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Author Topic: about input impedance...  (Read 5297 times)


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Re: about input impedance...
« Reply #15 on: May 20, 2004, 04:59:59 PM »

I have an old 1930's Shure 705A Crystal microphone that's intended to drive a really high impedance input.

Just to give some perspective on the impedance issue, this mic's datasheet gives response plots into 500K, 1M, 2M and 5M input impedances (nothing like a regular mic pre).  The low end response is highly variant.  From about 250 Hz to 7K this mic is nearly ruler flat (+/- 2dB or less), but below 250Hz things really start to happen.  Into a 500K input impedance, it's about -5dB at 80 Hz and -10dB at 40 Hz (still pretty respectable).  Into a 5M load it's only about -2dB at 40 Hz and maybe -3dB at 30 Hz !!  This from a "harmonica" mic!

Of course nobody has anything with a 5M load, except maybe an acoustic guitar DI box.

Just illustrating that, depending on the mic in question, input impedance can have a pretty big effect.

FWIW the higher the input impedance, the more susceptible the cabling/etc. will be to picking up radiated interference.  Properly balanced this shouldn't make a significant difference.

jonny boy

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Re: about input impedance...
« Reply #16 on: May 25, 2004, 11:00:54 AM »

I do alot of re-amping and i have never had a problem with re-amping any guitar. But you do of course have to change the impedance, i would suggest one of the line 6 PODPros...they do a great job for amp-modelling and re-amping.Also at the last NAMM show Radial Engineering featured the X-amp an active re-amp box with both direct and transformer coupled outputs for driving two amps without any  ground loops.

hope this helps
jonny k


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Re: about input impedance...
« Reply #17 on: May 26, 2004, 09:10:29 AM »

egg_ wrote on Thu, 20 May 2004 18:56

 I know you can buy a re-amp box ... would a passive DI box in reverse also work? It essentially does the same thing, right

Yes, it should work, the ones i've tried, did. Disclaimer: there can be passive DI's that don't work, and i'm not to be held responsible for that.

Now another impedance question. I have an old ribbon mic, looks like an RCA 74B, that needs a high line level preamp and is unbalanced. I would like to use it on recording, since it has a special aura hovering around it (it was used on plethora of classic '50's records), but i need recommendations on a suitable preamp. Obviously it doesn't feel at home in the same chain as, say, Neumann U87.

I'd be most grateful on any information.

Was that Your shoulder i stood on?


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Re: about input impedance...
« Reply #18 on: July 18, 2004, 09:53:03 PM »

I've been experimenting with impedance matching via a new Summit pre (2ba-221)that has continuously variable adjustment between 100 and 10k ohms and the general rule of thumb for me has been 4x the source (e.g. a 200ohm condensor mic with 800ohms input). After I pass that threshold I can't discern much, if any,  difference all the way to 10k. Am I missing something? I must admit my ears are just starting to get "trained".

Robert Wilkins
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