Development, Inc.
Red-Eye Instrument Preamplifier Technical Information
Frequently Asked Questions
Why is there no input level control?
I was able to simplify the Red-Eye Preamp and eliminate an input level control by providing
lots of headroom. The Preamp is specified to handle 1 Volt peak-peak Instrument Input
signal levels without distortion. That's about 3 times what most passive pickups can
generate. The Red-Eye Preamp can actually handle about 2 Volts peak-peak and has no time
constant associated with recovery after being overdriven. In other words, it does not "block"
or "gasp" if overdriven...only the offending signal peak will be neatly clipped and will
cause no longer-term effects.
How is the gain managed as the signal moves through the Red-Eye Preamp?
A fixed amount of gain is applied by the Red-Eye Preamp's high input impedance field-effect
input amplifier. The following levels are output relative to an Instrument Input
reference level of 0 db. (Output levels are specified with the Boost Button off).
- From the Instrument Input to the Effects Output the gain is -1 db. (In other words
the output voltage level is 80% of the input)
- From the Instrument Input to the Balanced XLR Output, the gain is -5 db. (In other
words the output voltage level is 33% of the input)
- From the Effects Input to the Balanced XLR Output, the gain is -5 db. (In other words
the output voltage level is 33% of the input)
The Boost Button affects both the Effects Output level and the XLR Output Level.
With the Boost Volume control set to minimum, the Boost Button adds +4 db to the output
levels. (This is the threshold of clearly hearing a boost in the signal) With the Boost
Volume control set to maximum, the Boost Button adds +9 db to the Output Levels.
How did you determine the output levels?
For the Effects Output Level, I did some testing of a few popular effects boxes
and pedals and discovered a number of them have little headroom. To make sure the
Red-Eye Preamp did not make matters worse and drive the Effects into overload, I reduced
the Effects Output level by 1 db below what the Instrument generates. Not much, I admit, but
it makes sure when the Red-Eye Preamp's Effects Loop is used the effects may sound cleaner
than without the Red-Eye Preamp.
For the Balanced XLR Output Level, the differential output voltage level to the XLR
Output is determined by the turns-ratio of the transformer I use. Essentially, the
transformer has three turns in the primary winding for each turn in the output winding,
which makes the output level 1/3 of the input level. That translates to 5 db below
the Instrument Input level. This output level is equivalent to a pretty "hot" microphone.
I'm sure there may be some simple mix boards that may not be able to handle this
fairly high level, but I designed this unit to have the highest signal-to-noise ratio
possible, and that means a hot drive level. Any good mix board will have an input level
control for each channel, so all inputs can be balanced and controlled to optimize
signal levels through the board. I'm counting on the sound technician to understand
how to do this!
How does the Red-Eye Preamp help piezoelectric pickups sound more like the instrument?
I wish I could claim magical insight on how to make musical instruments sound more
acoustic, but there's really nothing tricky in the Red-Eye Preamp. Of course,
piezoelectric pickups require the very high input impedance offered by the Red-Eye
Preamp's field-effect transistor input circuit. This prevents "loading" of the pickup
and keeps the frequency response of the pickup more nearly flat. But basically, the
preamp is just simply a very HiFi circuit. Flat response beyond the audio band, extremely
low distortion, lots of headroom, and very low noise. There is no "trick" EQ or
compression or anything else messing with the signal from the instrument...I simply
try to preserve the signal from the pickup precisely and exactly through the entire
signal path.
The lower frequencies are so clear when using the XLR output to the mix board. What
makes this difference?
If there's a secret, it's the balanced output transformer. I must have bought over
20 different transformers to test. Some were quite expensive (over $50) and others
were less than $2. I was testing for flat frequency response, low, smooth phase
shift and low intermodulation distortion. I did not get to test the well engineered Jensen
transformers because they were all too big to fit into the Red-Eye Preamp case.
One small transformer stood out way above the rest. Its low-frequency response extended
smoothly down to below 20 Hz and the high-frequency response went cleanly above 40
KHz. This transformer allowed me to couple the high performance of the electronic
preamplifier to the XLR output without losing any fidelity. Turns out, this transformer
was not even designed for audio use...it was normally used in computer modems. Good bye
to $50 audio specialty transformers. I don't think there is a competitive DI on the market
using a transformer that is this good."
How come the Red-Eye Preamp has a Treble Control, but no other EQ?
Part of the answer is that the Red-Eye has flat response to 18 KHz, and you can hear the
scrachiness of a fiddle bow. Many other DI boxes don't reproduce those high frequencies
at all. Musicians, especially fiddle and mando players, like to adjust the treble
brilliance to suit their particular instrument's pickup. The bass response of the
Red-Eye is flat down to 20 Hz and most musicians like the way that sounds.
How is the Red-Eye Preamp resistant to neon sign buzz and cell phone noise?
The Red-Eye has a filter circuit that cuts the radio frequency ranges generated by that
kind of noise. The Red-Eye has lots of headroom too, so that any such noise does not overload
the Red-Eye's input circuits and generate intermodulation distortion noise that might propagate
through additional stages of Red-Eye noise filtering.
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