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Thread: Impedance and Level matching tips?

  1. #1
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    Nov 2017
    Sofia, Bulgaria
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    Question Impedance and Level matching tips?


    A proud VHT 2502 owner here

    There's something that's been bothering me for quite some time...
    How does one ensure the correct level is coming to an amp from a preamp?
    I've got several preamps, each with a different output level selector switch and, obviously, an additional output level select knob. I know the difference between signal level categories like Mic, Instrument, Line Amateur and Line Pro but I have no idea how to actually measure the output of a given preamp in dB.

    There's also the impedance thing - Each preamp has it's own output impedance and I can't find information on the input impedance of the 2502 anywhere on the web. Does someone know how much it is and effectively what a load it is seen as by a given preamp?


  2. #2


    Regarding the signal level between the preamp and power amp, there are really only three factors that you have to watch out for: noise, saturating the front-end of the power amp, and overloading the power amp to the point of damage. Given that the 2502 is all-tube and your preamps are at most line level, I would assume the last thing is not of concern here. The noise factor will increase slightly as you further attenuate the output of the preamp since you have to add more gain on the power amp to compensate, but at these levels honestly it's pretty negligible, especially if your preamp signal is already pretty noisy as is expected with distorted guitar. Conversely, increasing the preamp signal too much may saturate the gain stages of the power amp, but since the 2502 is all-tube, I wouldn't be too concerned with this. In fact, many would consider this saturation desirable. I think you're over-thinking this. Just set your preamp out to line level and adjust the knobs until it sounds good.

    Regarding the input impedance of the power amp, I wouldn't be too concerned with it. If you're curious, you can use an AC voltimeter to measure the signal level of a candidate preamp with just the meter as a load, and then using the power amp and the meter as parallel loads. Then using the known output impedance of the preamp and the impedance of the meter, you can calculate the input impedance of the power amp. But like I said, it seems pretty moot to me. I would expect the input impedance of the 2502 to be pretty high, and the output impedance of your preamp to be pretty low, which would mean little signal loss from the load of the power amp front-end. The line level interface between the preamp and power amp is neither high power nor high frequency, so you don't have to be worried about overloading the preamp output with too low a input impedance, nor with matching the impedance of transmission lines.

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