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Dirty-South Blues Harp forum: wail on! >
Volume pot 250K but Pedal input 1 meg?
Volume pot 250K but Pedal input 1 meg?
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indigo
401 posts
Oct 28, 2017
9:40 PM
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I've installed quite a few pots into mikes over the years
without understanding the theory behind it, more like red wire goes there etc kind of thing.I've used 250K for dynamics and at least 1 meg for a crystal as is usually advised.
But it came to me the other day that when i plugged in a high impedance mike (no v/c) into my harp break (1 meg input) it sounded fine.
So recommended resistance for a dynamic V/C is 250K but it works perfectly well into a 1meg input?
So why can't you use a 1 Meg V/control on a dynamic?
This stuff makes my brain hurt but is there a simple(ish) answer to this?
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Andrew
1708 posts
Oct 29, 2017
2:27 AM
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If your mics are for unspecified devices, where you don't have any idea what the input impedance is, you are better off erring on the low side (if the mics are dynamic) just in case a device has a lower input impedance than expected. Perform some experiments. Try a 25k pot and see what difference it makes. Not a lot, I'll bet.
The thing is to imagine a chain of pedals. The low impedance output of one feeds straight into the high impedance input of the next. You don't need any impedance matching between them.
Last Edited by Andrew on Oct 29, 2017 8:36 AM
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MindTheGap
2377 posts
Oct 30, 2017
1:44 AM
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If you're after a simple principle then Andrew's last sentence hits the spot. It's called Impedance Bridging.
In harp-land things are made more complicated (of course!) because of the unusual variety of ways of wiring the VC. See the recent thread, referring to Lone Wolf's three alternative diagrams. If you wire your VC as a variable resistor rather than a potential divider, then there are other considerations than the this simple principle. In which case, best to stick to your rule of thumb.
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barbequebob
3451 posts
Oct 30, 2017
9:44 AM
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The 250K pot is correct for dynamic mics but 1meg is far too high and it becomes basically an over glorified on/off switch because of an impedance mismatch. If that was on a crystal or ceramic mic, 1meg is the minimum correct pot.
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Sincerely,
Barbeque Bob Maglinte
Boston, MA
http://www.barbequebob.com
CD available at http://www.cdbaby.com/cd/bbmaglinte
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indigo
402 posts
Oct 30, 2017
9:53 PM
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Thanks for the replies guys but i think i didn't state my original question clearly enough.(beer o'clock probably)
Ok,you have a dynamic mike so the accepted wisdom is that you need to put in a 250K pot to get the best out of it.
But if you plug the same mike (without a pot) into a (say) a Harpbreak) with a 1 meg input (and its own volume control) it all works fine.
So in one case the mike is 'seeing' a 250K impedance but in the other it is 1meg.So what is the difference between a pot in the mike to a pot in a pedal?
@MTG
Yep i saw the post re L/w diagrams which is what got me on this path in the first place.
The 'variable resistor' option looks to be ideal,in that you have control of the volume over a wide sweep.
Can you explain the 'other considerations'
Thanks(and whoops it's beer o'clock again).
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MindTheGap
2378 posts
Oct 31, 2017
1:15 AM
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Ok I think I see what you mean. I don't know the HB circuit, but it's likely that the volume pot in the HB isn't simply a potential divider, but a control on the gain of an active amplifier. In which case the input impedance is the same whatever the setting (actually I think it's 10M rather than 1M). So it's not the same as 'pot on the mic vs pot on the HB input'.
So then you ask why is it ok if the mic sees 250K and still ok if it sees 1M (or 10M in fact).
Re this 'seeing an impedance' idea, the answer here is Impedance Bridging concept. Although everyone says Impedance Matching, 'matching' doesn't mean 'make equal' in this context. The idea is to make the source (mic) impedance low, and the load impedance (amp/pedal input) high. If you do that, it doesn't matter much what value VC pot you use, the sound will be the same.
So if you have a dynamic mic with a typical output impedance of 50K, and a HB with an input of 10M, that's fine - 50K is 200 times smaller than 10M. If you then put a 250K VC as a potential divider on the mic, hardly any difference. If you put a 1M VC (as a potential divider) on the mic, then the ratio is 10, but that's OK too. But maybe not OK into a different pedal or an amp with a lower input impedance.
Next...'other considerations'. If you use the pot as a potential divider, either of LW's first two diagrams, then the situation is quite simple. It won't matter much what dynamic, Hi-Z, mic you attach it to. Again you choose a pot value that's a lot higher than the mic's impedance. In that case the pot is (approximately) taking a signal volume, dividing it, and passing on to the pedal. This is what Andrew was saying about chains of pedals, low into high, low into high...you can then treat them as separate units.
But if you use LW's third diagram, a variable resistor, then the impedance of the mic, VC and amp are acting together to divide the signal voltage - so you would get different results depending on what amp/pedal you plug it in to. The only certain thing is that when fully 'ON', you're getting the full mic signal into the amp, which is a good thing. On any other setting, it's hard to predict. As Bob says, it may just appear to switch off. At the other end of the scale, it may only shave a tiny bit off the volume.
It's hard to measure these impedances - they give one number but really it's an AC quantity that varies with frequency. Plus the amount of attenuation you want is subjective. So best to try it and see with a particular mic amp/pedal combination I'd have thought.
Phew :), does that cover it?
Last Edited by MindTheGap on Oct 31, 2017 1:23 AM
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indigo
403 posts
Oct 31, 2017
1:26 PM
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Your're right.Phew is a good way to put it.lol
But i think i can get my head reasonably around the subject now and will stick to my usual wiring system for pots.
Thanks for the great reply,helped alot.
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