Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Rickinbeachcrest on September 12, 2021, 10:26:44 am

Title: Multiple inputs, same impedance, why?
Post by: Rickinbeachcrest on September 12, 2021, 10:26:44 am

This is something I have been wondering about for a while now.  Two inputs with the same impedance. I can see two with different impedance on each one. 

Thanks for any enlightenment you can give me.
Rick

Title: Re: Multiple inputs, same impedance, why?
Post by: sluckey on September 12, 2021, 10:34:19 am

Quote from: Rickinbeachcrest on September 12, 2021, 10:26:44 am

This is something I have been wondering about for a while now.  Two inputs with the same impedance. I can see two with different impedance on each one. 

Many amps were built like that. It's the choice of the manufacturer.

Title: Re: Multiple inputs, same impedance, why?
Post by: tubeswell on September 12, 2021, 03:08:34 pm

Many Fender amps are wired with 'Hi' and 'Lo' impedance inputs - with the Hi input having a Vout of 96.7% (1,000,000R/1,034,000R) and 1M input impedance, and the Lo input having Vout of 50% (68k/136k) with 68k input impedance. But, if you have jacks plugged into both inputs at the same time on these amps, then the Hi/Lo input jack switching is 'bypassed' and both impedances are almost the same (one is 68k higher than the other, but this in in the context of a total input impedance of 1,068,000R vs 1,000,000R).

Title: Re: Multiple inputs, same impedance, why?
Post by: PRR on September 13, 2021, 11:23:24 am

I think the Fender Hi/Lo is more like 99.9% 1.068Meg 1.034Meg one way and 50% 136K the other way. When used solo.

There used to be amps intended for multiple players, with a bunch of 100K as mix resistors.

EDIT: need new abacus

Title: Re: Multiple inputs, same impedance, why?
Post by: tubeswell on September 13, 2021, 06:50:30 pm

68||68


OiO
  ~

Title: Re: Multiple inputs, same impedance, why?
Post by: pdf64 on September 14, 2021, 06:20:47 am

Regarding the input resistance, for the Hi input, I don’t see that the grid stopper’s 34k effective resistance comes into play? 
Rather the input impedance is the plain old 1M grid leak resistor, but with the total grid leak resistance being 1M034.
Of course that’s at lowish signal levels and frequencies, where the effects of grid conduction and Miller capacitance are negligible.