Class D is just Dandy!

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.

Let’s simplify this. I wrote this:

The biggest issue with input/output impedance is the change in frequency response....(followed by supporting discussion with links to external sources)

You replied:

This is mostly misleading or outright false.

Now your latest posts seem to be supporting my argument. I’m not sure you even read what I wrote before you claimed it was a lie or misleading, but you launch into discussions about how Atmasphere preamps are different (which while informative does not make your case that I was wrong).

So I challenge your "statement of fact" as having anything which directly refutes my statement with evidence.


Best,

E

@atmasphere

Wrote:

As we can see, if one is to point at a tube preamp and blame it for a change in frequency response,

I don’t "blame" preamps for changes in frequency response. I say high output impedance causes frequency response changes which vary based on the load. This is an irrefutable fact based on simple serial circuit analysis. Anyone with a basic understanding of AC circuit analysis would conclude the same.

It is also irrefutable that in general, tube preamps have higher output impedance than solid state. That is different than saying preamp X sucks, which is what you seem to have read.

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.

A - I never limited myself to 1 kHz
B - It makes no sense to talk about changes in frequency response if i was talking about a single point
C - The coupling cap is a major if not THE major contributor to output impedance. However it is not the only issue. I never said it was. The factors that go into any devices output impedance is more complicated, but includes the coupling cap if any.

There is no logical way to make both of these statements true at the same time:

• Tube preamps don't have a problem with output impedance
• The problem with tube preamps is the impedance of the coupling cap
  

Since the first includes the second, you can't have both of these be true at the same time.

Best,

E