Mic Pre Impedance test

[sr1200]

is there a way to measure the impedance of a mic pre?  Can i use a multimeter or do I need to build some kind of test load?

 

[Andy Peters]

is there a way to measure the impedance of a mic pre?  Can i use a multimeter or do I need to build some kind of test load?

You can determine the input impedance by looking at the schematic, or by opening the unit and tracing the signals.

-a

 

[sr1200]

reason im asking is that I have a build that the impedance is selectable, but I have a strange feeling that something got reversed and where it indicates impedance "A" its really "B" and vice versa.  Was hoping there was a way to plug something in with a load on it and measure the imp...

 

[JohnRoberts]

You can crudely determine the input impedance termination by using a sine wave voltage source and feed the input through a known fixed impedance (say 1k ohm). With the known voltage at far end and measured voltage at the preamp end of  the 1k you should be able to calculate the impedance from the voltage drop.

For a balanced differential input you need a floating sinwave source and two 1k resistors, but you should be able to determine whether your loading is backwards by shorting the other input to ground and measuring one at a time.

JR

 

[sr1200]

JR, in an unrelated topic, is the resotune thing yours?  And if so, are you going to be at NAMM?  My friend, a DW/Sabian endorsee, would love to check it out.

 

[JohnRoberts]

JR, in an unrelated topic, is the resotune thing yours?  And if so, are you going to be at NAMM?  My friend, a DW/Sabian endorsee, would love to check it out.

[veer]
Yes I am RESOTUNE...(Chief Orb)

No I will not be at NAMM, more than ten years away and counting... boorah.

I had booths at a couple PASIC drum shows but they made NAMM seem civilized (and quiet).

Tell your buddy he can contact me through my website but I am not very big on giving away free sh__

Professional drummers are a key demographic for me. If they can't afford a high tech solution who can?

I do have many happy customer with DW kit... nice drums. I'd endorse them too.  8)

I discuss how the DW "timbre-match(R)" relates to RESOTUNE resonance targets for tuning to notes, in my website FAQ section.

JR

[/weer]

 

[CJ]

mic pre  impedance,

you mean input Z

you can measure the inductance of the input xfmr with a 20 dollar meter and get  close answer,

5 Henries = about 150 ohms, 10 H = about 300, 16-20 would be 500/600 ohms,

or measure DCR for a rough estimate,

here are some measurements i took off some mic pre's:

Quad 8 with Jensen JT 110K: 35 ohms

Western Electric  with DuKane xfmr:  12 ohms

API with Cinemag 1:7  23 ohms

Langevin Am16  67 ohms

Telefunken V76  5.8 Ohms

Telefunken V72 with Peerless K-241-D xfmr  9 ohms

most of these inputs have about 10 to 40 henries of L on the input

you might have taps on your input trans for multiple input Z,

 

[sr1200]

k CJ, thats interesting...  45.7 is the reading at the supposed 1200 ohm setting and 11.7 is what im getting at the 300 setting.  would be cool to come up with a formula that works so that one could psudo check without too much hassle.

JR - im sure he wouldnt expect free by any means (doesnt get drums or cymbals for free... just dirt cheap compared to average joe haha) but I read through the site, very informative and hell, if that thing works that well, i think id be interested for one for the studio... God knows the POS drumsets I get thru here could stand a good tuning that doesnt take 53 hours.

 

[CJ]

i averaged all the mic pre values and get 25 ohms per 600 ohm winding,

the Langevin is kinda high, so throw that out and you get about 18 ohms per 600 Z.

an easy to remember number that will get you close is 20 ohms per 600.

different transformers have different structure, which affects DCR,

split coils like UTC are lower, bi fi like dukane are lower, so hard to be exact.
now i have to crank Moterhead to de-gauss all the pre's,  :-*

 

[MagnetoSound]

now i have to crank Moterhead to de-gauss all the pre's,  :-*

You poor thing.  8)

 

[Rob Flinn]

The i/p Z of the transformer is also related to the Z it is feeding into, due to reflected impedance. 
Depending if the trafo has a fixed resistor strapped across it's secondary, & you have the trafo spec it may be possible to guesstimate the i/p Z using the reflected impedance rule to some level of accuracy.

 

[sodderboy]

I was taught the following by George Alexandrovich:
Add a 5 or 10K pot turned CCW, linear is best, in series with an oscillator as source, feeding the load (input Z under test).  Measure level across the load, lets say it is 500mVAC.  Increase the pot, or series resistance, until you have a 50% drop across the load 250mVAC.

Remove the pot and measure the resistance.  That will be your impedance.  The slick thing is that you can use different frequencies and get an impedance map of the input.

It is the same idea as John's, but without the math.
Mike

 

[emrr]

Yep, and measuring for a variety of frequencies is necessary with transformer coupled inputs, not so much with transformerless, maybe just the bottom end if you are looking at cap related roll-offs on the bottom. 

 

[JohnRoberts]

Thanks I don't like repeating myself.  Direct measurement will be more reliable than going through all the transformer calculations, or trying to impute Z from winding DCR.

I actually built the voltage drop method of measuring impedance into my old TS-1 design.  There was a 51 ohm series resistor in the sine wave output, before the dB meter pick up, and a look up chart in the book/user manual.  This made it relatively easy to measure impedance wrt frequency. With X dB drop = Y impedance.

JR

 

[RuudNL]

Add a 5 or 10K pot turned CCW, linear is best, in series with an oscillator as source, feeding the load (input Z under test).  Measure level across the load, lets say it is 500mVAC.  Increase the pot, or series resistance, until you have a 50% drop across the load 250mVAC.

This is the method I learned at school a long time ago. I used it ever since and it is IMHO the easiest and most reliable way of determining an input impedance. Especially if you use a digital voltmeter and ohmmeter!

 

[JohnRoberts]

JR - im sure he wouldnt expect free by any means (doesnt get drums or cymbals for free... just dirt cheap compared to average joe haha) but I read through the site, very informative and hell, if that thing works that well, i think id be interested for one for the studio... God knows the POS drumsets I get thru here could stand a good tuning that doesnt take 53 hours.

Again contact me directly to discuss friends and family pricing.

Of course it works but I won't feed the veer here...

while I do love talking about my children.

JR

 

[CJ]

remember that R and X are 90 degrees to each other,

so a 5K pot resistance and a 5 K reactance add up to root 2 times 5 k, = 7K not 10 K,
but this should not change the reading, just do not use current times R to get the voltage,1

when you put that much dcr in series with the transformer, other stuff happens to the waveform that causes error but we just need a rough estimate.

you can also inject a  signal straight into the primary of the input transformer,

put an ammeter in series with the generator, use 1 volt at 100 hz,

measure the AC current,

let us say you measured 1 ma AC, ohms law says 1 volt/1 ma = 1 k,
(when dividing a voltage by ma, just use the number in ma and add a "k" to the answer)

so we know the X of the inductor at 100 hz, we only have one variable left, inductance,

X=2 pi f L

1K = 6.28 * 100hz * L

1K = 62.8 *  L

1K / 62.8 = L

L = 16 henries,

input Z at 20 hz will be

6.28 * 20hz * 16H. = 2K

we use 100 hz to measure exciting current because things get non linear below 20 and above 1 K when the core quits.

 

[JohnRoberts]

remember that R and X are 90 degrees to each other,

so a 5K pot resistance and a 5 K reactance add up to root 2 times 5 k, = 7K not 10 K,
but this should not change the reading, just do not use current times R to get the voltage,1

when you put that much dcr in series with the transformer, other stuff happens to the wave form but we just need a rough estimate.

you can also inject a  signal straight into the primary of the input transformer,

put an ammeter in series with the generator, use 1 volt at 100 hz,

measure current,

The impedance of a mic preamp input termination should be resistive or 0' phase.  Pure inductance or capacitance would +/- 90' .

Lets not make this more complicated than it is

JR

 

[CJ]

when you add the 5 k pot to the input circuit...jus sayin...

also, Q of input transformer without pot, use 20 ohms for transformer DCR:

2 pi f L = 1 k at 100 hz

Q=1K/20 ohms dcr

Q=50

now add pot:

1k/5200=0.19

so you have changed the Q of the circuit from 50 to 0.2, or a factor of 250,

so your test conditions are 250 times different than the actual circuit,

 

[CJ]

ok all you have to do is alter the pot method so that the resistance is very small,

you can even use a variac if you are careful.

test circuit illustrated below>

we want to measure the xfmr at a flux density close to the actual operating conditions,

we need more current for this, more than hand held meters can supply,

and we want the voltage across the sense resistor to be much smaller than the voltage across the inductor, so select R1 such that V2>>V1,


just pump in enuff volts til the current stops rising, (voltage across R1 stops rising), then divide that voltage in half, that will get you a similar test condition to the actual circuit,

make R1 0.1 or 1 or 10 ohms so you can calculate current easily,

or buy a 20 dollar meter on evilbay,

so procedure for circuit below:

1. inject a voltage that you think is 1/2 below the max that the pea amp can take,
2. measure the voltage across R1
3. calculate the current I=V1/R1
4. calculate the reactance XL=V2/I
yor input Z at the test freq will be the reactance,