SPL Iron Model 1520 Mastering Compressor
By Barry Rudolph
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Mix Magazine "Compression And Compressors"
Unique Internal Signal Path
Both of Iron's Input and Output controls are ALPS rotary switches with six positions each. By using a combination of the three-position C&K toggle switches with: +, -, and 0 (unity) positions, there is up to a +/- 12dB of gain range in 2dB calibrated steps. I liked that the VU meter always reads gain reduction or output level even while in hardwired bypass. The VU meter switches between GR and either VU 0dB, or VU +10dB ranges--which is perfect for setting hot levels to pro DAW interfaces.
After setting input level (Iron is strapped for 0dB not +4dB), the incoming audio signal is "monitored" by a PerkinElmer VTL5C1 Vactrol opto-isolator feed-forward peak limiter that's NOT in the audio path. When input levels exceed +15dB, the VTL5C1 reduces it by shunting the signal to ground. Then a custom-made Lundahl transformer splits the signal to feed the cathodes of a variable-mu remote cut-off 12AU7 connected in parallel with a carefully matched, sharp cut-off 12AX7. Called a variable-bias limiter/compressor, there are two tubes per channel with the control voltage applied to their grids simultaneously.
The control voltage is derived from a combination of: the Threshold setting, Attack and Release time setting, the Side-Chain EQ section (if switched in), one of three Tube Bias voltage toggle switch choices, and one of six Rectifier choices.
The Rectifier section has six different rectifier diode/capacitor combinations in this order: two Germanium choices with 1mf or 2mF caps; an LED with 3.3mF; Silicon with 330nF, another Germanium with a 220nF cap; and a Silicon/Germanium combination with a 100nF capacitor.
The first five choices have response curves that go from 0.1ms to 5 seconds. The last position uses a Germanium and Silicon combination for the fastest response time of 0.2ms to 300ms. SPL recommends the middle LED rectifier as a good starting position; I found faster rectifier Attack/Release characteristics when switching towards the right and slower rotating to the left.
There are five sidechain EQ positions: Off (actually a 20Hz HPF is in circuit all the time), EQ1, EQ2, EQ3, EQ4 and Ext. Designer Wolf Neumann came up with the four sidechain equalizer curves empirically during his mastering sessions using Iron over the last three years. I referred often to the included curve drawings in the manual--they are not typical sidechain curves.
The Ext position switches to a rear panel 1/4-inch TS unbalanced side-chain input for connecting your own side-chain EQ or introducing any signal to cause gain reduction. I tried this path by connecting a bus output from the studio's console to cause Iron to duck down a backing track for a voice-over and it worked well with the ability to control the fade up time using the Release control.
You can also use the tip and sleeve on the Ext jack to send and return to an unbalanced equalizer to applied your own side-chain EQ. I found it odd that the sidechain circuit is an unbalanced path over a single TS jack. There is also no ability to link multiple Iron compressors for surround mixing. If nothing is plugged into the rear jack, switching to Ext offers a way to pass audio through Iron for its sound without any compression.
Mastering Finishing Touches
For stereo linking, Iron's Channel 2 controls Channel 1. All settings are linked except for the Input and Output rotaries. Iron uses a single toggle for choosing either of two output EQs. AirBass is a smooth "smiley curve" with gentle shelving boosts starting from 5kHz up to 15kHz and from 200Hz down to 50Hz. The Tape Roll-Off filter is a subtle roll-off that is down -1dB by 25Hz and -2dB by 15kHz. These EQs are placed after the Lundahl output transformers and before SPL's 120-volt DC Audio Rail SUPRA output op-amps.
Auto Bypass is for "hands-free" A/Bing between hardwire bypass and Iron in circuit. Automatic bypass is adjustable from 2.5 seconds to 12 seconds and, once engaged, it continues to cycle back and forth until you turn it off. I think that the large bypass buttons should also light up when in/out circuit just as when manually toggling Iron's bypass button in/out.
In the beginning, arriving at a compression setting involves some experimentation and in time, I got an understanding of how all the different parameters interact and affect the sound.
I found when auditioning the sonic differences between the Rectifiers, it was easier if I set the unit's Attack time to position 2 or 3 (1 being fastest) and the Release time to fastest position 1. Starting this way, the compression action is mostly dependent on the Rectifier's time constant. The manual has a chart of approximate time values using any of the six rectifiers along with each of the six, Attack/Release time positions.
The first three Rectifier positions produced a clear attack with the main differences being the 1mF being the fastest recovery and the slowest at 3.3mF for the LED. Generally for program compression, the LED rectifier emphasized the low frequencies in a very solid and flattering way.
My first use was for a drum bus compressor. Even with extreme squashes, it is hard to get Iron to distort audible. Iron fattened up the size and width of the drums in super Hi-Fi way using 1 to 10dB of compression using Side-Chain EQ1 preset. This preset is flat in the low frequencies but its 700Hz peak caused more compression of the boxy part of the (drum) room and the dip at 2.4kHz going to nearly flat by 5kHz kept the high frequencies open and transparent. Amazing!
Iron inserted across the stereo mix bus in Pro Tools 12 HDX set as a leveling amp was a perfect fit for a hip-hop track that already had minimal dynamics. I set the Attack time to position 2 or 3 and Release time to position 3 or 4 and used the Germanium/2mf rectifier. Using side-chain EQ3 that boosts the sub bass but has a dip at 175Hz, I went with as much as 9dB of gain reduction and the track pumped like it was going to burst at the seams! Awesome!
I tried it on a lead vocal. Side-chain EQ4 has a lift in the bass causing a little more compression overall (and sometimes necessitating a new Threshold setting) but it also has a broad dip at 3kHz followed by a shelf boost starting at 4kHz. This curve caused Iron to compress more whenever the singer got shrill and reedy-sounding yet kept the sound open and transparent. Iron would be my first choice for compressing any singer.
SPL's Iron allows sculpting compression at the circuit design level. I could customize this compressor very specifically to produce the perfect style and amount of processing for any job. Iron is a powerful instrument for mastering, mixing or recording and it's difficult to get a bad sound on any source from full stereo mixes to vocals to drums to acoustic guitars. Incredible Kit!
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