[Harp-L] RE: Favorite Mic + impedance matching for crystal mics

[Tom Skailes <tom.skailes@xxxxxxxxxxxx>]

Richard/Mike/Buck

I'm not sure if any of this relates to your original question Richard, 
but hope it helps you, or anyone else out there looking for info.

I have a MC151 element Blues Blaster from the 80s and have spent some 
time looking at impedance issues as this mic came with a 500k volume 
pot. From what I've gleaned, crystal element mics like to see a high 
impedance but can cope with anything from 2Meg up. But what I've learnt 
since is that mic technique, particularly achieving an airtight cup and 
playing less 'cleanly' was what helped get the gritty sound I was 
looking for. The impedance thing does give the mic an enormous fat 
bottom end, which adds to the gumbo and is great if you're into that ;)

If you look to your electronic circuit theory you will note that the 
volume control, which is a resistor, and the resistor on the amp's input 
act in parallel, so the overall input impedance of 1Meg pot and a 1Meg 
input is 500k. (the formula is (R1 x R2)/(R1+R2) ), so you can see that 
if you have a 1Meg resistor on the amp's input, even if you could find a 
volume pot with infinite resistance, the best you could achieve taking 
the two together would be a shade under 1Meg, which is a poor match for 
the crystal.

Regarding Buck's question, most tweed Fenders had input impedance of 
1Meg, although some very early circuits had higher, and I do wonder if 
Little Walter ever went to the trouble of getting the input resistor on 
his amps changed - but then, maybe he didn't use a crystal mic. The 
tube/valve PA amps, such as the Masco PA amps do have ultra-high 
impedance inputs though (15Meg on the Masco MA17 for example), and 
they're supposed to sound great for harp - maybe LW used one of them...

So you have two options:

1. replace the input resistor with a 5-10Meg resistor, and use a 5-10Meg 
vol pot, giving you 2.5Meg to 5Meg overall depending on the values and 
combinations. 2.5Meg is fine for a crystal and will give you full tone. 
I've got 5.6M on my RI Bassman's input (channel 1, I've left channel 2 
left as 1Meg), and 6.6Meg on my VHT Special 6

2. build an impedance matching/dropping box as suggested by Mike - 
search for Jayphat in the Harp-L archives - this presents a high 
impedance to the mic, but sends out a low impedance signal that can then 
go through pedal chains or low impedance inputs with no worries. This 
has the advantage that you can leave your amp's input resistor 
untouched, and if you use effects pedals, if you put the box at the 
front of the chain it will then avoid any cumulative input resistance 
issues across the pedal chain. I have built a couple of these - the 
first had no resistor across the input, and hence technically had 
infinite impedance, but in practice for me it didn't work well because 
effectively this leaves the op amp unbiased and it seemed to start off 
sounding great and after 30 seconds or so started to get distorted (but 
not in a way that was good) - BTW I didn't try Mike's suggestion of 
holding the op-amp to 1.0 x gain by linking input to output, and this 
may well overcome the biasing issues I had with my circuit, but I wanted 
to have the ability to get a little extra gain if I wanted it. At one 
stage I also looked at building this circuit into the shell of the mic 
itself, as it can be made very small, but try as I might I couldn't fit 
a single 9v block battery into the shell, let alone two. My second one 
was built to the approx Jayphat spec (which uses a JFET rather than an 
Op-Amp as its gain stage, which purportedly behaves more like a 
tube/valve), but with some variations based on other articles I found on 
the web regarding preamp and buffer circuits - which is what this 
circuit does, or can do. I have a 10Meg resistor across the box's input, 
and a 10Meg pot in my Blues Blaster now (in the UK 5Meg pots are not 
readily available - I found a 10Meg on RS Online (if you're in the UK) 
but it's pricey). I run this box off a single 9v battery and it works 
fine. Doubling up batteries to 18v would give me more gain, but I don't 
need it. Currently I can run it anything from about 0.5x gain to 1.5x 
gain. I can send you info and links on impedance matching boxes if 
you're interested.

Having built the box, mainly to allow me to use a delay pedal that only 
has a 500k input resistance, I have now taken to just playing my mic 
straight into the amp via the 5.6 or 6.6Meg input - as I find that gives 
the fattest sound, and saves me messing around with boxes and batteries.

Things to watch - on Fenders and probably other similar amps, the second 
input is usually low input resistance and also halves the input signal 
'volume' (typically ~140k resistance in Fenders I believe) and hence 
will be a poor match to a crystal mic, but would work fine with a 
crystal used through an impedance matching box.

Anyway - have fun. Of course, if you get a dynamic element, or an MC127, 
you'll have none of these impedance issues anyway ;)

Regards
Tom