Real World Gear: Small But Sometimes Mighty (Part 1)
Testing, features and application of compact mixers
By
Ivan Schwartz
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Most everyone working in pro audio has need of a small, inexpensive
mixing console for any number of applications. It might be for every
day use if input needs are limited. Or it might be for a small club,
church or corporate gig, or a situation where addtional inputs are
required for a certain project. Or perhaps an interface between
a piece of consumer gear and an all-balanced “mega console” is required.
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Let’s see if each model can pass some basic tests, and also, how the
respective feature sets apply to actual conditions.
With so many models to choose from, it was necessary (and practical!)
to narrow down the choices. So we elected to look at a range of small
mixers that all have faders (although only two use full-size faders) for
adjusting input levels, and that have four (or more) microphone inputs
on XLR connectors. The decision also included elimination of units with
built-in graphic equalizers. (I hope no comment is necessary regarding
that decision!)
Thus we arrived at a list made up of “utility” mixers from a variety of
manufacturers, some well known to most, and some a bit more obscure. Our
reach stops well short of smaller “pro” mixing consoles such as the Allen
& Heath GL2200, Crest XR-20, Midas Venice and others.
A REPRESENTATIVE SAMPLE
As outlined above, the point here is to look at what’s potentially useable
in a pro environment to augment larger consoles, handle utilitarian
tasks, or stand alone at smaller gigs.
Units selected are a representative sample and don’t necessarily compete
directly with each other. Therefore, they can’t be compared strictly on
price, because features can vary tremendously.
The testing of each model can likely be conducted by most readers of this
magazine. For a basic test of proper grounding design, I used a “Pin 1
Hummer” to check grounding between Pin 1 of the input (and output), XLR
connectors and the chassis. While this used to be a common problem in
mixers, I’m happy to say that all of the mixers tested passed without
a problem. (To get instructions on building your own Pin 1 Hummer, go
to the Jensen Transformers web site, www.jensentransformers.com.)
To check clipping with various gain settings, a RaneGain test set was
employed. Yes, an oscilloscope would more accurately show the onset of
clipping, but the RaneGain does a nice job of indicating when clipping
starts to be audible, which is what matters most while mixing.
An interesting problem found in all but two of the mixers is the ability
to get audible signal clipping with no visual indication. Typically this
was achieved by running a very hot signal just below channel clipping
and bringing the fader up to maximum, yet having the master output fader
set fairly low.
This was done with a single input, so multiple hot channels would only
exacerbate the problem. One of these days all mixers will have bus clip
indicators BEFORE the output faders!
QUANTITATIVE MANNER
To evaluate mic preamp linearity at different gain settings, SIA SMAART
Live 5 was used along with an Audio Control MP-400 as a source switcher,
and a pink noise generator. The transfer function and phase response of
the mic preamps were checked at various gain settings. All tests were
run well below clip levels.
As expected, there was little variation between units. However, there’s
no doubt that they all sound a little different. This is an area where
a number of factors are hard to test in a quantitative manner.
It’s always amazed me that what should be a simple gain stage the mic
preamp can sound so different in similar products. There weren’t any
earth-shattering differences, but it’s still interesting to note that
they all had their own “sound.”
To check summing amp linearity at different levels, again Smaart Live
5 allowed viewing of the transfer function and phase response from input
to output. This was done to see if there were noticeable differences in
signals run with low input fader and high output fader, and vice versa.
Again, all tests were run well below clipping, and all of the mixers seemed
to perform about the same.
At first glance, the biggest difference between the mixers is the tone
controls, or equalization (EQ). In products of this caliber, the most
noticeable variances in EQ are typically the filter bandwidth and the
pot taper as it relates to the filter circuit. These parameters most affect
the interaction between the person, the mixer, and the act of EQ’ing.
There certainly are circuit topology issues as well, but that is beyond
our scope here. In all of the units, the EQ worked. Behind that flippant
answer is a reality; when testing EQ, all units did what they should have
done. Yes, there were perceived tonal differences. But when filter bandwidths
were similar, and curves were matched, they also sounded quite similar.
AWAY FROM BAD HABITS
Phantom voltage was checked with a TerraSonde Audio Toolbox Plus (ATB+).
Phantom voltages varied from 45.4 volts DC to 53.4 volts DC, so all of
the evaluated units would work with just about any phantom-powered mic
or direct (DI) box on the market. It’s nice to see product designers getting
away from the bad habit of trying to use lower phantom voltages.
Signal path polarity was tested, also using the ATB+. Does a positive
polarity signal applied to an input produce a positive polarity signal
at the master output? In every mixer, the result was... Positive! (Pun
intended.)
Maximum input level to the mic input before steady visual clip indication
on that channel, at the minimum preamp gain setting, was also checked
with the ATB+. A 1 kHz sine wave served as the test signal. The visual
clip indicators typically came on at 3 dB before audible clipping, although
this varies slightly with each mixer.
However, if the clip indicator for a channel is steadily glowing, typically
you’ll not want any greater signal applied to that input. This test produced
the most noticeably different results.
Interestingly, it was the modern generation of small mixers that did away
with input pads. Years ago, even entry-level Yamaha mixers had pads, and
now they’re almost a thing of the past.
With hotter output dynamic mics, condenser mics and DI boxes, it’s even
more important to pay attention to how much signal is sent to the mic
inputs. Either that, or keep some in-line pads handy.
The remainder of the evaluation involved a look at features, and the overall
impression of each mixer. Note that only three of the eight units are
specifically designed for sound reinforcement applications, while the
rest are “multipurpose” and therefore have recording features that at
times make for a less-than-ideal layout for live use. This is important,
as we’re looking at this genre for live sound applications only.
Some of the units are only available with internal digital effects processors.
No comments were made on effects, since some units had them and some didn’t.
Now, on with the show.
Based in the Seattle area, Ivan Schwartz provides market and product development services to equipment manufacturers. He has worked for over 25 years in mixing and both portable and installed sound system design and implementation, and was involved with console design with Midas, DDA and Mackie. Ivan can be reached at ivan.schwartz@verizon.net
November 2003 Live Sound International