SET vs OTL

Paulfolbrecht, since you asked- in our amps there is only one stage of gain, not two. Most SETs I've seen have either two or three. This is because there is usually some gain in the output, while in our amps there is none.

Crossover distortion, FWIW, is not a function of the 'phase splitter', it is a function of the push-pull operation of the output section. A number of crossover issues stem from output transformers, and some of them stem from class of operation. In a class A push-pull amp, carefully designed, you will not get any crossover distortion at all.

You are right in that if there are crossover issues, they can often appear at low power only, masking low level detail. This is something that simply does not happen in our amps, and it is easy to hear how they have plenty of low level detail, actually more than SETs running at similar power levels do.

You are also right in that harmonic spectra is not the defining issue regarding sound of OTLs and SETs. I should point out here though that most OTLs employ negative feedback, often fairly large amounts, while hardly any SETs use feedback. Our OTLs use little (1 or 2db) or none, so in a way they tend to have more in common with SETs sonically than they they do with a lot of push-pull amps.

Larryi, just for the record, an OTL that has a direct-coupled output does not have to have either a coupling cap or a servo circuit. They can be built to be so stable that the DC Offset can always be very small and only need adjustment occasionally, once every 3-6 months.

The multiple power tube issue is shared by SETs with multiple tubes, Push-pull with multiple tubes and of course transistor amps with multiple output devices. In all cases, it is possible to build the amplifier so the multiple output devices are not a defining characteristic.

Atmasphere,

I agree that multiple output tubes is really only a theoretical source of less than perfect sound (my SET has parallel output tubes). I am merely pointing out that "theoretical" concerns can be raised about any design choice.

A friend of mine has a direct-coupled OTL that has performed flawlessly for years. I would worry about some kind of catastrophic failure, such as an output tube shorting. Can that happen and is it a realistic concern? Wouldn't a servo circuit be the best approach to maintaining good sound while assuring safety?

Thanks, your posting are very informative.

I'd strongly disagree with Paul's comments re: SET amps not sounding "lush" vs OTLs.

While my earlier comments re: the "romantic" sound of SET amps were primarily intended to describe 300B SETs (and more specifically the Cary monos I own), I find some elements of this "lush" quality in all of the SET amps I've heard. I would specifically include the Cary 805s I also own, and more specifically include the 805s when used with the 211 output tube.

I have never gone the "flea power" route, so I can't comment on those, but I've heard 300B based amps from several manufacturers and ALL of them sounded warm tonally, so it ain't just Cary. As to OTLs sounding "whitish", I've heard that, too. My early production S-30s could sound this way when paired with the wrong speaker.

Marty

Incidentally, nothing wrong in my book with "lush" sounding amps. That's precisely the reason I own two pairs of SETs.

Marty

Martykl, no properly built single-ended 45 amp is lush in any sense into appropriate speakers.

Have you heard any? Which do you call lush?