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Dirty-South Blues Harp forum: wail on! >
Amp input impedance?
Amp input impedance?
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Bugfan
6 posts
Jun 10, 2014
7:58 PM
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Hoping for some advice from those in the know....
My vht special 6 combo has lo and hi input - 100k and 1 meg...pretty standard from what I have read.
I also read that for crystal or ceramic mic the input load needs to be 5 meg or you will get tone loss.
I read that for a CR element that ideally about 150k to 250 k amp input is best for the same reason.
I have a CR shure element mic (99B86)and a Ceramic astatic element mic ( a 564-1).
I have followed the VHT mod forums and have modified the amp to have a 5 meg input on the hi input .
Can I ?, or do I even need to?... modify the Lo input to match the ideal 150-250 k load that's best for the CR element mic? There is a parallel arrangement of resisters off the lo input and I can't find any mention of anyone modifying them. Or do I simply just use the high input - now 5 meg.... it seems to give the CR mic a more brittle tone that has a volume related distortion more than the lo input. I can't decide wether I like it that way or not!
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5F6H
1784 posts
Jun 11, 2014
3:11 AM
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The load doesn't "need" to be 5Meg for crystals, there is a subtle but tangible difference between 1M & 5M, I prefer 5M for just about any mic, with a stage sized amp - it's a shade fuller and slightly more responsive. This is probably best identified on larger amps as you shouldn't have a problem pushing the VHT into a region where it is responsive anyway.
CR/CM & dynamics work fine with 5M, but lots of folk also like running them at lower (but still Hi-Z loads), e.g. low gain/#2 inputs (68K to 100K). It is assumed that mics with lower dc resistance elements (1K to 2K) don't need such big loads, but we're talking AC impedance here & if it sounds different, it is different.
I also prefer higher loads if using a pedal betwen amp & mic.
Some amps are wired so that one input has no load resistor (not the Special 6 & typical Fender inputs), this can kill the preamp tube if you use a crystal element as the input is effectively open & the preamp tube goes into runaway. Not an issue with CM/CR/dynamics.
My, possibly unique view, is that it is the nature of the harp signal that is relevant here, harp not having the attack & decay of a plucked string benefits from a slightly hotter input. Old Ampeg accordian inputs also used 5M inputs, I suspect for this very reason (guitar can start to sound harsh, even oscillate with very high loads).
I wouldn't get too hung up on it, to be honest. If you're comfortable doing the work, suck & see. You won't damage anything playing with loads between 100K & 5M.
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www.myspace.com/markburness
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Greg Heumann
2740 posts
Jun 11, 2014
8:37 AM
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Mark has given you 95% of the info you need. When you use crystals keeping the impedance as high as possible definitely helps. CM/CR/dynamic is MUCH less sensitive and usually perfectly happy into 100K.
There is a tradeoff however. The higher the impedance, the more sensitive the "system" (mic+cable+amp) is to electro-magnetic interference - AKA "hum". Whether this affects you will have to do with your particular environment.
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/Greg
BlowsMeAway Productions
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Bugfan
7 posts
Jun 11, 2014
6:43 PM
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Excellent thanks guys . That is about the perfect answers to my questions thanks. The ceramic definitely sounds better now into the higher impedance. The CR has an interesting break up to it ( when in the high input as opposed to the low 100k input) that I think I like more as I cup harder and play louder.With a Dan electro slap echo delay in between, it accentuates that breakup on the loud sounds even more.
I really appreciate the advice thanks again guys. Just learnin and trying to figure it all out.
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