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DIY Stompboxes => Building your own stompbox => Topic started by: Mr.Kite on September 27, 2020, 04:41:01 PM

Title: Variable Impedance Control (Mic Preamp)
Post by: Mr.Kite on September 27, 2020, 04:41:01 PM
Hi,

I built this (https://www.youtube.com/watch?v=r1yPMDHiRuc&ab_channel=DIYRecordingEquipment (https://www.youtube.com/watch?v=r1yPMDHiRuc&ab_channel=DIYRecordingEquipment)) Variable Impedance control and I'd like to hear your opinion about the design\wiring.

I tried it with an SM57 (used to mike my AC15) with a Presonus Tube Pre V2 (valve section disengaged): it alters the tone slightly, losing some detail, gaining some warmth. Also the transient response seems a bit slower with lower settings, and it starts to lose quite a bit of output with very low settings (under 200-300 Ohm or so).

Basically it inserts a variable resistor (a 5k Log pot in my case) between the + and - signals of the TRS\XLR connection, in parallel with the input impedance of the mic preamp. Sorry for the dumb question, but is this the correct wiring for an Impedance Control in your opinion?
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: Mr.Kite on May 30, 2021, 10:14:02 AM
BUMP!

Since I've built this box I didn't use it very much, it seems to lower the volume and nothing else...but I'm not sure about the wiring: please, help me to figure out how to wire this thing!

I don't know if the variable resistor goes between Tip and Ring, Tip and Sleeve or Ring and Sleeve of the TRS connector. Right now I have it between Tip and Ring...

Basically I want to add an external variable impedance control to my preamp. I've already experimented with a similar device to use with my guitar, liking the results. In this case, the variable resistor inevitably goes between Tip and Sleeve, and it works very well.

Title: Re: Variable Impedance Control (Mic Preamp)
Post by: antonis on May 30, 2021, 02:23:24 PM
Basically I want to add an external variable impedance control to my preamp. I've already experimented with a similar device to use with my guitar, liking the results. In this case, the variable resistor inevitably goes between Tip and Sleeve, and it works very well.

So, what's your query..??
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: Mr.Kite on May 30, 2021, 05:31:34 PM
So, what's your query..??
I don't know if the variable resistor goes between Tip and Ring, Tip and Sleeve or Ring and Sleeve of the TRS connector. Right now I have it between Tip and Ring...
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: Rob Strand on May 30, 2021, 07:19:13 PM
Quote
I don't know if the variable resistor goes between Tip and Ring, Tip and Sleeve or Ring and Sleeve of the TRS connector. Right now I have it between Tip and Ring...
If your connector has a ring it's highly likely the mic is wired across Tip and Ring.   The ground/sleeve is wired to the coax shield.  The mic should work into a guitar amp (but it's not wise to plug the mic directly into a pedal) [actually maybe not, it depends on how the amp sockets are wired].

In this case you wire the loading resistor across the Tip and Ring.
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: Mr.Kite on May 31, 2021, 09:47:50 AM
Thanks, that's what I did, but the effect seems extremely subtle: the bigger difference is the output volume...

I'm not going to plug the mic into a guitar amp, just into a mic preamp: I thought this could be a nice additional tone control when recording but it doesn't seem very effective...
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: fryingpan on May 31, 2021, 10:57:46 AM
If I understand your query, the way I see it, a variable impedance control for a balanced signal should be a dual-gang pot between the two signals and ground, otherwise you're only altering the input impedance for one of the two signals (and the most obvious result is: loss of volume).
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: merlinb on May 31, 2021, 11:14:57 AM
In this case you wire the loading resistor across the Tip and Ring.
Correct.

a variable impedance control for a balanced signal should be a dual-gang pot between the two signals and ground,
Incorrect
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: fryingpan on May 31, 2021, 11:51:49 AM
In this case you wire the loading resistor across the Tip and Ring.
Correct.

a variable impedance control for a balanced signal should be a dual-gang pot between the two signals and ground,
Incorrect
Why? The idea of a balanced signal is that you get two signals in antiphase, so that when you invert one of the two you have two equal power signals and cancel out the common mode. If you add a resistor between the two signals (tip and ring are the two signals, while sleeve is the ground) aren't you feeding the two signals one onto another?
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: merlinb on May 31, 2021, 03:02:54 PM
aren't you feeding the two signals one onto another?
Yes, which is the whole point: current flows from one phase to the other. If you want the signal source to see a particular impedance, it needs to be between the phases. Nothing flows in the ground, it's a dead end, just a metal casing around the microphone.
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: iainpunk on May 31, 2021, 03:21:43 PM
In this case you wire the loading resistor across the Tip and Ring.
Correct.

a variable impedance control for a balanced signal should be a dual-gang pot between the two signals and ground,
Incorrect
Why? The idea of a balanced signal is that you get two signals in antiphase, so that when you invert one of the two you have two equal power signals and cancel out the common mode. If you add a resistor between the two signals (tip and ring are the two signals, while sleeve is the ground) aren't you feeding the two signals one onto another?
>aren't you feeding the two signals one onto another?
yes, that's basically the whole idea of impedance in this case. the center of the resistor would have no signal at all.
if you put resistors on the signal's to ground, they might get really hot with phantom power. and another thing is imperfectness of the impedance, if they aren't perfectly the same, common mode will be injected in the signal.

i really don't see the point of this project tho, since all it does is act like a worse than standard volume control that takes out different frequencies more than the rest, depending on the impedance of the source. this thing can only make your sound worse...

cheers
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: fryingpan on May 31, 2021, 03:37:22 PM
aren't you feeding the two signals one onto another?
Yes, which is the whole point: current flows from one phase to the other. If you want the signal source to see a particular impedance, it needs to be between the phases. Nothing flows in the ground, it's a dead end, just a metal casing around the microphone.
What about the resistance to ground in the input stage of an amplifier (generally speaking)? If the input resistance of a semiconductor device is infinite (for all intents and purposes) it's that resistor to ground that sets the input impedance.
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: merlinb on May 31, 2021, 05:01:25 PM
What about the resistance to ground in the input stage of an amplifier (generally speaking)? If the input resistance of a semiconductor device is infinite (for all intents and purposes) it's that resistor to ground that sets the input impedance.
Sounds like you're now talking about an unbalanced circuit, where the signal source is connected between hot and ground (like in most guitars). i.e. the signal source is connected across the resistor.

In a balanced preamp the input impedance is determined by whatever resistance is between the two phases.
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: fryingpan on June 01, 2021, 06:02:45 AM
I see. Right. Anyway the variable impedance control is a gimmick, and it has no use whatsoever with preamplified microphones. It can be somewhat useful with dynamic or ribbon microphones as the increased load will cause some (technically unwanted) tone shaping, just like a guitar in a low-Z preamp will sound dull, but the idea is that you have some quick and dirty tone shaping that just "happens". Compare this to the output impedance of a power amp. A high output impedance will cause colouration as the power amp will output more power around the impedance peak (and towards the rising slope in the treble) leading to a scooped sound. There is little to suggest that a "zero" output impedance power amp with equalisation cannot sound like a high impedance one, but the difference is that the equalisation basically depends on the speaker's impedance. Switching speakers "switches" eq too, without any intervention on the part of the user. This is certainly unwanted in hi-fi (and bass, due to the prevalence of ported cabs), but maybe desirable (for ease of use) on guitar.
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: iainpunk on June 01, 2021, 02:57:13 PM
I see. Right. Anyway the variable impedance control is a gimmick, and it has no use whatsoever with preamplified microphones. It can be somewhat useful with dynamic or ribbon microphones as the increased load will cause some (technically unwanted) tone shaping, just like a guitar in a low-Z preamp will sound dull, but the idea is that you have some quick and dirty tone shaping that just "happens". Compare this to the output impedance of a power amp. A high output impedance will cause colouration as the power amp will output more power around the impedance peak (and towards the rising slope in the treble) leading to a scooped sound. There is little to suggest that a "zero" output impedance power amp with equalisation cannot sound like a high impedance one, but the difference is that the equalisation basically depends on the speaker's impedance. Switching speakers "switches" eq too, without any intervention on the part of the user. This is certainly unwanted in hi-fi (and bass, due to the prevalence of ported cabs), but maybe desirable (for ease of use) on guitar.
another thing is that there is impedance dependent distortion and/or non-linearity in power amps with high output impedance. i find that my 16 ohm cab gives earlier onset of overdrive than my 8 or 4 ohm cabs. my amp is a single ended current mode amp, 'normal' voltage mode push pull amps, like 85% of guitar amps would have an earlier onset of distortion with lower impedance cabs, like 4 ohm. microphone internal amps also have this effect, next to shaping the frequency response, the impedance change will aslo determine the onset point of distortion.

this idea is a total gimmick, which worstenes every aspect of your signal. i highly discourage its use.

cheers, Iain
Title: Re: Variable Impedance Control (Mic Preamp)
Post by: fryingpan on June 01, 2021, 04:51:56 PM
To be fair, one of the ART preamps (not the best, but not the worst either) has a variable impedance control, and considering the cost of adding it to a preamp, it's one of those gimmicks that it can't hurt to have (at worst, you leave it in the maximum position - which is what you should do most of the time).