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Author Topic: Classic Tube Amp Sound  (Read 7675 times)

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ultrabike

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Classic Tube Amp Sound
« on: August 06, 2013, 07:47:36 AM »
Just to share some stuff that some of you may already know...

"Classic Tube Sound" Vahalla + HD600

I have spent some time with Schitt's Valhalla + HD600 combo and I really like it. I was expecting this Class A, single-ended triode, no feedback, non-inverting, 40W consuming, 7 Ib, 32 ohm, "objectivist" blasphemy to color and distort things up warmly, romantically, liquid-ly, and whatever-ly quite a bit. Well, not so...

While it did seem to sound a little warmer to me, it did so in the mosquito fart range. I did not perceive resolution loss, or at least it wasn't obvious. Music was delivered nicely and enjoyably in a hi-fi way.

I re-did some of the measurements with the Valhalla amp and without (also to double check my last results in the HD600 thread) and found that indeed no distortion or "objectively" hideous stuff was going on behind my ears:

FR with vs with out Valhalla (right - yellow is with Valhalla)



Distortion without Valhalla (right)



Distortion with Valhalla (right)



Note I'm not even being very fair to the Valhalla amp, since the Focusrite amp is feeding it...

"Classic Tube Sound" Valahalla + HD558

O...MIE...GAWD!!!  p:8 p:8 p:8 A very different story goes on with the nominally 50 ohm HD558. While frequency coloration due to amp and headphone impedance was expected, relatively speaking resolution went from lo-fi to infra-fi. It was not unpleasant, but things got a bit too muddy there. This made me wonder about the distortion numbers of this Valhalla + HD558 combo...

FR with vs with out Valhalla (right - yellow is with Valhalla)



Distortion without Valhalla (right)



Distortion with Valhalla (right)



Distortion numbers went up quite a bit for the unhappy Valhalla+HD558 combo indeed.

So...

These impressions and results make me wonder about the perception by some that tube amps inject distortion and coloration no matter what. Given what I'm hearing (and measuring), it seems to me that it all depends on the overall pairing of the system components... also known by some as synergy. In this case, if someone experiences the Valhalla (or similar tube amp) with a particular headphone, results could be sublime or catastrophic depending on what was used... In this regard I feel Schiit has done a wonderful job in specifically stating:

"What headphones does Valhalla work best with?
Beyer 600 ohm models and Sennheiser 300 ohm headphones are best–think high-impedance when you’re thinking abo ut Valhalla. Yes, we have plenty of people using Valhalla with low-impedance headphones, but the best match is with high-impedance cans."

From network theory, the coloration in the FR was expected. However, the way non-linear distortion behaves and interacts (amp and headphone) was not. The distortion of individual components do not necessarily just add up, and the way the Valhalla interacts with the HD600 and HD558 maybe a good example of that...
« Last Edit: August 09, 2013, 03:37:19 PM by purrin »
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Hroðulf

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Re: Classic Tube Amp Sound
« Reply #1 on: August 06, 2013, 08:39:17 AM »
Ah, so damping factor does work, eh?
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ultrabike

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Re: Classic Tube Amp Sound
« Reply #2 on: August 06, 2013, 08:41:52 AM »
Quote from: RudeWolf on August 06, 2013, 08:39:17 AM
Ah, so damping factor does work, eh?

Well, the damping factor naturally affects FR. I maybe wrong, but AFAIK it does not explain well the distortion performance.

EDIT: I meant non-linear distortion...
« Last Edit: August 06, 2013, 09:15:21 AM by ultrabike »
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Hroðulf

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Re: Classic Tube Amp Sound
« Reply #3 on: August 06, 2013, 08:46:51 AM »
Shouldn't it also affect the square wave due to lessened control of driver inertia?
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ultrabike

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Re: Classic Tube Amp Sound
« Reply #4 on: August 06, 2013, 09:00:22 AM »
Quote from: RudeWolf on August 06, 2013, 08:46:51 AM
Shouldn't it also affect the square wave due to lessened control of driver inertia?

Yup, if it affects the FR, it will affect the square wave response. However, it still doesn't tell you much about the non-linear distortion behavior.

AFAIK damping ratio is a quantity that arises from the linear transfer function of a system... Whether it is the Zload / Zsource definition, or the second order linear RLC (cascaded) circuit model definition, damping is still a linear model definition.

There might be some formula(s) out there that relate/combine the non-linear distortion contribution of the headphone and the amp, and give the whole rig non-linear distortion result... but I'm honestly not aware of it....

EDIT: It could be that damping does indeed have an effect on non-linear behavior, but dunno what that relationship is...
« Last Edit: August 06, 2013, 09:19:49 AM by ultrabike »
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gurubhai

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Re: Classic Tube Amp Sound
« Reply #5 on: August 06, 2013, 09:52:46 AM »
I am not familiar with Schiit Valhalla but seems like its just the output tubes running out of steam for the Low Z load that the HD598 presents.
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Hroðulf

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Re: Classic Tube Amp Sound
« Reply #6 on: August 06, 2013, 10:16:20 AM »
Yeah, there's much ink spilled on the damping factor significance, but I haven't been able to find a sufficiently simple explanation for me to comprehend. Also I'm unsure if the explanations I've come across extend to directly driven single driver solutions as much as multi-driver crossover systems.
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Re: Classic Tube Amp Sound
« Reply #7 on: August 06, 2013, 11:26:00 AM »
Very interesting, thanks! The change in D2 for the HD600 doesn't look too bad. Based on what I'd heard/read about tubes, I was expecting worse! I've always wondered if a properly designed tube amp would still have a noticeable increase in harmonic distortion (again, based on what people have told me about tubes and distortion), and that does not look to be the case. (This is not a topic I know too much about, so excuse my ignorance!)
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Re: Classic Tube Amp Sound
« Reply #8 on: August 06, 2013, 02:16:12 PM »
Quote from: ultrabike on August 06, 2013, 09:00:22 AM
AFAIK damping ratio is a quantity that arises from the linear transfer function of a system...

While the ratio is linear, the transfer function it describes is second order harmonic oscillator.
As in here, zeta (ζ) is the damping coefficient:  http://en.wikipedia.org/wiki/Damping_coefficient

Transistors, diodes and vacuum tubes can be reasonably well described in V/I by two second order functions, one for positive part, the other for negative. (The correct one is actually exponential.)
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Re: Classic Tube Amp Sound
« Reply #9 on: August 06, 2013, 04:05:42 PM »
I have always noticed that the hd555/595/558 that I owned would never sound good out of integrated amps or OTLs. It maybe the impedance curve crossing over from low impedance all the way to around 250ohms
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