Re: [Harp-L] Volume control issues

[Stephen Patrick Schneider <schneisp@xxxxxxxxxx>]

Mark Burness hipped me to a common issue with volume controls last year: If you have a volume control on the mic, that presents one impedance load to the mic element; but a typical guitar-style amp will have another impedance load resistor on its input, usually what's called a grid load resistor, which typically is a 1 meg resistor.  Say you have a vaunted 5 Meg volume pot installed on your Astatic crystal mic: The problem is that the two resistances to ground are not in series, adding up to 6 Meg; they are electrically in parallel instead, and the electronic math means the effective overall impedance load your mic sees will be about 860K.  That is a lot less impedance than the 5 Meg you thought you were getting when you installed that VC.  The element 'sees' both resistances together, as the overall impedance.
The problem with the lower impedance load is that vintage hi-Z mic elements may lose valuable signal/tone to ground in audible amounts if the impedance load isn't high enough (the input impedance ~impedes~ the signal's path to ground, is how I understand the term here).  Signal loss, in other words.  The Astatic MC-151 crystal element has the dubious distinction of being probably the all-time champion at that; when genuine Astatic crystal mics are criticized for sounding thin, the likeliest reason is some form of the impedance issues mentioned above--good stuff is bleeding off to ground.  Astatic recommended 5 Meg of grid loading, back in the day, for the MC-151, and my experience has been that at least 2 Meg total is needed to get a truly-filled-out tone & output.  The simple way to get that on the typical guitar amp is to change that 1 Meg grid load resistor on the input to a 5 Meg one, so that with your 5 Meg VC pot, the parallel resistance is 5/2 = 2.5 Meg.  Guaranteed fat for those who can play fat, if everything else is in order.
It took me years to grasp that anytime you have a signal travelling along its merry way thru a circuit, and it crosses a resistance to ground, even a seemingly huge resistance to ground like 5 Meg, ~some of the signal is lost to ground~.  Even when a volume pot is turned all the way up, it's usually still losing some signal to ground, despite the pot's overall resistance, because the pot's resistance 'track' is connected to ground at one end.  If what's lost is a crucial bit of signal that you really wanted to hear, you will not like the impact that path to ground had on the sound.  Different freqs can be lost to ground depending on the situation, it seems a complex deal to me, but in simple terms, desirable amplified harp frequencies can sometimes be lost simply due to having a volume control on the mic, especially in conjunction with the issue of the amp's input impedance.
Shure CM/CR elements are much less affected by this issue than some other classic elements, which is one of their virtues, but they do bleed off a bit to ground if a VC is used.  There is disagreement about how much is tolerable or even desirable: One expert says to use a 150K pot, another says 5 Meg.  Either one is oversimplifying the issue, given that the amp's input impedance factors into the overall impedance the element is gonna see with a VC, but the CM/CR are very tolerant of impedance mismatches (by mismatch, I mean less or more than optimal impedance, if people could agree on what optimal impedance is).
Even the hyper-sensitive Astatic MC-151 tolerates one common mismatch well: Running the vintage JT-30VC with its stock (way-undersized) 500K volume pot into a big Fender amp like a 59 RI Bassman or a Super Reverb works pretty well if you need to cut thru a mix onstage.  You can turn the bass knob up to compensate somewhat for the lows/low mids lost to the impedance mismatch and some players prefer that sound.  Probably the touring harp pro's rig of choice in the 1990s, from what I saw then.  If you do match up the impedance in that situation (5 Meg VC on the mic, 5 Meg grid load resistor on the amp input), you wind up running the bass knob way, way low on the amp.
So if you think you are hearing some loss of tone/output with a VC on the mic, you may not be imagining things.  It's a trade-off: The ability to modulate your volume onstage via the VC may be worth it, but it's very dependent on context and personal taste.
If you do have a VC on your mic, it is worthwhile to review the input impedance situation with the amp(s) you use the mic with.  You may get an improvement in tone/output by modifying the amp's input for more impedance (usually by changing the one resistor mentioned above).  If you use a vintage Astatic MC-151 crystal element with a VC, it's probably crucial to get the impedance as well matched as you can, or at least to try it like that to see whether you like it.
Other vintage mic elements are harder to generalize about; I once took an early Harp Commander, which has a 1 Meg/5 Meg input impedance switch on its input, and tried some different mics, just flipping the switch to see whether the mic changed tone/output.  It seemed like only the earliest mic element designs (probably pre-1955 or so) benefited audibly from the higher input impedance: CM/CR a little bit, Astatic MC-127 ditto, some early Electro-Voices like a 623 and 605 were a little meatier.  A Shure Slim-X crystal really benefited significantly, but a Shure R7 crystal didn't (definitely look into that if your Slim-X has disappointed you).  None of the other mics improved as dramatically as the average vintage MC-151 crystal does.  The pathetic Kobitone crystal used in the current BluesBlaster did not seem to benefit from high input impedance.  However, I only had one example of many of these mics to test so YMMV.
If you can get an amp tech to let you A/B test with your amp by playing, 5 Meg grid load resistor vs. 1 Meg, you'll know for sure whether the difference is audible/useful.
For studio recording, if you can afford to have one, it would probably be good to have one mic with a good vintage element and no VC or switch or even connectors if possible--just hardwire the mic cord directly to the element.  Getting away with that live, onstage, is another thing entirely: I remember Walter Higgs talking about 'harp player semaphore', trying to keep a hardwired mic from feeding back live.  He was onstage at the time, waving his arms around, with such a mic.  Context-dependent, as I said.  Where I live, players who don't use volume controls seem to play too loud or too soft too much of the time.  Their technique/judgment may be partly to blame, but though the lack of a VC seems to aid/abet those failings IMO, the real culprit is the bands playing too loud.  The reason the pioneers didn't use switches or VCs was that they often didn't need to.
Mind you, I'm not an electronic engineer or pro tech, just a guy who doesn't like to hear people diss MC-151 elements.  I hope the above is comprehensible; it's an attempt to simplify the matter as I understand it: minimizing the loss of tasty signal to ground.  Apologies if it occupies an entire digest; or if a good explanation has already appeared onlist.
Harmonica content: This is one issue that can affect the delivery of your harmonica content when you play amplified harp.