[Harp-L] Impedance questions and some (probably) crazy thoughts on volume controls in crystal mics

[Tom Skailes <tom.skailes@xxxxxxxxxxxx>]

Hi Guys

This may cover some old ground, so apologies if that's the case.

I'm just getting round to building a Jayphat impedance matching box so I 
can run a delay/echo pedal with my MC-151 blues blaster crystal mic. 
Been thinking about achieving a good high impedance to the crystal 
element and had a number of thoughts/questions that it would be great to 
get some feedback/advice on - I haven't looked at circuit theory for 
over 30 years so you'll have to forgive my ignorance ;)  :

1 - my blaster has a 2 Meg vol control pot in the Rod Piazza 'Hot Rod' 
circuit configuration (I think) with the capacitor. I have a 5.6Meg 
resistor in the input of my Bassman RI. Because the pot and the resistor 
are in parallel, my understanding is that when the pot is full on, the 
effective impedance to the crystal is 1474k. The first question is, when 
the vol pot is turned to say half, the circuit layout suggests that this 
presents part of the pot resistor in series, with the remainder in 
parallel with the amp's resistor, so at the exact halfway point I 
(probably wrongly) calculate an impedance to the crystal of 1Meg + 848k 
- is this correct, or does it remain constant, regardless of the vol pot 
setting?

2 - On the JT30 site I found an article that mentioned putting a 
resistor in parallel across the output from the vol pot as a means to 
make a linear taper pot more 'audio' in its operation. The author of the 
article stated that the crystal would see the full impedance of the vol 
pot, whilst the amp would see the impedance of the resistor after the 
pot. To me this doesn't sound correct - but I could well be wrong - my 
thoughts are that a 5Meg pot with a 1Meg resistor following in parallel 
across the outputs, coupled with the typical 1Meg resistor in the input 
of a guitar amp would at full pot volume result in an impedance to the 
crystal of 454k, whilst at the halfway volume point, provided my maths 
is correct, 2.5Meg + 416k (result of 2.5M/1M/1M in parallel)? Which is 
right?

3 - The volume pot can dull the tone of the crystal and some people 
suggest removing the vol pot from the mic circuit. With my Bassman that 
would give me the full 5.6Meg impedance to the crystal, or on a typical 
guitar amp input 1Meg. This leads to my first probably crazy question - 
to avoid the problem of parallel resistors dropping the impedance to the 
crystal, would it be possible/sensible to remove the amp's input 
impedance resistor from the circuit, based on the (probably erroneous) 
assumption that the vol pot in the mic effectively replaces it. I'm 
thinking this might be OK when the mic is connected to the amp, but 
might cause excessive squealing or hum when the mic jack is pulled from 
the amp socket. What's the story on this?

4 - As per 3, what would the effect be of dropping the 5.6Meg input 
resistor out of the Jayphat circuit - would anything get fried if the 
Jayphat was accidentally turned on without a mic jack connected? My 
understanding is that the J-FET in the Jayphat has essentially infinite 
impedance. I'm guessing that this might only be beneficial for crystal 
elements, if at all. This is related to a second article on the JT30 
site which suggested putting an op amp inside the mic between the 
element and the pot, so that the crystal saw a massive impedance, but 
the amp saw much less - this sounds a bit like trying to cram the 
Jayphat circuit plus two 9v batteries between the crystal and the vol 
pot, with the vol pot as part of the Jayphat circuit - has anyone tried 
this?

5 - Final crazy question/thought: Is it possible to wire the vol pot in 
the mic so it's a variable resistor in series with the crystal element? 
I'm guessing that at full 'off' on the pot, sound would still come 
through, the level of which would depend of the ratio between the pot 
and the amp's input resistor. Would this work? In this configuration I'm 
guessing that the minimum impedance presented to the crystal would be 
the value of the amp's input resistor, and the maximum would be the sum 
of the pot plus resistor???

Anyway, as I said, there is probably some/much wrong/muddled thinking 
amongst the above, and I'll probably just end up building the Jayphat as 
per the schematic on the Harp-L site, but it all got me thinking.

Regards
Tom