Matching preamps and amps (1 Viewer)

[JohnDW]

Can anyone help me with these concerns - in my plans to use a Sony STR 4aes as a pre pro driving separate amp(s) I have come across concerns that indicate some amps have high impedances (22 ohms ?) and some preamps dont get enough signal gain (20 db being low and 70 being high ?) to drive certain amps. Especially since I will dobtlessly be using an amp from another manufacturer, what do I need to know/look for to make sure amp and preamp will match up in these regards (as opposed to tone, etc other more subjective qualities)

 

[Saurav]

The general rule of thumb is to try and ensure that your amp's input impedance (which will be in kilo ohms or Kohms) is around 10 times the preamp's ouput impedance (which will be in ohms). These specs should be listed with the other specs of the equipment. Make sure you look at impedance numbers, and not voltage numbers or sensitivity numbers or anything else. Signal gain should be less of a concern, IMO, the chances of a mismatch there are pretty low.

 

[JohnDW]

using your metric, a amp input impedance of 22 kOhms (citation 7.1) being driven by a sony 4aes output of 2 volts at 1kOhms is a 22x differential - is this too large, or is the answer you want a 10x or more differential ?

 

[Saurav]

It should be 10:1 or more. 1K is kinda high for the output impedance of a solid state device, they're usually in the few hundred ohms range. For instance, my NAD integrated amp's output impedance was 200 ohms, I think. In terms of differential, the larger the better, up to a point - it makes the job of the preamp easier. This is until you get up into fairly high input impedances, say 50K or 100K or so - then, the output capacitance of the preamp can interact with the amp's input impedance to cause rolled off bass. Since very few preamps will list output capacitance, you might as well not worry about this anyway
Gain - your amp should have a sensitivity figure, like 0.5V or 1V or 2V. This is the input voltage needed to drive it to full output. If your preamp's maximum output voltage is close to this, or greater than this, you're fine. For instance, if your amp reaches full output at say 1.5V and your preamp's max output is 2V, then you know that you'll get full power at 75% of your preamp's volume control range, which is pretty decent. If your preamp's peak output is 1V, you'll never be able to get full power out of your amp, and that's not so good. Likewise, if your preamp's peak output voltage is 15V (a ridiculously high number, chosen for the sake of illustration), then you'll hit your amp's power limit at 10% of your volume control's range, which is again not good - a slight turn of teh volume will make the amp blast at full power, and you won't really be able to make fine adjusments to the volume.
So, rule of thumb - as long as your preamp's maximum output is close to your amp's input sensitivity, you're fine. If they're off by too much, it might be inconvenient because it might not get loud enough, or might be too loud, depending on which figure is greater.
Hope that makes sense.