pre-attenuator

[gh0st]

Let me see if I have this right.  The resistor in series with the signal, the first one is the 68k, and the one that goes to ground from the input of the attenuator is the 47k.   

Is that right?

yup. I'll fiddle with the wiring in case I've done something dumb and report back, thanks for the input so far.

[Grainger49]

I edited my post that had the formulas.  I inserted the values that are calculated, not the reciprocals, but the final number for the parallel combination.

[gh0st]

Fixed!

Pure human error >_<

When I switched the location of the resistors, I inadvertently connected the 47k resistor between ground and the wrong side of the 68k resistor - it's supposed to go next to the pot, not be connected directly to the input from the RCA. Doh!

I *think* that my maths was right in the previous post with respect to how I'd actually wired it; 47k parallel to 168k = ~36k, instead of 47k parallel to 100k, with 68k in series = ~100k.

Live and learn! With the beast back up to full strength, the volume range is back to being improved, and the noise is better than with them installed at the RCA inputs, so all in all, I'm chalking it up as a win.

Now to get some time on the 7236 and 12bh7... thanks for helping folks

[Grainger49]

Dom,

Checking what you have above your math is right.  But it is the wrong circuit.  That is why I asked for verification.

With it corrected the input sees almost 100k ohms regardless of the attenuator position.

[Laudanum]

Dom,

Checking what you have above your math is right.  But it is the wrong circuit.  That is why I asked for verification.

With it corrected the input sees almost 100k ohms regardless of the attenuator position.

Hey Grainger.  The input impedance being determined by the measurement between lug 3 and lug 1 (input and ground) of the attenuator, correct?

[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.