Tube Power Amp Reviews

Jonathan Scull: The question I want to ask is, Just who do you think you are?

Bruce Rozenblit: [laughs]

Scull: Just kidding. But really, here you are claiming to have fixed all the problems in Futterman's OTL design.

Rozenblit: Yes, exactly.

Scull: Well, that takes balls...

Rozenblit: Yes.

Scull: What defining moment launched your audio building career?

Rozenblit: Well, years ago I picked up a little EICO tube amp, their version of the Dynaco Stereo 70. I'd never even heard tubes before, and it was just unbelievable.

Scull: You weren't even a tube guy?!

Rozenblit: Right, I'd never heard tubes before, and thought it was a ridiculous waste of time to even think about such old technology. But when I heard them for the first time, I said omiGod! And then Glass Audio magazine came along, and that had a lot to do with it too.

Scull: The question must be asked, Bruce: As fine an idea as your OTL circuit may be, why didn't anyone think of it before?

Rozenblit: I'm a degreed electrical engineer, okay? That means a hell of a lot. There aren't very many people in audio these days who can say that.

Scull: Well, perhaps having an EE degree isn't all upside. It also means that you've been indoctrinated, if you know what I mean.

Rozenblit: Well, it gave me the tools to learn. Futterman accomplished some wonderful things, and I have great respect for him. But there are probably a thousand guys who could have done what I did if they'd just applied themselves to it.

I'll tell you what I think—a lot of it was this incredible infatuation with the Futterman mystique. Like he was some kind of a God. And he did get some of it right. When you walk into a design project with that kind of blind allegiance to a cause, it's like those right-wing fanatics. You know, no matter what you tell them, they're going to believe what they want to believe. They're blinded to what's really going on. And I think that had a lot to do with it too. It's about open-mindedness. It's about being creative. It's about letting yourself go and trusting your intuition.

Scull: Well, your design obviously works...

Rozenblit: Right. I've got thousands of hours of field time on the amps, and nothing's ever happened. Nobody's ever damaged a speaker and none have blown up.

Scull: Since you're running in low-bias class-AB, what about crossover notch distortion? Isn't that why everybody pushes their designs into class-A—to avoid the zero crossing point?

Rozenblit: Yeah, that's what they say.

Scull: Well?

Rozenblit: Well, you have to take an opposite tack to get an OTL to work. That's one of the tenets of the design.

Scull: Don't get Zen on us, Bruce.

Rozenblit: [laughs] You have to approach this thing with a blank slate. What do I have to do to make it work? I knew I had to get the heat down because OTLs were just burning themselves up.

Scull: Are you saying that crossover distortion is not an issue at low power levels?

Rozenblit: Right—the only time you'll see crossover distortion is when you drive the amp into clipping [(footnote 1)]. But by then things aren't linear anyway, so what's the difference?

Scull: You use a cathode-follower topology in the amps...

Rozenblit: Yes, they provide a unity gain stage on the output. That's my outlook: I use whatever works. It's all in the implementation, how you string the different pieces together. That's engineering. A lot of guys won't use particular design elements. They'll say you can't use feedback, you can't do this, you can't do that. But I say, why not? If it works, use it. Because otherwise you're cutting yourself off from being creative.

Scull: Let's talk speakers for a moment. We had very good luck using the Joseph Audio RM-50s on the T8s. They don't drop below 7 ohms and have a relatively flat impedance curve. But I understand your amps will work into a variety of "normal" moving-coil speakers.

Rozenblit: Well, for the monoblocks, any 8 ohm speaker at about 86dB sensitivity or better is fine. And actually, any 4 ohm speaker of at least 89dB sensitivity or better is okay too. Just stay away from speakers that get down much below 4 ohms. In any case, the amp will make a solid 50W into 4 ohms.

Scull: Are most of your customers using moving-coils?

Rozenblit: I'd say about 80% have dynamic speakers.

Scull: What are some of speakers they use?

Rozenblit: Well, Von Schweikerts, Gallos, Merlins, E.S.P.s, and the big Sound Labs are popular.

Scull: Not bad. I understand you're coming out with a preamp?

Rozenblit: Yes, it's a new circuit. I won't do anything that's a me-too design. It's based on a grounded-grid gain stage. It's very simple and uses just three tubes. It's going to come out at $1395, and I hope to have it out by the Fall.

Scull: And you've written a tube book?

Rozenblit: Yes, it came out June 15th. It's called The Beginner's Guide to Tube Audio Design. It's for audiophiles and hobbyists who want to learn how tube circuits work, and even how to design their own. It's written in very simple, easy-to-read language. There are formulas in it, but it's all fully explained.

Scull: And where would that be available?

Rozenblit: It's being published by Audio Amateur Publications, and it's available through all the usual audiophile channels for $24.95. It's a small book, about 100 pages, so you don't get bogged down with a lot of hazarai. It's very straightforward.

Scull: So interested parties can call, fax, or e-mail to get your white paper on OTLs?

Rozenblit: Yup, people do it all the time. And I love talking to them. They like talking to the designer—it gives them confidence. And you know, a lot of them wind up buying!

Footnote 1: I may be missing something here. Classic crossover distortion, which occurs when one set of output devices in a class-B design hands over to the other, is constant and is therefore an increasing proportion of the signal as that signal reduces in level. It is thus at its smallest, expressed as a percentage, when the amplifier is on the verge of clipping.—John Atkinson