Calrec PE1549 - My take

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Well-known member
Jul 21, 2012

Finished up my lunchbox sized two channel trial with this design - everything works ok. I also added a high and low pass filter. More details can be found at my blog.



Here is the text minus images:

I have been working on a console project (in fits and starts) for a while now and decided to use the Calrec PQ1549 design as vetted by Gyraf for the project. I’m packaging this as yet unbuilt console in a series of 5U height modules (218mm) that fit into a Schroff europac enclosure with custom 5U sides. The modules are 8HP wide or 1.6″/~40mm allowing for 10 channels in each 19″ rack plus a 4U blank.

This equaliser is the first module I’ve built in the format and decided to start with a 2 channel lunchbox laser cut from bamboo.

The equaliser doesn’t differ from the original Calrec design and subsequent Gyraf transcription, though all the switching is implemented on relays. Overall dimensions are 6″ by 8″.

I broke the signal after the input amplifier and added relay switched High Pass and Low Pass filters lifted from the Harrison 32C console. It was vetted in a previous project but I altered the C50K pots as used in the original to C100K and used vanilla Alpha RK16 pots instead of the horribly overpriced and poor quality Omeg units from my first build.

The module has a built in positive and negative voltage regulators for powering the opamps. This was done to reduce crosstalk in the final rack of 10 units. The 34pin IDC ribbon cable carries +21.5V, Ground and -21.5V generated from a very basic external power supply consisting of a transformer with 15VAC secondaries, a bridge rectifier and some bulk capacitance. The regulators them selves are a stock standard LM317 and LM337 combo mounted to massively oversized heatsinks, a cheap purchase from a surplus house in quantity. With all relays and LEDs on the unit consumes 240mA at +21.5V and 90mA at -21.5V. I’m hoping that by keeping the voltage regulation on each module I’ll avoid heat issues by spreading dissipation across the unit as well as avoiding the need to  build a high current power supply with regulation.

The unregulated 21.5V rail is also used to power all of the relays, in this case I’m using NEC EC2 24V models that happily operate with anything over 18V or so. The front panel switches mounted to a secondary PCB allow for control of the relays without any wiring. 8 of the 9 are driven using a ULN2803 darlington driver IC with the remainder making do with a BC550.

Front panel spacing is 0.8″/~20mm between the knobs – perhaps a little tight for my liking but for a channel equaliser in a large desk its all fairly standard.

I/O is mostly on ribbon cables, a 34Pin unit carried power and runs perpendicular to the front panel, whilst audio signal is in a 10 pin ribbon cable for connection to a XLR pcb. Power supply and Audio Grounding is kept separate throughout the PCB and each of these are broken out to an M3 terminal for a robust connection when installed in the final rack. The final unit will have a grounding bars drilled out for the 1.6″ module spacing running across the rear of the modules to facilitate easy grounding.

The overall feature set looks like this:

High: Gain, Frequency, High Q Switch, Shelf Switch

High Mid: Gain, Frequency, High Q Switch

Low Mid: Gain, Frequency, High Q Switch

Low: Gain, Frequency, High Q Switch, Shelf Switch

High Pass/Low Pass: HP Frequency, LP Frequency, Bypass Switch

The unit worked without issue save for the relay control switches, I wound up getting them all backwards. Its rare than something works right out of the cage so I was happy.

The lunchbox was a slapdash affair, designed quickly so I could hop onto another order and keep costs down. All faces are 3mm bamboo ply with the top and bottom panels being triple thickness. I drilled out holes and hammered in M3 spacers (with a spot of glue dropped into the hole) for the module mounting. I was planning to screw the modules directly into the wood but since this is a prototype the repeated assembly/disassembly would destroy the wood before too long. The enclosure was wired up with basic XLR connectors, power comes in from the external supply on a 4 pin XLR connector on the rear and is wired directly to a 34 pin IDC header.

The power supply was a true zero purchase build, I even hand cut and folded the sheet aluminium that went into the enclosure. The donor for this was the cover of a Tektronix R7603 oscilloscope I keep in the shed for spare parts. I used a 20VA transformer from my bins with two 15VAC windings and was hoping to get something over 20V when I put it all together. However this was undersized for the job and when powering both units with all relays/leds on the DC coming from the capacitors drops to 18V with plenty of ripple which means my 17V regulators on board wind up with no headroom to regulate and things didn’t sound good. Rather than purchase an appropriate transformer, I just lower the rails to +/-15V which works fine.

I’m not showing the inside of the power supply because its a bit scrappy, but it is safe.

Overall happy with this project. I’m going to dump the unit at a friend’s studio and see what they think of it over a longer period before pulling the trigger on the 10 channel rack. I enjoyed the metalwork and really need to buy some proper shears and a brake as I cut the enclosure with a hacksaw and did the folding using a hammer and a brick – decent tools would have made that much smoother.


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