Class D is just Dandy!

The biggest issue with input/output impedance is the change in frequency response. This is especially bad with tube pres as they usually have a high output impedance. Driving a low impedance input amp can affect the overall response and deviate from ideal. With purist tube pre's even the volume control setting can affect things because they lack additional buffer stages that would prevent this.

This is mostly misleading or outright false.

Output impedance **might** affect frequency response (in some cases, certainly not if the output is direct-coupled) but also affects distortion.

Tube preamps do not necessarily have high output impedance. That depends a lot on the design of the circuit! If it employs feedback, its output impedance will be fairly low and driving a 10K impedance should be no worries- check with the manufacturer (our preamps don't employ feedback and 600 ohms or less is no worries)! In a purist tube preamp, the volume control won't affect things (other than volume) despite there being no buffer; its all about how well the design is executed!

As an example, we make two tube preamps that have balanced outputs (and were the first in the high end audio world with such) and the load that they can drive makes no difference- the frequency response is flat, owing to a direct-coupled output. They also support driving 600 ohms as they were intended to support the old school balanced standard. Yet because they lack loop negative feedback, their output impedance is relatively high (compared to most solid state preamps), yet their distortion is very low driving any amplifier made with a balanced input. I've used our MP-1 to drive loudspeakers directly; how many solid state preamps can do that? 

Just sayin'.

@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