Approaching the sound of Shure SM7 with an SM57 (Unudyne III) and passive components

At your risk.


Just as cooking uses food ingredients, spices etc, electronics use resistors, capacitors, inductors ie passive devices and active devices like transistors, electron tubes etc.


Pleiades does nor have an SM7 microphone, neither an SM57. But it has a Unidyne III which is the father of both, (first such mic capsule impementation?) and it is made in USA. It had been bought at eBay.co.uk as non working and it works flawlessly. No PayPal item significantly not as described case had been opened to claim that it actually is in great working order.


Here is some SM7 story by Shure:
https://www.shure.com/en-US/performance-production/louder/sm57-on-steroids-the-shure-sm7-b-story


This family of mics including the SM57 and SM58 have a high frequency rise from similar capsules.


But the SM7 uses a switchable LRC filter to reduce the broadband accentuation and make flat frequency response:


https://pubs.shure.com/view/guide/SM7/en-US.pdf?clean_category=User%20Guide


As can be seen in figure 4 of above link, this is done by 11mH in series with 220nF in series with an unknown resistance which must be the wire resistance of the coil creating the inductance L.


By inputing the numbers to the known resonance equation:
https://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml
we get about 3.5KHz and this is where the broadband notch attenuation is centered.


Whenever a Pleiades (130Ω,40mH) filter had been used with Unidyne III in order to attenuate bass heaviness, the high frequency peak had been left exposed, prominent and gave the impression of a harsh sound.


Today has been the time to play with that.


By looking at the Shure 545 Unidyne III datasheet we see that the broad band peak is centered around 6KHz. It was nevertheless decided to stick with a 11mH inductor and play with capacitance values and then series resistance.


Fortunately 11mH already existed as a very small Magnetec ring nano perm core with about 30 turns of orange wire wrap wire.


A few Philips orange MKT capacitors were taken from another drawer. Values at 100nF, 100nF, 68nF etc.


And a vintage NOS resistor of 240Ω.


Signal path:


Male singing, speaking or hissing noise - Unidyne III - Pleiades (130Ω,40mH) gentle slope low cut filter - Pleiades LRC band rejection filter - Sony TC-D5 Pro - Sennheiser HD 580


Firstly, no Low Cut, and 11mH in series with 100nF had been connected across or in parallel with mic's output.


The mic sounded warm without loss of high treble. By gently hissing to the mic it is just the sibilant content or noise that disappears.


Then a 330mH or 160mH was also connected as bass cut. Sound was bad and had a tubby quality with or without the LCR. No 600mH inductor is at the moment available.


The gentler slope Pleiades (130Ω,40mH) had been connected and the bass performance on singing voice became very nice. But treble peak had been left exposed.


Then the 11mH,100nF filter had been connected and the sound became smooth.


The 240Ω series resistor had been added to reduce the effect of the filter. Still a very nice and natural sound.


Then 68nF had been connected in parallel with 100nF making a total 167nF capacitor. Singing voice is very natural.


It should be well worthwhile placing this LRC filter inside a Neutrik module XLR box or barrel adapter.


It may also be worth using a 1:2 or 1:3 input transformer after the filters as a mic preamp passive activator or a Pleiades V6 battery powered electron tube activator front end active amplifier for possibly the ultimate in resolution or low noise.