use turntable pre-amp with mic?

[harvey]

Can I use a turntable pre-amp with a mic?

Recently, I bought a Shure SM10A "head-worn" mic (*) at a garage sale.
Because it is low-impedance (which means its signal level is not high
enough to plug it directly into my PC sound card), I need to have a
pre-amp in between the mic and the computer.

I already have a turntable pre-amp for use with the moving magnet type
of cartridge.

The output level of the mic is listed as -65 dB and that of the phono
cartridge as 3.5 mV.

Here's my question: Can I safely use the turntable pre-amp with the
Shure mic? My concern is that I not damage the pre-amp (or the mic).

Any audiophiles in the audience?


(I can handle connecting everything together; my question concerns
any possible adverse consequences of doing so.)

[Mark Turner]

I'm looking around for some info but see one potential problem right off. The pre-amp probably does the RIAA equalization which the mic may not need.

[Mark Turner]

http://remixmag.com/tech/studio_tools/remix_mixed_signals/

Quote: Though most turntables' connector cables (usually RCA) may be the same as other line-level devices, due to the limitations of vinyl as a recording surface, phono preamps make use of technology known as the “RIAA compensation curve.” This compensation EQ radically boosts low frequencies while attenuating highs.

[harvey]

Thank for the info, Mark, and the link looks like a good source.

Since the RIAA EQ curve doesn't change mid-range frequencies (much)
and voice is mid-range, perhaps this filtering will not be a problem for
voice applications, such as recording audio books.


I forgot to say that the pre-amp is by Music Hall, model MMF-1.
There's nothing about it on the Music Hall Web site.

[Planish]

I wouldn't expect it to work very well.

Here's the resulting frequency response curve (the solid line) that the RIAA eq does.

Note that even at 6 kHz, it reduces by about 9 dB.
At 200 Hz it's already boosted by 8 dB
50 Hz is boosted by 20 dB.

For comparison, standard analog telephone lines are supposed to be flat (well, within something like +/- 3 dB) between 300 Hz to 3 kHz. If you want to simulate a "phone voice", just apply a bandpass filter that rolls off everything below 300 and everything above 3,000 Hz. Typical FM radio and TV audio frequency response (within +/- 2 dB, in Canada) is about 50 to 15,000 Hz. AM Radio is more like 50 - 10,000 Hz.

I think that even if you were to come up with an "Inverse RIAA Equalization" to process it after recording, you'd have a lot of bass clipping and low-frequency noise during the recording itself. Even if the low-end excursions were reduced (by the inverse filter) they'd still sound clipped, as if you were talking through a kazoo. You would not be able to clean that up.
Also, the (software) inverse filter would amplify the high-frequency noise (of the analog-to-digital converter hardware) along with the voice, and it would sound hissy. Especially so if you had the record level reduced to prevent the bass clipping.

I haven't actually tried it, but this is what I strongly suspect would happen.

[harvey]

This has been a good educational exercise, since I hadn't known about
the RIAA EQ curve ... or the fact that most stereo system amplifiers
with a phono input have circuitry on that input to filter the signal according
to the RIAA curve. So now I know (why) you have to be careful about
what device you plug into which jack on a stereo amplifier.

However, all this doesn't answer my question, which is about the safety
of plugging the Shure mic into the Music Hall phono pre-amp. I don't
want to damage either one. If I can be reasonably certain that it's safe
to do so, I'm now intrigued to test this set-up, to compare it with the
low-cost Labtech headset mic I've used for voice recording.


-------

Something for audiophiles interested in vinyl records:

"The New Way is simply a whole new approach and new way of thinking
regarding the way you play and record your treasured vinyl", based
on using "a Flat preamp without RIAA EQ to get our audio into the PC".
http://www.enhancedaudio.com/newway.htm

[Planish]

However, all this doesn't answer my question, which is about the safety
of plugging the Shure mic into the Music Hall phono pre-amp. I don't
want to damage either one. If I can be reasonably certain that it's safe
to do so, I'm now intrigued to test this set-up, to compare it with the
low-cost Labtech headset mic I've used for voice recording.

I just had a look at the Shure SM10A specs. It's a simple dynamic mic, so there's no phantom power DC voltage like on a condenser mic, or high-level output. It may not sound good, but it should be very safe, in terms of not blowing up either the preamp or the mic.

The output level of the mic is listed as -65 dB and that of the phono cartridge as 3.5 mV.

Umm... that's a bit vague. "dB" by itself is just for comparing relative levels, like saying "12.9%" or "150%", except that it's based on a logarithmic scale. Plus values are greater than some reference value, negative values are less.

the specs for the mic say:
Sensitivity (1,000 Hz at 8 mm (5/16 in.)
Open Circuit Voltage: –65.0 dBV/Pa* (0.45 mV)
*1 Pa = 94 dB SPL

The "open circuit voltage" means as measured by a high impedance meter (as if it was not plugged into anything), and it's given for a particular sound pressure level as measured in Pascals (same unit as for barometric presure).
At any rate, that reference sound results in 0.45 mV, which is much less than 3.5 mV. About 18 dB less.

Because it is low-impedance (which means its signal level is not high enough to plug it directly into my PC sound card)...

Impedance has little to do with absolute voltage (or signal) levels, in that you can have equipment with "low impedance outputs" that may be putting out voltages that are much higher than other equipment with low impedance outputs. Eg. professional tape decks might have low Z (low impedance) outputs, but standard line levels of +4 dBu; whereas a consumer-grade tape deck might have Low Z output at line levels like -15 dBu. Both would be plugged into the High-Z inputs of the next stage in the chain, but you wouldn't want to feed consumer-grade equipment with pro equipment, unless you turned it down by 19 dB.

It's a bit like comparing the RPM of two electric motor with their ability (horsepower) to handle heavy loads, or water pressure versus gallons per minute. (No, that's not quite an accurate analogy either. Pressure versus Gal/min is more like voltage versus current. Whatever.) There are reasons for choosing other impedances than "high"(typically 10,000 ohms and up) and "low" (50 ohms or less) but they mostly have to do with issues like rejection of RF interference, or minimizing power losses during transmission over long distances, or prevention of reflected signals in high frequency applications. It's a Black Art.

BTW - batteries do not drop in voltage when they die; rather, their internal resistance (impedance) increases, which reduces their ability to supply current to a mid-to-low impedance load.

At any rate, what the "low impedance of the mic" does mean, however, is that it is appropriate for plugging into a high impedance input, which your PC's sound card audio input is very likely to be.

Something for audiophiles interested in vinyl records:

"The New Way is simply a whole new approach and new way of thinking
regarding the way you play and record your treasured vinyl", based
on using "a Flat preamp without RIAA EQ to get our audio into the PC".
http://www.enhancedaudio.com/newway.htm

That might be useful, considering that the RIAA EQ is not the only one to be used, historically. "Final Vinyl" recording software has something like a dozen different EQ curves to select from, depending on the manufacturer/publisher of the record. Could be particularly important if you have really old records from the '30s and '40s.

FWIW - Whenever you see the term "integrated stereo amplifier", the "integrated" part means that it has a phono preamp with EQ integrated into the circuit, and therefore has dedicated phono inputs.