Anybody have experience building an OTL amp?

[jduffy]

I've been reading about them and they seem interesting. Wondering what people think about building them and the sound.

Thanks.

[poynton]

Not yet..... but as soon as I get off this boat, I'm going to be playing around with a couple of ideas from another thread I started.

Andy

[Alastair E]

There are a lot of very experienced guys here that have made OTL amps. I have made a Circlotron type OTL using the 6C33C-b output tubes. It sounds (At least to my ear) one of the best amps Ive heard...

Some OTL builders prefer the SRPP type amps as the power-supply is much less complicated than the Circlotron with its two separate floating supplies.....

Im sure TubeTVR (Hans) will be along soon, He has great experience and has a website describing his OTL work.

[woody]

Alastair,
In the circlitron amp of yours what did this floating power supply
look like? Could you post the circuite?

[Wavebourn]

I experimented, was dissatisfied, later increased voltages/decreased current and added an impedance matching transformer, after that used MOSFET source follower loaded by current source and got much better result, after that loaded SS voltage follower by a SS voltage to current convertor and concluded that it is the best sounding amp I ever made.

[Nixie]

How about a switch-mode output transformer, Berning style? The transformer then works at high frequencies with its nonlinearities out of the audio band (and is cheap, just a mid-sized potted-core ferrite). Reviewers seem to love Berning's amps, and I believe smoking_amp from this forum has built a version. This is different from a class-D amp, since you still have a class A tube amp doing the amplification, whereas the switcher simply does impedance conversion and when properly designed, has significantly less impact on the sonics than an output transformer. It really seems the best of both worlds here. Too bad you can't sell it afterwards as it's patented.

[tubetvr]

Hi,

As Alastair mentioned I have built OTL amps based on what some call the "inverted Futterman" output stage, different output configurations for OTLs are mentioned here http://members.aol.com/aria3/otlpaper/otlhist.htm

With the inverted Futterman configuration you get reasonably low output impedance even with few tubes, with 2 6C33C tubes you can easily get 25W in 8 ohm and the ouutput impedance before feedback is then around 11 ohm which can be lowered to very low values with feedback.

Some difficulties when building OTL amps:

Finding good caps for the output stage, they should be of excellent quality with very low ESR, the cost of these caps is often a substantial part of the total for the whole amp

The difficulties mutiply in increasing order when you try building higher power amps, tubes connected in parallell need to be derated in order to get reasonable life time, (I think lack of proper derating is one reason why many commercial OTL amps have had reliability problems).

If you need more than a couple of watts dont even think about running the amp in class A, there are no tubes available that can give more than a few watt in class A and even then the amp will dissipate waste amounts of heat. Class AB1 works very well and there is then no need to run the tubes above max ratings.

With regard to the sound of an OTL amp it is unfortunate that some have had bad experience when trying to build one. I have heard numerous amount of different amplifiers and I can't say that I have heard any that is better than a well designed OTL amp.

My current amp can be seen here http://www.tubetvr.com/otl.html I have built 4 of these and no one have had any reliability problems, my own amp has been working now more than 6 years with only one tube failure, (one noisy driver tube due to decreased isolation between cathode and heater).

Regards Hans

[Nixie]

US patent 5,612,646.

Use pat2pdf.org instead of the patent office website.

[jduffy]

Thanks for all the great info you guys. I'm going to keep studying up on them the next few months.

Has anyone here built from scratch one of the Transcendent Sound OTLs?