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[carlos del valle]

Hi there, not sure if this is the right place to post this, but...


I've found this cool technics hifi speakers on the street. they don't work, but the cones seem to be in neat condition. I want to make a mic out of the 5" mid range speaker, and the wiring is perfectly placed and labeled + and -, with an impedance of 16 ohms. I just want to know what kind of connector should i stick at the end of the cables, if there would be a way of grounding it if i could put an xlr, or it had to be a mono jack, and what source impedance would it have. could i just plug it in a mic input? line input?

thanks a lot!

[rankus]

The easiest / best way to this is to wire it up with a 1/4" jack on instrument type cable (Shielded two strand)...  Then use a Direct Box to match impedance to XLR...

[carlos del valle]

of course, DI. the mistakes are there to learn from them... Thanks!

[sonny]

I used an old 5" driver from sum shitty speakers and wired it up to a XLR using the -ve as the ground as well (like any unbalanced lead) and it works fine plugged straight into a mic pre.  I generally have to pad it of wind bottom end out, especially when using on a Kick.

[James Perrett]

rankus wrote on Fri, 06 October 2006 18:23

The easiest / best way to this is to wire it up with a 1/4" jack on instrument type cable (Shielded two strand)...  Then use a Direct Box to match impedance to XLR...

Actually speakers have a really low impedance so a DI box isn't the best way to go. If anything you need a reverse DI box to bump up the output voltage a little. I would just wire the speaker terminals to pins 2 and 3 of an XLR through a 2 core screened microphone cable with the screen connected to pin 1.

Cheers

James.

[carlos del valle]

so, the 16 ohms is actually the output source impedance of the "mic"?

[electrical]

The loudspeaker used as a michrophone is a dynamic impedance source, with a nominal impedance of 8 or 16 Ohms, or whatever it measures as a resitive load.

You do not need to use a direct box to "match" its impedance to a microphone input, as its source impedance is already an order of magnitue lower than a conventional microphone (50-500 Ohms). Worse, you will be throwing away much of the signal through the transformer.

Make a cable that connects the two conductors in the loudspeaker to pins 2 and 3 of a standard XLR connector, and then plug it in like any other microphone. Do not do this if you cannot shut-off the phantom power, as the loudspeaker will act as a near dead short to the phantom supply.

In that case, use a similar (but opposite gender) special-made XLR cable to connect the loudspeaker to a passive direct box at the low-impedance (XLR) jack. That is, you use the direct box to raise the source impedance. Then connect the unbalanced 1/4-inch jack of the direct box to a second direct box to re-balance and isolate the loudspeaker. Connect the balanced output of this (second) direct box to the microphone input.

Draw a diagram if you need to. It's a lot simpler than it sounds.

[Andy Peters]

electrical wrote on Mon, 06 November 2006 19:23

Make a cable that connects the two conductors in the loudspeaker to pins 2 and 3 of a standard XLR connector, and then plug it in like any other microphone. Do not do this if you cannot shut-off the phantom power, as the loudspeaker will act as a near dead short to the phantom supply.

Actually, phantom shouldn't be a problem.  A loudspeaker looks and acts like a dynamic mic, which has no problem ignoring phantom power.

Pin 2 and pin 3 are both biased up at +48 V.  The voltage difference between the two pins, and therefore the two terminals of the speaker, is 0 V, so no problem.

-a

[electrical]

Andy Peters wrote on Tue, 07 November 2006 00:20

electrical wrote on Mon, 06 November 2006 19:23 Make a cable that connects the two conductors in the loudspeaker to pins 2 and 3 of a standard XLR connector, and then plug it in like any other microphone. Do not do this if you cannot shut-off the phantom power, as the loudspeaker will act as a near dead short to the phantom supply. Actually, phantom shouldn't be a problem.  A loudspeaker looks and acts like a dynamic mic, which has no problem ignoring phantom power. Pin 2 and pin 3 are both biased up at +48 V.  The voltage difference between the two pins, and therefore the two terminals of the speaker, is 0 V, so no problem. -a

Absolutely right. I was thinking (in the case of the phantom power) of an un-balanced connection to the mic input, where pin 3 was tied to pin 1. That's the forbidden connection.

I've been away too long. My "bad."

[carlos del valle]

Let me see if i'm getting the interesting part of this.

So, you use a DI box to match high impedance sources (around 15Kohms) to the low impedance mic input (something like 2000 or 3000 ohms), but if the nominal impedance of the source is smaller than 1/10 the mic input impedance, you don't need to match it...

[electrical]

carlos del valle wrote on Wed, 08 November 2006 13:47

Let me see if i'm getting the interesting part of this. So, you use a DI box to match high impedance sources (around 15Kohms) to the low impedance mic input (something like 2000 or 3000 ohms), but if the nominal impedance of the source is smaller than 1/10 the mic input impedance, you don't need to match it...

A microphone input expecting to see a nominal source impedance of 150 ohms won't normally object to a lower impedance than that. There are certain specific amplifier topologies incorporating an input tranfromer within a feedback loop that do underperform with very low source impedances (<20 Ohms), and David Josephson has written about this case in the past, but for most microphone pre-amplifiers, low impedances are not as much of a problem as high (>1.5k Ohm) impedances.

Besides which, running a 16 ohm source through a step-down tranformer as used in a direct box would lower the source impedance, exaggerating the mis-match.

The transformers in the example I give are used to balance and isolate the loudspeaker "microphone." The resultant source impedance (assuming the same transformer is used for step-up and step-down) would be the same as the original loudspeaker.

[ShakesTheClown]

How would speaker/magnet size affect the sound of the transducer?

[carlos del valle]

ok, I think I got it...

Thanks

I have to check the input transformer within a feedback loop thing, tho. Completely new concept for me...