[brj]

Hi all.

Apologies for asking a question that could easily be answered with a measurement, but I have neither speakers nor any type of load cell with which to do so at the moment.

I'm looking to better understand the behavior of an amp's input impedance with respect to signal frequency.  Most manufacturers report a single input impedance value, but I rarely see the conditions reported.

My understanding is that the despite the sometimes dramatic load variations imposed on an amp by a speaker's impedance and phase angle behavior with respect to frequency, the input impedance curve of a well designed amp having a sufficiently capable power supply should remain fairly flat.  But how universal is this in practice?

(I'm pondering this in light of line level vs. speaker level crossovers.)

Many thanks!

[mgalusha]

Not sure how much it changes in many amps. I have noticed that JA measures it in Stereophile for some devices, a quick look on their site turned up this for a Lamm amp

The unbalanced input impedance was 39k ohms at low and middle frequencies, dropping inconsequentially to 27k ohms at the top of the audioband. The balanced input impedance was twice these values, as expected.

. I seem to recall other gear he's measured exhibiting similar behavior, the impedance drops as the frequency goes up.

Not hard to measure though I've only checked it at 1kHz myself.

[brj]

Thanks, Mike.

Prior to posting, I'd actually already checked the Stereophile review for my amp and read this:

The input impedance of the balanced XLR jacks was 29k ohms, close to the specified 30k ohms; the input impedance of the single-ended RCA jacks was higher than specified, at 20k ohms. Both figures were constant across the audioband.

And just because I love a good plot, I also sent a note to the manufacturer of my amp requesting one, though I have yet to hear back. 

Given the review comment, I wasn't really worried about it with my current amp.  I was really more curious in a general sense because I hadn't considered it before.

(This all started when I began to ponder how much value there was in knowing the design of the speaker-level crossover of my speaker as I start looking at implementing a line-level crossover.  Given the massive differences in the impedance characteristics, the answer is obviously "not as much as you might like!" )