Speaker Suggestions for 300b SET Amp

Both the tweeter and midrange compression drivers incorporate Kilpsch’s best- effort phasing plugs to maintain coherent wave fronts, and both have been updated to titanium diaphragms for extended response and lower distortion

The phase plugs are integral to coupling in the throat of the speaker.

Having researched this issue myself now for quite a while, if I were running a SET and looking for good sounding HE speakers to match on a budget, that can play ANY kind of music well, Klipsch Heresy III, hands down, easy decision. These were originally designed for flea powered tubes amps of the day ~ 50 years ago and are still around (track record) and the latest versions I’ve heard sound wonderful, none of the bad things one often hears from audiophiles about high efficiency horns apply. See the various online reviews for more evidence of this.

The throat and overall shape of the horn is essential to it working properly; these days computer modeling should have sorted out any anomalies that caused by past errors in this part of the design. Thus modern horns should be as smooth and revealing as any other speaker technology. 

If 7 watts won’t do it, you’ll need a lot more power due to the logarithmic characteristic of the human ear.

The hotter tweeters were the result of simply pushing the amps too hard.

The fatal weakness of so called circutron otl push pull power amp design is that the power grounding have to go through both legs‘ cathodes, thus power path mingle with the signal path, introducing distortion and noises.
Furthermore, after some experiment,  i found circutron push pull otl amps have very low damping factor, it produced worst sound when driving my quad 57 among all otl design amps.
Atmosphere uses circutron design, so it has very high distortion and is terrible for driving difficult loads such as quad 57s.
Transform coupled tube amplifiers on the other hand uses inductance to their advantages when driving difficult loads.

Obviously this person has not used our amps with Quads. We have a lot of Quad customers. The opening statement is false; you can literally run the amp with no filter caps and it will sound pretty decent. So the first paragraph is outright false.

The damping factor has entirely to do with the number and type of tubes involved, how they are driven, the power supply voltages and bias points, and finally how much negative feedback is employed, if any. We don't use any, and this resulting in an output impedance that is 1/2 that of other OTL output circuits running open loop. I regard the 2nd paragraph as highly misleading as it is apparent that whatever amp he used (if indeed he did use an OTL at all), it was not one of ours.

Our customers with Quads say its a match made in heaven. The 3rd paragraph is false but may be based on the premise of a homemade amp of some sort.

We get about 0.5% THD at full power with most of our designs- a bit lower with the larger amps, with about 0.04% or less IMD; not bad for zero feedback! Care must be taken to not accidentally ground the speaker terminals via the test equipment, else the distortion skyrockets. This is a **very** easy mistake to make. If byang12 really did try to measure the distortion of a circlotron OTL, its very likely he made this mistake. I've seen several respected magazines do it as well and I've done it myself.

I am sick and tired of someone trying to promote his own amp by demoting other type if amps, specially singleended transform coupled low power high quality amps.
Eg.  Transformless push pull amps produces extream high odd order distortions and extreamly sensitive to power supply quality,while he declaires single ended amp has high ODD order distortion,which is false statement at best.

I have to chime in on this one, just to set the record straight. The text after 'Eg.' is false.

Transformerless push-pull amps (of which our are an example) do not produce 'extreme odd ordered harmonics', and in our case are actually less sensitive to power supply quality as opposed to single-ended circuits. This fact about balanced circuits is well known. This is easy to demonstrate- we can actually run one of our amps without any filter caps in the power supply of the output section and it will actually work fairly well. The topology does cancel even orders due to cancellation in the output (in our case, the cancellation occurs throughout the amp).

Because our amps are fully differential, like any fully differential amp, the primary distortion component will be the 3rd harmonic.

With single-ended circuits (tube or solid state) the primary distortion will be the 2nd harmonic.

The 2nd, 3rd and 4th harmonics are considered musical by the human ear and contribute to 'warmth', 'fulllness', 'bloom' and so on. The higher ordered (5th and above) do not. The 7th contributes to a metallic quality. The ear is far less sensitive to the lower orders. so if you are pragmatic, you design the amp so that higher ordered harmonic generation is avoided, while doing what you can to suppress the lower orders, as long as that does not hurt that lack of higher ordered content.

Our amps do generate higher ordered harmonics like any other amp does, but at a level that tends to be considerably lower than in a single-ended circuit. We suppressed the higher orders by simply keeping the signal path simple (one gain stage) and by separating the power supplies to avoid intermodulations. The idea was to get low distortion without feedback; at full power one of our amps properly set up will make about 0.5% THD, while IMD (which is highly audible) is less than 0.02% at full power. That's not bad for a zero feedback amp. 

What I have said about SETs making odd ordered content is true (although I didn't mention anything about that on this thread), but I did not say that this is **all** they do...  I've often pointed out that the distortion of SETs is primarily the 2nd harmonic, and that if you use them at low power their distortion decreases to unmeasurable (and also inaudible).

This is where the 'inner detail' of SETs derives as distortion can mask detail (and many push-pull amps actually have increased distortion at lower power levels). For this reason, you generally want a speaker for an SET that is efficient enough that the amp never makes over about 20-25% of full power; in that way you avoid the odd ordered harmonics that will show up at power levels above that (as well as higher ordered even harmonics, both of which are used by the ear to sense sound pressure). This is to maximize transparency.

When that guideline is violated and the speaker is of insufficient efficiency, the SET will appear to be very 'dynamic'. But its a false dynamic, simply occuring out of how the distortion interacts with our physiology, and transparency will suffer.

I prefer that any audio system sound like music, and that is how you go about it with SETs. If the speaker lacks the efficiency, the amp will fall right flat on its face. I have this idea that you want to honor your amplifier investment dollar as best as possible so the amp you bought can strut its stuff. I do apologize if anyone thinks that this sort of advice is intended to denigrate SETs, its not. Its just statement of fact so you can get the best out of the amp. 

@wolf_garcia

When you drive any single-ended amp hard, it starts to make odd ordered distortion. That distortion tends to be on the transients, and our ears use those harmonics to sense sound pressure. So the result is single-ended amps tend to sound very dynamic (REAL LOUD) but unless you actually have have high efficiency speakers (and I think a Heresy qualifies) if you measure the sound pressure you will see its really not all that loud. It just *sounds* that way due to the distortion.

Most 300b amps are good for about 7 watts.

The distortion goes up and begins to include the odd ordered harmonics at powers over about 20-25%. So you might have about 2-3 watts of really usable power- over that, you don't really get to experience what the amp is all about (despite many remonstrations you may hear to the contrary; keep in mind that many people with SETs often put them on speakers that lack the efficiency needed; this is **the most** common mistake made with SETs)!

So I would not consider any loudspeaker that does not involve either a high efficiency full range driver (like a Lowther or Feasterex) or horns. The exception would be if you are in a very small room or only doing near field listening. In most average rooms, you will want a speaker that is **at least** 98 db 1watt/1 meter.