[kyrill]

WHat is the sonic effect when a 20 watt tube amp (PP triode) only has to output between 0,01- 0.1 watt?

I have a modded Jadis amp which i belief may even sound better than a pure SET because of its mods done in Holland  and in Australia , but can i use it for an extreme 108 db sensitivity driver?
I dont want to sell the Jadis, it is too beautiful. One its mods effects is: Induced Permeability for the output transfomers. http://www.customanalogue.com/jlti_el34.htm

[JoshK]

No idea.  Have you considered the linearity of the operating points you've chosen?  That matters more than a lot of other things, I think.

The Japanese have been choosing more conservative operating points for tubes for decades and it works.  They like to run 300B's for instance at 5-6 watts instead of 8 and the op.point is more linear there.  But this isn't changing it to a fractional watt.

It sounds like you are doing something to the output iron.  Are you running an offset current through it like Dave Slagle likes to do with his SE OPTs?

[JoshK]

Sorry.... I think I misunderstood your question.  You are asking what does a 20w amp sound like at a fraction of a watt?    I thought you meant to modify your amp to only put out a fraction of a watt.

Have you seen those distortion curves of amp that the rags like to publish.  Usually a SS amp is a bit concave in that distortion goes down for up until a couple watts then it starts to climb until its rated power.  IIRC, that has to do with NFB.   Depending on how your amp is operated, you may or may not have negative FB.  THD lines for non-NFB amps usually are more monotonic, so distortion is really really low at a fraction of a watt.   Remove the NFB if you have it and put the tubes in triode mode.

[Niteshade]

Wouldn't the limitations of a fractional watt amplifier outweigh even the most positive attributes of such a project?

[kyrill]

Hi Josh,
yes i have a 22 watt triode PP tube amp with no overall FB ( like in SET)
but I want to know if i can use it for a 108 dB sensitive mid/tweeter ( 60 watt max)

Blair, i find yr sentence cryptic, what do you mean?

: )
K

[JoshK]

Yes you can use that amp.  It will be cruisin' easy and the distortion will be really low.

[kyrill]

Nice

It is this horn driver that took my attention here on AudioCircles http://www.prosoundservice.com/Product_Details.html?m9:item=B%26C-DE250 as it has no metallic diaphragm and from several sources I read how musical it is

as a side question Josh, what happens if you use such a sensitive driver but without horn? do you have to equalise it for a flat response?

[Niteshade]

I do not see a reason to build an amplifier that has such a low output power. It's great to use power tubes at a fraction of their rated power, but there's no reason to carry it to such extremes. I like at least a couple of watts even with 100db efficient speakers. The extra headroom sounds great.

[Steve]

WHat is the sonic effect when a 20 watt tube amp (PP triode) only has to output between 0,01- 0.1 watt?

I have a modded Jadis amp which i belief may even sound better than a pure SET because of its mods done in Holland  and in Australia , but can i use it for an extreme 108 db sensitivity driver?
I dont want to sell the Jadis, it is too beautiful. One its mods effects is: Induced Permeability for the output transfomers. http://www.customanalogue.com/jlti_el34.htm

Hi Kyrill,

At that low power, .01 watts, both tubes are run class A (each tube conducts over the entire 360 degree cycle), even though the amp may be listed as class AB. It may not be ideally biased as in a pure class A amp but I don't think any problem. May not have much rotation on the volume control if that is a concern.

Hope this helps and happy listening.
Steve

[kyrill]

ah nice : )
thx ppl

[jb]

WHat is the sonic effect when a 20 watt tube amp (PP triode) only has to output between 0,01- 0.1 watt?

The issue is not maximum power output but overall gain. I have 110 dB horns that I drive with a 20-watt, reduced-gain SET. At normal listening levels, the volume knob it at about 1 o'clock. The combination plays effortlessly. I also have a 1.5-watt SET that plays just as loud as the big amp but not as effortlessly. The big amp has both more finesse and greater authority.

I have a modded Jadis amp which i belief may even sound better than a pure SET because of its mods done in Holland  and in Australia...

I doubt that. Not with 108 dB speakers. All push-pull amps suffer from crossover distortion that comes from splitting the single-ended input signal into a complementary pair and from when the pushing half hands off to the pulling half and vice versa. The crossover distortion is insignificant at full output, but it will be more apparent at lower levels. When the smallest musical details call for microwatt output levels, the S/N will suffer. That’s why SETs and horns are the ideal combination.

[JoshK]

Crossover distortion happens at full power, not low power.  In low power, one side isn't being cut off, both are in class A, so there is no crossover distortion.

[jb]

Crossover distortion happens at full power, not low power.  In low power, one side isn't being cut off, both are in class A, so there is no crossover distortion.

As I said, crossover distortion comes from two sources: One being the push-pull handoff and the other being the splitting of the input signal. The latter happens at all signal levels. If at normal listening levels both halves of the PP circuit are operating class A at miniscule levels, which is the case with high efficiency speakers, why bother with PP? The only benefit of PP is higher power. Why pay the price if you don’t need the power?

[jon_010101]

The major sources of increased distortion at low power in solid state amps are 1) noise and 2) crossover distortion.  High power solid state amps often have a lot of gain, so the system's noise floor may be more apparent.  If run class B, some will exhibit crossover distortion - which really isn't good, especially since it is entirely avoidable.  Most tube amps do not show small-signal crossover distortion, since the A->B transition might occur in the 1-to-a-few watts range, typically, for "bigger" amps.  They might show some noise, of course. 

Single-ended amps, unless grossly overpowered, will have higher distortion even at low power than equivalent push-pull amplifiers - the difference is a bit of 2nd order distortion vs. a smaller bit of 3rd order.  Personally - I prefer the push-pull sound (unless we're talking "big" single ended amps, Ongaku, etc., driving efficient speakers, where distortion can remain acceptably low even at listening levels).

If you only need a small amount of power, one thing that can be done to improve a push-pull amp's performance is reducing its gain.  For example, the Quicksilver Horn Mono amps have incredible noise performance and very low distortion due to very low gain and a bit of feedback.  They also take quite a bit of input voltage to drive to clipping... but with efficient speakers they should never need to.

Crossover distortion is not a product of splitting the phase (barring a few strange exceptions) ... it is due to turning off half of the output stage as it transitions to class B.  This affects solid state amps at very low power (since it is possible to achieve class B easily, vs AB), and class AB tube amps typically in the 1-to-a-few watt range.  Class A push pull amplifiers do not have this problem, since the output devices never get shut off at large signal 

FWIW, in typical listening, I don't think I ever exceed 1W output on various 10-60W push-pull amps I own.  My average listening is in the 0.01-0.1W range.  My amps that tend to sound best under these conditions, however, are class A (push-pull) - which allows that the amp's nonlinear distortion (minus noise) is approximately minimized at small signal. 

[JoshK]

...and the other being the splitting of the input signal. The latter happens at all signal levels.

That is not crossover distortion, it is imbalance in phase splitting that leads to 2nd order distortion.  If the order of this distortion is small it actually leads to a balancing out of the harmonic spectrum of distortion, since the PP amp cancels out most of the 2nd, leaving mostly 3rd.

[Steve]

...and the other being the splitting of the input signal. The latter happens at all signal levels.

That is not crossover distortion, it is imbalance in phase splitting that leads to 2nd order distortion.  If the order of this distortion is small it actually leads to a balancing out of the harmonic spectrum of distortion, since the PP amp cancels out most of the 2nd, leaving mostly 3rd.

To back up Josh's viewpoint, there is no crossover distortion in any stages operated in class A operation, just imbalances when using pp. With that said, the phase splitter must be designed with extreme care or there will be loss of musical accuracy.

Hope this helps.

[kyrill]

if not with extreme care i assume, we do have a kind of "distortion" which triode configs dont have

why is there not a name for this kind of distortion"?

[JoshK]

Just an FYI, push pull and triodes are not mutually exclusive.  One can easily use triodes in a push pull stage, do strictly class A, or both.  One can even do push pull pentodes in strictly class A.  It is just not often done, due to people wanting more and more power.