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Mike Rivers

Aqua Marine

Posts: 2,525 Member Since:13/10/2012

#22 [url]

Oct 4 14 9:08 AM

intaud wrote:

I guess “high input impedance” is relative.  I selected input impedance based on maximizing voltage transfer and S/N, and reduced loading.  Using simulation I found after about 10k the law of diminishing returns prevailed.  With a source impedance of 250 ohms, and using the 10x “regular” (?) input impedance, the S/N for 2.5k ohms was about 1 dB worse than 10K ohms. 

If you are saying high input impedance can have an adverse effect on the microphone’s frequency response then I need to better understand this.  How does 10k (or 30k) change the curve compared to the optimal input impedance (whatever that is)?
 

The load impedance that the microphone sees affects the amount that the mechanical resonance of the ribbon motor assembly is damped. Since most mics have a resonance within the audio frequency range, this is incorporated into the design of the mic. There's a certain amount of leeway, of course, but it's one of the things that contributes to why a mic sounds different when used with different preamps. If the preamp input impedance is too low, the ribbon will be overdamped causing a dip in the frequency response. If the impedance is too high, the ribbon will be underdamped, resulting in a peak in the frequency response.

If there's a transformer between the ribbon and the mic output (there almost always is), the incuctance and capacitance of its windings forms a resonanat circuit which also must be properly damped to avoid irregularities in the frequency response. So that's a secondary effect of load impedance on the "sound" of the mic.

Different mics have different resonant characteristics so one size doesn't quite fit all. For what it's worth, the AEA TRP (ribbon mic preamp) has an input impedance of 15k, or maybe it's 18k ohms, because that's what seems to be what works best with their most popular mics. Back in the early days when everything had an input impedance of 600 ohms, people would take the input transformer out of their Ampex 300 and connect their RCA 44 directly to the grid of the preamp tube, presenting it with a load of a megohm or so.

Modern active condenser mics have their pre-preamp designed to optimally damp the mic's ribbon and effectively isolate it from the preamp. This is today's solution (not a bad one, actually, if done right) to the "which preamp is best for this mic?" And that mic? And the other mic? 



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intaud

Tin Man

Posts: 11 Member Since:13/02/2014

#23 [url]

Oct 6 14 10:08 PM

My main interest in this discussion is in regards to passive ribbon microphones with output transformers. It seems the latest thing is Active Ribbon mics. To me this kind of takes away from the attraction to ribbon mics and their simplicity. I want to avoid active electronics as much as possible in the signal path.

I can see the need to consider electronic damping for moving coil dynamic microphones due to their mass. I’m kind of surprised preamp input impedance can affect the damping of ribbon microphones with their relatively little mass. I understand the transformer’s peaking frequency response due to resonance if not properly loaded. What I’m curious about is why this resonance is not compensated for by the microphone designer? A simple Zobel RC network tuned for the particular transformer could smooth out the peaking. The capacitor should be small. I believe this is commonly used in transformer based preamp designs.

Considering all this the one thing that’s confusing to me is the ribbon microphone specification for recommended preamp input impedance. It’s typically stated as a greater than value (i.e. “> 1500 ohms”). This seems to imply don’t be too low and the higher the better, to minimize loading and maximize voltage transfer. If concerned with damping and maybe resonance why not specify an optimal value or range?  At any rate maybe I’m trying to make a mountain out of a molehill, just say anything in the 1.5k to 3k range should be fine for most ribbon microphones and call it a day.

Thanks,
Dave

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Mike Rivers

Aqua Marine

Posts: 2,525 Member Since:13/10/2012

#24 [url]

Oct 7 14 6:31 AM

The ribbon in a mic may have very low mass, but (at least with the classic ribbon mic design) it's also very loosely sprung so it can move easily. The resonant frequency of the ribbon is quite low, often down in the 10 Hz range. There is often some internal adjustment, sometimes mechanical, sometimes electrical, sometimes both, to get the sound that the designer is after when the transformer is terminated somewhere in the ballpark of what he wants.

Remember that in a ribbon mic, you have a step-up transformer, which, for the preamp load impeance being reflected back to the ribbon, it's a step-down, and the impedance ratio is the square of the voltage ratio. So, just for an example, if it's a 10:1 transformer, a 1000 ohm preamp input impedance looks like 10 ohms across the ribbon. With active electronics between the ribbon and the connector, the ribbon always sees the same electrical load, so regardless of what happens on the other end of the mic cable, the ribbon always behaves as designed.

You probably are making a small mountain out of a small mole hill. While in any given situation, you can change the sound of any dynamic mic by varying its load, if you get used to what your mic sounds like with your favorite preamp, you'll lean to work like that. The ribbon motor doesn't generate a lot of voltage, so saving as much of that as you can is a good idea, but have some faith in modern electronics. Most modern preamps have about 60 dB of gain and at times, you'll need more than that from a "barefoot" mic if you want to get a decent record level. You'll end up with a better signal-to-noise ratio by putting a specially designed low noise amplifer ahead of the preamp to give you 20 dB or so of quiet gain that's also optimized for the mic. That's what you get with an active ribbon mic or an outboard pre-preamp like the Cloudlifter. But you don't want to put just any old amplifier there. By the way, somebody makes an active ribbon with a switch to bypass the internal electronics if you don't need the gain.

If you need a rule of thumb, I think it's reasonable to expect that the sound of a mic won't change very much, either in level or frequency response, when it's loaded with 3k ohms or higher. You might notice the effect of a 1k load with some mics, and not others. And for what it's worth, many years ago I reviewed a CAD Trion dual ribbon mic and it was so floppy that, while it didn't do the high end any good, I liked it better when loaded with 300 ohms when used on an acoustic guitar.



For a good time, call mikeriversaudio.wordpress.com

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intaud

Tin Man

Posts: 11 Member Since:13/02/2014

#29 [url]

Aug 14 16 9:51 PM

Thanks Brad!

Being a somewhat speciliaized preamp it's been slow but steady, which is probably a good thing when starting out.  It is gaining traction in classical and jazz applications where transparency and low noise are desired.

Dave

http://www.integeraudio.com
 

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noahsnyder

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Posts: 5 Member Since:10/05/2016

#31 [url]

Jun 9 17 10:33 AM

Hi Dave,
 I heard your preamps recently on some orchestral recordings and they sounded great. I'm impressed.
    Best,
        Noah

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