Preamp input impedance.

  • Thread starter Thread starter Steenamaroo
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Steenamaroo

Steenamaroo

...
Hey all,

I've just picked up an old 30-50ohm ribbon mic which hasn't arrived yet.
Now, I'm obviously gonna check it out first and see if there is a problem, but I'm thinking something may need to be done to raise the impedance of the mic, or indeed, lower the mic pre input impedance.

I know i can buy a transformer for the mic but my question is, is the latter possible as a cheaper alternative?

I have preamps that are racked modules from an old console; I added phantom power, line inputs and pads myself so I'm happy to tinker, but I figured someone on here might know if what i'm asking is even possible?
The pres are ward beck m490g. They have 60-12 input transformers.

The specs say preamp input impedance is 750ohm and I'm thinking making it switchable to around 150 would be about right?


If this is a goer I can post schematics of the pres but for now I'm just enquiring.

Thanks in advance.
 
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I would guess that there is little mechanical damping effect of impedance on a ribbon, because the ribbon's impedance is so low compared with the transformer primary. So you will give up about 1dB of output going from 750 to 150 ohm with a 30 ohm source, and that's about it.

The only way you'd want to increase the output impedance of the ribbon is to rewind its secondary, which is another way of saying buy a new transformer. That could be worthwhile as a 30 ohm source is probably very low sensitivity; increasing that should help with SNR.
 
Thanks for replying MSH.

Ok, i think I follow you.

Are you saying altering my preamp isn't going to do what i thought?

Out of interest, when you buy a pre that has Lo-z mic input option, will it just have such a transformer built in to raise the impedance of the mic?
 
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Steenamaroo said:
Thanks for replying MSH.

Ok, i think I follow you.

Are you saying altering my preamp isn't going to do what i thought?
Click to expand...

Correct. It might alter the sound of the mic due to the reactive source impedance of the transformer, but given that you are still pretty far from a matched load I don't think the effect would be very large. It will have essentially no effect on the mechanical system, as loading a moving-coil mic can.

Out of interest, when you buy a pre that has Lo-z mic input option, will it just have such a transformer built in to raise the impedance of the mic?
Click to expand...

You can't change the mic source impedance unless you change something about the mic. However, by varying the load on the mic you can change the performance of the system. Variable input impedance preamps will have either a variable resistance (switch or pot) or multitap transformer at their input.
 
Ok, so you can adjust preamp input impedance with variable resistance or with a transformer, but if i read you right the former isn't really beneficial to an old ribbon mic?

If that's the case I'll pick up a transformer when the time comes.

Thanks MSH.
 
If the transformer increased the ratio by hitting a different tap, that *could* help with noise performance, but there are practical limits to that approach. It wouldn't be as good as a proper transformer in the mic.
 
Thanks for those links. That EDCOR jobs has the right price. Shame the postage is twice that again.

I'll shop around for some UK alternatives.
 
I believe Canford distributes Lundahl, but they don't list the LL2913.
 
Got a good deal on an old reslo. Heard very mixed reviews on them but if I'm honest I always just really liked the look.
Might end up being an ornament! lol.
 
I want one of those ornaments. lol
I'd love an old eagle mc70 on the shelving right outside my studio door, beats a lava lamp any day.
Not that i hate lava lamps i opted for a big plasma plate instead.
 
My guitar shop friend always says he used to get an electric shock of the old reslos.
Anyone else heard of this?
 
what kind of tranny is recomended with reslo's the period ones or a modern cloudlifter/ fethead type?
 
Cloudlifter and Fethead are amplifiers (active components), not transformers (passive).
 
kip4 said:
My guitar shop friend always says he used to get an electric shock of the old reslos.
Anyone else heard of this?
Click to expand...

This thread makes it apparent that I'm no guru,lol but I guess something in his setup isn't wired correcty/safely.

I've read a bunch of people saying the difference is night and day between trying to use this mic stock and using a transformer (linked above), but that still doesn't mean it's good.

Curiosity has got the better of me, but I did hear a voice sample and a few drum samples that I liked, so we'll see.
 
my friend was probably smoking way too much pot in the 60's.
theres a couple of trannys on cpc steenamaroo not sure they are suitable but then as msh points out im no expert hope this helps
Browse for Products | CPC
 
Steenamaroo said:
Thanks for those links. That EDCOR jobs has the right price. Shame the postage is twice that again.

I'll shop around for some UK alternatives.
Click to expand...

Dang, I wouldn't have expected something that small to be so expensive to post. I just sent some small circuit boards to France and Finland, and it was about US$5 in postage. But I also sent a MXL V69 to Germany and that was steep. (US$79 or so)
 
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