High Z mic (Elvis style) /high Z mic input (old Wards amp) why or why not?

This just got me curious. (Impedance 101) I am sure this is covered somewhere, but the meta-meta wasn't all that clear to me.

I have an EV731 cool looking old Elvis mic. (which sounds pretty cool anyway...) is switchable between 150 ohm and 25k ohm.  I also wound up with an old Wards amp around the same time, (I'll be posting a refurb project on that when I get a round tuit.) The mic inputs are unbalanced and go straight to a pentode with something like a 1Meg grid resistor.

When I was a kid , I figured out that the "H" was louder.  I don't have the amp available right now to try this out, but wondering about the theory on all this. Why would 25k be an option, and when would I have wanted to use that?

Hello Blue Jinn,
I also have some vintage Mics with a switchable high/low Impedance.The Problem is that both Outputs are unbalanced and I want to use them in a Studioenviroment so I pulled the Tranny out and connected the Capsule ( a dynamic Capsule put out a low impedance, balanced Signal by Nature ) direct to the Plug. With a Xlr Plug , i put the hot Signal to pin 2 ,the cold to pin3 and if the Mic has a  metal Housing i connect pin 1 to the Case or i leave  Pin 1 n.c. The advantage is that you have a balanced,low impedance Signal that you can send over a longer Distance. If you have a High Impedance Micpreamp and like the original sound of the Mic,you can connect the pulled Tranny between the Mic and the Amp much closer to the Input of the Micpreamp with less Problems. I put the Tranny in a small Metalbox and can decide how to connect the Mic,with or without the Tranny.The Disantvantage is that you have a Levelloss,of course,because you don t have the gain step up from the Transformer. These kind of Mics where mostly designed for very short Cablelenghts. High Impedance Mics are nice to plug into a Guitarramp or a vintage Tapemaschine with a high Impedanceinput and with this step up Transformer build in the Mic you get a much hotter Output. Lets say the Capsule has a Impedance of 250 ohm and the Tranny is a  1:10 you get 20dB more Level and a Impedance of 25 kOhm.
Greetings
Lothar

> the "H" was louder.

Maybe not.

Depends on the input it goes to.

Say the basic mike is 150 Ohms. To get a 25K output it has a step-up transformer in it.

Same as the step-up always found on the input of a modern tube preamp designed for low-impedance mikes.

Square-root of 25K/150 is 13, so it is a 1:13 step-up transformer.

Do you know your Voltage Dividers?

There's Mike impedance and Input impedance. Draw your two resistors, try the different values for each, figure out how much signal gets through.

When the 25K side is plugged into a 1Meg grid resistor, 97% (1,000,000/1,025,000) of the signal hits the grid.

But if plugged into a 2K input (typical of modern low-Z mike inputs) we get 2,000/27,000 or 0.074 or 7.4% of the stepped-up signal. The loss is about 13:1. Nominally the output is the same in L or H. (In fact I suspect frequency response will suck on H because the transformer is designed for open grid, not 2K.)

If plugged onto a 600 Ohm input (now unfashionable but there's a few around), the H position gives 600/25,600 or 2.3% of the un-loaded H position. Since this is 1:13 step-up from the 150 Ohm output, output is 30%, compared to the L position which gives 600/750 or 80% of the un-loaded L position.

So now H is softer.

Another point, mentioned by Lothar, actually shown in the data sheet. 100 feet of cable on this 25K output is like turning your Treble knob right down. (Which confirms that the "25K" rating is correct; use 30pFd/ft and do the R-C math.) The L connection can drive 166 times as much cable for the same treble loss, or 16 times as much for "no" treble loss (allowing for these mikes not doing 20KHz well). Even at 20 feet you want to use the L setting if you want full 10KHz response.

Why both L and H?

Many microphone types have been used in PA. Carbon mikes first, which need a special transformer and a fat little bias supply. Then hi-class work used condensers with a 3-stage amp in/near the mike, simple mixer connection. Ribbons, which had to have a transfo at the capsule. Dynamics which are naturally ~~~10 Ohms but learned to drive as 37 Ohms to 600 Ohms. Note that none of these simply connect to a tube grid (as they ultimately must). And then there was a fad for Crystal mikes which "can" connect directly to a grid. In fact the lack of transformers and head-amps made crystal mikes THE way to go for some years, even though they are fragile and either modest output or crappy sounding.

Given all the different types of mikes used 1930s-1950s, many mixer makers opted to just give you the tube grid and let YOU figure out any transformer needed.

Even after extreme impedances faded from the market, there's a big cost-savings by not installing transformers "standard". You see 1950s-1970s PA mixers with transformer sockets. And if you need the big 4-in amp for power but only need one podium mike, the buyer saves money by only buying and stuffing one optional transformer.

So E-V was just covering their market.

This mike's "L" output is not enough voltage to drive a tube grid well. However you may already have a transformer.

With the 1:13 step-up and a hi-Z load, this mike works great into tube grid without added transformer (unless the line is long enough to trim your highs).

Thanks for the background on this!

Will be fun to try that with an instrument input. And that Wards amp if I get around to restoring it. There is a funny backstory of complete ignorance there I'll relate sometime...

The backstory on the mic is kinda funny; I had a summer job as a custodian at a church. the pastor had this mic laying around outside his office, told me to trash it because it didn't work. I naturally took it home instead, and the only thing wrong with it was a a wire had come loose in the cable's XLR plug. (Apparently no one thought to try another cable.....)  A quick fix.