The problem of low input impedance of Topping amplifiers

[solderdude]

To complicate things, for some unknown reason, Stereophile tests pre-amp outputs into a 600 Ohm load. (as one of it's tests)

Its a standard load from the wired telephone era and also used in professional broadcast gear (load and source impedance with coupling transformers)
It is probably a standard load option from the used test equipment.
It is also probably the lowest 'line level load' one can come across (in the pro-world) so when it measures fine on 600ohm (which most opamps can handle) it will work fine on any higher (resistive) load as well.

 

[MaxwellsEq]

It is also probably the lowest 'line level load' one can come across (in the pro-world) so when it measures fine on 600ohm (which most opamps can handle) it will work fine on any higher (resistive) load as well

When I started in the studio world there was still quite a lot of 600 Ohm gear in use, so we had to test with 600 Ohm loads. It's probably unnecessary to check it now, but I agree, a well designed device should not find 600 Ohm difficult to drive.

 

[KSTR]

In the end, the bottom line is that Cambridge Audio isn't doing themselves a favor when they don't state (the trivially simple to derive or measure) output impedance and maximum recommended load in their specs, notably if the output stage were somewhat non-typical and picky with load resistance (like from too small DC blocking caps or whatever) -- which is of course something we don't know...

 

[MaxwellsEq]

I bumped into some measurement data by soundstagenetwork.com showing some amplifiers' damping factor varies across the signal frequency spectrum. Here are the plots of 2 solid-state speaker amplifiers:

Benchmark AHB2
View attachment 409033

Rotel RT-6000
View attachment 409034

So I think frequency-dependent output impedance is not uncommon, indicating internal capacitive load.

This is wholly irrelevant to your use-case.

Power amplifiers have to be able to raise high voltages across the low (4 to 8 Ohm) speaker load whilst also supplying high currents. Your preamplifier needs to only produce 2V and a few milliamps into a load which is at least a hundred times easier to drive. There is also no back EMF and no extreme reactance from crossover and motor components. So no-one talks about damping factor for line-level outputs, because there is nothing to damp!

 

[philomusique]

This is wholly irrelevant to your use-case.

Power amplifiers have to be able to raise high voltages across the low (4 to 8 Ohm) speaker load whilst also supplying high currents. Your preamplifier needs to only produce 2V and a few milliamps into a load which is at least a hundred times easier to drive. There is also no back EMF and no extreme reactance from crossover and motor components. So no-one talks about damping factor for line-level outputs, because there is nothing to damp!

I should've made my point clearer. Although power amplifiers are not relevant in my use case, the plots show that output impedance of solid state amplifiers can vary depending on the signal frequency.

In fact, I suspect a constant output impedance value in the specification of an amplifier, either a phono stage or a power amp, is very likely a nominal number, unless there is zero capacitive and inductive loads in the output stage.

So I think in general in an audio signal chain (except the speakers), we want each stage to have a very low output impedance and its next stage to have very high input impedance, to minimize the variations in the frequency response.

 

[MaxwellsEq]

I should've made my point clearer. Although power amplifiers are not relevant in my use case, the plots show that output impedance of solid state amplifiers can vary depending on the signal frequency

That's ONLY because the load is very, very, very low.

 

[philomusique]

In my use case the Topping preamp L70 has an input impedance of 2k ohm. *IF*, a big if, the output impedance values of Cambridge Audio Alva Duo are as reported by their customer support, vary between 200 ohm to 600 ohm. Presumably this variation is frequency dependent, due to conductive loads at the output stage.

A back-of-envelop calculation shows that the frequency response of the Alva Duo -> L70 combination can vary around 1.5 dB.

Of course this is based on the big assumption that Alva Duo's output impedance is between 200 - 600 ohm.

Suppose I connect Alva Duo to an amp with 20k ohm input impedance, the frequency response variation will be less than 0.2 dB.