Hi all,
I recently finished my W492, and wanted to take a few minutes to document the build. I've modified the core EQ pretty heavily with a view to using primarily for 2-bus / mastering work, and also built a number of other functions in, so hopefully this will be useful to some folk down the line. Huge thanks to everyone who's posted on this project across the site, and of course to Audiox - I'm definitely perching on the shoulders of giants here!
Features:
- Dual mono Audiox W492 4-band EQ with fully parametric mid bands, plus Harrison Ford Filters HPF / LPF
- Rotary switches for frequency (24 steps/band) & gain (12 steps/band, switchable +/-)
- Channel components matched to high tolerance (less than 0.1dB difference between channels at all settings - except when boosting or cutting 15dB on the high shelf, when there's 0.2dB difference!)
- WIMA filter caps for low shelf and low mid bands, Styroflex for high mid and high shelf
- Switchable mid/side matrix (a la Wayne Kirkwood)
- Per-channel bypass (inside mid/side matrix) and master bypass
- Vintage Jensen output transformers (switchable per channel - relaxes the sound very slightly)
- Switchable gain range per channel (x1 / x0.5)
- Switchable frequency range for low mid (x1 / x0.5) and high shelf (x1 / x3) bands
- Variable bandwidth for low mid and high mid bands (Q = 0.45 to 3.6)
- THAT1246/1646/LT1058 ICs used in HPF/LPF
- Backlit plexiglass logo badge 8)
- Panels and NRG case by Frank
It sounds superb - very clean and precise, low distortion, and quieter than I expected given the amount of wiring involved (sub -94dBFS). There are 13 PCBs including filters, M/S matrix, power supplies, relays etc, and all the front panel controls are wired off-board, so hooking everything up took a good few days... The combination of fully parametric mid bands and shelves plus filters makes it really flexible, and I'm very happy with the styroflex caps for the high end in particular.
I spent a long time planning, listening and testing with this one - a good couple of years of reading and planning, and dozens(!) of iterations of front panel design. With all the potential for issues with such a complex build I was more than usually nervous when firing it up for the first time, but debugging was mercifully straightforward this time - the biggest issues were a bad solder joint in a SMT frequency switch, and popping on some switches (see below). In terms of mods to the W492, here's what I did following a ton of research, breadboarding and measuring (all component references are to the Audiox schematic, and I used REW for measurements):
Variable bandwidth
10K pot (RP) with 3k3 series on CCW (RB2) across U3A (low mid) and U5A (high mid), wiper to ground via 330R series (RB3). This involved running wires to one side of each of R16, R17, R26 (low mid) and R31, R32 and R41 (high mid) - I left little loops in the resistor leads to allow this when populating the boards. One caveat - the centre frequency wanders a little as the bandwidth is changed from wide to narrow (e.g. a few Hz at +15dB at 1k), so there may be a better way to implement variable bandwidth on this circuit, but after using it extensively on a variety of material it doesn't feel like a major issue. And ideally I would have preferred rotary switches to pots for stereo matching, but front panel space didn't permit that so I matched the pots instead as best I could.
Gain switches
5k resistor ladder with 5k resistor from CCW switch position. The +/- (boost/cut) switch swaps the CW and CCW poles going back to the EQ board, so the CCW position on the switch is always flat. Note that the shelves popped loudly at first when switching between boost and cut - putting a 1M resistor from each end of the switch to the wiper reduced this to manageable levels.
Gain range
For low & high shelves:
R8, R45 = 620R = +/-18dB gain range
R8, R45 = 1k2 = +/-8.5dB
For the low and high mid bands:
R15, R30 = 220R= +/-15dB
R15, R30 = 4k7 = +/-7.5dB
Frequency range
High shelf x3: swap C16 & C17 28.8nF for 10nF (lovely air boost)
Low mid x0.5: use empty C8 and C10 pads to switch in an extra 100nF at each point
Here's some photos from the build:
Testing bandwidth mod (lots of wires coming off the PCB!):
http://ringlefinch.co.uk/wp-content/uploads/2018/11/4-W492-bw-testing.jpg
Populating the internal control panel:
http://ringlefinch.co.uk/wp-content/uploads/2018/11/5-W492-control-panel.jpg
Control panel done:
http://ringlefinch.co.uk/wp-content/uploads/2018/11/6-W492-control-panel.jpg
Assembling and wiring it all up:
http://ringlefinch.co.uk/wp-content/uploads/2018/11/7-W492-populating.jpg
And all racked up!
http://ringlefinch.co.uk/wp-content/uploads/2018/11/8-W492-racked.jpg
I'm really pleased with how my W492 has turned out - it's been a major labour of love, and a great exercise in learning about state variable filters (and pushing the bounds of established designs...). Hope some of this helps someone further down the road!
Andy