Instruction Manual

To use this HIGH FREQUENCY LIMITER you probably don't need a serious technical manual
but some features are worth explaining all the same.
The "basis" for this design was a vintage high freq limiter used primarily to "cut" vinyl records.
The RIAA curve specifies a large amount of high frequency boost that is later removed (along with hiss
and other noise) in playback. This is fine except that to get louder records excessive highs could produce
a nasty distortion and reduce a $5000 cutting head to a smouldering mess. There were several different
high frequency limiters available to deal with this but one device in particular solved the problem in an
elegant way. With this unit, the signal was put through only a few passive parts to achieve its purpose.
The variable component was a special high quality transformer with a separate winding that controlled
its high frequency response. The technical term is "saturable reactor". We use this technique in our HIGH
FREQUENCY LIMITER and follow it with our professional line amplifier to provide make-up gain and
a low impedance output. We also use modern op-amp circuitry in the control circuit (sidechain) but only
a class A vacuum tube circuit is appropriate for the audio. We wanted it to remove nasty sibilance, not
create more. No VCA's and no op-amps in the audio path.
While we use the original name for this circuit, it is somewhat inaccurate in todays terminology.
The circuit is actually a sliding filter or voltage controlled filter (VCF) except without the distortions
typical in VCF's or VCA's.
There are two "sensing" circuits. The HI-MID circuit is a tunable band-pass filter. The HIGH
circuit is a tunable high-pass filter. The user can select which sensing circuit is most appropriate for the
problem faced. When the offending frequency passes through the filter and is "over" the THRESHOLD
then the filter begins "sliding" down from above 20 kHz until the offending frequency is reduced enough
to be below the threshold. This is indicated by the meter and LED.
There is one input and two outputs. The input is floating transformer balanced in parallel with
a 22K resistor. Both outputs are low impedance unbalanced. PIN 2 hot on XLRs. There is a fair amount
of confusion that unbalanced outputs are not good with balanced inputs. Not true. A good balanced input
should cancel as much hum as it can with either balanced or unbalanced signals. However, there are
many balanced line drivers that have problems driving transformer inputs (at low frequencies) and
unbalanced inputs or long lines (instability or HF oscillation). You should have no problem with hum
or any instability with this unit. There are grounding options with the terminals on the back panel.
The MONITOR OUTPUT is a 1/4 inch mono jack with a special purpose. There is a switch on
the front panel that selects the source to be monitored. In NORMAL the source is the same as the XLR
output, in SIDECHAIN it provides a way to hear the sensing filters and tune them accurately to the
problem frequency. The THRESHOLD knob will control the volume of this output. The XLR does not
have this switched signal and is intended to be fed straight to tape safely, even if the engineer is "tuning"
The BYPASS switch is not a hardwire bypass. The signal bypasses the De-essing circuit but still
goes through the input transformer, tubes and RE-ESS circuit.
The RE-ESS control is a simple 12kHz shelving EQ with 6 dB cut and boost. We named it that
just for fun. The best place to DE-ESS is individual vocal tracks usually after EQ and compression.
Sometimes the right amount of de-essing unfornunately removes some of the "AIR" on a track. That
name was taken. The RE-ESS control is set up to return some of those "nice" highs after the "bad" highs
are buried. This control is active even in BYPASS. If you are going to use this HIGH FREQUENCY
LIMITER on a mix you will notice two things. First - setting the threshold is difficult because high hats,
snares and other brite sounds will trigger some de-essing. Second - the RE-ESS control is pretty damn
effective in fixing that first thing. It is also useful during recording as a "safe" hi frequency boost (or cut).
It can be better than most EQs because it doesn't add TIM distortion as a by-product. Some feel that the
tube/transformer circuit adds the "right" warmth and not some cheezy distortion. Check it out.