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abbey road d enfer

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OneRoomStudios said:
I'm looking into some custom pots now. If I end up going down that road, it might be worth getting custom dual concentric pots, which would make this viable in 500 series format.

If that were the case though, I'd need to make a choice - either limit it to one variable mid band (so it would be HP, Low Shelf, Mid band, High Shelf, LP), and have room for an output volume control, or give it two mid bands, but make the gain fixed at unity while flat. There's only so much room on a 500 front panel.
Who needs gain control on an EQ?  :)
 

OneRoomStudios

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Ok, in that case, I have a rough schematic/pcb layout. Here's where I've landed feature/function-wise:

500-series, single channel

OEP transformers in and out (they're cheap and solid)
DOA output stage
IC opamps for filter stages - I decided to go with OPA2134 instead of DOAs (can't fit 5 DOA's and 2 transformers in 500 series)

4 concentric controls:

top inner knob: low pass filter, switchable between 6dB/oct or 12 dB/oct (down to 6kHz at 6dB, & 7k5Hz at 12dB)
top outer knob: Bax shelf boost/cut above 6kHz (-3dB down point at full cut)

top-middle inner knob: frequency for wien-bridge mid-band (650Hz  - 8kHz)
top-middle outer knob: boost/cut (+/-12dB)

bottom-middle inner knob: frequency for wien-bridge mid-band (145Hz  - 1K6Hz)
bottom-middle outer knob: boost/cut (+/-12dB)

bottom inner knob: high pass filter (up to 270Hz), fixed 12dB/oct
bottom outer knob: Bax shelf boost/cut below 250Hz (-3dB down point at full cut)

Plus, there'd be a bypass switch (bypassing filter stages, but still running through xfrms and output DOA).


This would (I think) result in a pretty flexible eq, with enough character to be worth building, at an affordable price. a "Volks EQ" if there ever was one.

What do folks think? The one trick would be reaching quantities for minimums on the custom pots.
 

OneRoomStudios

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Ricardus said:
I would build the sh*t out of these.

I'd call that positive feedback.

Also, I have enough room on the pcb to leave space to use either standard opamps (i.e. OPA2134) for the bands, or use Sparkos Labs SS3602 discrete opamp to make the whole thing discrete. Depending on how pricing works out on the pots and metalwork, you could probably build up a pair of these fully discrete with transformer i/o for less than most of the cheaper transformerless/IC EQs out there. I'm still waiting for word on minimums for the custom pots from TT Electronics.
 

Ricardus

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I've seen those Sparkos opamps, and they're a bit pricey so I'd go with standard opamps, myself. It might be fun to build one with them just to see what they sound like, but I suspect it's not necessary.
 

SWAN808

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are you using 50k reverse log, log or linear pots on the high and low pass filters?
 

OneRoomStudios

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The plan was for dual reverse audio pots for the frequency selection. I got a quote for concentric pots with dual rev audio on the inner shaft and linear center-detent on the outer shaft.

Unfortunately, it doesn't seem like there's enough interest in a project like this to hit the minimum quantities.
 

SWAN808

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OneRoomStudios said:
The plan was for dual reverse audio pots for the frequency selection. I got a quote for concentric pots with dual rev audio on the inner shaft and linear center-detent on the outer shaft.

Unfortunately, it doesn't seem like there's enough interest in a project like this to hit the minimum quantities.

ok thanks, I was wondering as I found that Bourns do a 4 gang reverse audio taper pot which could be used in a stereo implementation...20k tho...

https://www.mouser.co.uk/ProductDetail/652-PTD904-2015PC203
 

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