OTL or parafeed?

[Jaime]

In the web he said this is an OTL amplifier.
SINGLE ENDED TUBE AMPLIFIER "TRSAT-20"
(FUNERAL BELL FOR NO OTL)
http://www.super-otl-audio.com/Description.htm

But, this is really an OTL?
It looks like a parafeed.

Best regards Jaime

[tubetvr]

What is described in the text is an ordinary cathode follower with a choke in the cathode circuit in order to reduce resistive losses. But it is difficult to know exactly what he is talking about as there as so many factual errors in the text...

Regards Hans

[dhaen]

The interesting thing is the use of an air-cored choke. I had considered winding air-cored transformers for a 6c33 amp, but with a single 6c33 the impedance is not low enough and the winding is massive.

[kevinkr]

Factual errors aside, the text is pretty hard to follow.

I will concede that his bias adjustment scheme is pretty clever in its simplicity.

I had a pretty bad experience with 6C33 in otl, described in detail here
www.kta-hifi.net under amplifier projects.

Considering the number of output tubes used both the damping factor and output power seem low. I guess if I were to attempt something similar I would use an autotransformer in place of that choke..

Kevin

[tubetvr]

As I have described many times before I have very good experience with using 6C33 in OTL circuits, however I wouldn't ever consider building a cathode follower as an output stage.

For me it seems that the the inverted Futterman type of coupling is the optimum solution for an OTL amp.

It has much lower output impedance than a cathode follower and of course not higher as is written in the article. It is also very easy to apply quite high amount of feedback while keeping high phase margin thereby achieving very low output impedance.

BTW, can anyone make sense of the text concerning amplifiers with and without feedback in the article? I can not, if any the described amplifier with adamping factor of 2 have very much higher output impedance than what is achievable using feedback.

Regards Hans

[Miles Prower]

BTW, can anyone make sense of the text concerning amplifiers with and without feedback in the article? I can not, if any the described amplifier with adamping factor of 2 have very much higher output impedance than what is achievable using feedback.

He's from Croatia, and it would seem that there's an "Engrish" problem here.

Measure-

ments show that typical loudspeaker nominally 8 ohms has variable impedance between 3 ohms and 30 ohms.

If the amplifier has voltage negative feedback, it can be considered that constant voltage generator drives the loudspeaker.

In reality it means 10dB power difference. To eliminate this variation of power, it was necessary to eliminate this negative feed-back

If you take 3.0 X V^2 and 30.0 X V^2, where V is a constant, the difference will be 10db, like he says. However, a "constant voltage generator" is a battery. While this could drive a loudspeaker, the result probably wouldn't be real interesting to listen to. He probably meant a constant current generator. Either way, NFB or no NFB, won't do anything for the change in impedance with signal magnitude and frequency that occurs with the speakers, especially since a big component of that is mechanical, not electronic.

I wonder what he does with that air coil. "Output tubes are fed with DC via voltage very massive coil. It is an air coil, with maximum linearity. Total load of the output". Very massive coil means lots of DC resistance ans stray capacitance. So how does he get the bandwidth he claims?

Output Power @ 8Ohms 20W

-Power Consumption 600W per channel

600W go in and 20W come out: that's an efficiency of 3.3%

Not very good for all those 6C33's.

[Giaime]

Originally posted by Miles Prower

600W go in and 20W come out: that's an efficiency of 3.3%

Not very good for all those 6C33's.

In fact it's easy to obtain more than 50W on 8ohms with just 4 6C33 per channel... that's Ciclotron!

see here http://www.audiocostruzioni.com/a_d/...otron-bebo.htm

[tubetvr]

In fact it's easy to obtain more than 50W on 8ohms with just 4 6C33 per channel... that's Ciclotron!

A circlotron is unnecessary complex and doesn't offer any advantages compared to an inverted Futterman circuit which also have lower output impedance.

It is quite easy to reach 80W or more with 4 6C33C tubes with very low output impedance and excellent linearity while still maintaining high reliability.

Regards Hans

[Giaime]

Originally posted by tubetvr


A circlotron is unnecessary complex and doesn't offer any advantages compared to an inverted Futterman circuit which also have lower output impedance.

It is quite easy to reach 80W or more with 4 6C33C tubes with very low output impedance and excellent linearity while still maintaining high reliability.

Regards Hans

Mmmm I see. Sorry, I wasn't saying that Ciclotron is best, because I'm not technical enought to know what are the difference between the two, I was just saying that everyone is obtaining more than 20W from 5 6C33

Could you point me at some simple links where OTL in general and Ciclotron/Futterman is explained?

[tubetvr]

Could you point me at some simple links where OTL in general and Ciclotron/Futterman is explained?

The best summary I have found describing different OTL circuits is here http://members.aol.com/aria3/otlpaper/otlhist.htm however I don't agree with their choice of a circlotron as the best circuit. Output impedance is very important in an OTL as it decides how many output tubes that will be necessary, an inverted Futterman will have about half of the output impedance compared to a Circlotron.

An advantage of the Circlotron is usually described as that it is balanced and therefore have low even order distortion, but that is true for all push-pull type amplifiers including the Futterman. That there are no real advantages together with the increased complexity is the reasons I prefer the inverted Futterman instead.

See here for a summary description of my OTL http://www.tubetvr.com/otl.html I get 25W from just 2 6C33C. I am working with a version that uses 4 6C33C and in that one I get 80W in 8 ohm with 2nd order distortion of only 0.2% without any global feedback, that is a proof of excellent output stage balance.

Regards Hans