Is this mic on ???

[JimJarvis50]

This thread could draw crickets, but I have an issue with a microphone that I can't seem to figure out. The microphone is a Shure, balanced, low impedence mic. This goes into an unbalanced wireless transmitter with both mic and line-level inputs. I can use the "hot" and "cold" leads from the mic, wired to a stereo 3.5mm plug (the required input format), and this works OK. However, I was hoping that I could increase the gain on the mic and get better performance. So, I got this pre-amp:

https://www.mcmelectronics.com/product/555-21010

It has both balanced and unbalanced inputs and outputs. I did a test where, for reference, I just ran the hot and cold leads to the transmitter (via the stereo 3.5mm plug). Then, I plugged the mic into the balanced preamp input and tried first the hot and cold leads from the balanced output and then the unbalanced output. I believe the unbalanced output is a mono signal from a stereo 1/4" jack (so that you can plug in headphones and hear something in both ears). As best as I can tell, no amplification is being applied to the signal with this preamp. With the gain control on the preamp, I can turn the volume down, but the maximum gain is no higher than the reference test (bypassing the preamp). There are other things I could say about the setup that could be factors, but let me stop here and ask if there is something obvious that I'm missing with respect to converting a balanced input to an amplified unbalanced output using this preamp.

Jim

 

[cvanc]

Looking at that MCM web page, the written specs for the gain of the preamp do not seem to match the markings on the preamp in the photo. The photo seems to show a gain range of +6dB to -60dB, which is low for a mic preamp. Further, you can lose a handful of dBs just choosing certain combinations of balanced/unbalanced connections. What are the markings on yours?

A true mic preamp should have a gain of +40 or even +60dB (or more).

 

[JimJarvis50]

Looking at that MCM web page, the written specs for the gain of the preamp do not seem to match the markings on the preamp in the photo. The photo seems to show a gain range of +6dB to -60dB, which is low for a mic preamp. Further, you can lose a handful of dBs just choosing certain combinations of balanced/unbalanced connections. What are the markings on yours?

A true mic preamp should have a gain of +40 or even +60dB (or more).

The unit actually says +6 to -50 dB.

Jim

 

[cvanc]

The unit actually says +6 to -50 dB.

Jim

A maximum gain of 6 decibels is awfully, awfully low to call that box a mic preamp. You're not noticing much gain simply because there isn't much to notice.

I'd return it if possible and look for a different answer.

 

[tfish]

Jim, This might be what you're looking for!

It comes with free shipping!

Tony

 

[sghioto]

Doesn't make sense to label the gain control from +6db to -50db that's an attenuator not an amplifier. Maybe they are referring to the input signal level that can be applied to achieve a line level output, but if you have +6bd or about 2 volts of signal who needs a preamp. Probably just defective.

Steve G

 

[JimJarvis50]

Jim, This might be what you're looking for!


Tony

I'm completely speechless ...

Jim

 

[fyrwrxz]

Jim- you haven't said much about your main amp. Pre-amps usually 'adjust' the output prior to any amplification and as such, don't usually add much 'usable' volume to the signal straight out. What your main amp sees is adjusted using the preamp to prevent frying the output transistors from ringing by being overdriven (constant clip) thus the negative db scale. (warning- I'm really old school so I could be fulla crap in the digital age!). Is this a SM57 or SM58?

 

[sghioto]

One must not overlook the specs of the wireless transmitter used. The level of audio input determines the frequency deviation or loudness of the signal which is limited to + and - 75 Khz. The mic input on the transmitter probably meets this requirement thus any additional amplification (via the preamp) will not make any difference due to the limiter circuit in the transmit unit.

Steve G

 

[JimJarvis50]

Jim- you haven't said much about your main amp. Pre-amps usually 'adjust' the output prior to any amplification and as such, don't usually add much 'usable' volume to the signal straight out. What your main amp sees is adjusted using the preamp to prevent frying the output transistors from ringing by being overdriven (constant clip) thus the negative db scale. (warning- I'm really old school so I could be fulla crap in the digital age!). Is this a SM57 or SM58?

It's a PG58. Not sure how this differs from the SM58 though.

Jim

 

[JimJarvis50]

One must not overlook the specs of the wireless transmitter used. The level of audio input determines the frequency deviation or loudness of the signal which is limited to + and - 75 Khz. The mic input on the transmitter probably meets this requirement thus any additional amplification (via the preamp) will not make any difference due to the limiter circuit in the transmit unit.

Steve G

Well, that's one of those details. The transmitter does have automatic gain control so you can't overload it. What I have noticed, though, is that an input such as music seems to result in an output level that is perhaps a factor of 5 greater than the voice input. So, I assumed that the level of the voice input could be increased quite a bit before an overload condition would occur. Perhaps this is not the case.

Jim

 

[bobkrech]

With an unbalanced microphone, signal is between the + and - wires which are pins 2 and 3 respectively on an unbalanced 3 pin connector.

To make a balanced microphone into an unbalance microphone connect pins 1 and 3 together and put on the sleeve connector. The signal + line, pin 2 on the balanced connector goes to the tip of the unbalanced mic connector.

A balanced amplifier actually has 2 amplifiers, one for the pin 2 + output and a second for the pin 3 - output. The internal configuration is that of a differential amplifier. If you only use 1 input as in an umbalanced input, the second channel has no signal to amplify so the overall gain is 6 db less than having signals going into both sides.

Two unbalanced mics wired together and input as a balanced mic will make an unbalanced stereo amplifier. One mic goes into input pin 2 and 1, and the second goes into pin 3 and 1.

Hope this make is a bit clearer.

Bob

 

[sghioto]

Jim, what is the brand and model number of the transmit unit?

Steve G

 

[billspad]

I'm not trying to be a smart ass but are you sure you turned on the power to the preamp? I mention that because 40+ years ago I remember seeing one with dead batteries do what you describe when I was working at a college radio station.

 

[JimJarvis50]

I'm not trying to be a smart ass but are you sure you turned on the power to the preamp? I mention that because 40+ years ago I remember seeing one with dead batteries do what you describe when I was working at a college radio station.

Well, that's another one of those details. I initially used the AC adaptor, but that didn't work when the transmitter was on a battery due to hum. I noticed that the voltage from the adaptor was actually 17 volts, even though the input says 12 volts. I don't know if converting over to 12 volts would matter (it shouldn't, but who knows).

Jim

 

[bill_s]

The gain control says +6 to -50, not -50 to +6 which would be an indication of gain. Instead that's probably showing the input level, turn it up for a lower input.

What's wrong, not sure. Could have blown it up with that high voltage adapter. Might not work with a mono plug in the headphone output, shorts sleeve to ground, but don't think you're doing that. I don't think running the balanced outputs to that transmitter is right, the output is almost surely not transformer balanced, they're really 2 separate amplifier outputs, for unbalanced from there use one signal and ground.

One way to get a boost on a high-impedance input is use a matching transformer (e.g. Shure A85F). This steps up the voltage along with the impedance. I've mostly seen them with bands, the boost is too much for most inputs when you scream into the mics, and a straight adapter cable works better in that case.

 

[billspad]

Well, that's another one of those details. I initially used the AC adaptor, but that didn't work when the transmitter was on a battery due to hum. I noticed that the voltage from the adaptor was actually 17 volts, even though the input says 12 volts. I don't know if converting over to 12 volts would matter (it shouldn't, but who knows).

Jim

17 volts is probably the no load voltage and it drops to 12 volts when you connect it to something. It shouldn't hum so there's a possibility you have a bad unit. I'd also check to make sure the wiring matches Bob's drawing. Digging up 40 year old memories again, I recall the balanced mics we used hummed a lot when something happened to the ground.

 

[JimJarvis50]

The gain control says +6 to -50, not -50 to +6 which would be an indication of gain. Instead that's probably showing the input level, turn it up for a lower input.

What's wrong, not sure. Could have blown it up with that high voltage adapter. Might not work with a mono plug in the headphone output, shorts sleeve to ground, but don't think you're doing that. I don't think running the balanced outputs to that transmitter is right, the output is almost surely not transformer balanced, they're really 2 separate amplifier outputs, for unbalanced from there use one signal and ground.

One way to get a boost on a high-impedance input is use a matching transformer (e.g. Shure A85F). This steps up the voltage along with the impedance. I've mostly seen them with bands, the boost is too much for most inputs when you scream into the mics, and a straight adapter cable works better in that case.

Well, I have very little idea what you said here, but I think you have identified the problem and possible answer. It's very likely that the transmitter input is high impedance. There is not spec that I can find, but the "manual" states that one should use a stereo microphone with a stereo plug input. I think stereo mics are typically high impedance, although why a stereo mic input is provided on the unit is not obvious to me.

Originally, the question I asked is how to connect a balance mic to an unbalanced input. The correct question might instead be how to connect a low impedance mic to an unbalanced, high-impedance input. The transformer seems to be the tool to do that.

One other question I have. The preamp specs state an input impedance of "10K ohm bal; unbalanced". Is it possible that it is not designed for use with a low impedance mic? Maybe the XLR input fooled me?

Jim

 

[bill_s]

Low impedance is a lot lower output voltage on a mic. True high impedance mics expect a loading of 50K ohms or more. 10K is acceptable for low cost condenser mics, which use a battery and preamp, and some low cost dynamic mics are also mid-impedance. Exact matching provides maximum power transfer, important when recording quiet sources such as a violin or person 50 feet away with minimal noise. There is some tonal change when not loaded as expected but this is not a substantial problem as long as the load is higher than the source impedance.

The specs say 44 dB gain unbalanced, 50 balanced. The labeling on the gain control is not like a normal volume control, which are usually labeled, when in dB, from -infinity to 0 dB (relative only). The maximum gain should allow a -50 dB source to produce a 0 dB output, in this case this is meant to be relative to line voltage, .775 volts for 0 dBu or 1 volt for 0 dBV. ( https://en.wikipedia.org/wiki/Line_level ).

XLR connectors don't necessarily assure the connection is truly balanced and can be line or mic level. However I'm sure your preamp is supposed to be able to boost the level. Either it is broken or connected or used improperly. I'm wondering if maybe you're connecting the mic as one balanced connection and shield, which produces output at all only because of the capacitance of the cable. Shouldn't be any problem with a normal XLR cable from mic to preamp. I don't know why they didn't provide a normal unbalanced output on the thing (MONO 1/4") or why your transmitter would spec a stereo mic. Might work better with a stereo cable between them (tip, ring, sleeve).

17 volts is probably the no load voltage and it drops to 12 volts when you connect it to something. It shouldn't hum so there's a possibility you have a bad unit.

A regulated voltage adapter should be used to avoid hum, which will have no difference in loaded and unloaded output. Does a LED light up on the preamp?

 

[JimJarvis50]

The transmitter vendor doesn't know the input impedance. However, they normally recommend the use of an electret mic. My understanding is that an electret mic would have a much higher output than the dynamic mic I have, and that's why I'm not getting the output I would like. I suppose the obvious answer here is just to get a battery-powered electret mic and declare victory. I still don't understand, though, why the preamp isn't working, nor do I know if an impedance-matching transformer would be effective for the mic I have (since it's not clear to me if the input is really high impedance).

I know I haven't answered all questions asked here, but if the info above prompts another question or a repeat question, ask away ...

Jim

 

[billspad]

The transmitter vendor doesn't know the input impedance. However, they normally recommend the use of an electret mic. My understanding is that an electret mic would have a much higher output than the dynamic mic I have, and that's why I'm not getting the output I would like. I suppose the obvious answer here is just to get a battery-powered electret mic and declare victory. I still don't understand, though, why the preamp isn't working, nor do I know if an impedance-matching transformer would be effective for the mic I have (since it's not clear to me if the input is really high impedance).

I know I haven't answered all questions asked here, but if the info above prompts another question or a repeat question, ask away ...

Jim

Way back at the beginning you mentioned a stereo mic. Electret mic's like the ones that plug into your computer have a "stereo" plug but one of those conductors is expecting to see a voltage to power the mic.

 

[JimJarvis50]

Well, I think I figured this out. I went and got an Electret mic (battery operated, not needing plug-in or phantom power). The output on it is much higher than the Shure mic. I arranged to be able to switch between the two to compare the levels. I started with the Shure, got a level, and then switched to the Electret. Got about the same level??? However, when I switched back to the Shure the level was initially much lower, but then rose back up to the baseline after a few moments. So, it looks like my problem is just the auto level adjust on the transmitter. The electret forces the auto level to attenuate the input, and then it takes a few moments to recover with a lower-level input. So, it doesn't help to amplify the mic output - the transmitter will just attenuate it down.

Knowing this, I again tried the preamp, and it actually works fine (into the line input instead of the mic input). The transmitted volume I can get with the preamp to line input is again about the same as the Shure to mic input, so there is no benefit to using the preamp. What is still odd though, is that I can set the receiver volume to give a soft output on the voice input (though the preamp and the line input). Then, music though the line input nearly blows the speakers. I don't understand why voice appears to be attenuated so much more severely than music. But, it doesn't appear that there's anything I can do about it.

Thanks everyone for the help.

Jim

 

[bill_s]

I was going to bring that back up and suggest trying the preamp through something else like a stereo. Auto level is slow to recover. The difference with music probably one of tonal balance and constantness. It might help to cut some bass from the mic. The Shure probably has better handling of breath noises at close range, but also has proximity effect bass boost which an omnidirectional mic (cheap electret) does not.

 

[bobkrech]

https://www.mcmelectronics.com/prod...mpaign=criteo+retargeting&CA_6C15C=1515958736 is another preamplifier from the same company specifically designed to level convert low output mics and cartridges to high level inputs for only $14.

• Features:
• L/R RCA RIAA equalized phono inputs
• L/R RCA phono line level outputs
• Dual 1/4" mic inputs (connecting to right input only will create mono mode)
• Front panel MIC/PHONO selector
• Red LED power indicator
• Rugged metal housing, black color
• Requires 12VDC, AC adapter included

• Input impedance: 50Kohm
• Input level: 0.5mV (mic, 3mV (phono)
• Gain: 50dB
• S/N ratio: >48dB
• Equalization: RIAA (phono input only)
• Output level: 150mV
• Output impedance: 10Kohm
• Dimensions: 1.4" (H) x 5.3" (W) x 2.8" (D)

Bob

 

[JimJarvis50]

https://www.mcmelectronics.com/prod...mpaign=criteo+retargeting&CA_6C15C=1515958736 is another preamplifier from the same company specifically designed to level convert low output mics and cartridges to high level inputs for only $14.

Bob

Yeah, I saw that one. Seemed like the one I got was perhaps slightly more directed at what I was trying to do, but that was just a guess. But then again, why would anyone buy such an obviously inferior unit when they could buy the superior $30 unit (from me) for, say, $14 ??

Jim

 

[JimJarvis50]

For those who might be interested ....

I was posing the mic questions because I'm trying to upgrade our club PA system. Our setup is asymmetrical, with about a 250 foot flight line that is all to one side of the LCO station. Our 12-volt/70-volt PA is showing its age and the available battery-powered replacements are pretty cheap. So, we decided to try a wireless system. The results are shown in the pics.

The transmitter is an FM unit that transmits over commercial FM bands. We have a stubby antenna and an antenna attenuator installed to get our range down to a few hundred feet. We won't be bothering anyone.

The satellite stations have a car stereo head (Kenwood KDC-152) that's inexpensive ($60) but has lots of power. The speakers are Pioneer TS-G1644R at $27 a pair. They work fine (don't bother with the horns). Yes, they're overbuilt, but what else would you expect from me.

I've tried these out at a couple local launches and they seem to work pretty well. One would probably be enough to cover our flight line, but with two, everyone at the launch and at the pads will be able to hear our launch and safety announcements (and our tunes between flights sound great!).

We'll see how this works over time, but so far, so good.

Jim