DEM Reissue mods questions

[Ben N]

So I just pulled my old DEM out of the crate of currently unused effects. I tweaked a couple of the trimpots (bias & clock) by ear, which had been messed with, replaced the 4558 input opamp with a TL072, true-bypassed it, and replaced the DPDT with a 3PDT for no reason other than my congenital inability to solder to an X-wing Carling without destroying it. I plugged her in and fell in love all over again (don't tell my wife). So I gets to thinking--would you mind thinking with me? Here's a schematic linky. There's plenty of room for a small daughter-board on the side of the main board, and for extra pots along the top. An easy mod is bringing the mix control out to the panel. But here are some other thoughts, half-gestated.

1) One reason I haven't been using her has been the annoyance of the AC power, especially now that I have moved from the US to 220V-land. However, just now I realized that all the power it really needs is enough to drive the 12VDC regulator, and in fact the export models took a 24VAC wallwart--which could have been as little as 14VDC. So, priority no. 1 is getting rid of the transformer and making this work with my pedalboard. Currently, I'm running everything off a 1-Spot, and I'd rather not have a separate wallwart for one pedal if it can be avoided. However, a quick search tells me that folks have not been too successful trying to feed these with a charge pump due to heterodyning issues. Would massive filtering help with this? Hey, I'm ditching a power transformer, I ought to be able to squeeze a couple of electrolytics and a choke in, if it would help, and there are volts to spare. Has anybody successfully pulled this off, or have a recipe for doing so?

2) If I feed this puppy 18VDC (through the connection in the Euro/Japanese version of the schematic, i.e. J4), can I just ignore the rectifier diodes, or do I have to take them out?

3) Among the other wonders of this thing are the lovely vibe/near-rotating speaker effect it can produce. So I think a LERA or similar would be grand, but the LERA as drafted by RG has two connecting points and replaces a 25k-250k pot, whereas the rate pot on the DEM is 10k, and connects to the circuit (not ground) at three points. Is there a way to hook a LERA into a DEM? And, is there a very low profile DPDT anyone can point me to that will fit between the pcb and the casing?

4) I also think that the rotating speaker effect would be enhanced by a light trem in sync with the flanger. What do you think about tapping the LFO at E10 (the filter matrix switch) and using it to control a switchable simple dual opamp plus optocoupler affair at the output?

Enough for now. TIA,

Ben

[DiscoVlad]

1) I've got one of these with the 24V wall wart, and where the power goes into the board is connected slightly differently to the schematic - the E1-3 Pads have been repurposed for the additional components, I'll dig some pictures up later.
Alternatively, you could always replace the transformer with a 220V one.
I have a few 18V pedals, and my DEM seems to work fine off that supply.

2) you can remove one of the rectifier diodes (D1 - 1n4001), D2 is there for reverse polarity protection so should probably stay, the addition of the Zener (also D1) and 180 ohm resistor are there to prevent the 78L12 from over voltage.

3) no idea, sorry.

4) Use another opamp as a buffer and it should be fine (unless it isn't...usual disclaimers apply). While you're there, give your add-on board some gain, to fix the volume drop.

[Ben N]

Hi, Vlad, thanks for the response. I thought about whether a buffer would be needed, but figured an FET-input opamp based trem would have a high enough Z-in on its own. Still, no harm in trying it both ways, and an add-on buffer would buffer both the flanger and the trem. I haven't really been troubled by the volume drop, but that, too, is an idea worth trying on the breadboard. Maybe leave one or both of the trem stages in-line (no bypass) and just switch the optocoupler in and out. Your power advice seems dead-on, thanks. I don't think I want to mess with a 220v tranny, but I would definitely appreciate pix of your board, thanks.

[DiscoVlad]

Ok, here's the pics of my DEM:
The Red arrow in both pictures points towards E2.

Trace side:


Component Side:


Hang on a minute, there's no R7 there! Does anyone know what it does?
I'm guessing changes the gain of the next stage?

These are the the connections to the board:

nb. The only "modifications" done were to run a ground wire from the E8 hole to the grounding tag bolted to the case, and replace the Vin Wire from the DC Jack. (I bought this as a faulty unit, and one of the power wires was broken).
I should probably have moved the red jack grounding wire from E5 to the ground tag too for proper star grounding, but didn't.

E5 and E8 are  board Gnd.
E4, E7 and E9 are the board input, and are connected to the Switch (green wire), and Input/Direct jack (white wire)
The Yellow wire E6 is the board output.  


Now, the wall wart conversion:
The main points are to remove D1 and the wires to E1, E2, and E3 from the transformer.
Your DC Jack should fit into the hole for the mains cable grommet.

No cuts to the board traces are necessary.

The 180 ohm resistor (1W or higher rated!) is connected to E1 and E3
The Zener diode connects between E2, and the stripe end of the old D2, it's oriented in the same direction as D1. This should be clear from the picture.

The V+ wire from your dc jack goes to the hole next to E2 where the other leg from the old D2 used to be.
The Ground wire from the DC Jack goes to either the chassis ground tag, 1/4" jacks, or whichever of E8 or E5 on the board is unused (depending on how much you care about grounding )

The EH supplied wall wart is 24V,100mA CENTER POSITIVE! and has 2.5mm plug. If you do anything different I'd strongly suggest labelling the jack.

Hopefully this is helpful to you.

[DiscoVlad]

Before I go blindly adjusting things... schematic notes say to adjust Trim 2 for "4kHz null at TP2" where TP2 is the board output wire. What does 4kHz null mean?

It's a 4kHz input signal obviously, but null?

edit:
I put the missing 4k7 resistor back in... and yeah... I think it sounded better without.

[Ben N]

Many thanks. It'll be a few days before I get a chance to crack her open again and compare, but it seems to make sense. My only question right now is this: I thought the 24v wall wart supplied by EHX was AC? Hence the rectification on board, right? In any event, I have no intention of using an AC wallwart, so I will take care to get the polarity right, and labeling is a good idea. I also gather that there is no real need for 24v, and that anything down to 14v should suffice to feed the 12v rectifier. I will probably use 17-18v, just because it's handy and useful to have on board.

Dunno about the test procedure. There are several resources scattered about the internets, including a post by our very own Mark Hammer on the old Ampage fx forum, about how to do the trims, both by ear and by scope.

[DiscoVlad]

Yeah, it's 24V DC. The plug polarity is backwards from Boss style though.