[James Romeyn]

The specs for the component serving as preamp do not list the source impedance for the pre-outs.   

Power amp specs...

Gain: 28.6 dB
Input impedance: 7.5 k-Ohms
Input level for clipping: 755 mV rms   

There is plenty of extra system gain. 

What are potential negative audible effects of a bad preamp/power amp interface?  I presume it's similar to a passive line stage, being soft bass and dynamics, neither of which are apparent here.     

I'm concerned mainly because the amp input impedance is the lowest I've seen.

[richidoo]

What preamp is it?

If it is solid state then the output impedance is likely <300 Ohms which is plenty low enough. If it is tube output stage then it could be higher, even up to 2-3 kOhms.   But like you said, if it sounds good, you're fine.

[James Romeyn]

Don't laugh...well, if you must go ahead.  I worded the OP to avoid this disclosure.  It's a 2003 Pioneer VSX-D912 HT receiver with pre-outs.  Earl Geddes used the 912.  For music I use only the pure analog 7.1 inputs, which sounded indistinguishable from my $7500 SST Ambrosia, which I placed with such as VTL's $17k 7.5 Reference and Krell's $15k Evo.

What do you suspect is the worst case (highest value) for the output impedance of the 912's preamp output?  What's a minimum recommended ratio for the impedance between the preamp and power amp?

Ever see such a low input impedance?       

     



     

[avahifi]

The typical output impedance for a solid state preamp is in the 560 to 1000 ohm range.  This will interface with any reasonable power amplifier.

Your power amp has a 7500 ohm input impedance.  This is no problem at all for a well designed solid state preamp.  However it might be an issue with a vacuum tube preamp which will have a much higher output impedance.

There are two issues in that case.  One is that the series output impedance of the preamp combined with the input impedance of the power amp form a voltage divider.  This will lower the effective gain of the system.  The higher the output impedance of the source is in comparison to the load impedance the more the gain will be lowered.

The second issue is drive current capability from the source.  The source must drive whatever distributed capacitance of the connection cables and the load.  The higher the output impedance of the source and the lower the input impedance of the source, the harder time the source will have driving the load.  This eventually could cause audible distortion.

However in your case I expect there will be no negative issues at all within the design capabilities of both units.

Regards,

Frank Van Alstine

[James Romeyn]

Thank you, Frank, for your reliable advice. 

[richidoo]

Manual says line outputs are 200mV / 2200 Ohms.

Thanks for explaining the two issues with impedance ratio, Frank. I didn't know it affected voltage gain.

A few years ago Odyssey's tube preamp Candella and Stratos SS amp had a similar impedance ratio to your setup and still had strong detailed bass. Their ratio has been widened since then.

[James Romeyn]

Manual says line outputs are 200mV / 2200 Ohms...

richidoo
Thanks for the link.  I have two printed hard copies of that manual.  Page 75 lists "REC 2.2 k (Ohm)" and "MONITOR OUT 75 (Ohm)".

Both the above specs appear under the heading "OUTPUT".  I'm virtually positive "OUTPUT" for "REC" is an error because:

• "REC" is usually an input
• Pioneer lists record output but not record input
• Inputs usually have higher impedance than outputs (see numbers above)

Monitor output is indeed a "line output" as you describe, but it is fixed level for recording, not variable level as are pre-outs.   

I looked closely and see no pre-out spec (maybe it's right in front of me, left my glasses upstairs  ).