[Pete Schumacher]

Thought I'd post up a recent data set of what happens to the response of a speaker when a low impedance solid state amp drives it compared to a low feedback tube amp.







Here we see a Clayton 300W monoblock compared to a 300B SET on the same speaker.  The Clayton is in blue, the 300B in red.  The largest and most audible difference can be seen where the impedance of the speaker drops to the 4 Ohm range.  There is a very noticeable drop in output from the 300B in that range which will be heard as a thinning of the sound, with vocals loosing a bit of "body."

This altering of the frequency response can be alleviated with some impedance conjugate filtering applied to speaker to make the impedance a much flatter line across the band.  Most speakers do not do such things and will show similar swings in their impedance.

This is why negative feedback is a very useful feature in amplifiers as it greatly reduces the amplifier's susceptibility to variances in speaker impedance.  It can also explain why some refer to system "synergy" between speakers and tube amps and is something to really pay attention to when choosing your final setup.

[RDavidson]

The Claytons are class A. Do they use any negative feedback?

Synergy applies to speakers and amps of all types, not just tubes.

[G Georgopoulos]

The Claytons are class A. Do they use any negative feedback?

Synergy applies to speakers and amps of all types, not just tubes.

class b/ab is even better at this than class a
why,part of the o/p impendance of class a
feeds also the ground return...

[Pete Schumacher]

The Claytons are class A. Do they use any negative feedback?

Synergy applies to speakers and amps of all types, not just tubes.

I don't have schematics for the Clayton, but I would be shocked if they weren't using negative feedback.

[Pete Schumacher]

class b/ab is even better at this than class a
why,part of the o/p impendance of class a
feeds also the ground return...

You can take a class AB amp, ramp up the bias and make it class A.

I'm not sure what you mean by part of the output impedance of class A feeding the ground return unless you're referring to single ended class A.  Class A can be push-pull as well as single ended.

[G Georgopoulos]

You can take a class AB amp, ramp up the bias and make it class A.

I'm not sure what you mean by part of the output impedance of class A feeding the ground return unless you're referring to single ended class A.  Class A can be push-pull as well as single ended.

c'mon pete you know what i mean!...

[fex02]

Pete- great thread !
feel free to share more of your knowledge!

[RDavidson]

More info is needed on the Claytons to fully understand the comparison......Actually more info is needed on both amps.
The tube amp is obviously single ended.
I would think that for more of an apples to apples comparison, the Claytons would also be single ended, which I'm guessing is part of the basis behind the comparison. Or was it just comparing an example of high power SS Class A vs "high" power tube Class A?
True, both amps are very likely using at least a little bit of negative feedback for the sake of controlling distortion, but this aspect alone still doesn't make it an apples to apples comparison, to draw a tubes vs SS conclusion with regard to speaker impedance. A more meaningful comparison would be to compare a single ended tube amp and a single ended SS amp with similar output and similar distortion characteristics.

[Pete Schumacher]

More info is needed on the Claytons to fully understand the comparison......Actually more info is needed on both amps.
The tube amp is obviously single ended.
I would think that for more of an apples to apples comparison, the Claytons would also be single ended, which I'm guessing is part of the basis behind the comparison. Or was it just comparing an example of high power SS Class A vs "high" power tube Class A?
True, both amps are very likely using at least a little bit of negative feedback for the sake of controlling distortion, but this aspect alone still doesn't make it an apples to apples comparison, to draw a tubes vs SS conclusion with regard to speaker impedance. A more meaningful comparison would be to compare a single ended tube amp and a single ended SS amp with similar output and similar distortion characteristics.

There's enough info in the graphs to draw some conclusions and it's based on the fact that the speaker does not present a constant impedance.  Because of that fact, these two amps produce quite different response plots using the same speaker as their load.  This wasn't posted to show that one amp is better than the other or to slam the obviously lower feedback SET compared to the SS amp.  Only that these things need to be taken into account when listening to speakers with different amps/topologies so that one can identify the sources for differences in sound which in this case would be an obvious change in response.

[RDavidson]

I understand. It was just a general test.
I just think the test would've been more meaningful / multi-dimensional if the two amps were similar, even though it wasn't necessary to make the very good point about impedance.

[konut]

What are the output impedances of each amplifier?

[BobRex]

Keep in mind that the 300B amp has an output transformer, while the SS amp doesn't.  While, yes, the transformer (or lack thereof) affects the output impedance, there's more to it than that.  I would think that an autoformer MAC would act the same way as the 300B, no? 

[paul79]

depends on the output transformer quality also

[Folsom]

Keep in mind that the 300B amp has an output transformer, while the SS amp doesn't.  While, yes, the transformer (or lack thereof) affects the output impedance, there's more to it than that.  I would think that an autoformer MAC would act the same way as the 300B, no?

It's actually about the amplifiers ability to sink enough current. Even when the amplifier sees 8ohm or whatever, if the other side of that transformer is harder to drive in a given frequency range it'll take more power. A difference might be if it's easier to produce more power at a given load impedance, to the amplifiers advantage.

Tubes are generally low current devices, but with transformers they may handle reactive loads a bit better, otherwise you might not even consider them on very reactive or non-sensitive speakers. They are probably much less susceptible to oscillations that can occur from reactive loads, which also eat current, as well.

[mcgsxr]

As a guy for whom much of the science goes over my head, it is a cool comparison, and does make for some interesting reading.

I own 4 ohm Totem Model 1's, and they have a reputation for being fairly hard to drive.

I have recently added a different, more powerful amplifier, and I am happier with the new pairing.

Thanks for the post!

[Pete Schumacher]

Keep in mind that the 300B amp has an output transformer, while the SS amp doesn't.  While, yes, the transformer (or lack thereof) affects the output impedance, there's more to it than that.  I would think that an autoformer MAC would act the same way as the 300B, no?

I've used some 805 based SET amplifiers whose feedback is taken from the output putting the transformer in the loop.  In that case it does lower the output impedance and makes them less susceptible to changes in load impedance over frequency.

[G Georgopoulos]

You can take a class AB amp, ramp up the bias and make it class A.

I'm not sure what you mean by part of the output impedance of class A feeding the ground return unless you're referring to single ended class A.  Class A can be push-pull as well as single ended.

to give you an idea of a class-a damping factor

df= RL || (Vout / Ibleed) / Zout

cheers 

[Pete Schumacher]

to give you an idea of a class-a damping factor

df= RL || (Vout / Ibleed) / Zout

cheers 

That doesn't factor in any feedback effects.

[G Georgopoulos]

That doesn't factor in any feedback effects.

ha 
Zout is already nfb factored
can you now see my point???

if you got a better formula let us see it 

one thing i'll tell you vapor speakers need alot of
silicon to sing not glass!!!!

[Pete Schumacher]

ha 
Zout is already nfb factored
can you now see my point???

if you got a better formula let us see it 

one thing i'll tell you vapor speakers need alot of
silicon to sing not glass!!!!

You obviously haven't heard Nimbus, Derecho, Arcus, or Perfect Storm George, or you wouldn't have made that last statement.   

We had Nimbus running on a 3.5W 2A3 SET and a 35W 805AT SET at RMAF a couple years ago and it filled a very large conference room without a problem.  And at Axpona 2014, we had Derecho running on 18W SET and were easily hitting 100dB in the room.

https://www.youtube.com/watch?v=li-NBI6kP2I

Your formula is of the form R || R/R.  R/R has no units, so I'm not sure it's completely correct.

Damping factor is simply the ratio of Zout and RL and it doesn't matter what class you're running.