3x Passive Audio Attenuator: One Hour Build!

Time for another easy one, and maybe my shortest AudiodiWHY post to date, which may not be a bad thing? Simple Passive Attenuator.

You can build something like it out of junk parts, but it might get used a lot!

No decal--skipping straight to FPE finished module looks like this:

I needed this module because I have gotten into Eurorack recently  and there seems to be not a whole lot of consistency in output P/P voltages. Some of them output (say) 2V P/P while for others it's about 5 or 7 or 10V P/P.

Maybe that "hot" output signal needs to be turned down so the next module doesn't distort--since we know it's never cool to smoke Doepfer. But the good news: it's easy to attenuate them hot signals using something like this:

I built this up out of junk parts and an "alubase" panel I had leftover from a PCBWAY order.

Here's the obligatory photo of the back:

Another bonus: I am a rare but devoted Frac guy, and Frac power supplies can get a bit over sized; they can stick out into the module next door when you beef 'em up, which you usually need to do. Putting a passive panel, with no PCB sticking out the back, next to the Frac's "wing" power supply keeps things from shorting out. Thank goodness!

Not much more to say here except it's fun to think up passive modules--i.e., no need for DC power. Can a Low pass gate be passive?  Yep. Can a distortion module be passive?  Yes--check out the previous post here.

Hours of fun, maybe not? but an hour of fun?  yep.

OK it took me less time to write this post than build this module but not much. Again maybe not a bad thing. See ya next time.

Quick One: Front Panel Express

I'm still looking for a way to DIY small quantities of professional looking front panels for projects, but until recently, no dice. I've written about using decals on metal, but that process is time consuming, hogs your oven ("sorry honey, no dinner tonight, I'm cooking my nerdy front panels"), and takes a fair amount of practice to get good results.

And, even when decaling comes out spot on, compared to the real deal, it's still a bit lacking. They're decals!

I may have found the answer--I finally broke down and got my first panel from Front Panel Express (in Europe, same sorta thing here).

Good news, it came out looking really good I think, and was easy. (No I don't work for or get any sort of promotional consideration from these guys).

Front Panel Express' "Hammer Cocktail" panel

Same thing--metal from PCBWAY and graphics via Adobe Illustrator and Lazertran

The panel you see at the top of this post cost me about $59 USD including shipping, tax etc., compared to about $10 each for bare metal and decals, that's a lot more piasters. It would have come down a bit per panel if I bought more of the same design. You know: "The more you buy, the more you save." Shipping is free for orders over $50 and shipping alone for the panel here was about $12USD.  A lot of shipping dough I think for a single 1oz scrap of metal....but it came in a beautifully packaged, carefully prepped box, no way it was going to get scratched or smooshed on the way to my shop and the whole thing came out, really, perfect.

To use Front Panel Express you have to use their software. No choices--it's their way or the highway. Sadly you can't say send these guys a gerber and they'll make a panel from that (which you can with PCBway--use "alubase" and black silk screening).

I tried this FPE CAD software a few years ago and failed horribly; I couldn't even get a 1u Frac panel sized, much less the drills and whatnot. But this time for whatever reason--maybe more time working with Eagle? I was OK. Turn on the grid, draw the panel, no sweat.

Good news is: FPE has the ability to import graphics from a variety of formats. This is very helpful! For me I prepped the artwork (including little circles for drills) in Illustrator, moved it into Photoshop for some touch up, saved it as a print quality PDF, then laid it over a blank 1U frac panel in Front Panel Express' designer app.  Then, used drills in the FPE designer to punch the holes right over the art. I was nervous that thing wouldn't line up when I got the panel (which would have been $50 odd bucks down the toilet?) but, worked great.

If you want to engrave your panels for a more Moog/Synthesizers.com type look you can do that too (how to here) but for me, graphics is just fine.

So now I'm going through other modules where the front panel came out a bit butt, and I'm finding it super fun to make FPE panels, since you can go hog wild with graphics. For instance, further immortalize Don Buchla's photo on your DIY "source of uncertainty."  Why not? I am going to convert more of my homemade panels to FPE over the next few months, it just looks more professional.

OK that's it for this one. Perhaps the mo-fessional part of me that wished I had Bob Moog's chops will kick in.  Until next time, don't breathe the fumes.

GABBAR--High Performance AR/ASR

There are those who think outside the box and there are others for whom there is no box at all, gabbagabi from the EM forum is the ladder!

gabbagabi and I started corresponding after the "FAITW" posts; we are both interested in high performance EG's. In his messages, gabbagabi said he had designed a AR/ASR that attacks around 1ms and is still  musically useful. He sent me his schematic and I (embarrassed to say) couldn't figure out how it worked.

Hmm.

The GabbAR uses a couple of opamp stages as comparators. There is a 100nF cap in there that blips the comparator to start the attack. For me AR's were always 2 diodes, a cap and a couple of pots, and sounded like, well, an AR.

I tried putting stages of gabbagabi's design into a SIM site but the results were--inconclusive?

Gabbagabi reassured me he strip-boarded this ASR a couple of times and it worked great. We traded Eagle files; even though gabbagabi usually uses a design program called "Sprint Layout" he ported his work to Eagle, which is what I've always used. Thanks dude! Also, gabbagabi wanted to fab what he calls a "humanized" PCB, meaning that the PCB is laid out to roughly match the schematic--which takes more PCB space, but makes understanding what's going on and troubleshooting/modding a lot easier. Something I've not seen before.

I sent a gerber of his humanized design to fab along with some other things I was working on, it came back a week later, and I built the GabbAR in a couple of evenings. Guess what? gabbagabi's design works and sounds friggin' GREAT.

It says AD CSLAMMY on the schemo--that's my EM avatar--but this is Gabi's work.

The AR does fine with +/-12V DC or +/-15 supplies, and its outputs scale based on the supply voltages. So for me, in Frac +/-15V land, the max output of the ASR is about 7.5V.  That works.

Gabi's "humanized" PCB layout....

You can mess with C7 and the pots (use audio taper, I think it sounds better) to get sub 3ms attack and to my ears--no click.Very nice! Of course bigger caps and/or bigger pots mean longer AR times. gabigabba suggests using 100K pots and 100uF for C7 for "normal" AR speed, or 1M/2uF for very fast AR.  For me, I ended up putting 10uF and 100uF caps for C7 on a switch and using 12mm 100K audio tapers. So: If you build this AR, you may want to mess with the values for C7 and the A and R pots.

One gotcha: make sure the transistors used for T2 and T3 match the schematic.  Depending on what you use, you may have issues. I used a 2N3906 which was silk screened correctly on the PCB but had to flip a 2N3904 to get this to work. Follow the BOM, bottom of this section.....

I also added an op amp buffer on a tiny daughter board. Now, I get inverting output from about 7.5V to 0V in addition to the native 0V to -7.5VDC and 0 to 7.5VDC "native" output signals. 1001 uses.

Testing....

More testing. Daughter board gives me an additional inverting output....

Once I had this EG up and running I tried to figure out how it works, for anyone truly interested please read my explanation docx here. I'm not an EE so please, feel free to comment or message me if you have other ideas about how this board works--it's tricky.

You can get the PCB and the usual downloadable dookie (BOM, gerber, etc) here.

Finished; UPDATE! I replaced this home made front panel with one from Front Panel Express a few weeks after taking this photo. Maybe one of these days I'll take a picture of the new and improved panel version? 

Going Forward: gabbagabi and I have been IMing again; it'd be nice to shrink this design so 2x or 4x  fit in a single board and (am I really saying this?) see if I can get this shoehorned into the smaller Eurorack format.

But that's for another post. In the meantime it was super fun working with gabbagabi on this. I hope to post more of his stuff down the road. So until next post--go ahead, take a whiff, it's June!