matching impedances

Discussion in 'AV Receivers' started by Aris A, Feb 19, 2004.

  1. Aris A

    Aris A Stunt Coordinator

    Mar 29, 2002
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    Hey guys maybe you can help me out on this...I have a ss amp which has an input sensitivity of 47K(1000mv)and I'm considering to match a tube preamp which has an output impedance of 600 ohms. Is the matching good or bad?
  2. Andre K

    Andre K Auditioning

    Feb 4, 2004
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    it's a perfect match, technically. These are the usual impedances for LF electronics (audio frequency). It's for power amplifier outputs (speakers!) and RF (high frequency, 10MHz an up) circuits you have to be careful.
  3. Chu Gai

    Chu Gai Lead Actor

    Jun 29, 2001
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    Boy, what a tough question. I'll try to give you an objective answer that touches on a few considerations.

    When one wants to match impedances, such as you're doing with a preamp and an amp, contrary to what many may think, it does not mean that the values should be equal. In consumer audio equipment, impedance matching is taken to mean that the source should have a low output impedance while the destination should have a high input impedance. Doing this, with the interconnect of your choice, allows the voltage produced by the preamp to be 'dropped' to the amp for further processing.

    The majority of solid state preamps have output impedances signficantly lower than the tube preamp you're considering. For example, a Parasound Halo P3 has an output impedance of 1/10 of your unit, or 60 ohms. The question then becomes what does this 10 fold disparity mean in terms of the sound that you're going to hear?

    Well the answer to this question depends upon the capacitance of the interconnect and the total length, which when multiplied together, gives you the total capacitance. The resistance of the interconnect (gauge) is pretty much irrelevent since it's small and can be neglected. OK then, let's crank some hypothetical numbers with some hypothetical components and let's calculate two things: frequency drop at 100kHz and phase shift.

    Long interconnect: 1000 pF
    Tube preamp: output impedance = 5000 ohms
    SS preamp: output impedance = 100 ohms
    SS amp: input impedance = 100k ohms

    With the ss/ss combination, at 100 kHz, the response will be down 0.001 dB compared to 10 kHz. Phase shift is approximately -4 degrees. Essentially fairly flat.

    With the tube/ss combination, at 100 kHz, the response will be down about 12 dB compared to 10 kHz. Phase shift is approximately -75 degrees.

    I'd say with your combination and modest lengths of interconnects, a few meters, you'd have a reasonably flat output going into the ss amp with a very slight, and probably impercetptable amount of high frequency roll-off, but nothing that a slight tweak of the treble couldn't compensate for.

    There will be other issues that will color your sound a bit but then that's either a bane or a benefit depending upon your preferences.

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