If my amp has a 51k input impedance, what should my preamp output impedance be ? ....

[cmalak]

Kev...yes, plenty of headroom for you and as Myles said if you are not hearing any tonal aberrations than you are fine.

Myles...yes you are right that those values were measured into a 600ohm load but my point is that you just want o make sure there is enough headroom that there is no chance of an impedance mismatch. Often you can run into issues when using a tube preamp with slightly higher than average output impedances and SS preamps that have very low input impedances (Burmester 911 mk III is a great example, with their input impedance via RCA is 22.9Kohms but only 1.88Kohms via XLRs). So I would never connect a tube preamp which typically have output impedance in the 100-400 ohms via balanced connections with the Burmester amp. If you do a search of the audio forums, the 10X rule (between pre's output impedance and amp's input impedance is often cited as being a minimum thresh hold, with many preferring 20X and higher ratios simply to ensure that there is no chance of an impedance mismatch).

In any case, my only point to Joe was to make sure to do the research (enquire with manufacturers) to at least take one potential mismatch variable out of the equation. That's all.

 

[microstrip]

IMHO, the x10 rule is just a rule of thumb, and as most rules of thumb it is a source of many cases of poor matching. The problem is more complex than it seems and includes aspects such as system (amplifier and speaker) sensitivity, capacitive or resistive coupling and topology of the output of the preamplifier.

The distortion figures of many preamplifiers increase dramatically when they have to supply current - most tube designs and ARC preamplifiers are typical cases of this behavior. Usually we can rely that that manufacturers know about their jobs and we can rely on their indications - they optimize their preamplifier for their own amplifiers and we can learn a lot from their specifications. Otherwise we must look at the measurement section of reviews or knowledgeable people with a lot of experience. But remember that an amplifier with a 12 dB lower gain will ask for 4 times higher current - the same effect as reducing the input load by a factor of 4.

Anyway 51k is a comfortable impedance for most preamplfiers. And the input impedance of the Burmester 911 mk3 is an enigma that no one until now could solve - how can an RCA/XLR adapter that does not include any transformer change the XLR impedance from less than 2k to 22.9K in unbalanced? How can people report great results using it in balanced mode with ARC tube preamplfiers? :huh:

 

[cmalak]

IMHO, the x10 rule is just a rule of thumb, and as most rules of thumb it is a source of many cases of poor matching. The problem is more complex than it seems and includes aspects such as system (amplifier and speaker) sensitivity, capacitive or resistive coupling and topology of the output of the preamplifier.

The distortion figures of many preamplifiers increase dramatically when they have to supply current - most tube designs and ARC preamplifiers are typical cases of this behavior. Usually we can rely that that manufacturers know about their jobs and we can rely on their indications - they optimize their preamplifier for their own amplifiers and we can learn a lot from their specifications. Otherwise we must look at the measurement section of reviews or knowledgeable people with a lot of experience. But remember that an amplifier with a 12 dB lower gain will ask for 4 times higher current - the same effect as reducing the input load by a factor of 4.

Anyway 51k is a comfortable impedance for most preamplfiers. And the input impedance of the Burmester 911 mk3 is an enigma that no one until now could solve - how can an RCA/XLR adapter that does not include any transformer change the XLR impedance from less than 2k to 22.9K in unbalanced? How can people report great results using it in balanced mode with ARC tube preamplfiers? :huh:

Microstrip...agree totally. One of the reasons why I am a fan of keeping preamp and amp matched from the same brand (just my own bias but that is one of the reasons I do it). Also, totally agree with the enigma around the 911 mk IIIs input impedance variance bet RCA and XLR. I am not sure how you can match that thing with anything but a Burm pre via XLR connections.