Looking for a simple 'presence' circuit...

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SSLtech

Well-known member
Joined
Jun 3, 2004
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Location
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...for an intercom. I've had to build three custom intercom stations into a 3-man film dubbing console, and the mics (Beyer cardioids, sleeve-mounted and acting rather more omni... though with slightly low output) sound flat-ish, but could certainly use a little prescence peak.

-If I talk into the Trilogy panel station that's nearby, and feeding the same Trilogy Orator comm. system, the signal 'cuts through' the mid-band noise in the machine room quite nicely. I suspect that they have a peakier mic, and then possibly a slight prescence lift. Here's a couple of their panels:

500-30-50_Med.gif

500-42-50_Med.gif


-Does anyone have a cheap-n-cheerful prescence-boost (but otherwise unity-gain) schematic?

I'm using SSM2017s as the front end preamps, and I'm driving SSM2142 line drivers, I could probably squeeze a single or dual op-amp circuit in there, providing that it doesn't get too physically intrusive.

Cheers,

Keef
 
I'm going to take advantage of a semantic loophole in your request and offer this simple passive circuit to be interposed between preamp and line driver. It gives unity output in the 5kHz "presence" band with loss outside that band variable between 0 and -12dB.

presence1.jpg


The trimpot at 50% rotation:
presence2.jpg


Trimmer at 100% rotation:
presence3.jpg


For a 3kHz center frequency, which might be more useful in a 'com, use .082uF and 33mH. The curves are of similar shape.

The Rsource and Rload are approximations based on the datasheets for the devices you're using. RL1 simply represents the DCR of the inductor.

If you have a few dB* of gain to spare in your preamp, give it a try. It's compact, cheap, easy to make, adds no noise and requires no power.

(*Most of the intelligibilty of speech is carried in the upper midrange, anyway, so you might be surprised to find that you don't have to increase the gain as much as the attenuation curves might lead you to believe).

Suitable inductors (magnetic shielded, plug-in radial):

Mouser # 434-02-183J (18mH), $1.20
Mouser # 434-02-333J (33mH), $1.20
 
How about API 553/Op-Amp labs type?


if you disregard output transformer, omit R1 and R2 on API 553
here for either peaking MF or Shelving HF with:-

http://homepage.mac.com/strangeandbouncy

With MF, you can use stock capacitor with stock inductor for @2k8Hz, or halve/double Cap for 5k6 or 1k4.

AND it'll be unity gain! :green:

ANdyP
 
By the way...

As drawn, the trimpot gives the effect of "turning everything else down", which may feel strange. If you want something that feels more like a boost, break the connection between the wiper and the end of the pot, and connect C1/L1 to the pot wiper.

If 12dB of presence boost is more than you need, use a 1K pot and simply increase your preamp gain by 6dB. Then the composite circuit will act as a true boost in the way you probably had in mind.

B2: I used Circuitmaker, student edition. I don't trust simulators in general, but it seems to work fine for simple passive circuits if you're careful to include the most important parasitics (such as series resistance of inductors).
 
That looks nice and simple!

I might put a trimpot in there and perhaps go for some numbers in the 2.5-3kHz centered range. There'll need to be three of them, one each for music, dialog and FX mixers, but I reckon it looks like just the ticket!

"NYD: -Active in the art of passive"

:green:

Keef
 
With the 33mH inductor I listed earlier, you can go a little ways to either side of 3kHz--without broadening or narrowing the curve too much--by changing the value of the cap.

2kHz = 0.22uF
2.5khz = 0.12uF
3kHz = .082uF
3.5kHz = .062uF
4kHz = .047uF
 

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