Class D is just Dandy!

Good comments by Erik.

In the situation Autre described in his post dated 4-29-2017 it is very unlikely that impedance compatibility issues were present.  The Rogue Sphinx has a very high input impedance (more than 100K according to Stereophile's measurements).  And although I don't know what the input impedance of the Peachtree Nova 125 SE is, it is most likely much higher than the output impedance of the solid state Onix CD player which was driving it.

Impedance incompatibilities between line-level source components, such as CD players, and integrated amplifiers such as those, or between preamps and power amps, are likeliest to arise when a tube-based output stage is driving a solid state input stage.

Regarding the 10x rule of thumb guideline which Todd alluded to, I would state it as follows:

The input impedance of the amp (or other component that is receiving a line-level input signal) should be at least 10 times the output impedance of the preamp or line-level source component that is driving it, at the frequency within the audible range for which that output impedance is highest. Which in the case of preamps or source components having capacitively coupled outputs (such as the majority of tube preamps) will usually be at 20 Hz.  And the output impedance at that frequency will often be far higher than the specified output impedance (which is usually based on a mid-range frequency such as 1 kHz), because the impedance of a capacitor rises as frequency decreases.

That doesn’t mean that there will necessarily be a problem if the guideline is not met. It depends on how the output impedance **varies** as a function of frequency. What it means is that there **won’t** be an impedance compatibility problem if the guideline **is** met.

If Stereophile has reviewed the preamp or source component, the measurements section of the review will usually indicate the output impedance at 20 Hz as well as at other frequencies.  But if only a nominal impedance can be determined, such as a manufacturer's specification that is presumably at a mid-range frequency, to be safe I would suggest a ratio of 50x or preferably even 75x.

Regards,
-- Al
 

Whew. Thanks Todd, Erik and Al. That is a lot to digest but I think I understand it a bit more now. This does a lot to explain the basis for such terms "system synergy" and "system compatibility". 

And now, back to our regularly scheduled topic of Class D amps are Dandy!

@autre 

"system synergy" and "system compatibility".

This is one of the reasons I was more then happy to go with a class-D integrated.😀

Cheers,

Scott

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'.