[Harp-L] Volume control issues.

[B Boggs <ceudoazul@xxxxxxxxx>]

Hi Stephen,
  Thanks for such an excellent post on volume controls.
  Very good information on the subject.
  The R7 crystals can even be hotter then MC151 and need the 5 MG ohm volume pot.
  And if the amp input has lower impedance at the input jack, then it will effect the sound.
  Get a mil-spec pot with impedance of 250- 500K Ohm for hot Shure CM and
  for crystals use 3- 5 MG Ohm. 
  Some builders use 100 K Ohm pots, and like the post comments, hot Shure CM hi-z elements are "forgiving".
  Without the matching resistance in the input jack of amp (mentioned in post below), then the amp will not get all the frequencies; and that is a separate issue.
  Not many amp builders are using such high resistance across the input jack(s).
  Thanks again, Steve, for taking the time to write your post.
   
   
   
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From: Stephen Patrick Schneider <schneisp@xxxxxxxxxx>
Subject: Re: [Harp-L] Volume control issues
To: harp-l@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 f!
 at 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.



       
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