Theremin amps

Posted: 7/17/2008 2:01:07 PM

From: Escondido, CA

Joined: 2/6/2008

I started with the following requirements:

1. Tube amp (well, the preamp section at least!)
2. 3 band EQ
3. Effects loop
4. Record or line output
5. Small power amp.

All of these elements are things that I have built in fairly recent times.

The tube preamp adds "warmth" (translation it has more even order harmonics ... thus adds a bit of its own character).

Since most theremins are line-level outputs, the tube stage will be a common-cathode stage rather than an amp.

Since we have this perfectly good tube extra section available, there's a guitar input that does have gain.

3 band EQ: I have a very nice EQ section I have designed that I use in my acoustic amp and guitar amps. The "middle" control has cut/boost adjustment and frequency adjustment. I can adapt a PCB I already have to use for this for the prototype.

Effects loop: Just a nice thing to have.

Small power amp: I use a TDA2040 in a headphone amp I build. It drives speakers quite nicely (at the expensive instantaneously draining the battery to nothing). So given an adequate AC-powered supply it will work well as the output stage. Most of those $60.00 guitar amps (as well as a few more expensive ones) use this chip or something very much like it! (I know that from various repairs of these amps.)

With the new transformer I should be able to get about 20W out of this design.

Thus the amp will have just about all the things people want for home use and the ability to run a connection to a larger PA system for playing at gigs.

Speaker: Just have to have a diamond-shaped speaker! So I have in mind to mount the amp at about waist height with a diamond speaker above it on an extension of the stand.

Posted: 7/17/2008 10:50:58 PM

From: Atlanta, Georgia

Joined: 4/24/2008

Common-cathode? Do you mean common-plate (a.k.a. cathode-follower)? Normally-biased common-cathode stages will afford voltage gain.
Posted: 7/18/2008 12:32:00 PM

From: Escondido, CA

Joined: 2/6/2008

Sorry ... yes cathode-follower (common plate).

Plate circuit has no series resistor, output taken off the cathode-bias resistor.

Posted: 7/18/2008 9:43:28 PM

From: Toledo, Ohio United States of America

Joined: 2/22/2006

And, what would one of these custom Amps cost a lesser being such as me, without tube electronics experience? I like the desciption of your design. Please, advise.

Good luck!

Posted: 7/22/2008 4:33:07 PM

From: Escondido, CA

Joined: 2/6/2008

Thanks for the interest Teslatheremin!

I haven't added up the Bill of Materials cost, but that usually isn't the expensive part, except for the speaker and power transformer.

Metalwork remains the big ticket item if you are doing production work. Not that I plan to do so.

After I build a prototype I would be willing to entertain building a few.

At the moment I have a layout for the power amp section, an already-assembled EQ board and am working on the layout for the tube section.


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