH7A White CF's are driving those.
Phase splitting is done with an ECF802 and the input is a trioded EF86.
All HT rails are regulated with valves (shunt reg.) but not the output stage.
They date from 1987 but still beat a lot of the competion hands down.
Very fast and the go up to 250 KHz without any problem.

Another one I use is a parallel 2A3 SE Intergrated amp I occasionally use on another set of smaller speakers for DVD and such.

Greetz,
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Old 22nd October 2002, 10:24 AM   #14
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Default Mmmm...sounds good..

You seem quite knowlegdeable on tubes though...Why buy something like the Croft when you can build something?
(Although I have to admit..something like a big OTL would be quite hard to build compared to a simple SET project..)

I suppose in '87 parts etc were a lot harder to come by...plus no internet etc...
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Old 22nd October 2002, 10:39 AM   #15
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Bas,

These amps were actually part of my company's (now defunct) demo material.
At that time I wasn't all that knowledgeable so I had to learn it the hard way.
I can honnestly say those where the greatest days of my life.
Later the company concentrated on distributing electron tubes worldwide (98% export) and consulting.
The only hard part in designing an OTL amp is the phase splitter since the output stage is inherently unbalanced.
All the stuff I designed was always for other companies so I never got around to building much for myself.
I did design quite a few OTL amps for some private customers,all of them still use them today.
That's very gratifying I think.

Cheers,

P.S.Getting good quality parts wasn't really that hard,just ordered them by fax.What was harder to do was sourcing the parts.
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Old 22nd October 2002, 10:57 AM   #16
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Hi Frank,
Interesting p(a/o)st...good to know..if I ever do something OTL I'll know where to post :-)
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Old 22nd October 2002, 11:16 PM   #17
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Bas,

Something I spot on your second pic intrigues me.
On that pic I see what I reckon are the bypass caps for the elco's.
(somewhere in the middle of the chassis)
Did you wrap copperfoil around them or am I wrong?
If so,what for?

Regards,
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Old 23rd October 2002, 07:31 AM   #18
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Hi Frank,

It is indeed copper foil..I read an interesting article on the site of Aiken amps saying that the outer foil of a capacitor should be connected to the lower potential (earth)...providing EMI protection.

Because nobody could tell me where the outer foil was on the Audyn caps...I wrapped copper foil around it and connected that to earth.

The copper foil is by 3m and provides shielding effectiveness (far field) is in the range of 60dB to 80dB (30 MHz to 1 GHz).

Funny thing is....i don't know how much is really helps in my application...but I just did it anyway...


It is also used as electrostatic shielding between transformer windings.
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Old 23rd October 2002, 10:28 AM   #19
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Default SHIELDING.

Hello Bas,

A quote from that article:

"Proper orientation of the capacitors will make the amplifier much less susceptible to outside noise, including hum, interference from fluorescent lighting, and tendency towards oscillations or frequency-response peaks and dips due to unwanted feedback from nearby signals within the amplifier."

Assuming the Audyn caps are constucted the same way as most other axial filmcaps the earthing point should be constistently at the input,i.e. the A of Audyn.
I stress that that is where it should be,field tests will show that not all manufacturers bother.
If you would peel away the plastic foil with brandname and specs
you may have found that band.
Sometimes you can actually feel it without peeling away anything.

If you have the necessary test equipment to run the test as described by Aiken it would surely tell the tale.
Building as they describe will provide a bit of extra immunity to RF
and EMI induced garbage.
Especially true in case of your design since it's in virtual "cabrio" mode.
That copper tape is relatively expensive too,isn't it?

Give it a try if you like.

Ciao,
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Old 23rd October 2002, 10:44 AM   #20
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Mmm then I have them the wrong way around :-( ..I suppose with the copper tape on now...it would not be worth my while to change it around?

Yes the tape is relatively expensive 27 Euro for a roll of tape...
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