impedance matching amp/preamp

I think I came across something like this before where 100K output impedance is the recommended input impedance for the amp to match the pre-amp's output. Not saying that is the case for certain in your case, but it is possible. 60K input impedance on the SS amp should match pretty well in general to most tube pre-amps I would say.

Would some one kindly explain why an impedance for input to an amp should be so high. I think of as the resistance to the voltage from the preamp making it extremely hard to drive the amp. If the preamp has a output impedance of 20 times that would that mean that the signal leaving the preamp has to be strong enough to to to overcome that impedament as well. Anyway I am clueless.

Mechans,

First, keep in mind two fundamental relationships:

Ohm's Law: Voltage = Current x Resistance

Power = Current x Voltage, which, by subsitution of the above relationship, equals ((Voltage squared)/Resistance)

(Both relationships applying to situations where the effects of inductance and capacitance are negligible).

Higher input impedance makes it harder to transfer POWER into a given load device, in the sense that more voltage is required to drive a given amount of current into that load device.

However, we are not interested in delivering power into the amplifier. We are interested in having the amplifier do the following:

1)Sense how much voltage is present at its input.
2)Provide an output voltage equal to the voltage that is present at its input times some gain factor.
3)Be able to supply enough output current to maintain that output voltage in the presence of a very heavy load, meaning a speaker whose low impedance will draw very heavy currents at typical output voltages. Thereby resulting in high power levels being delivered to the speaker.

A theoretically ideal line-level interface, such as between preamp and power amp, will be driven by a component having zero output impedance, and will be loaded by a component having infinite input impedance. The infinite input impedance will result in no current being drawn, while the input voltage will be sensed and amplified as described above.

If we hypothetically think of the preamp as developing an output voltage "ahead" of a resistance equal to its output impedance, then the fraction of that voltage which will appear across the input of the amplifier will decline as preamp output impedance rises and/or power amp input impedance falls. That is in itself of no significance as long as the values remain within reasonable limits -- the only difference it will make is that a small adjustment in volume control setting will be required.

However, the reason for wanting input impedance to be 10 or 20 times or more greater than output impedance is that neither impedance is likely to be perfectly constant as a function of frequency (particularly the preamp's output impedance, and particularly if it has a coupling capacitor at its output). If the variations of preamp output impedance as a function of frequency are not negligibly small with respect to power amp input impedance, the frequency response of the interface will be significantly non-flat.

Hope that helps.

Best regards,
-- Al

Without getting into technical details, the analogy of how a drum skin works to produce sound is a good one to help understand what is going on.

A drumskin that is too loose will have less impedance to resist the strike of the drumstick which can result in a loose, distorted sound. Tightening the drumskin produces a tighter, less distorted sound. How tight the drumskin should be is a matter of taste, however there is some point at which too little tension produces poor sound quality

Eletrical impedance in a circuit is like the tension on a drumskin. it enables voltage fluctuations to register and be transferred correctly. If impedance is too low, the effect can become similar to that loose drum skin and result in higher distortion ie less accurate detection and transmission of the input signal.

Thank you for both explainations. I think I will only understand it fully when I find a way and the time to immerse myself in the Physics. I seemed to understand these concepts with much better facility thirty years ago. It is annoyingly frustrating for me to have only a dilettante's knowledge of electronics.