Input impedance and passive tone on bass

[Owen]

I am in the very privileged position of working in an FE college which has just provided us with a new studio. It is lush, but that is beside the point. Yesterday we were testing various signal paths out and we plugged a passive Sandberg P into a Neve 1073 500 series, a Cranborne Camden 500 series and a Warm Audio WM12. Obv they were all different flavours of nice. But what confused me somewhat was that the passive tone control on the Sandberg had virtually no effect into the 1073, but did exactly what I expected it to into the Camden and the WM12. I suspect this has something to do with this impedance load being presented to the bass by the preamp input, but I could well be wrong. Can anyone throw any light on it?

 

Sadly, my fave was the 1073. My tastes are simple but expensive.

[Bill Fitzmaurice]

Impedance probably explains it. The Neve is designed to operate with low impedance sources. A passive bass is a high impedance source.

[Owen]

There is a hi z / low z option on it which did not seem to make a huge amount of difference. But it sounded MIGHTY fine.

[tauzero]

The input impedance could well be the cause. The tone control is just a variable resistor and capacitor in series bleeding a greater or lesser amount of treble, connected to one end of the volume pot wiper. The lower the input impedance, the less effect the tone control will have. I think (not certain about this and I may not have fully thought it through) that if the volume on the bass is turned down, the tone control would have more effect as the part of the volume control track between wiper and the active end of the track would be effectively added to the input impedance of the amp. 

Edited September 26, 2024 by tauzero
Clarification of which volume

[Bill Fitzmaurice]

1 hour ago, Owen said:

There is a hi z / low z option on it which did not seem to make a huge amount of difference. But it sounded MIGHTY fine.

The high Z is 4k. A bass amp input impedance is typically 1 meg.