"Gain" from impedance matching transformer (?)

keith.rogers

keith.rogers

Well-known member
Calling @ecc83 ...

How can a passive device spec state that "Gain is 21 dB," like this Shure XLR to 1/4" (balanced low to unbalanced high) impedance matcher?


I'm guessing it's simply the voltage increase, at the expense of some current loss in the transformer, so all that's being reported is the difference dBU or dBV calculation, but is that really "gain" in the usual sense?
 
Gain is related to changes in voltage, so with a transformer, the winds ratio gives an accurate gain figure, if, and only if the impedance matching is correct. The spec is correct. Of course, increasing the voltage comes with a compromise, current available drops by the same ratio. Only the attached kit will determine if it works well.
 
For Shure to baldly state a 'gain' of 21dB is a little disingenuous? The voltage ratio can be calculated from the root of the ratio of secondary to primary impedance, root 133 = 11.5 which indeed gives 21.2dB of lift...but, IF the mic sees a load of 300R there will be an attenuation of 3.5dB already and another loss on the secondary side unless the load is very high e.g. a 1 meg guitar input.

My son uses a similar transformer to boost a '57 into a looper so he can loop acoustic guitar. Works quite well but EFF knows what it is really doing to the response!

Dave.
 
Heres a mp3 spoken word, 545 set to HighZ setting on mic.

Using a ISA One I went through the 600, 1400, 2400, 6800 ISA Z range. (The hsssing is a bit of a mystery as it doesnt seem to do that in Low Z.?)

But Shure seems to have their specs right, but maybe not explained real well....or I dont understand all of it.
 

Attachments

  • 545 Hiz into ISA One impedance test MP3.mp3
    2.1 MB
Yes, Shure's bare spec of a 'gain' of times ten IS correct assuming zero source R and infinite load, in the real world it will be far less. In any case, such step up circuits should really have the secondary loaded with an RC 'Zobel' network to damp the L/C resonance and that will further reduce gain. Still, the traffs are really intended as a problem solver and do that pretty well in most cases.

Dave.
 
I remember 515 and 545 with Hi-Z and they never sounded dull from memory - but there again, maybe I just couldn't hear more than 8K or so through the system.
 
my experience wasnt dull....it was noisey, hsss...as in the sound clip I posted.
not sure what causes that, one article said something about impedance mismatch....
 
CoolCat said:
Heres a mp3 spoken word, 545 set to HighZ setting on mic.

Using a ISA One I went through the 600, 1400, 2400, 6800 ISA Z range. (The hsssing is a bit of a mystery as it doesnt seem to do that in Low Z.?)

But Shure seems to have their specs right, but maybe not explained real well....or I dont understand all of it.
Click to expand...
None of those input impedances are really "high" in the sense of what the A85F is converting a low impedance mic to/for. The switchable 545 is interesting, but it did require a different cable, presumably terminating in a simple 2-conductor plug, i.e., unlikely you'd go into an XLR input with a high-impedance mic, right?

But, the 545 is spec'd only at 2dB higher output (-55dB vs -57dB) in the high impedance mode, so while the expected input impedance wasn't obvious to me, the suggested A95 transformer (high-to-low, i.e., the reverse of the A85F), has an input impedance of 48kΩ. So, the original 545 high impedance setting was something you'd use with an old VocalMaster PA and those screw-on mic connectors.
 
If you listen to the mp3 clip, its obvious the loudness gets really loud on the last ISAOne impedance setting 6800...and the 545.
I dont know why or if it matters, but it got loud for me.
But the hssssing noise made it worthless , so I put it all back on Low Z.
The booster mic things dont add that hssssing noise....strange?

yes the term High Impedance throws a lot of us, we think 150-<600 is Low....therefore everything above 600 is High, which is wrong.
The High Z is 50k to 2meg+....and no one talks about the middle range 1k to 3k much. not sure why? 1k-3k is most interfaces, mic preamps Ive owned. all I gather is it was aimed at LDC for some reason.

in electronics what little I recall you wanted Matched Impedance for best transfer , but that wasnt audio...Im barely recalling it at all as I never used that electronic schooling in my job (40yrs).....circuit designs and all that was a different field.

interesting stuff though, imo.
 

Attachments

  • 545 Hiz ISA One pic.png
    545 Hiz ISA One pic.png
    54.9 KB · Views: 2
Last edited:
".and no one talks about the middle range 1k to 3k much. not sure why?" This came about CC because of the development of transistor and later op amp mic pre amps. It tuns out that the optimum source impedance for a bipolar transistor is around 1-2k (depends somewhat on the collector current, sort of) for lowest noise. In the last decade or so it has become popular for hi end pre amps to have higher input impedances, 10k or even higher since (it is said) these enhance the sound quality of dynamic mics and especially ribbons.

Of course, what the mic pre input ACTUALLY 'sees' is at the mercy of the actual microphone's internal circuitry and that can be simply the diaphragm coil or a complex of transformer, inductor and capacitance for EQ e.g the SM7b. Plus a DC resistance.

Dave.
 
I had just caught something about that Dave... and tube stuff with "matching impedance" then to the 1948 Bell Lab...and Transistors came and with 10x's Load Z for lower noise etc... Shure site mentions POWER transfer with matching Z and Low to High with VOLTAGE transfer.

The link Keith posted mentions voltage gain of the A8SF transformer. Ive got a couple of those things around here and not sure why? lol
Theres the resonant frequency of the low impedance mic thats confusing too.

I get the part where LDC became a norm and 1k-2500k (or <10k per Shure comment) becomes popular......High Z is still another application.

Ive heard some say dynamics are zero noise because they have no electronics in them, but that sound clip I posted has a huge amount of shssssshhh noise using the HighZ mic to Low(mid) Load Z....and the gain jumps up a lot...as the ISA One Z is higher.....which makes me think the "matching impedance" is maximum POWER transfer? is that right?



LOW - LOW
LOW- HIGH
HIGH - HIGH
HIGH - LOW.... smoke a doobie
 
Dynamic mics certainly do produce noise, but not a lot. The DC resistances in the VC and transformers produce thermal noise. I don't have the resistance for a '57 (say) to hand but I doubt it is less than 100 Ohms? That gives a noise voltage of ~-133dBu. Say we have 60dB of 'noise free' preamp gain? The pre amp noise output will therefore be -73dBu, good but not staggeringly low. In most cases however, the 'noise' due to the 'room', people, external ingress etc will dominate. VERY low noise capacitor mics are in fact limited simple by the random hits of air molecules!

"Matching" came from telephone technology and later RF. If you send telephone signals over long distances and don't match send and receive impedances then you get 'reflections' sound signals coming back atchyer. Most of you are old enough to have seen 'ghosts' on analogue television? Same principle and in CCTV you must terminate the video cable (usually at 75 Ohms) or you get similar picture anomalies. S/PDIF we all know need a 75R cable but does not seem all that fussy about it?

However, for audio transfers under about a km we need not worry about 'matching' and going low to high gives a noise advantage, little voltage loss, less HF loss and a better Common Mode Rejection Ratio.

And. a little OT. We speak of 'matching' a speaker to a valve amp? We don't in fact but that's another can of worms!

Dave.
 
is that why the 545 in Hi Z got so loud when the ISA One impedance went to the 6800 setting? closer to Matching?
and I still dont get the reason for the horrific hssssshhhhh sound in my sample in Hi Z mode?

I ordered a different version of the A85F thing, this one is XLR on both ends and supposedly will better match the Low Z mics to the standard 1k-3k mic pre impedance.
Somehow.?


definition:

The Simplest Way To
• SOUND BETTER!
• LOWER THE IMPEDANCE FOR YOUR SM57,SM58,
• OR OTHER DYNAMIC MICROPHONE,
• AND SOUND BETTER
•
• HEAR AN AUDIO SAMPLE AT
• FIREHOUSERECORDING.NET ON THE LOWER-Z PAGE
•
• THESE MICROPHONES WERE DESIGNED WHEN 600 OHMS WAS THE INDUSTRY STANDARD FOR PREAMPS
• CURRENT CONSOLES AND DAW INTERFACES ARE 1500 OHMS, 2000 OHMS, 3700 OHMS, ETC.
• THAT'S WONDERFUL FOR YOUR CONDENSER MICS, BUT YOU'RE MAKING YOUR DYNAMIC MICS MOAN IN PAIN
• STOP SQUEEZING THE FREQUENCIES THAT YOUR MICS ARE CAPTURING
• LESS RESISTANCE MEANS FULLER TONE.
•
•
• Every studio or live engineer should have half a dozen of these around for their dynamic mics.
• You might not prefer it every time, but then again, you might.
 

Attachments

  • Lower Z.png
    Lower Z.png
    151.6 KB · Views: 2
I would simply contact Shure support with the exact model 545 you have and ask them how to use it in high-impedance mode, i.e., what if that's what you seem to want to do. The specs I found for that mic said it needed different cable wiring to use in high impedance mode, but there have been a few different models.

The "SD" version does show a +23dB increase in output in high impedance mode, but that is suggesting that it's looking for something a lot higher than 6.8k, which is not really "high" impedance, but probably targeted for something at the high end of low-impedance mics.

pubs.shure.com

545SD User Guide - Shure

Online user guide for Shure 545SD wired microphone.
pubs.shure.com pubs.shure.com

 
Right. I was just interested in learning more on the Impedance stuff and the Impedance Changes Effect.....
Actually Im not even using the Shure's anymore much with a slight effort to silence the room, the MXL V67G has been really enjoyable and easy to work with.
I'll probably move the Shures to the drum and guitar amp room.
 
At the very simplest (almost over simplified), it’s a voltage divider with the impedance of the source as the top R and that of the load as the bottom.

Vout = Vin * Rbottom / (Rtop + Rbottom)

As Rbottom (load impedance) gets big compared to Rtop (source), the ratio gets bigger, the output is attenuated less. If it got noticeably louder when the load impedance got bigger, the lower ones were definitely too low. We usually shoot for a 10 times mismatch because then the divider works out to less than 1db attenuation, which is generally considered to small enough to live with.
 
keith.rogers said:
Calling @ecc83 ...

How can a passive device spec state that "Gain is 21 dB," like this Shure XLR to 1/4" (balanced low to unbalanced high) impedance matcher?


I'm guessing it's simply the voltage increase, at the expense of some current loss in the transformer, so all that's being reported is the difference dBU or dBV calculation, but is that really "gain" in the usual sense?
Click to expand...
I dont know if the ohms is measured at some freq?
But I have two of these and dead-unplugged no sound, the resistance is 50ohm low XLr side and 1.4kohm on the high side, 1/4' mono.
The only thing inside this barrel is a transformer.

add: I ran the 545 into the Z plug thing, and then compared a standard mic in to the mic/instrument in using the plug thing...and the inst HiZ was a lot louder, but in my setup theres a lot more noise which I am leaning to is RF noise? but either way.....I suppose there is a GAIN increase...but its not anything I would use.

When I use a Cloudlifter or Cathederal Pipes booster I dont get any of the noisy crap, the boosters are a nice clean boost.

I dont plan on using an instrument input HiZ for any dynamic mic duties...lol
Its starting to seem like comparing New tech rubber car wheels to caveman rock wheels... as for HiZ mic , Ill use an old texas slang...El Paso!
No wonder those impedance things have been around forever in my junk drawer and not needed.

I think the LOWER-Z I ordered will be a good product , I dont know whats inside it a resistor? a transformer?...for preamps that dont have impedance knobs. Only $14 so, its similar to the Shure A85F, but better I will predict. Just having XLR on both ends is better for starters.
 
Last edited:
I have to admit that I’m a bit confused by your written description of what you’re doing. Probably my fault.

BUT most of the time a HiZ instrument input on a preamp or interface also includes a kind of hidden “gain” over a line or mic input. In my interface, pushing the button to go from line input to instrument gives a 9db boost without touching anything else. Course, the actual mic inputs are 6db up from the line inputs, too. This shouldn’t be significantly noisier than just turning up the gain knob by however much, but theory and practice often differ.

BUT if the HiZ connection is an unbalanced connection, that definitely could increase noise, especially if it’s not wired properly.
 
From memory, the Shure 545's that had alternate impedances had a 4 pin Tuschel type connector and they enabled the selection of impedance. Mine had low on an XLR and high on a ¼"TS cables.I cannot remember if the XLR was balanced or unbalanced?
 
You must log in or register to reply here.
Back
Top