Blue Jinn

Calculating Shure Transformer rating HOWTO
« on: August 27, 2009, 08:25:33 PM »
Hello,

I have perused most of the transformer impedance/gain postings here and am hopefully on the right track. I have a Shure M68 mixer want to use parts from and the case for another project. Want to re-use the transformers which is only listed in schematic by part number.

Schematic located here:

http://www.freeinfosociety.com/electronics/schematics/audio/shurem68.pdf

There are voltage measurements on the schematic. At the XLR connector .74mV (which seems consistent with a nominal dynamic mike output) and just after C1 9.1mV. If this is correct, my math would say there is a 12.3 voltage gain, for  ~1:12 winding ratio? The input impedance for the unit is stated at 300 ohms, so taking the voltage ratio, and squaring it I come up with 300:43k or so for the impedance ratio.

This seems to be a decent step up ratio for a mic pre. Don't know how they will sound, but want to try it out. Thoughts?

Thanks


not my thing. I am here just to unbalance the equation --pucho812

 http://www.bluejinn.org


rodabod

Re: Calculating Shure Transformer rating HOWTO
« Reply #1 on: August 27, 2009, 08:40:43 PM »
Numbers look reasonable. Note the spec. for the Hi-Z input impedance which is also a rough giveaway.
Quote from: tv
punchy fat bastard chip

leswatts

Re: Calculating Shure Transformer rating HOWTO
« Reply #2 on: August 27, 2009, 09:06:23 PM »
My guess is they reflect 600-1kohm from the input stage. I'd have to check the circuit more to know for sure.

I Have a ton of these transformers too...about 50. I actually still use M68s for talkback in the studio.

As far as the quality...I don't know. I wouldn't call the m68 exactly hifi. Used on Woodstock, but you know how that sounded. Might be good for color.

Knowing the primary inductance would be telling. They might be ok...they have good mu metal cans at least.

Are you hoping for sound color or hifi specs?

Les

CJ

Re: Calculating Shure Transformer rating HOWTO
« Reply #3 on: August 28, 2009, 12:04:13 AM »
slap em in there.

sometimes stuff that is totally not right for the job ends up giving you the "new" sound.

like the guy who mixed live sound for marley, everybody thought he was nuts with that bass fader, but look!
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

shabtek

Re: Calculating Shure Transformer rating HOWTO New
« Reply #4 on: August 28, 2009, 12:41:19 AM »
http://www.groupdiy.com/index.php?topic=29752.0

m67 transformer in nyd 1bottle, probably the same thing?
http://i248.photobucket.com/albums/gg189/shabtek/DSCN2479.jpg

edit: its the silver can with the hot-glue on the right
and 'sounds' like what yer after in a 1 bottle
« Last Edit: April 16, 2015, 02:55:09 PM by Ethan »
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

PRR

Re: Calculating Shure Transformer rating HOWTO
« Reply #5 on: August 28, 2009, 01:09:38 AM »
> for  ~1:12 winding ratio? The input impedance for the unit is stated at 300 ohms, so taking the voltage ratio, and squaring it I come up with 300:43k or so for the impedance ratio.

Close enough. Looking at input clip levels suggests 1:15, but that's not very different.

Work the other way. Looking into Q1, is it 43K?

R8 150K loads the input, but is bootstrapped to over 1Meg.

R4 1Meg is Miller-effected by the ~~1:11 gain of Q1, call it 100K.

Q1 Base is about Hfe times 680, these were hi-gain parts, call it 170K.

1Meg||100K||170K = 59K. Which equals the 60K spec rodabod pointed out.

For 300r input claim, that suggests 1:14 ratio.

The noise voltage/current is skewed by resistors. My guess is that OSI is 5K-10K. This says it is not matched for Best Noise Figure. Which agrees with my memory (they were quiet but not silent) and the specified 0.7uV noise where we'd like <0.3uV.

But all documentation is dubious. Shure even says +/-15%, but BITD it could have been pretty far off and we would not know/care.

1:12, 1:14, 1:15..... Whatever: the ratio is perhaps too high for low-volt BJTs. As tube inputs, 43K-59K secondary makes more sense; tubes have "no" current-noise. You want to keep your grid capacitance well below 100pFd, which suggests the less-high-Mu triodes. Or pentode.

> Don't know how they will sound

Up to a point: inoffensive, not sparkling. Economical utility iron. Sound/dollar, a fabulous deal. If the music cooks (and the iron is not over-strained), the track will work. If the music sucks, these won't un-suck it.

As the manual says, hot condensers "may" overload. The first limit is the 30mV clip point from a 11V B+ and 12*11= 1:130 gain. In modern studio practice, that is "will" clip. The iron alone can take more. How much more depends on your bass content. And surely Shure did not design-in any more headroom than necessary.

Blue Jinn

Re: Calculating Shure Transformer rating HOWTO
« Reply #6 on: August 28, 2009, 01:18:54 PM »
Thanks for all the replies and the infusion of knowledge!

I was thinking of recycling a trafo or two in to something like the NYD 1  bottle and giving it a listen.

I also am thinking of taking this box and making a quick and dirty set of mic and guitar pres in one box to go along with a cheap (i.e. 10 year old) multi channel sound card setup for a singer songwriter type going off to college. Nothing great, but decent enough in a small space and price. 

Paia and muzique.com have some simple PCB's that look like they'd work, and I wanted to use one or more of them with a transformer input. Thought I'd try the Paia kit (US$10) first and perhaps modify and reuse one of the transistor stages so I'd still have a volume control. Goals here are *reasonable* quality, easy build, and easy price. 

:-)
not my thing. I am here just to unbalance the equation --pucho812

 http://www.bluejinn.org

leswatts

Re: Calculating Shure Transformer rating HOWTO
« Reply #7 on: August 28, 2009, 02:43:59 PM »
They might load pretty well into a jfet input op amp for a simple fairly quiet mic pre. I think i'd try loading the secondary a little higher...like 150k or so, if they behave. I think their primary inductance is about 1 Henry, so they would still have some low end.

Les


 

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