The AM Forum

Hi,
NOTE; Going to the ACE hardware and asking for a few feet of audio chain was wrong...
hi

Trying to connect a new mic, does not work, think I have a clue as to why.
But I am baffled by the metrics, and can't tell if the ancient Shure M67 preamp
has enough gain for the mic I am using.

Here is what the spec says for the Shure M67 preamp/mixer:
(http://i180.photobucket.com/albums/x257/fish1_07/am/collins%2032v-2/m67b_zpsonuhaizy.png)

I suspect that the M67 does not have enough gain for the mic, but can't make sense of the mic specs
so as to compare with the M67 specs.

Shure SM7B microphone documentation:

SM7B dynamic microphone
The output level of the   SM7B is -59dBV/Pa.   (???!!!**&&%%)
   
For typical speech applications, three inches from the grille, the SM7B requires at least +60 dB of gain at the microphone preamp.
Many modern microphone preamps, which are designed for the “hot” output level of condenser microphones, provide   only 40 to 50dB of gain.

When selecting a preamp for the SM7 for voice-over work, make certain the preamp has a minimum 60 dB of gain.
Here's why:

a) With an acoustic input of 84dBSPL - typical speech level at 3 inches, the output level of the SM7 is -69dBV.

b) The mic preamp must increase the mic signal level up to about 0dBV - line level.

c) -69dBV of mic signal, increased by +60dB of preamp gain, equals an output level of -9 dBV. Not quite to the 0 dBV level, but close enough to work with most line level inputs.

Many modern mic preamps assume the mic is a condenser mic with a "hot" output level.
Thus, it is common to find mic preamps or mic mixers with gains of only 40 to 50 dB.
This is not enough gain to bring the SM7 signal up to line level.



How can I tell if the ancient Shure M67 gain is adequate for the new SM7B mic?
73
Frank
KJ4OLL

Let me see if I can help.  I was in broadcast for way too long, so I have been down this road before.  The Shure mixer is a battleship, I have never seen a mic it would not adequately amplify, including E V RE-20's, Shure SM57, 58 and SM7B's, as well. You should be able to connect the SM7B straight into the back and go. Set the gain control for half way and start there.  NOW be sure the TONE - MIC switch on the back (Channel One only) is set for MIC of course). The master gain will probably need to be about 4 or 5, and you should be good to go. If it does work, send me an email to my "real email address on my profile page, not message mail, and I will send back my toll free phone number and we will work thru it.  Remember in wiring the XLR plug, GROUND is always on pin 1. Depending on who taught you hot is either 2 or 3, and low is the other pin.
In short the mixer has plenty of gain unless it is damaged or has been tampered with.

This might help as well:

Converting mic output voltage
You can manually convert between values easily by remembering two things: that a 6dB change is always a halving or doubling of the previous voltage value, and the reference voltage you're working with. The reference will be 1v for dBV, or 0.775 for dBu.

Of course - it is much quicker to just use the proper formula! In Excel use the following "=20*LOG10(voltage/1)"

What if you want to determine the voltage output of a mic rated for -50 dBV/Pa?

1v = 0dBV
1v - 6dBV = 0.5v = -6dBV
0.5v - 6dBV = 0.25v = -12dBV
0.25v - 6dBV = 0.125v = -18dBV
0.125v - 6dBV = 0.0625v = -24dBV
0.0625v - 6dBV = 0.03125v = -30dBV
0.03125v - 6dBV = 0.015625v = -36dBV
0.015625v - 6dBV = 0.0078125v = -42dBV
0.0078125v - 6dBV = 0.00390625v = -48dBV
0.00390625v - 2dBV = 0.0031623v = -50dBV = 3.16mV/Pa
So we could restate the sensitivity to say that the mic produces 3.16mV when exposed to a 1kHz tone of 94dB-SPL at the capsule.

Volts, millivolts, microvolts
1 volt (v) = 1000 millivolts (mV) = 1,000,000 microvolts (µV)

0.001v = 1.0mV = 1,000.0µV

From:  https://support.biamp.com/General/Audio/Microphone_sensitivity

RANE has a good chart as well:

http://www.rane.com/note148.html

Phil - AC0OB

This mic has standard wiring: pin #1= ground/shield, #2 positive, #3 negative. The mixer has 2 outputs; one to feed the microphone input of another device (low level) and the other to feed a line input (high level). To add to what Bob said, input #4 can be either mic or line so make sure the switch in back is in the mic position. The binding posts numbers are the same as the pin numbers of the "mic" output.  Note the data relative to "output" and you can see that the one labeled "Microphone" is only 6 millivolts.  You don't say what you are feeding, a PA?  Do you get a reading on the meter?  Note it has a switch so you can read either +4 or +10 DBM.  The +4 is typical of the output of consumer gear...recorders, etc.  The +10 is broadcast standard.  This mixer is rated at +18 off the binding posts; and that's a lot!  In any event, if everything is working properly, you should have more than enough gain to do the job.

Here is what I was trying to get working.
(The JDI direct box is a passive audio transformer, used for musical instruments.)
http://www.radialeng.com/jdi.php

Turns out the "JDI direct" is also like a "roach motel" for my signals.
Only way to get a waveform through it is to drive the JDI with signal levels beyond what I have available.
OR
I have the M67 output configured wrong (yep), by using the "MIC" output, instead of the "line", loaded w/ 600 ohms.

SHURE SM7B MIC <--> SHURE M67 MIXER <--> DEQ2496 EQ <--> JDI DIRECT BOX <----> COLLINS 32V2 AM TRANSMITTER MIC IN

So here is plan "B", which I hope to try this weekend.
SHURE SM7B MIC <--> SHURE M67 MIXER <--> DEQ2496 EQ <----> COLLINS 32V2 AM TRANSMITTER MIC IN

There is also a plan "C", if these other options get too weird.
SHURE SM7B MIC <--> SHURE M67 MIXER <--> CBS Labs AUDIMAX <--> CBS Labs VOLUMAX <----> COLLINS 32V2 AM TRANSMITTER MIC IN

taking line output (around 1 V) and directing it into the 32v mic input will overdrive it ... just use a resistor from line level to input (I find 100k works well for solid state mic in, may need some to ground say 10k for tube mic in)

if you are a purist and want balanced to unbalanced as well add a 1:1 xfmr ahead of the pad ...unnecessary .... the direct box is a bad idea as it was designed to work at millivolt levels and would further increase V  going balanced to unbal

Try plan "D" first.

Balanced mic output to balanced mic input; Shure SM7 to M67 and then with a scope or AC meter, make sure you are getting line level output on the M67 without any loading on line level output.

Phil - AC0OB

Dump the direct box.  It has no place in your application.  In addition to broadcast experience since 1962 (as of Sunday when I turn 70) AND recording experience since 1965, (I founded Full Compass in the early 70s), what you have to do is quit messing with everything except the mic and the mixer.  The 32V2 typically was built for use with a high output crystal mic unless I'm wrong.  In any case, judging from your questions about relative levels, I suggest you start dialing back that audio chain because what you're describing is something you could use on a broadcast transmitter, not on a Ham rig!  I also suggest you ignore all the specs / levels / data and play it strictly by ear. Don't get hung up with numbers.  If it sounds good, milk it!

Everything boils down to the parameters of the audio input of the Collins.  Anything you want to put between the mic and the transmitter will over drive the modulator.

What to do.  For one thing, if you use either output of the mixer into the Collins, one (microphone) might be too low and two (line) will be too high.  Of course, if you want to set up an audio chain, the Shure mixer can drive any device with a high-level balanced input.  Once you start cascading devices, you're wading into a netherland of impedance and level matching.  The more devices you add the better the chance of picking up stray RF and the more noise and distortion you face.  Bottom line: thinking in terms of broadcast audio and processing then trying to apply that to your transmitter is like mixing apples and oranges.

You need to match your fine Shure mic to the transmitter first.  You might try coming off the M67 mic output into a 600 ohm to 50,000 ohm audio transformer, plug it into the Collins mic input and see what happens.  Any way you try to slice it, the more devices you try to use, the harder it will be to make everything work because what comes out of the last processor in the chain can't have any more output voltage than the Collins mic input is designed to handle.  Then there's the standard caveat; less is more.  In recording, I found that over use of any processing device ruins the product. The same hold true for AM Ham radio.  The exception would be AM broadcast where loud ruled back in the 60s and over-processing meant loud.  Not with ham radio.

Of course, you could always modify the Collins to accept a line-level input if it doesn't have one, which would mean digging into the guts and by-passing the first audio stages.  That would eliminate concerns about over-driving, but unless you know what you're doing, you can really screw up the transmitter's audio chain.

Thanks for the clarity and "level set"!

Adding twin turbos and Nitros Oxide injection to the 1961 VW Beetle....again....

The mania to build...

Where
Is
My
MEDICINE!!

I'll simplify and try it.
Interestingly, the JDI box is a Jensen audio transformer for matching impedance, (but at high power levels).

 

tnx for the info about the jdi direct box ... jensen is certainly a name in the biz...

a happy medium (for me at least) is to use enuff vocal processing to be heard on noisy bands yet not splatter or sound unnatural ...

reports here seem to be good with a symetrix 528e driven by a Sennheiser 421 left over from my studio daze ...

most important is getting each stage swinging the right level

The Shure M67 will work just fine.
Don W4DNR

I built a couple direct boxes to use in my studio.  One plugs into the speaker output of a music amp...8 ohms and drops the level to mic level at 600 ohms.  There is a switchable pad for matching.  The fewer devices between the mic and transmitter the better.  The common processing in AM broadcasting was a compressor followed by a limiter. Top-40 stations of the 50s / 60s used a lot of leveling which greatly reduced the dynamic range of the music.  Louder was better and maximum constant modulation was common practice.  With Ham radio you want just enough punch without splatter.

Started looking for a 600 ohm to 50,000 ohm audio transformer

So far, located this one, pretty close.  http://www.surplussales.com/item/_ta/467737.html

Check this out:

https://www.edcorusa.com/wsm_series (https://www.edcorusa.com/wsm_series)

Be sure to select the correct drop down list.

Al

PS: just noticed that there is no choice for your higher Z

I have used the UTC A-10 transformer for this application, good quality, excellent frequency response, and configurable for multiple input impedances.  My father used several as mic input transformers in a hi-fi preamplifier, and later a stereo mixer for recording and sound reinforcement.  Check these links.....

http://www.ebay.com/itm/UTC-A-10-transformer-/321971348291?hash=item4af6fce343:g:14gAAOSwKtlWkUFH

Current bid:US $20.00


http://www.ebay.com/itm/1-UTC-A-10-input-/351622665090?hash=item51de581382:g:YQwAAOSwFqJWjeZZ

Current bid:US $5.58

If you fail to find what you are looking for, please contact me offline.  I know I have some in my transformer stash.

interesting thread.  

My understanding is that mics are usually tested, and sensitivity rated, at either 74 or 94db SPL.   I'm trying to refurbish a couple really beat up D104s and measure the response (http://s670.photobucket.com/user/mikeinkcmo/library/Radios/D-104?sort=3&postlogin=true&page=1) when I'm done.   This is a work in progress, over the next however many days, but the thing that surprised me is the low output level of dynamic elements VS Crystal mics.   Its about a 30db differential.

The Shure mic SM7B is a lot more  mic than you need for Ham radio...deep pockets
I use one in my Flintstone, (Music show on WBCQ , 7490, Fri, 7-8pm) studio and the output level it low..........-60 I have the mic strip gain turned up all the way. Shure admits that it is not as hot as the popular condenser mics and not as hot as the Electrovoice RE's
I doubt if that Shure mic mixer is up to the task.
I use a DBX mic processor for the SM7B and its gain control is turned all the way up for 60dB gain. This is a very good quality processor with no noise or hum or hiss in its outpoot.
Playing with matching transformers is opening up another can of worms.
The mic works into the proper piece of audio processor or quality mic strip in a broadcast console or it doesn't.

Fred

No substitute for good ol' Empirical Method.

Shure SM7b & M67 preamp on top trace, 500mv/division
Shure 444D bottom trace, 50mv per division

https://www.youtube.com/watch?v=4OwdEsfHlUo

After reading a bit, it looks like the audio transformer is designed to handle impedance mismatches,
while maintaining signal level.

In the case of the signal from the M67 is line level, 1V.
The 32V-2 wants a signal of some small number of millivolts.

So I don't need to maintain signal level, I need to attenuate the signal to use the SM7B, Way too hot.

Might need just a resistor network instead of an audio matching transformer???
What am I missing here?

Good youtube vid.
The M67 is looking for 600 ohm output. You can turn down the master output control or build an attenuator. I do not know the input impedance of the 32V.
I hope you're watching a 'scope for modulation or other methods of monitoring...not looking at meters.
Any plans for processing? like a limiter? EQ? The SM7 might be a little bassy. I do not know what mods have been done to your 32V.
The scope you used looks pretty nice

fred

Or bypass the first audio stage of the 32V - feed the Shure to the high side of the audio gain pot.

Unless the 32V has been modified, the input impedance to the first stage is 1.2M ohm.

That means you want to use a high output, high impedance mikes such as D-104's or dynamic mikes whose output impedances' are greater than 50k ohms.

Comparing different mikes with different output impedances and with no defined loads is meaningless.

Phil - AC0OB

Thanks, the scope really shows the differences in signal levels between the different mic technologies.

No mods to the 32V-2, standard mic input circuit.

Use the REA software & sensors to monitor modulation, not sure if the REA thing is just a toy or actually providing real data.

There is a "Behringer DEQ2496 EQ/RTA Mastering Processor " in the rack for use with this, but it is not on-line while I tinker with the basic signal.

The EQ goes back in-line after the attenuation for the 32V-2 input is figured out.
The SM7B has some mechanical switches on the back to modify the frequency response curve, but they don't have the range of values that the EQ is capable of.
If the mic switches produce good reports w/ mechanical switch settings, the EQ won't be used, but EQ is available if needed.

Steve , K4HX has the best solution to bypass the audio freq restrictions and extra amplification in the 32V.
The REA monitor is the real thing!! Broadcasters are using this as a viable way to monitor their station. It may not be a rack mounted Belar mod monitor. The display and info are accurate. Adjust phasing of the mic for the most positive peaks.
And use a good receiver in your shack and good headphones to tailor the audio EQ. It would be nice to find out what bands and times to work you.



Fred

Why try and inject line level audio into the mic input on the 32V?

As Steve said, feed it into the high side of the audio gain pot or into the cathode of the second audio stage. No need to buy a matching transformer. The Shure M67s were widely used in remote service in broadcast. They are nice units.

Al VE3AJM

Here is a schematic I think Steve was thinking of if you are intent on using a 150ohm dynamic mike.

BAT is the balanced-to-unbalanced transformer that is switched in to by-pass the first audio stage.

Three components are added: The BAT, C*, and XLR connector.


Phil - AC0OB

OK, then see page 2 of the pdf file.


Yep. DO the basics and get the input audio levels to the 32V set before putting any junk in between.

Cool!
Thanks very much, trip to Skycraft Surplus later in the week to find resistors.

Owing to the fact that your M67's VU meter is calibrated for near 600 ohm loads, the loading resistor value on the M67 should be 680 ohms.

Phil - AC0OB

I went through the same hassle when I attempted to use a dynamic mic with my DX-60B.  The first problem is going from balanced to unbalanced as in the mic input. The second is trying to convert a low output mic to high output as found in a crystal type and the third problem is matching impedance.  I almost went nuts because nothing seemed to work.

You can go into a mic preamp / mixer to boost the gain as with the M67 but then what?  If you feed the transmitter mic jack, it's unbalanced.  The mic output of the M67 won't match the Collins input as far as voltage is concerned. It would be too low.  By rule of thumb, it's totally possible to feed a very high input impedance like the 1 meg+ of the Collins with something lower, say 50K or more.  In my studio, I have some gear that is unbalanced and about 50K input and I actually feed it with an unbalanced 600 ohm source!  My entire studio is unbalanced and is totally functional and QUIET.  Another rule of thumb for consumer gear is use 47K ohms input as a benchmark.

Theoretically you could try taking the 600 ohm output of the mixer, take the #2 binding post as your "hot", short #3 to ground and unbalance the output...a common practice, and then feed directly to the Collins.  That eliminates the balanced-to-unbalanced problem.  Next you would have to insert a volume control or cut the master on the mixer down to nearly zero.  Driving a high impedance input with a low impedance source always works provided both are unbalanced.  That would solve the level problems.

With an input impedance on the Collins of over 1 megohm, and a crystal mic such as an Astatic D-104's impedance of only 5,000 ohms, you don't need any math to see how unbalanced, low to high works.

All you have to do is worry about levels.  Just keep in mind, there is nothing sacred about balanced audio lines unless they are really long.  You certainly can feed the output of the last device in the chain to the volume control inside the Collins, but guess what?  It would be unbalanced.  If you went that route, you wouldn't have to do anything but short out one side of the balanced output of the last device in the chain and have at it...you wouldn't need anything in between regardless of impedance.  Just don't try to drive a device with low impedance input with one with high impedance output.

On the M67 you have two level controls: the Mike Level (internal 50k ohm pot) for each of the four mic channels and the output level (Master control pot) to control audio output levels, plus the gain pot on the 32V; Plenty of level control available for the 32V.


Actually, the D-104's output impedance is 1 to 3 Megohms depending on the vintage:

http://amfone.net/Amforum/index.php?topic=8975.0

Historically, most older transmitter speech amplifiers used 470k to 1Meg or greater unbalanced inputs.

K4KYV feeds his mike into a HiZ speech preamp balanced from the balanced output of his D-104. There is no rule that says you have to use an unbalanced connection for a D-104.




Phil - AC0OB

In reference to Phil's statement;

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/D-104/D104%20balanced1%208_zps8dbl2do3.jpg)

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/D-104/D104%20balanced%208_zps8gr8e1cz.jpg)

I just use an Astatic D104, with a simple mic pre-amp (http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/Station%20KE0ZU/Controller/StaConR5%204_zpss4uyxsmh.jpg) driving a transformer coupled output to whatever transmitter I'm using.  FET input Emitter follower, level control, 60db gain in 20db chunks, 150 Ohm balanced/un-balanced output.   Allows me to use virtually any conventional transmitter and/or microphone combination.  

This is what Tim HLR did with my 32V1 and I get good audio reports.  Most cannot tell the difference between my 32V1 and my class E transmitter.  My AF system is 1) EV RE15, 2) dBx 286s and the output goes in the "line input" of the 32V1.

Al

I stand corrected.  The 5,000 ohm impedance apparently is the output of the amplified version.

There's no beatin WA1HLR, Timmy. The guy has a golden ear for audio and can run circles around most of when it comes to RF.
Anybody get in any QSO with Tim lately??

Fred

I heard him the other nite on 3.885 with K4KYV and W4BVT, but was working on the Viking II-CDC, and by the time I got over to the Ham station, the atmospheric noise made it impossible to contact anyone.