Preamp Impedance

Looking at NAT Audio preamps, one model (Utopia), which I have heard and liked, has an output impedance of 70 ohms. A newer model (Magnetic), which the designer says is much better has an output impedance of 250 ohms. Now I can see that into a NAT valve power the newer pre might sound better, but will the higher impedance be a problem for usual suspects SS power amps, (the input impedance of some starts at 10k). Can that higher impedance negate the improvement in other stuff?

Tough for me to demo both

A pre-amp with a output impedance of 250 Ohms should not have any trouble driving an amplifier with a input impedance of 10k Ohms.

Stereophile magazine does an additional pre-amp test into a 600 Ohm load (why? I don't know) but at 600 Ohms most pre-amp start to have problems.

John Roberts (a pro audio circuit designer) early this month in another forum wrote:

The modern scheme of low output/high input impedance is called a "bridging" termination or interface and generally described as a 1:10 ratio, which also delivers a less than 1dB insertion loss (voltage). Very old school terminations used to be the same impedance to insure maximum "power" transfer, these were the legacy 600:600 ohm interfaces. Modern design has pretty much evolved beyond transformer interfaces using power transfer to instead maximize voltage transfer which is best at lower source and higher input Z.

Thanks - how does one look at how much power does a preamp have to drive a power amp. Is it just the impedance ratio?

All the preamp has to do is develop enough voltage to drive the power amp to full output. The output/input ratio should be at least 1:10 so in the example you provided of a 250Ω into 10kΩ that is a 1:40 so is of no concern in general.

Knowledge in the making. Thanks for starting this thread. I never experimented with pre/pro. Over the years had four integrated amplifiers. Grundig, Pioneer, Sony and now Denon.
I have read some time in the past that impedance ratio should be 1:10. If less the audio quality is not vibrant.

Also like Julian very sensibly pointed out care should be taken not to overload the max. Input voltage of the power amp by the output voltage of the preamp. Knowledge in the making for me.

Have fun and hope you have a great holiday.

There is more, actually. Apparently the figures manufacturers publish is upto 1khz, so at 20khz the impedance might be actually much higher, and with some valve preamps can go up to 4.5kohms. This is usually the case with coupling capacitors at the output. Lower impedance means more current and with such preamps the interconnects matter less. With my AR Ref 3 + AR Ref110 combo, the RCAs were interconnect sensitive while the XLR was not.

These two links explain it well http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1261767359
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1341955108&read&keyw&zzat+cable


Which is why NAT highlights that "NAT is really proud to present line stage with no coupling capacitors or inter-stage transformer on signal path, yet zero feedback concept."

bonzo said:

There is more, actually. Apparently the figures manufacturers publish is upto 1khz, so at 20khz the impedance might be actually much higher, and with some valve preamps can go up to 4.5kohms. This is usually the case with coupling capacitors at the output. Lower impedance means more current and with such preamps the interconnects matter less.

Click to expand...

Looking at Stereophile measurements of some rather expensive vacuum tube pre-amps. The output impedance of most, stays about the same from mid-range to treble. It's the low bass that has the raising impedance.

A coupling capacitor being in series with the next unit's input impedance, will have no effect on the load the pre-amp see's. But too small of a coupling capacitor will cause bass roll-off.

The capacitance that can be problematic is the parallel capacitance of some very long or boutique interconnect cables.