Microphone question...

[RolandSWL]

Hi All,

 Ultimately, source impedance should match load impedance for maximum power transfer. How about a microphone

with an impedance of 300 ohms

plugged into a transmitter with an input impedance of 5 Megohms? No problem?

Thanks, Roland

[W3RSW]

Gain will be way down for several reasons, not just the impedance power mismatch.
Odds are that a 300 ohm mike is highly inductive  
That will make some interesting RLC frequency responses and bandpass changes due to several conflicting and mutually coupled LC circuitsbetween the mike and the input audio stage, etc.

Also a typical 300 ohm mike puts out 40 to 50 mV whereas the 5 meg xmitter input may have been designed for a crystal mike putting out " many" mV's ( think of the VW ad with the older ladies, "many miles.")

But hey, try it. You'll probably have to crank up the first stage audio gain into the hum fun region, but you'll get audio. Oh yes, you'll get audio.

[W7TFO]

So use a suitable input transformer...

73DG

[MikeKE0ZUinkcmo]

My station mic is a standard D-104 and I run it into this buffer/amp and switch it between several tube and solid state rigs.   Works well with all of them.   Never have done any "tests" to see what the output Z looks like but no bad reports.

[RolandSWL]

Thank you for your answers.

RSWL

[flintstone mop]

Gain will be way down for several reasons, not just the impedance power mismatch.
Odds are that a 300 ohm mike is highly inductive  
That will make some interesting RLC frequency responses and bandpass changes due to several conflicting and mutually coupled LC circuitsbetween the mike and the input audio stage, etc.

Also a typical 300 ohm mike puts out 40 to 50 mV whereas the 5 meg xmitter input may have been designed for a crystal mike putting out " many" mV's ( think of the VW ad with the older ladies, "many miles.")

But hey, try it. You'll probably have to crank up the first stage audio gain into the hum fun region, but you'll get audio. Oh yes, you'll get audio.

I agree
An extreme mismatch like that would be horrible frequency reponse.

[RolandSWL]

oh and how do I add the roger beep?

[w4bfs]

... Ultimately, source impedance should match load impedance for maximum power transfer.

a true statement, no question

a lot of people in electronics have trouble with this ... not always are you needing to transfer power ...sometimes you just want a voltage swing signal that you can get without interferences to apply...so here goes

if you are not transferring power it is just fine to follow a low source impedance with a high load impedance ...it definitely is not fine to follow a high source impedance with a low load impedance .... the sources must be rated or be applicable to the application... for example

1. a 12ax7 preamp stage has a 100k Ohm plate resistor and we want to capacitively couple to a volume control pot ....what minimum value pot should we use ?  the output impedance of the preamp is around 50k Ohm and if you load it down with a low impedance load you will kill the stage gain and likely distort the signal ... a simple rule is an order of magnitude so if the output impedance is 50k Ohm then using a 500k Ohm pot will not excessively load it and importantly you can use an even higher value pot and load even less ...aotbe

2. a cathode follower uses a 1k Ohm resistor to ground for the output signal .... do we have to load it with a 1k Ohm impedance load ? .... it depends .... for low freq work into non-reactive loads, no HOWEVER rf into rf loads that are reactive that is a problem that will need analysis

getting the idea ?   .... you can always apply circuit analysis techniques to a proposed application and test your hypothesis

[AB2EZ]

As Beefus says:

In most low-level audio applications, you are not trying to maximize power transfer from the source to the load. The power is miniscule (1V rms across 600 ohms =>0.0017 watts) , so maximizing power transfer (efficiency) is not an issue.

What you are trying to do is to produce enough voltage across the total load on stage X (which could be the microphone) to drive the input of stage X+1, and to the minimize noise and distortion added in the path between the output of stage X and the input of stage X+1.

Typical (but not all) audio products (like preamplifiers and audio processors) are designed to have an input impedance of 10,000 or 20,000 ohms; and are designed to have an output impedance of around 100 or 200 ohms. Therefore, the output port of stage X is definitely not "matched" to the input of stage X+1.

As Beefus, et al. point out: what you want to avoid is connecting the output of a high impedance output source to the input of a (relatively) low impedance input sink. This will cause: the source output signal to be significantly attenuated (vs. when the sink is not attached), the attenuation will typically be frequency dependent (rolling off the "highs" or the "lows" or both... depending upon the equivalent circuit of the source), and the signal-to-noise ratio will be degraded.

Using a transformer to produce a higher impedance for the microphone to look into is one possible solution. Using a microphone preamplifier that has a sufficiently high input impedance is a better solution in many cases. For example, "condenser" microphones include a built-in FET-based preamplifier with a very high input impedance (at audio frequencies), and a low output impedance.

Stu

 

[RolandSWL]

Thanks for the info. Would removing the preamp in the D-104 TUG-8 make it a better match for Hi-Z tube transmitters? The Astatic specs indicate that the TUG-8 model has an output Z of 5k ohms.

http://www.preservationsound.com/wp-content/uploads/2012/07/Astatic_Catalog_197.pdf

RSWL................

[w4bfs]

what Stu said ...  

the D10 4 preamp will make more audio output & distortion & noise .... these last 2 are relatively small by comparison to the audio out ... be wary of overdriving the first stage

depending on how much gain and input Z of the tx will affect your best path ...

[AB2EZ]

Roland

The preamplifier that is built into a D-104 stand has characteristics (too low an input impedance) that tend to roll off the low frequency response of a typical D-104 crystal microphone capsule. In your case... if you are going to feed a typical boat anchor transmitter (like a Ranger, for example), you are better off to bypass the D-104 stand's built-in microphone preamplifier. The stock input impedance (at audio frequencies) of a typical boat anchor's built-in microphone preamplifier is 1Meg-ohm. This is several times the input impedance of the D-104 stand's built in preamplifier.

Why does the D-104 have to look into such a high impedance?

The equivalent circuit of a D-104 crystal microphone cartridge is a voltage source in series with a .001uF (or somewhat larger*) capacitor. This type of signal source (a voltage source in series with a capacitor) may seem strange/unfamiliar to some... but that is how a crystal behaves when you squeeze it with an incoming sound pressure wave.

The combination of the 0.001uF series capacitor and the impedance looking into the preamplifier input forms a frequency-dependent voltage divider... that rolls off low frequencies.

If the impedance looking into the preamplifier is 1M ohm (resistive), then the 3dB (low frequency) roll off frequency is 1/[2 x pi x 1,000,000 ohms x 0.000000001 farads] = 159Hz.

If you want the combination of the microphone and the preamplifier to have a lower 3dB cutoff frequency, then the impedance looking into the microphone preamplifier has to be higher (e.g. 5M ohms to lower the cutoff frequency to 32Hz).

 *Depending upon the specific vintage of D-104 crystal microphone cartridge... and whether the grounded side of the cartridge faces toward the back plate of the cartridge holder or toward the front screen of the cartridge holder... the value of the equivalent circuit series capacitor can be higher than 0.001uF.

In my shack, I feed my vintage D-104 microphone into a stand-alone preamplifier with a 10M ohm input impedance and (I believe) a 100 ohm output impedance. ["Countryman type 85"].

For example: http://www.ebay.com/itm/COUNTRYMAN-DT-85-DIRECT-BOX-Free-Fast-Shipping-/301742696334?hash=item464144278e

As discussed in prior posts, this low-output impedance preamplifier can drive the audio input of any transmitter or audio processor (a high input impedance boat anchor or low/moderate input impedance modern device) I have.

Stu

 

[KD6VXI]

Roland......

Place a 4.7 Meg resistor in series with each lead in the mic housing.   This will give you a 9.xx Meg input Z,  the bass response comes up appreciably,  and the aforementioned overdrive to the first transistor is eliminated.

I tried mosfet inputs,  and they worked great.   They also occasionally fried.   Cheap fix.   Once I tried this method,  no looking back.

I'm sure it has its drawbacks,  like floating the element,  probably more susceptible to hum.....   But I've not noticed at legal limit.

--Shane
KD6VXI

[W3RSW]

Now I'm confused.
Roland originally asked if a 200 ohm low impedance mike could be used in a very high impedance transmitter mike input.  Two very high 4.7 meg resistances in the 200 ohm mike lines will do wonders for attenuation.

Back in the day I'd've advised Roland to try an old 50c5 or equiv. audio output transformer backwards for a first cheap trial,  3.2 ohm Mike side to 5k transmitter side.
Unless of course he can obtain a good Jensen transformer closer to desired impedance ratio.

Anyway Roland, the transformer voltage step up mentioned by 7fto is a reasonable choice to try. If you do it be sure to shield the high resistance side and mount the xformer close to the transmitter input jack to reduce rf and hum pickup.

[AB2EZ]

In the context of interfacing a D-104 to a boat anchor transmitter which presents a 1M ohm input impedance .... which is the 2nd of the two questions that Roland asked in this thread

I think that Shane's suggestion is an interesting alternative. I would be reluctant to float the normally-grounded side of the microphone capsule (for a variety of reasons). However, adding a 4M ohm resistor in series with the microphone input (located close to the microphone connector) would:

a. Reduce the low audio frequency cutoff by around a factor of 5 (i.e. from 159Hz to 32Hz)

b. Attenuate the voltage across the input of the 1M ohm preamplifier by a factor of 5 [i.e. 1M ohm/(1M ohm + 4M ohm) = 0.2]

Since a D-104 cartridge produces a relatively high audio output voltage, and since there is usually more than enough gain available in the transmitter's audio chain... this would work... without having to modify the transmitter.


Yet another solution, that I have used, is to add a .004uF capacitor across the D-104's output (preferably located close to the D-104). This will have the effect of changing the equivalent circuit of the D-104 to a voltage source (0.2 x as large as before) in series with a .005uF capacitor (5x as large as before). That would produce the same result as in a. and b. above.


Stu

[W3RSW]

"OK Fine!"  
Understood.

[KA2DZT]

Typically a low impedance (300ohms) mike requires a mike input xfmr.  It would be a balance input, two wire plus ground. A xfmr like a UTC A-10 would work if you can find one.  I use a UTC mike input xfmr pulled from an old RCA studio mixer.  The xfmr has two input windings that yields 150 ohms with the two windings in parallel and 600 ohms in series.  The secondary is high impedance 80K-100K which can be connected directly to the grid of the first stage.  No grid resistor or very high grid resistor can be used.

The input xfmr also gives a high step-up in voltage to the grid.

Fred

[MikeKE0ZUinkcmo]

Ultimately, source impedance should match load impedance for maximum power transfer. How about a microphone

with an impedance of 300 ohms

plugged into a transmitter with an input impedance of 5 Megohms? No problem?

Not so long as the amplitude of the mic audio is the proper level for the transmitter.