Preserving zero feedback
 

Would a passive high-pass filter between a preamp and amp preserve zero feedback, e.g., a fully balanced Marchand passive high-pass filter between an Ayre KX-5/20 and VX-5/20? Obviously the balanced cables that run between the preamp and amp preserve zero feedback.

I was up until 6 this morning reading about the virtues of Ayre's zero feedback design and reviews of the VX-5/20. I'd like to high-pass my KEF Ref 1s at 80 Hz, but not at the expense of the Ayre sound I so enjoy.

Filters
 

I am not familiar with the Marchand filter, but a filter has no feedback per se.
I do think that active filters (like a Bryston or JL Audio crossover) can often out-perform passive designs.
Are you certain of the high-pass frequency (80 Hz)?

The use of feedback or in your case zero feedback is accomplished within each individual component and is not related to the interaction of amp, preamp and speakers.

Yes it would.

I've now talked with Gary at Ayre and Phil Marchand, both of whom think a Marchand fully balanced passive high-pass filter could be inserted between the Ayre KX-5/20 preamp and VX-5/20 amp with no harm to the Ayre sound. So I've ordered the building of a 80 Hz 24 dB/octave filter. Its arrival is probably some weeks away at which time I'll reset the low-pass to the subs to match the high-pass crossover and slope.

Usually, zero feedback refers to zero global negative feedback, so there's typically local or nested feedback networks in a ZNF amplifier. Use of negative feedback in a preamp is far less problematic, because the preamp will have its output impedance fixed by a resistor, and thus will never be exposed to a load impedance that creates hf stability problems. That said, use of op-amp based filter networks really isn't desirable in a high-end system, so a passive filter is probably the best choice.

I hope so. I received a notice this morning that the Marchand fully balanced passive filter I ordered is being shipped. It's a fixed 80 Hz with 24 dB/octave slope filter I plan to insert it between my KX-5/20 and VX-5/20. I'll adjust the Velodyne SMS-1 bass manager to low-pass to the subs at 80 Hz rather than the 40 Hz I'm currently using and rerun its acoustic room correction with the new configuration.

This is true in most cases. In the Ayre products, however, there's zero feedback both globally and locally in any portion of the circuit. It's always been a very strict philosophy at Ayre. At any rate, everyone is spot-on that the use of the filter should be just fine in the system.

Thanks for the encouragement, Ryan.

Thanks for this info. It's very challenging to design a truly ZNF amplifier, and harder still to make it look half decent on a test bench.

For the uninitiated, there is definitely merit in the approach, particularly in terms of transient behavior, slew rate, and unconditional stability. This adds up to an amplifier that gets you closer to a live listening experience.