This little project came about when my good friend Zumdish visited recently from The Celestial Department Store (Fifth Quadrant). 😊
The great DBX 120X-DS subharmonic synthesiser that I’d been amusing myself with until a year or two back had been put into storage due to EQ incompatibilities with the overall system when listening to music in “hi-fi mode” in which both the 120X-DS and an analogue bass equaliser were switched to their respective bypass modes.
For sub-synth to work properly, bass EQ MUST be applied downstream of the synthesiser to target the generated subharmonics as well as the fundamental bass frequencies independently of one another. Up-front EQ cannot target the subharmonics as they don’t yet exist. Also equalising the fundamentals pre-synthesis would be ineffective as (say) a 40Hz cut filter to address a room mode would diminish the generated 20Hz subharmonic.
In my system the sub-synth relied on stereo analogue GRAPHIC bass equalisers (2 by ESP P84s) with the stereo output split two ways to four channels (front stereo speakers and 2 rear subs – stereo maintained). That was fine for the heavy deep bass impact/effect of the DBX, but “Hi-fi mode” without sub-synth and relying on up-front PARAMETRIC EQ was compromised by the two-way split in which only two channels of the DAC were exploited, and the very same compromised EQ that was applied to the front speakers was also fed to the rear subs.
Superior bass EQ for hi-fi mode is achieved by exploiting 4 channels of the DAC and applying individually tweaked and different PARAMETRIC EQ to the fronts and subs. Ideally hi-fi mode has the main speakers receiving one set of EQ filters and the subs receiving their own different set of filters, as the speakers excite room modes differently.
A multi-channel DAC (in my case a Focusrite Scarlett 18i20) provides ample channels for this. That hi-fi mode is where I’d been for a couple of years with the DBX and P84 in storage.
But I missed all the fun of the DBX
It wasn’t possible to have the exciting sub-synth mode on the one hand, and a properly equalised hi-fi mode on the other. Well, that was until I mentioned the problem to Zumdish with the idea of creating a special switcher that would route the signals “this way or that” depending on whether “hi-fi mode” or “synth mode” was desired. I simply explained to him the logic and with a pop of his mouth, he created a schematic for a relay switcher, and before I knew it he’d designed and fabricated a PCB! LOL!!! That Zumdish sure ain’t no slouch!
The PCB has five Panasonic DPDT gold contact communications TQ2-12V relays (10 switches) and passes four DAC channels straight through in hi-fi mode (or when switched off as the relays default to this mode when no power is applied), but when flicked over to synth mode, input channels 3 and 4 are ignored, channels 1 and 2 are routed to the 120X-DS and the dual P84s and the stereo outputs of the P84s are looped back to the PCB where channel 1 is split to outputs 1 and 3, and channel 2 is split to output channels 2 and 4! Just what the good doctor ordered! 😊
The playback software then has two Zones set up. One being “Upstairs Hi-fi” with 4 channels all including the best and dedicated parametric bass EQ filters for the fronts and rears separately, and the other Zone being “Upstairs Synth” has just 2 channels and no bass EQ at all because that is handled downstream by the P84s post-synthesis!
The PCB routes the positive side of the unbalanced signal from 4 inputs to 4 outputs when there is no power applied to the board. That’s most of the time. The RCA shells (on the zero Volt line) are all tied together to the heavy copper bus wire around the RCAs. The thin Teflon coax shields are all soldered to the bus wire, but the PCB ends are cut off and heat shrunk over. The RCA shells have washers to isolate the zero Volt line from the case. The case is Earthed to a power strip. This is vital because the unit takes 12V DC from a noisy external SMPS plug pack which injects noise into the case via the barrel socket on the rear panel. The plug pack is a standard double insulated, non-Earthed type. In any case great care was taken in the DC delivery from barrel to PCB. There is a reverse polarity protection diode which drops about half a volt and 100nF capacitors to the input barrel’s negative terminal form a pi filter around the diode. SMPSs MUST have a small load at all times to mitigate noise, so a 560 Ohm resistor is fitted across the barrel’s terminals. The DPDT rotary switch on the front panel has one pole connected to a pair of front panel mode-indicator LEDs. The other pole simply applies 12V DC to the PCB whereupon all relays activate into Synth mode. No matter the position of the rotary switch, at least one LED is powered drawing 20mA, but being a BBM type switch, there would momentarily have been no load on the SMPS – hence that resistor across the barrel terminals. There is a 1N4148TA small signal diode across each of the relay coils to suppress fly-back voltage spikes upon release of the relays.
Those Earthed bars of copper are from scrap and provide ballast so that the box doesn’t flip over when all 12 interconnects are attached.
The switcher works exactly to design and doesn’t inject any noise into the system whatsoever. Not even a pop on release of the relays. If anything, an ear-to-tweeter test suggests that there is even less general noise in the system than previously. Just a slight white noise from the tweeter where there was previously a minor buzz. This might be attributed to the grounding bus around the RCAs, but I doubt it.