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

joeinid

joeinid

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My Job 225 has a 51k input impedance. How critical is the output impedance of my preamp? Would 220 ohms or less be sufficient?
 
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Many follow the 10 X rule, so if your pre is 220 ohms then you would like to see the amp at around 22K or greater......less than the 10 X could result in the bottom (bass) rolling off. I'm sure there is a bit of give and take. With your amp at 51K I wouldn't think there be any issues.
 
Thanks Jeff. I have not noticed any issues with anything so far but wanted confirmation. I appreciate your "input" :)
 
The GAT should have no problem as it's output impedance is 100 ohms. That was more of an issue in the old days with a 10 ohm input resistance and 500-1000 ohms or more output resistance. Dave Wilson covered it many years ago in TAS. Biggest tip offs you have an issue is loss of dynamics and low end.
 
Myles B. Astor said:
The GAT should have no problem as it's output impedance is 100 ohms. That was more of an issue in the old days with a 10 ohm input resistance and 500-1000 ohms or more output resistance. Dave Wilson covered it many years ago in TAS. Biggest tip offs you have an issue is loss of dynamics and low end.
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Thanks Myles. Yes the GAT is quoted as being under 100 ohms and sounds superb with the Job and every other amp I've tried so far. I was thinking of trying something like the McIntosh C50 on the Job because I want the tone controls for some situations.
 
joeinid said:
Thanks Myles. Yes the GAT is quoted as being under 100 ohms and sounds superb with the Job and every other amp I've tried so far. I was thinking of trying something like the McIntosh C50 on the Job because I want the tone controls for some situations.
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Years ago, ran the small Goldmund amp that used the Job circuit quite successfully with the Cj ART S. 3 (with an output impedance of 500 ohms) without any issues.
 
Myles B. Astor said:
Years ago, ran the small Goldmund amp that used the Job circuit quite successfully with the Cj ART S. 3 (with an output impedance of 500 ohms) without any issues.
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Great! Thank you. I'll see what CES has to offer than I'll make a decision.
 
Joe...the only thing you should be aware of is the output impedance of the preamp is usually measured at a particular frequency, usually 1KHz and usually the output impedance changes at different points on the frequency curve. So I would want to have a minimum of 10X spread between the highest point of the output impedance and the input impedance of the partnering amp. So for example, the stated output impedance on my ARC Ref 3 is 600ohms via XLR and 300 ohms via RCA connections. So the 10X rule would suggest an input impedance of 6Kohms (on XLR) / 3Kohms (on RCA) would be sufficient. However, the output impedance of the Ref 3 rises to as high as 1437ohms (balanced or via XKR) and 625 ohms (single-ended or via RCA) at 20 KHz. So via balanced connections, I would want an amp with at least a 15Kohm input impedance. Which is why ARC recommends an amp with a minimum of 20Kohm load for the Ref 3 (actually for most its preamps because presumably they all measure similarly in terms of output impedance). Having said all of this, in your specific case, sounds like there is more than plenty headroom and if you are not experiencing any tonal imbalances (i.e., the preamp-amp partnering) is delivering a tonally neutral presentation (no specific part of the frequency spectrum being highlighted or being sucked out) than sounds like you are fine.
 
Thank you Cyril. Awesome explanation and it makes perfect sense. I did not realize output impedance was frequency dependent. Thanks!


cmalak said:
Joe...the only thing you should be aware of is the output impedance of the preamp is usually measured at a particular frequency, usually 1KHz and usually the output impedance changes at different points on the frequency curve. So I would want to have a minimum of 10X spread between the highest point of the output impedance and the input impedance of the partnering amp. So for example, the stated output impedance on my ARC Ref 3 is 600ohms via XLR and 300 ohms via RCA connections. So the 10X rule would suggest an input impedance of 6Kohms (on XLR) / 3Kohms (on RCA) would be sufficient. However, the output impedance of the Ref 3 rises to as high as 1437ohms (balanced or via XKR) and 625 ohms (single-ended or via RCA) at 20 KHz. So via balanced connections, I would want an amp with at least a 15Kohm input impedance. Which is why ARC recommends an amp with a minimum of 20Kohm load for the Ref 3 (actually for most its preamps because presumably they all measure similarly in terms of output impedance). Having said all of this, in your specific case, sounds like there is more than plenty headroom and if you are not experiencing any tonal imbalances (i.e., the preamp-amp partnering) is delivering a tonally neutral presentation (no specific part of the frequency spectrum being highlighted or being sucked out) than sounds like you are fine.
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Yes, Cyril is absolutely right, and the lowest the closest and more constant that output impedance is at 1 kHz, 5 kHz, 10 kHz, and 20 kHz, etc., the better it is.

Eg.; (Amplifier section):

- 30 milliohms at 1 kHz
- 45 milliohms at 5 kHz
- 55 milliohms at 10 kHz
- 75 milliohms at 20 kHz

=> From 1 kHz to 20 kHz the variation is from 30 to 75 milliohms - or 45 milliohms difference (very modest rise) in output impedance between those chosen frequencies, which is very good and means a deft use of feedback and an adroit balance between open- and closed-loop gain.
 
So based on the measurements above & the output of 10 ohm from a pre to mono-blocks that have a 50k ohm input, is this a good match?
 
NorthStar said:
Yes, Cyril is absolutely right, and the lowest the closest and more constant that output impedance is at 1 kHz, 5 kHz, 10 kHz, and 20 kHz, etc., the better it is.

Eg.;

- 30 milliohms at 1 kHz
- 45 milliohms at 5 kHz
- 55 milliohms at 10 kHz
- 75 milliohms at 20 kHz

=> From 1 kHz to 20 kHz the variation is from 30 to 75 milliohms - or 45 milliohms difference (very modest rise) in output impedance between those chosen frequencies, which is very good and means a deft use of feedback and an adroit balance between open- and closed-loop gain.
Click to expand...

Output impedance unit is ohms not milli-ohms; for preamps like Joe's GAT it's 100 ohms and for the Rowland Consummate 150 or 300 ohms.

Also, I couldn't find any pre with a 30 ohm output impedance?

Most manufacturers believe 10:1 ratio is a minimum with some believing 20 or 40:1 being even better. Some feel the higher the better.
 
cmalak said:
Joe...the only thing you should be aware of is the output impedance of the preamp is usually measured at a particular frequency, usually 1KHz and usually the output impedance changes at different points on the frequency curve. So I would want to have a minimum of 10X spread between the highest point of the output impedance and the input impedance of the partnering amp. So for example, the stated output impedance on my ARC Ref 3 is 600ohms via XLR and 300 ohms via RCA connections. So the 10X rule would suggest an input impedance of 6Kohms (on XLR) / 3Kohms (on RCA) would be sufficient. However, the output impedance of the Ref 3 rises to as high as 1437ohms (balanced or via XKR) and 625 ohms (single-ended or via RCA) at 20 KHz. So via balanced connections, I would want an amp with at least a 15Kohm input impedance. Which is why ARC recommends an amp with a minimum of 20Kohm load for the Ref 3 (actually for most its preamps because presumably they all measure similarly in terms of output impedance). Having said all of this, in your specific case, sounds like there is more than plenty headroom and if you are not experiencing any tonal imbalances (i.e., the preamp-amp partnering) is delivering a tonally neutral presentation (no specific part of the frequency spectrum being highlighted or being sucked out) than sounds like you are fine.
Click to expand...

But those values were obtained by JA into what he says is an unrealistic 600 ohm load. Also how much of that Also, How much of that variability is attributable to ARC IIRC using a cathode follower that Joe's GAT doesn't use?

So like you said, a small variation will have little effect and one can hear if things are awry.
 
Myles B. Astor said:
Output impedance unit is ohms not milli-ohms; for preamps like Joe's GAT it's 100 ohms and for the Rowland Consummate 150 or 300 ohms.

Also, I couldn't find any pre with a 30 ohm output impedance?

Most manufacturers believe 10:1 ratio is a minimum with some believing 20 or 40:1 being even better. Some feel the higher the better.
Click to expand...
So based on this the match between my preamp & mono amps is very good Myles? Preamp 10 ohm, amps 50 kohm.
 
Thanks Myles, no issues with the sound at all, absolutely loving that. Just learning with enjoyment.
 
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