Pre-Amps, Mixers and One Slow Learner

[Barry Rivadue]

With digital I tend to learn things butt-backwards, and my latest question has to do with pre-amps and mixers, terms I've repeatedly read about but did nothing about, until now. I have a Shure SM58 mic, a DVX100 and a computer with a Sound Blaster card. Now, here are some very elementary questions:

* Explain the function of a pre-amp and/or mixer
in regard to a dynamic microphone, and/or
input from a DVX100.

* What are some recommended pre-amps under $200?

* What is this "insert" feature mentioned in some
pre-amp specs?

* How do these devices work with a computer and, say,
Vegas?

Thanks for any info!

[Jay Massengill]

A preamp circuit is required to raise the very small mic-level signals up to line-level. Mixers with mic inputs have a preamp circuit built in to each of those inputs.
A standalone preamp has one, two or eight inputs and performs the same function as a mixer preamp, but without the other routing, mixing and control features. A preamp will also supply phantom power for those mics that need it. Your dynamic mic doesn't need phantom, but it does need a good preamp to cleanly raise its level since that mic has a very low sensitivity.
There are several ways to look at "best". Do you mean most practical or best sounding? A small mixer with good preamps will be the most practical, but good small mixers like Mackie tend to be over $200 except for their lowest models.
There are lots of musician oriented standalone preamps available for that price.
An insert jack allows you to send the preamped signal out to an additional device, like a compressor for example, and then return the signal to the original device for further routing, mixing, processing... On Mackie mixers this jack also allows you to send a copy of the preamped signal somewhere else without interrupting the signal flow through the original device.
Using one of these devices with a computer and software such as Vegas allows you much greater control and higher sound quality than sending the mic directly into your computer sound card. Unless your sound card or audio interface is a high-end unit that has good preamps already built in.

[Bryan Beasleigh]

You have an excellent preamp built into your DVX100.

Here's some reading for this evening
Location Sound by Dan Brockett

A whole herd of articles covering every aspect of production sound

There will be a test ;-)

[Barry Rivadue]

Wow, thanks so far, everyone.

I figured the DVX100 was okay on this. My Shure SM58 I suppose would then need a preamp; all this is primarily for a studio setup, and not for location.

[Mike Rehmus]

<<<-- Originally posted by Barry Rivadue : With digital I tend to learn things butt-backwards, and my latest question has to do with pre-amps and mixers, terms I've repeatedly read about but did nothing about, until now. I have a Shure SM58 mic, a DVX100 and a computer with a Sound Blaster card. Now, here are some very elementary questions:

* Explain the function of a pre-amp and/or mixer
in regard to a dynamic microphone, and/or
input from a DVX100.

You don't need anything but an XLR cable to enjoy quite good sound out of the SM58 and the camera.

I' don't have answers for the two snipped questions.

* How do these devices work with a computer and, say,
Vegas?

If you capture audio from the microphone via your camcorder, then your sound card does not enter into the capture equation at all. And if you master back out to DV, or DVD, it doesn't play any role either (other that allowing you to listen to the audio). This is true for most NLE setups I know of.

If you go out to an analog video recording setup you either have built-in analog out from your NLE or you have an outboard DV to analog converter so the sound card is avoided.

You can run your audio throught something like Sound Forge without any degredation by the sound card. It only operates as an audio output device and is not in the audio computational chain.

Note that the Sound Blaster, like most other consumer sound cards, will work OK until you want pristine silence during quiet periods and want to be able to hear the very good results from your sound editing.

As soon as you want to capture audio directly into your computer then the sound card will play a significant role in the quality of the audio you capture.

Thanks for any info! -->>>

[Barry Rivadue]

All this certainly clarifies things; thanks.

[Aaron Koolen]

Beas. That Location Sound link is fabulous!

Cheers
Aaron

[Robert J. Wolff]

Jay,

I'm stuck in a position of using an RF system, vs., an on board mike, for wild life. The Mackie pre amp compact field mixer, seems to me to be my best bet, for @ 8 c notes. I think after examination of the size and weight, the portablitlity of the unit makes it worth my investment.

Do you or any one else have a better idea? I don't suppose that there is all of that much difference when using RF in the field, far away from spurious signals???

[Bryan Beasleigh]

"The Mackie pre amp compact field mixer:
I can't find any reference to a field mixer on the mackie site.

Most of what they make is far too fiddly and requires 120 volt input. A field mixer is sturdy, battery powered and has the absolute minimum of control. There aren't dozens of pots and sliders to confuse the issue. The maxhies and behringers look to me, something like what Red Green's nephew carries.

[Jay Massengill]

I'm not aware of that Mackie either. Can you give us some more details on what you're looking at?

[Mike Rehmus]

You do have to take some care with RF and portable mixers. Some of them have some problems with RF leakage causing internal noise .

My Shure FP32 will do very wierd things if I plug into its output with a Sennheiser plug-on transmitter. I've heard that a blocking or maybe filter cap needs to be added to between the mixer and the transmitter.

[Robert J. Wolff]

OOPS!! Copied from wrong site! It is a Sound Device item. Sorry.

Below is the one I have been looking at for field use. Any thoughts?



Suggested Retail: $855 USD


MixPre Features



Dynamic range exceeding 110 dB

10 Hz to 50 kHz audio bandwidth

Maximum of 66 dB of gain per input

Premium Lundahl input transformers

Discrete 6-transistor balanced output drivers

Sealed, conductive plastic potentiometers

Seven-segment, sunlight-readable GaN LED meters

Phantom power, 48 volts or 15 volts

High-pass filters @ 80 or 160 Hz, 6 dB per octave

Inputs hard panned to left, center, or right outputs

High-power headphone monitoring

"Unclippable" input peak limiters via two-stage opto-isolator circuit with adjustable limiter threshold (per input)

Dual mono or linked stereo limiter operation

Two internal AA batteries

External 5 to 17 VDC powering

Power LED indicates low battery power

RF filtering on all inputs and outputs. Free from "Pin 1" grounding problems

High-strength, extruded aluminum chassis with protective metal end panels

All-metal connectors solidly connected to chassis





MixPre— left view




MixPre— right view







_


The MixPre from Sound Devices is a studio-quality two-channel, portable, stereo microphone mixer. Its impressive audio performance and comprehensive features, including pan switches, built-in slate microphone, 1 kHz tone oscillator, and headphone monitoring, make it ideal for the front end of any studio or field production system.


Radio, television, and film production engineers value the compact size and ability to withstand extremes in the field. The MixPre combines rugged mechanical and electrical construction, compact size, and high-quality components. The MixPre gives no-compromise performance for any application.

User Guide (276k, English)

User Guide (198k, Spanish)

Fact Sheet (352k)

MixPre Circuitry (100k photo)

360° View (requires Flash 5+)




Specifications




Gain:
(s/n 0503_ or greater), Mic Input to Output, per channel, continuously variable


Output
Gain Range

LINE
off — 66 dB

TAPE
off — 56 dB


Equivalent Input Noise:
-126 dBu (-128 dBV) maximum (150 ohm source, flat weighting, 22 - 22 kHz bandwidth, gain control set at 50% or higher, phantom power off)


Output Clipping Level (Line):
+22 dBu minimum with 100k ohm load, +20 dBu minimum with 600 ohm load


Dynamic Range:
110 dB minimum


THD + Noise:
0.05% maximum (from 60 Hz to 22 kHz @ +4 dBu output level, 22 Hz - 22 kHz filter bandwidth, gain control at 50%, phantom off)


Inputs:
Transformer-balanced, 2k ohm input impedance


Outputs (XLR):
s/n < 0507 - impedance-balanced, 120 ohm output impedance,
s/n >0507 - active-balanced, 120 ohm output impedance


Outputs (Tape):
Unbalanced, tip-left, ring-right, sleeve-ground, 2.4k ohm output impedance


Power (Internal):
2 AA alkaline batteries, 6 hours life typical with +4 dBu signal into 600 ohms, no phantom power


Power (External):
5 - 18 VDC via 4-pin Hirose connector (pin-4 positive, pin-1 negative), voltages above 14 VDC cause no damage to unit, but will open an internal poly fuse. Poly fuse will reset when voltage is removed.


Power LED:
Green indicates power and good battery. Red indicates power and low battery. LED turns red when approximately 1 hour of battery life remain (with phantom power off). Green with external DC power.


Frequency Response:
20 Hz - 30 kHz, +0.1, -0.5 dB; -1 dB at 5 Hz & 50 kHz (relative to 1 kHz with 150 ohm source, gain controls set at 50%)


Input Clipping Level:
-10 dBu minimum


Output Clipping Level (Tape):
+11 dBu (2.75 V RMS) minimum with 100k ohm load


Common Mode Rejection Ratio:
100 dB minimum at 80 Hz, 60 dB minimum at 10 kHz


Channel Separation:
Greater than 80 dB at 1 kHz (gain controls set to 50%)


High-Pass Filters:
80 Hz or 160 Hz (switch selectable), 6 dB per octave


Phantom Power:
48 V through 6.8k resistors or 15 V through 680 ohm resistors (switch selectable)


Limiter:
Thresholds adjustable per channel from -10 dBu to +18 dBu, 10:1 limiting ratio, 5 mS attack time, 100 mS release time, Amber/Red LED indicates limiting/clipping, Dual mono or stereo linked


Internal Voltage Rails:
±15 V, regulated


Polarity:
All inputs to all outputs, non-inverting. Pin-2 of XLRs "hot" to unbalanced inputs and outputs.


Metering:
Seven segment GaN (Gallium Nitride) LED meter with three intensities, peak responding


Operating Temperature Range:
0 to 70 degrees C, 32 to 160 degrees F


Dimensions:
43 mm x 94 mm x 140 mm (h x w x d), (1.7" x 3.7" x 5.5")


Weight (unit only):
0.8 kg, 2.0 lbs.


Weight (packaged):
1.2 kg, 3.0 lbs.


Certifications:
meets FCC Part 15 Class B, eligible to bear CE mark




© 1999 – 2004, Sound Devices LLC All rights reserved. Privacy statement.

[Bryan Beasleigh]

The Mix Pre is one of the better choices and is well known to this forum. It is also marketed by Shure as an FP24. I own a Mix Pre as well as a Sound Devices 302.

[Aaron Koolen]

Bryan will be able to confirm this, but my understanding is that the mixpre does not accept line level inputs. That's fine as it's meant to be a mic preamp anyway, but if you're looking to use it with a feed from a desk or some other line level source, you'll need a pad. That's why I'd prefer something like the 302 - but yes it's WAY more expensive.

Bryan, is this true?

Aaron

[Oleg Kaizerman]

thats corect , sd skipped the line input and mike outs as well
no stereo support (ms decoding)
individual chanel monitoring is also missing .
good preamps ,10db att when you work with condenser and that might be not enouth when you pullthe signal all way up with low output mikes ( i dont sure if that was changed in the last models), no t (tonador)powerimg
weri limiting mixer -exelent expander for 302 or 442
the 302 is the way to go if you planning to do with more then 1 mike