Transformer upgrade for sm57?

Does anyone know the ratio of the transformer in the '57?
Is there any mileage in upgrading it?
A reduction in hi-gain hum? Flatter response? Lower self-noise?

I'm using the mic in high gain (70+dB) situations and am enjoying the tone but not the noise.....

Any help would be greatly appreciated.

Andy

No such thing as a free lunch. The output of the capsule is only so much, and given its impedance and Johnson noise, even with a impossibly low noise from the preamp at that gain you'd still get a bunch of noise out of the capsule. A transformer will not help that part - it amplifies the voltage but also the impedance... and the Johnson noise.

What sort of cat is it? Maybe you can switch to a larger cat? :green:

Not a small cat, but close.....gentle voiced singing from 12 inches away......meeeoww.

Just wondering if I'm losing any level in the transformer.....and whether anyone has ever made a jensen/lundahl/?? replacement.....there's gotta be an improvement of some sort to be had there somewhere....space permitting.....

And I still don't know the ratio anyway! ;-)

Cheers

Andy

Try measuring the impedance of the windings to figure out ratio. Jensen has info about on their website. I think it's in the schematics section.

Measuring the DC resistance of the windings won't tell you the turns ratio... not with any certainty.

As an example, imagine a 1:1 ratio transformer with the secondary wound on top of the primary. Even though the primary and secondary have the same number of turns, the diameter of the turns in the secondary is larger, which means a greater length of wire and higher DC resistance. It's not uncommon for a 600:600 ohm transformer to have, say, a 50 ohm DCR primary and a 60 ohm DCR secondary. But the turns ratio is still 1:1.

The situation gets more complicated with high ratios, or cases in which the primary and secondary are wound with different gauges of wire.

The way to measure turns ratio is to inject a signal into one winding and measure its amplitude on the other.

Dave is correct, but I was referring to the use of a sinusoidal voltage, i.e. 1kHz, hence impedance. There is info out there on measuring (rough) impedance/turns ratio with a 1kHz signal and AC volt meter. Unfortunately I can't recall this exact procedure at the moment.

[EDIT][quote author="NewYorkDave"]The way to measure turns ratio is to inject a signal into one winding and measure its amplitude on the other.[/quote]
Exactly!

> No such thing as a free lunch.

Correct. And Shure already ate all the free breadsticks.

> Lower self-noise?

The SM-57 is transformed to about 300Ω, on the high side of mike impedances, to get maximum output (and S/N) with typical mike preamps. You are not going to find a significantly better ratio.

Are you SURE the noise is the mike, not the preamp or the room? Self-noise of a '57 is around 20dB SPL, and many rooms are noisier than that.

> A reduction in hi-gain hum?

50/60-Hz hum is a different problem. Get away from power lines and AC cords.

Pitchless "hum" is the room ventilation or distant traffic noise coming through the wall. Or maybe flicker noise in the mike preamp, though this is usually overwhelmed by room rumble.

Recall that most speech/singing has a specific lowest tone. Most female singers, you can low-cut at 220Hz and lose none of the singing (I have met exceptions). Male singers can be strong to 80Hz, but a steep 70Hz low-cut can really clean up the sound if room-rumble is a problem. Perhaps the worst problem is small-chest males who want to sing chest-tones, but can't power-boom. A single-D cardioid or bi-di mike is their friend, boosting nearby low tones without boosting distant room-rumble. Even so some electronic bass boost may be needed to give the ballsy sound they think they should have, and then you have to have a low-rumble room (or a booming bass track to smother room rumble in the vocal track).

> Flatter response?

Why use a '57? It is very good for many situations. If it isn't, get something else.

There are a host of '57/'58 imitators, with similar tonal sound, and several use super-magnets for improved output. I mostly use the AudioTechnica imitation '57/58s: the cheap ones are frankly inferior to a '57 but good enough for live jazz. The good ones (from A-T and everybody else) are nice variations on the '57 and some are higher output.

IIRC, the capsule in the '57 used to be about 10Ω, so the tranny musta been √(10Ω:300Ω)= 1:5. Note that it isn't just the ratio: a 1:5 150Ω:4KΩ transformer will give terrible response and noise. 10Ω:300Ω is going to be a custom winding. And for all I know, Shure may have changed to a transformerless capsule in the 20 years since I last opened one.

PRR,

the capsule in the '57 used to be about 10Ω

Click to expand...

You was very close

I find a spare 545/R45 (SM57) capsule and a transformer on my workshop and done some measurment.
I measure a impedance (at 1 kHz) of 13 ohms and Shure say in a old paper for the R45 capsule, it shall be between 12 to 14 ohm.

The transformer have approx. +12 dB step up with 1kohm secondary load, and the frequency respons (between the -3 dB points) are 10 Hz to 50 kHz

--Bo

Thanks Bo et al.

I guess I'm gonna have to bite the bullet and just try an 'upgrade' transformer in there and see what it sounds like.....

The closest I saw after a quick look on the Jensen site was the jt-13kja, which I appears to have over double the bandwidth, among other things and would appear to be a suitable upgrade - although it may have to be external! ;-)

Cheers.

Andy

Opened my 57 today and it seems that the transformer is capsulated into epoxy. How about replacing the transformer with an active circuit? Rode's D-POWER doesn't seem to fit inside but there's still room for one or two circuits on small pcbs (put the first gain stage before the tranny space, then drill a hole thrugh epoxy and solder the output stage circuit into XLR connector). Any ideas what kind of circuit to use there? Rode uses an opamp there, all SMD circuit. Very low input impedance and balanced outputs are required. How about the ESP project#13 followed by an inverter for (-) output signal? Also p#93 circuits might work well.