INPUT/OUTPUT IMPEDANCE

If you know you should be within the 1/10 ratio, then congratulations, you have somehow managed to retain useless information I have long since forgotten. At my advanced age I can only dimly recall it seemed to matter once. Long ago. Maybe. Dunno. No. Yeah I am sure now. I did think it was important enough to think about. What I cannot for the life of me remember is how many years of it not actually mattering it took before I started to forget why I thought it ever mattered in the first place. Sorry. Did you say something about over thinking something?

Although the output impedances of the AV7705 appear to not be specified by the manufacturer, according to a post I found at another forum those numbers are 800 ohms (RCA), and 200 ohms per leg (XLR, which means 400 ohms for the XLR outputs per the usual way of defining balanced impedances).

Those are the kinds of numbers I would expect for this type of product. And given that it is solid state the output impedances are unlikely to rise significantly above those numbers in the deep bass region (as happens in the case of many tube-based preamps, which can result in rolloff and/or undesirable phase shifts at low frequencies when the load impedance is too low), or at any other audible frequency. So given the input impedances of the power amp you stated the bottom line is no problemo!

Enjoy! Regards,
-- Al

Thanks to both for the quick response! Much appreciated. Now, I just need a head shrink for the Audiophelia Nervosa I have... 

The 1:10 ratio does matter as you want to minimize voltage loss at the load input. With most amps/preamps on the market this isn’t an issue because they conform. But to claim this should be completely ignored or best forgotten is nonsense. Last year I was discussing with Keith Herron whether he felt the VTSP-360 would be a good match with my amp and the FIRST thing he wanted to know was the input impedance of my amp. Why would an audio component designer/manufacturer ask a question like that if it didn’t matter at all and was completely unimportant?

antho768 OP
I don’t want to over think this, but I just pulled the trigger on a power amp rated at 10k(RCA) 15k(XLR) input impedance. Problem is I don’t know the output impedance of my pre/pro(Marantz av7705).

The 7705 being solid state "should" be fine into that 10kohm load.

Cheers George

The 1:10 ratio does matter as you want to minimize voltage loss at the load input.

Very true, of course. However in itself that voltage loss would only result in a very slight reduction in overall system gain. A much more significant issue arises if the output impedance varies significantly as a function of frequency, AND does not satisfy the 1:10 ratio at all frequencies. In that situation the result can be both frequency response irregularities and undesirable phase shifts at some frequencies.

As George alluded to that is most likely to be an issue in the case of tube-based components, many of which use a coupling capacitor at their outputs. The impedance presented by a capacitor increases as frequency decreases, so the output impedance in the deep bass region can be much higher than the specified output impedance, which is usually based on a mid-range frequency such as 1 kHz.

To cite some examples, Audio Research recommends a minimum load impedance of 20K for almost all of their line stages and preamps, and 60K in the case of a few older designs. And the manual for the fantastic Herron phono stage recommends a minimum load (line stage input impedance) of 50K "for optimum performance," although I recall a member here stating that Keith Herron verbally indicated that 20K is likely to be satisfactory for most users.

Regards,
-- Al

I love the way people remove posts after asserting something truly ridiculous creating "plausible deniability"...lol

Thanks everyone for your replies. So I'm wondering if I should return the amp and get the Parasound a52+. The amp I am receiving tomorrow is the Anthem MCA 525. I originally was waiting for the Parasound, but noticed this thing and was impressed with the build and specs. I know with Parasound, their impedance is in the 90's(k). 

What are your opinions.

Anthony

I have no knowledge as to how the Anthem and Parasound amps compare sonically.  But as I and George indicated earlier impedance compatibility is not an issue in this case.

Regards,
-- Al

Either is likely fine from an input impedance perspective. 

Usually it is tube pre-amps that have higher output impedance that some SS amps can handle better than others typically with input impedance ~ 40 to 60 Kohm or higher. 

Anthem amps are products I have heard recently that I would consider owning. Have not heard Parasound but those are also highly regarded in general. Either should work well and sound good it seems in your case.

If you think you would ever consider using a tube preamp with the amp you choose I would choose the amp with the higher input impedance which sounds like the Parasound, all other things being equal.

As George alluded to that is most likely to be an issue in the case of tube-based components, many of which use a coupling capacitor at their outputs. The impedance presented by a capacitor increases as frequency decreases, so the output impedance in the deep bass region can be much higher than the specified output impedance, which is usually based on a mid-range frequency such as 1 kHz.

In the old days tubes got around this problem (in balanced systems) by using an output transformer. This enabled the output to also not reference ground, something that is *very* tricky to do without a transformer! We developed a circuit that allows for direct-coupling of the output in the balanced domain but otherwise acts much the same way as an output transformer does, although wider bandwidth and less distortion. So there are tube preamps that can drive loads like this with flat frequency response; in fact can drive loads as low as 600 ohms.

Ok but practically to keep tube preamp options open it’s much safer to go with an amp explicitly designed to handle that which spec wise means higher input impedance than 10kohm more like 60-100kohm typically with most products I have seen.
Bel Canto ref amps are a good example. Older models with stock Icepower modules only had 10k input impedance and users with tube preamps reported poor results. Newer models since cost more but added custom input section with 100kohm unbalanced input impedance. Those like the ref1000m amps I use with ARC tube pre-amp are a match made in heaven.

Who does it? or did it? Thanks...

@oldhvymec  The best-known example IMO is Ampex in their 351 electronics, which were the tape electronics used to make many early stereo LPs like RCA Living Stereo and the like.