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The good, the bad, and the ugly OTL amplifiers
The good, the bad, and the ugly OTL amplifiers
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Old 5th November 2006, 11:53 AM   #21
Burnedfingers is offline Burnedfingers  United States
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Quote from the Transcendent audio site for the purpose of looking at the feedback aspect of the OTL amplifier.


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Let’s look at the Transcendent T8 as an example. It uses 8, EL509 tubes in push-pull. Each 509 has a plate impedance of about 150 ohms. 150 ÷8 = 18.75 ohms. This is unacceptable. Negative feedback is employed to achieve proper performance. The amplifier uses 33 dB of negative feedback which reduces the output impedance to 0.4 ohm thereby achieving outstanding woofer control and the ability to drive 4 ohm speakers. This specification was verified by Stereophile when they reviewed the amplifier.
Note: the 6AS7 has a plate impedance of 280 ohms so if my math is correct 280/20 sections is 14 ohms which is further reduced by the negative feedback that is applied. The feedback is 23db.

So, in your opinion is this amplifier capable of driving a 4ohm load that I would encounter with my speakers in reproducing low frequency?
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Old 5th November 2006, 11:58 AM   #22
Burnedfingers is offline Burnedfingers  United States
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Quote from the Transcendent audio site for the purpose of looking at the feedback aspect of the OTL amplifier.


Quote:
Let’s look at the Transcendent T8 as an example. It uses 8, EL509 tubes in push-pull. Each 509 has a plate impedance of about 150 ohms. 150 ÷8 = 18.75 ohms. This is unacceptable. Negative feedback is employed to achieve proper performance. The amplifier uses 33 dB of negative feedback which reduces the output impedance to 0.4 ohm thereby achieving outstanding woofer control and the ability to drive 4 ohm speakers. This specification was verified by Stereophile when they reviewed the amplifier.


Quote:
Different tubes will provide similar results. The configuration of the OTL output stage does not matter whether it is series connected push-pull or balanced. The controlling factors here are the number of output devices and the plate impedance of the devices. A 6AS7G is a dual triode with a plate impedance of 280 ohms. An amplifier that uses 8 such tubes will have an output impedance of 280 ÷ 16 = 17.5 ohms. A fourteen tube amplifier will have an output impedance of 10 ohms.
The amplifier I am considering has 10 6AS7GA's in it per channel thus reducing the output impedance to 14 ohms. This coupled with 23db of negative feedback should allow the amplifier to drive low frequency without a problem in my opinion.
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Old 5th November 2006, 01:15 PM   #23
Shoog is offline Shoog
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Just an opinion,
I would speculate that most of the sonic advantages of a OTL amp can be achieved by transformer coupling a few 6AS7's into a quality toroidal output transformer. Run the 6AS7's at high current and low voltage. This type of setup gets around the inherent limitations of an output transformer, because high current low voltage has a much lower stepping ratio than a normal output tube which drastically improves the frequency response. Of course you will be running PP.

This is a very rarely tried configuration (because suitable OT are rare and 6AS7's are not liked by any but the OTL crowd) so most people cannot express an honest opinion of its sonic merits.
Running an OTL amp just doesn't make sense on so many levels, when there are good alternatives out there.

Shoog
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Old 5th November 2006, 02:31 PM   #24
Spasticteapot is offline Spasticteapot  United States
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Quote:
Originally posted by Shoog
Just an opinion,
I would speculate that most of the sonic advantages of a OTL amp can be achieved by transformer coupling a few 6AS7's into a quality toroidal output transformer. Run the 6AS7's at high current and low voltage. This type of setup gets around the inherent limitations of an output transformer, because high current low voltage has a much lower stepping ratio than a normal output tube which drastically improves the frequency response. Of course you will be running PP.

This is a very rarely tried configuration (because suitable OT are rare and 6AS7's are not liked by any but the OTL crowd) so most people cannot express an honest opinion of its sonic merits.
Running an OTL amp just doesn't make sense on so many levels, when there are good alternatives out there.

Shoog

I disagree. I'm a big fan of OTL amps, and own several.

(Of course, they're all solid-state.)

I would wonder if inexpensive tubes (availible by the truckload on eBay) could be substituted. With 32 tubes/channel, you'd need a huge power transformer, but think of the output power!
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Old 5th November 2006, 04:36 PM   #25
coresta is offline coresta  France
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Hi Erik the best : look at
http://www.tubebuilder.com/images/sc.../6c33c-cir.gif

in fact , mine is different : 5814 // as input, twice 5687// as driver , and E186F as curent source . and 10 6080wa as output tubes !
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Old 5th November 2006, 04:53 PM   #26
SY is offline SY  United States
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The good, the bad, and the ugly OTL amplifiers
Quote:
6528 (not good in OTLs)
What's the issue with them?



bf, from those impedance numbers, I'd guess that these speakers are not good OTL candidates.
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"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 5th November 2006, 08:48 PM   #27
coresta is offline coresta  France
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Not linear as 6336A: The cut-off point is met very quickly. 6528 are perfect for SET .
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Old 5th November 2006, 09:09 PM   #28
Giaime is offline Giaime  Italy
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Quote:
Originally posted by burnedfingers


Gamie,

Have you ever heard Cornwalls? Anyway they are Cornwall in size and that is about it. They are a highly developed design by DJK that totally kicks the Klipsch to the curb. They test almost ruler flat also. I refer to them as a Cornwall clone because they look almost the same. In terms of performance they are so much smoother and higher efficiency.

I'm interested in your opinion don't get me wrong. Have you heard any OTL amplifiers in which you could lend a opinion based on this information?
My name is Giaime, and I was referring to original Klipsch Cornwall speakers. And I usually don't talk about things I don't know or heard.
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Old 5th November 2006, 09:31 PM   #29
ErikdeBest is offline ErikdeBest  Switzerland
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The good, the bad, and the ugly OTL amplifiers
Hi Coresta

Thanks for the info! But I thought you did not deal with novals in your own projects? And the 5814, don't know, but being similar/substitute for the 12AU7/ECC82 they should not be that linear?

Erik
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Old 5th November 2006, 09:42 PM   #30
Burnedfingers is offline Burnedfingers  United States
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Quote:
My name is Giaime, and I was referring to original Klipsch Cornwall speakers. And I usually don't talk about things I don't know or heard.
Sorry about getting your name wrong.
Quote:
My name is Giaime,
Quote:
My name is Giaime
I'll try not to forget
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