mistermullens
10 posts
Sep 08, 2017
7:11 AM
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I recently purchased a Shure 545, and switched it from lo-z to hi-z. Yesterday, I purchased a On Stage "Hot Wire" XLR-1/4 Hi-Z cable and my amps are feeding back except at low volume. I used my standard SM58 with the same cable, and didn't have any feedback until the amp was dimed. Am I overloading the amp input with two Hi-Z mic & cable?
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MindTheGap
2332 posts
Sep 08, 2017
7:26 AM
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Have you been reading Martin's recent threads about mics and hi/lo-Z and so on? If I've read you correctly this is in the same territory.
Martin was using a lo-Z mic into his pedals/amps and worried that he should be running it hi-Z because that is 'correct'. However, the signal voltage from hi-Z is much stronger than lo-Z and so it can be too strong for an amp - making it feedback-y and twitchy to set.
The standard advice is, having increased the mic signal by going Hi-Z, to REDUCE it again but swapping to lower gain tubes. Or buy a (quite expensive) attenuator to reduce ('pad') the mic signal.
Or you could use Lo-z mics with an 'incorrect' XLR-1/4" cable. And if the sound is nice, job done. For some reason, this is seen as a bad thing to do.
I suppose the merit of using lower gain tubes is that there are few to choose from, so by trial and error and educated guessing you may find one that works well. It's also possible that the manufacturers of the mic attenuators have found a value that is in the right range for a typical guitar amp and hi-Z mic, so it just magically works right.
Last Edited by MindTheGap on Sep 08, 2017 7:35 AM
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mistermullens
11 posts
Sep 08, 2017
7:44 AM
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"Or you could use Lo-z mics with an 'incorrect' XLR-1/4" cable. And if the sound is nice, job done. For some reason, this is seen as a bad thing to do."
When I did this with my SM58+Hi-Z cable, it really did sound great. Plenty loud as well.
As a guitar player, I'm liking this to using an 4ohm output into an 8ohm speaker. My understanding is most Fender amps can handle the mismatch, but I'm not sure about the Hi-Z/Lo-Z mismatch and if it will cause damage to equipment.
I'll try and find Martin's thread.
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timeistight
2194 posts
Sep 08, 2017
8:07 AM
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"Or buy a (quite expensive) attenuator to reduce ('pad') the mic signal."
I got my Lone Wolf Mojo Pad Deluxe for $59 at SPAH. Lone Wolf is selling the older version for $43 on their store. I wouldn't call either "quite expensive".
On the other hand, Butterfield ran high-impedance 545s into big, loud amps throughout his career and managed to avoid excessive feedback. I've never heard of him gain reducing his amps in any way. He did prefer mics with on-off switches.
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MindTheGap
2333 posts
Sep 08, 2017
8:32 AM
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timeistight - $43 is quite expensive vs $0 :) You could buy a harmonica for that. I bet the Lone Wolf thing is good though, for the reason I said.
Whatever PB did, you have to admit that the standard advice these days is to make a tube guitar amp more harp friendly primarily by using lower input gain. And it's correct advice. But it's not the only way to do it.
E.g. the amp that Giles Robson recommended in the thread about 'safe bet home amp' comes in guitar and harp flavours. The harp-flavour has a lower gain preamp tube. Not only that, but definitely that.
Last Edited by MindTheGap on Sep 08, 2017 8:37 AM
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1847
4432 posts
Sep 08, 2017
8:43 AM
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MindTheGap
2335 posts
Sep 08, 2017
11:41 PM
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timeistight - thanks for mentioning the Mojo Pad Deluxe. That does look useful, with two levels of attenuation and some kind of tone switch too.
I guess the next development might be a continuously variable attenuator, AKA a volume control! But one that you can set and leave for a particular mic and amp, rather that one on the mic to vary in performance.
Last Edited by MindTheGap on Sep 08, 2017 11:51 PM
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MindTheGap
2336 posts
Sep 08, 2017
11:49 PM
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mistermullens - 'will it cause damage to equipment?'. Not in the way that you mean by putting the wrong impedance speaker on your output would. But I wouldn't rule it out - with heavy clipping on the input stage you'll be could be putting a square wave into the power stage. That's 'max power' and I could imagine it stressing the components in an old amp. That's beyond my practical knowledge, but I wouldn't do it myself.
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Martin
1259 posts
Sep 10, 2017
7:58 AM
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@mistermullens: Here is one of my recent and extensive treads where helpful people are trying to teach me some fundamentals about electronics in a harmonica context:
http://www.modernbluesharmonica.com/board/board_topic/5560960/5496590.htm
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timeistight
2197 posts
Sep 10, 2017
8:12 AM
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MTG wrote, "I guess the next development might be a continuously variable attenuator, AKA a volume control!"
Nathan told me me that they rejected using a volume control because of impedance issues.
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MindTheGap
2337 posts
Sep 10, 2017
9:02 AM
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OK, I can understand that. The nice thing about the Lone Wolf kit is that it seems to be just right for the purpose.
That said, Greg Heumann makes a volume control and it says on his page that David Barrett has tested it with all the types of mic.
I don't know what the recipes are - although the Mojo pad is advertised as reversible, suggesting a T-Pad. Which would be more awkward to make continuously variable. I don't know what the benefit of having it reversible is, other than you can connect it in the dark.
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MindTheGap
2338 posts
Sep 10, 2017
9:13 AM
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...thinking about it, the real panacea would be a buffer pedal that could either attenuate or boost. Then you could match any mic to any amp if it had enough range and the right kind of taper.
I wonder does their 'Clean Cat' pedal attenuate or just boost? You'd want unity gain at 12 o'clock with a nice centre detent. Then attenuate to the left, boost to the right.
Last Edited by MindTheGap on Sep 10, 2017 9:14 AM
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timeistight
2199 posts
Sep 10, 2017
9:35 AM
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The Mojo Pads are T-pads. Here's an article Randy Landry wrote about pads:
http://tightcup.com/how-pads-can-benefit-harp-players/
I've not used the Clean Cat but the Harp Tone+ does boosting and cutting and throws in two bands of harp-friendly EQ.
The reason I like the pad is because it's passive. You can leave it in your gig bag for when you need it without worrying about batteries or power supplies.
Last Edited by timeistight on Sep 11, 2017 4:40 PM
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dougharps
1562 posts
Sep 10, 2017
9:40 AM
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I wouldn't use it with a crystal or ceramic element due to impedance issues, but my inexpensive Fish & Chips EQ from Danelectro has a volume slider separate from EQ sliders that can boost or cut volume. It works fine with High-Z dynamic mics.
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Doug S.
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MindTheGap
2339 posts
Sep 11, 2017
12:27 AM
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timeistight - thanks for the link. That's a good article - Randy spells out the options: tube swaps, VC on the mic or a pad. With the practical pros and cons. He doesn't mention using a lo-Z mic with an XLR-1/4" cable though :)
He also says that the voltage divider (e.g. a typical mic VC) is uni-directional, whereas the T-Pad is bi-directional. Correct, and that's important in some circuits but I don't see why that's important here.
Also he says that there's a frequency roll-off. I'm surprised about that with a purely resistive circuit at audio frequencies. But this is practical electronics and I defer to he who knows. A long time ago I worked on measurement circuits on HV, hi-frequency systems - and there you had to compensate for the small amount of inductance of the resistors. But that was much high than audio frequency.
Re the Danelectro, I couldn't find any data on it (the website is all about must-have guitars) but it's likely that the input impedance is very high, so ought to be fine with any kind of Hi-Z mic, not just dynamic. But the proof of the pudding is in the eating.
Last Edited by MindTheGap on Sep 11, 2017 12:34 AM
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