Passive preamplifier with GS phono

The main objection to passive preamps is that they have a large "driving impedance."

They tend to be best matched with valved amplifiers, which have a high input impedance.

When used with a solid-state amplifier, its input capacitance, which is essential to prevent slewing distortion (after Otala), joins with the large driving impedance to form a high-frequency filter. It might therefore result in some loss in how "lively" the sound is. Some call this a loss of dynamics, but it is better stated as being soft on transients.

Some may prefer the laid-back signature, but how laid back will it be?

If it is a "pot in a box," and that is all it is, then the worst driving impedance occurs at the potentiometer's electrical midpoint and on an audio taper volume control pot that appears at the 3 pm setting (approx.). It is where the resistance on either side of the wiper is half the pot's value.

Using Kirchhoff's circuit laws, looking out from the power amp's input, both resistances are seen as being in parallel, and so the worst driving impedance is one-quarter of the pot's value. If 20k, then the driving impedance is 5k. This assumes the driving impedance of the source selected on the passive preamp is suitably low.

If set to say 11 am or 12 o'clock position, the bottom half of the pot is lower resistance, and it appears in parallel with the higher resistance of the upper half. By calculation, it can be seen that any other setting than the electrical midpoint results in a smaller driving impedance. In other words, it is less of a problem at normal listening levels.


However, other power amplifiers might use smaller resistances and larger capacitances, meaning the cut frequency might venture into the audible spectrum.

Another thing to watch is interconnect length and interconnect capacitance, which also acts as a low pass filter.

A passive, provided it is not very expensive, can be considered a starting point.

Graham Slee wrote: The main objection to passive preamps is that they have a large "driving impedance." They tend to be best matched with valved amplifiers, which have a high input impedance. When used with a solid-state amplifier, its input capacitance, which is essential to prevent slewing distortion (after Otala), joins with the large driving impedance to form a high-frequency filter. It might therefore result in some loss in how "lively" the sound is. Some call this a loss of dynamics, but it is better stated as being soft on transients. Some may prefer the laid-back signature, but how laid back will it be? If it is a "pot in a box," and that is all it is, then the worst driving impedance occurs at the potentiometer's electrical midpoint and on an audio taper volume control pot that appears at the 3 pm setting (approx.). It is where the resistance on either side of the wiper is half the pot's value. Using Kirchhoff's circuit laws, looking out from the power amp's input, both resistances are seen as being in parallel, and so the worst driving impedance is one-quarter of the pot's value. If 20k, then the driving impedance is 5k. This assumes the driving impedance of the source selected on the passive preamp is suitably low. If set to say 11 am or 12 o'clock position, the bottom half of the pot is lower resistance, and it appears in parallel with the higher resistance of the upper half. By calculation, it can be seen that any other setting than the electrical midpoint results in a smaller driving impedance. In other words, it is less of a problem at normal listening levels. Using my 70's power amp circuit as an example, it has 10k of its own input resistance as part of a low pass filter with 440pf of capacitance, so it cuts at 36kHz. If I add 5k of resistance - making 15k - then it cuts at 24kHz. That might be audibly acceptable. I designed it to be so. (https://www.hifisystemcomponents.com/forum/1970s-design-indulgence_topic4543_post67245.html#67245) However, other power amplifiers might use smaller resistances and larger capacitances, meaning the cut frequency might venture into the audible spectrum. Another thing to watch is interconnect length and interconnect capacitance, which also acts as a low pass filter. A passive, provided it is not very expensive, can be considered a starting point.