Class D is just Dandy!

@atmasphere

Perhaps you feel your preamps are less susceptible than most, which I could believe. I did not make my statements absolute. If you’d like to submit FR charts at different loads like Stereophile has and at different volume settings this would prove your preamps outstanding exemplars.

I have to say that for you to call my statement in its entirety misleading or false when anyone can pull up page after page of examples in Stereophile measurements of tube preamps showing variance in their frequency response based on difference in the impedance they are driving is pretty gutsy.

Anyone can also compare solid state preamps to this and see that in general they perform better by this measure.

So I hope that maybe you misread the generalities and nuances I was making and re-think your reply or produce engineering principles and data which makes all the other data and the basic electric principle of impedance in series and voltage division a quack science.


Best,

E

I'm sure you make fine products, but would you then please explain the variance in all the tube preamps in the Stereophile section if not due to output impedance issues?

My prior post was a correction and not an attack. I was merely pointing out where the problems were and stated why, basic engineering principles included. More are below.

We won't allow Stereophile to review our products (we don't agree with their editorial policy which seems to be tied to their advertising and I know this from direct experience) which is one example of why if you limit yourself to their pages, you won't get the full picture.

This has more to do with the choice of coupling capacitor at the output of the preamp than it does the output impedance!

Please note that this phenomena has to do with solid state just as much as tubes.

Of course, the ultimate indicator is a graph of the output impedance vs. frequency. If you see it rising as it approaches 20Hz, this **might** indicate a loss of bass impact depending on the input impedance of the amp. The general rule of thumb is a 10:1 impedance difference between the two; as long as you hit that margin with the amp you have in mind its likely no worries.

The output impedance curve of our balanced preamps looks the same as their frequency response curves; we cut them off at 1Hz. So regardless of the load its driving, the preamp will have flat response from 1Hz to over 200KHz. Ours are not the only tube preamps with direct-coupled outputs that have ever been made- as a result you can't just assume that if it has tubes that it will have troubles making bass into a solid state amp with a 10K input impedance or the like. Generalities are often misleading that way.

My prior post was a correction and not an attack. I was merely pointing out where the problems were and stated why, basic engineering principles included. More are below.

You called my statement:

mostly misleading or outright false.

That’s not a correction. If you had said "I feel distortion is a bigger issue" then it’s a matter of opinion as to which is more audible. If you want to say I wrote falsely or in a way to mislead back it up.

You then state:

This has more to do with the choice of coupling capacitor at the output of the preamp than it does the output impedance!

What, exactly, are you claiming is a result of the choice of coupling cap? Distortion or output impedance?

The general rule of thumb is a 10:1 impedance difference between the two; as long as you hit that margin with the amp you have in mind its likely no worries.

This supports, not undermines, my statement.

E

Let me go back to what @atmasphere objected to.

The biggest issue with input/output impedance is the change in frequency response...

I was very specific in what I was talking about. I did not say "the biggest issue with tube preamp sound quality." I said the issue with "input/output impedance."

If you see another issue with input/output impedance to be more important, please show your work.

Best,

E

A statement of fact is not an attack. You are taking this personally.

Here’s the work:

What, exactly, are you claiming is a result of the choice of coupling cap? Distortion or output impedance?

Here’s a formula for calculating cutoff frequency:
F=1,000,000/CxRx 2Pi

Normally you see this formula with a 1 instead of a million; I used the latter so that f is in Hz (-3db point), R is in ohms and C is in uF.

The coupling cap at the output of a preamp, in concert with the input impedance of the amplifier used determines the cutoff frequency.

example: a solid state preamp has a 10uf output coupling cap. The input impedance of the amp is 10K.

1.59Hz=1,000,000/10K x 10uf x 2Pi

We can see from this example that if a tube preamp has a 10uf coupling cap that it too will have a cutoff of 1.59Hz into the same amp.

This means there will be no appreciable phase shift at 20Hz so bass impact will be unimpaired, since the cutoff is 1/10th the lowest frequency to be played. A cutoff at 20Hz will mean that phase shift exists up to about 200Hz. The phase shift will cause the system to sound lean.

Many tube preamps **do** have such large coupling caps unless the designer has not done their homework (or has figured out that the larger the coupling cap, the more coloration it imposes, and so has elected to limit the capacitor size so as to get greater transparency). As a manufacturer you can’t forecast to what amps the preamp will be paired.

The size of the coupling cap will not affect the output impedance unless one is able to graph the impedance curve; if rising at lower frequencies the culprit will be the output coupling cap and otherwise not the output impedance of the preamp.

Now how much **distortion** the preamp makes can be affected quite a lot by the load that it drives. That is likely the more powerful argument for being careful about what preamp drives what power amp. Tube preamps often have very low distortion; in most cases its a good idea to have them drive a higher impedance so as to take advantage of that fact. Our preamps again are an exception- they regard 10K as an effortless load.

Please note:

This supports, not undermines, my statement.

yes, this is evidence of how this was simply a statement of fact and not an attack.

The biggest issue with input/output impedance is the change in frequency response...

I was very specific in what I was talking about. I did not say "the biggest issue with tube preamp sound quality." I said the issue with "input/output impedance."

As we can see, if one is to point at a tube preamp and blame it for a change in frequency response, the factor is not the output impedance (which is often only stated at 1KHz), its the coupling cap at the output. That is a bit different from ’output impedance’ and that is why I placed the correction.