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

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

A lot of questions Tom...

point 3 - I would recommend NOT removing the input resistor on the amp, if you 
were to plug in a crystal mic without a volume pot, without a delay pedal, the 
amp's input grid would see an infinite load and send the tube into thermal 
runaway. You theoretically can remove the resistor if you ALWAYS use a 
delay/volume pot, it's just not really a wise move. 


If you have a device between the mic & the amp input, the output impedance of 
that device becomes the input impedance of the amp, the 5.6Meg now helps to 
prevent signal bleed to ground between the device & amp. Theoretically, once 
over a certain value, the amp's input impedance should,'t make any difference 
whether 1M or 5.6M...but I assure you that it does (AC impedance & dc resistance 
are not the same thing.

A Jayphat style device will also work with all other kinds of mic, it is not 
something that is exclusive to crystals. 


If you plug your 2M mic (vol ful up) into a 5.6Meg input, directly, then yes the 
overall load is ~1.5M, less if the pot is turned down, there is no way round 
this, the resistance of the pot's wiper to ground, in parallel  5.6Meg is always 
the lowest common denominator. Adding resistors in series with the signal from 
mic pot to amp, or from element to pot does not change this, it just adds series 
resistance in the signal path which may dull tone. This is relevant to your 
point 5, depending on the signal voltage, putting the pot in series with the 
signal may dull off tone...it may not?

If your delay robs tone seriously, then a Jayphat will help the input impedance, 
but not affect anything later in the delay circuit that might be screwing up the 
tone (there are a lot of components in a delay, the input is just one aspect). 
You should not NEED a Jayphat to get your amp working satisfactorily with a 
delay, though it might be a useful thing to have. Select a delay pedal that 
doesn't screw up the tone too much (anything you plug into, between mic & amp 
has some effect), assess this effect by plugging straight into the amp, then 
through the delay - don't just turn the delay on & off. Some players actually 
like a delay that softens tone slightly, others want to keep things tight. Most 
players do not use Jayphat style devices, though I'm not trying to put you off 
building one.

Cheers, Mark.


________________________________
From: Tom Skailes <tom.skailes@xxxxxxxxxxxx>
To: harp-l@xxxxxxxxxx
Sent: Saturday, 19 February, 2011 14:10:37
Subject: [Harp-L] Impedance questions and some (probably) crazy thoughts on 
volume controls in crystal mics

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