pre-attenuator

[Grainger49]

Input impedance is at the RCA jacks.  That is, what the source sees.  With the pre-attenuator the input impedance is determined by the whole combination, pre-attenuator and stepped attenuator.

My post 2 above is assuming, as stated earlier in the thread, that the attenuator uses a pair of resistors totaling 100k for each step.  I don't remember if it has been verified or not.

 

[gh0st]

Input impedance is at the RCA jacks.  That is, what the source sees.  With the pre-attenuator the input impedance is determined by the whole combination, pre-attenuator and stepped attenuator.

My post 2 above is assuming, as stated earlier in the thread, that the attenuator uses a pair of resistors totaling 100k for each step.  I don't remember if it has been verified or not.

That's correct - the attenuator measures ~100k input-to-ground regardless of position, so adding the small resistance of the input wiring, this is what's measured at the RCAs. Now that I've fixed the pre-attenuator circuit, this ~100k at the RCAs is restored, which is the desired effect.

 

[Laudanum]

OK ... just making sure I had it right with the pre-attenuation.  The resistance still remains constant regardless of pot rotation when measuring from input to ground of the attenuator ... or at the RCA jacks.

 

[Grainger49]

Desmond,

Yes, The pre-attenuator is sized so that the total is right at the original stepped attenuator resistance (impedance).

Within the attenuator there are two resistors that sum to 100k at each step for each channel.  For instance a 67k and a 33k or a 47k and a 53k.  The input, source impedance stays at 100k while the voltage sent on to the tube grid (input) drops with each step from the top (wide open).

You probably have seen that already.

 

[Laudanum]

Yep, got it Grainger.  Thanks.