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Ha. Fight of the giants has started!

No fight here. I have the utmost respect for Merlin and others on this forum. The Engineer's Thumb compressor is a great design; I have one on my pedalboard right now. The pellucid and the diode compressor fill a niche which I thought was missing.

I'm using the "OTA in the feedback loop" approach from the ET, and the "2 transistor peak detector" sidechain approach from the DOD280, Dynacomp etc. I just married them up to make an original circuit from bits of other designs. But it really is very quiet I promise  :)
1. low noise and a NE5532 biased via a 1M resistor are not entirely consistent.

Thanks for your comments. I'm thrilled people are paying such detailed attention!

From a noise perspective, the 1M input resistors are in parallel with the guitar pickups at audio frequencies, typically about 5k - 15k,  which reduces the noise considerably. I agree 1M alone would be noisy as hell, as you could hear if you ran the device flat out with the input open circuit. The compressor is really very quiet; quieter than any of my other compressors (I've got about a dozen) with the exception of the Empress which is about the same. Noise in compressors is tricky to quantify because it depends on gain reduction, output volume, input impedance, bandwidth of measurement etc but it really is "low noise"  :) I can run this into my amp with the gain cranked and I don't hear any obvious increase in hiss when I kick the compressor on, unlike the Dynacomp which sounds like a steam kettle under the same circumstances. Bear in mind that with little or no signal, the LM13700 is off, and contributing no noise to the circuit, so it's just the noise gain of the opamp based on the sustain setting. NE5532 is a quiet opamp when driven from low impedance sources. The voltage noise and current noise equal out about 22k ohms input impedance. With my strat pickups the voltage noise dominates. If you switch the opamp for a TL072 you can clearly hear the hiss level increase compared with the NE5532. I really like the NE5532 and NE5534 opamps for audio - they're cheap, quiet, and can drive quite low impedance loads with low distortion. I only use TL072s when super low input bias current is absolutely necessary.

It's interesting you don't need to current sharing resistors according to the datasheet, because the voltage on those pins differs by about 10-15mV on the bench, enough that one of them would hog most of the current if I hard wired them in parallel (~60mV per decade re Ebers Moll). I guess they even out as the current increases? Generally I use current sharing resistors anytime I have bipolar transistors in parallel, as in this case (the bias pin looks like 2 diodes).

I've never really seen much benefit in the linearizing diodes. It makes biasing harder, and the few times I've tried it, I couldn't get the oft quoted 10dB in noise reduction at the same distortion level.

You're absolutely right re R11-R13, trimmer balance here would be better. I just decided to run a "balanced" signal into the OTA because I already had an inverted signal to drive the transistor peak detector, and it seemed sensible to use it. Whilst not perfectly balanced, the common mode input signal will be much smaller than if I didn't drive it this way, so you don't really lose anything.
Building your own stompbox / Re: Help With Op-Amps
« Last post by GibsonGM on Today at 05:00:01 PM »
Welcome to the forum!  :)
You could duplicate the above on my behalf, couldn't you Sir..???  :icon_redface:

From Antonis and myself, Welcome! :)

 Like that?
Building your own stompbox / Re: germanium transistor cache
« Last post by mac on Today at 04:29:00 PM »
Would you guys buy 10 packs of 2sb 54, 56 if the price was right?


And 2sb172/75/76 Matsushita too.

2sb77 Hitachi, another excelent jap germs.

In a few years they'll be more valuable than Bitcoins :-)

is the input cap 0.047 or 0047? i can't quite tell.. if its 4.7n, i'd go bigger.

to clip just one side of the wave, maybe try a shotke and led or germanium and led in series off the output cap? add like, a 50k trimmer or so between the 2 diodes n ground, and you should be able to dial in just a bit of half wave rectification.

i dunno if it would work in your circuit as well, but i find ge's drop too much output and cause a lot of fuzz, and led's tend to neuter distortion but are loud n buzzy, but the combo of the two in series is really musical and a bit louder than some other alternatives i've tried.

the led clips at about 1-1.5v, the ge at .03, and the thing is leds get weird on the note decays... they kinda fizzle out, or change pitch, or just cut out... but the ge keeps conducting and makes the decay a lot more smooth n natural. and tho ya gain some bite and compression, you don't lose too much output.

a single diode somewhere may make it come to life. i like 4739 reverse biased from how hemmo did the bazz fuss for a nice hint of asym clipping too.

hope it helps bro
rock on!
the best place for resources is wayne joness's excellent site,

that dude is both a gentleman AND a scholar.
anyways, i was recently gifted a semi-working gr700 guitar synth with the 24 pin cable, a gr202 guitar synth controller, and a gr300 that just needed an led replaced.

the 700 worked perfectly at first, til i hit a patch with a random sample and hold. at first, i thought the hold pedal function had stuck, but cleaning it had no effect.

so did some research, which led to the 80017a chips being faulty, and that they could well be repairable.
what it is is a couple of dip chips with printed resistors and smd caps on a ceramic substrate thats sealed in a rubbery plastic kinda stuff that in some cases is still gooey inside 40 years later, kinda like the insulation that turned to sticky goo in old japanese guitars or the black foam insulation that would rot into hideous foul goo...

anyways, over time, the stuff becomes more and more conductive til it causes all kinds of weirdness.
supposedly, if you can get it off, the chips will be "restored".

and it does, in fact, work that way.

anyways, the prevailing wisdom is to scrape off some of some of the goop on the top and sides of the chip.
the theory is this will let solvent in to help dissolve the goop. ya soak 'em for a day or so, then start chipping away at it.

to dissolve the goop, use acetone. pour about 4 fingers into a glass jar with a tight fitting lid and set aside for a minute.

anyways, me never being afraid to let the magick smoke outta stuff, decided to see if there may be a better way, for me, i found one. i don't advise you do this, caveats apply, acetone is nasty shit, as is the goop on these... if ya mess it up, mess yourself up, poltergeists start flying out of your television set or whatever, i'm not responsible.
wear gloves and protective glasses if ya do it my way, as shit will be flying off and shrapnel in the eyes sucks.

i took bog standard single edge razor blades and hacked away at the epoxy goop til i could see the white card beneath. i did this on the top of the card, the sides of the card, and as much of the bottom as i could get at. i didn't @#$% with where the pins were coming out.

i used a small file to grind off the rest i couldn't get with the razor blades. i found a single edge was good for about one card. the junos and gr700 use 6 cards a piece. they DID break, and pieces DID go flying in my face, and bounce off my glasses. but i was determined, insane, possibly a bit of both.

once i had clear margins around the chips, with the bumps facing me, i used the razor wherever possible to start lifting the epoxy to chip at least some off.

look on wayne's page at the stripped chips to see where NOT to do this. i didn't, and had to buy two replacements <tho my bassist, jolly roger fixed 'em at work for me>... one where i had sliced one of the smd caps in half ... it still worked...

and the smd caps are 680p @50v by the way, cuz they're not marked on the schematic roland made available...

and another where later, as i was cleaning goo between ic leads, i inadvertantly lifted a leg and f'd up a pad... he fixed that one, too. its what he does.

one more i had actually sliced one of the caps off completely. it still worked, but had a bit of fuzz to the filter on that one string. thats been fixed now too.

so i gotta couple spares... may be handy. also updated it to the newest firmware, or ordered it anyways and will be installing it when it comes in.. but i digress...

so anyways, socket where the chips on the board went, too. makes life a lot easier. you need 12 hole sip socket strips, and use your iron on hole #11 to push the pin/socket out. that way you have a proper key and it will fit the circuit board.
i found to desolder, use a strong sucker. a little fresh solder on the tip of the iron, heat the joint, suck the solder out, and use some needlenose to wiggle the pin around and it should come free fairly easily. do this 11 more times per chip.

took me 4 hours and myriad bingers to accomplish. but i'm slow.

anyways, so ya got your chips out, ya got the edges scraped clear, ya chipped off some of the crud on the bump side of the chip... the non-bump side is safer, but ya really wanna get some solvent in there..

next, soak that shit. throw 'em all in your jar of acetone  together, and seal the lid of the jar.
let 'em soak over nite. the reccomended time is 12-48 hours. i was figuring on 72 hours if needed.

when i checked them the next day, you could see where it was coming loose and beginning to bubble off.

me, being the impatient sort, hearkened back to my days of making parts for WMD's for the goobermint and thought, "man, i wish i had a tumbler"....

a tumbler is like a washing machine full of ceramic stones, and water with a bit of solvent in it. ya throw parts in, and it deburrs them, and polishes them to a nice uniform satin finish in industrial situations.

so i decided to just use the chips for the stones, and began aggravating the jar...errr... agitating the jar.

kindq like you'd swirl your fav beverage "on the rocks" while mindlessly doing other stuff, watching the other idiot box, blatherskiting with friends on the phone or whatever.

anyways, by about 24 hours, due to the aggravation of the agitation, the epoxy crud was really beginning to come off the backsides of all the chips, so i did a test one and found i could literally just pull the crud off with my fingers in one piece.

then i used a dental tool, some tiny files i bummed off my dentist that she uses for doing root canals, and started making sure i got as much of the goop out from between the pins of the ic's as possible. you may need to re-dunk the chips to get it all to loosen up, as the acetone evaporates, it starts to get hard again.

small sharp screwdrivers and single edge razorblades pretty much ruled. if ya use the razor, you can, if careful, juuuuuuuustslice between the legs of the 2 chips, and then drag out the goop between the legs and under them. lotta work... but in the end, good.

also make sure ya get off all the goop between the legs that plug/solder into the pcb inside the synth. once you do, install the chips into the sockets you installed, and be rewarded with a once again functioning juno or gr700 synth. you will have to recalibrate, but that info is on wayne's site, too in the service manual.

dissasembly isn't as bad as it sounds... couple nylon multiconnectors and screws, and the whole board comes right out. well designed and easy to work on.

anyways, thats my experience with doing this. it worked, it was positive, and for what it is, the 700 is kinda cool

if ya wanna play berlin's "take my breath away" in particular. it just SOUNDS LIKE the 1980's.

the gr300, being a poly analog, is much much cooler.

there's a reccomended mod to make the guitar's hex fuzz stronger, where you change two resistors from stock values of 56k and 2.2k to 100k and 5.1k.

but i don't like it... you do bump up the output of the synth by 12db, but the filter gets harsher and gnarlier sounding and loses the sweetness and ability to really dial in as many sounds in my opinion from the guitar.
i have owned a lot of analogs over the years <2600, mini and micro moog,prophet 5, juno 6, 60, 106, eml 101 etc etc> and this mod, to me, as a synthesist, is a great mod for a guitarist, but too much for my taste.

i would recommend instead splitting wayne's values to 68k and 3.3/3.6/3.9k instead. that will give about half the boost, and may make the filter response a bit sweeter while still driving it harder and about doubling the output.

but anyways... i hope some poor bastidge who gets one of these someday finds some of this info useful.

rock on, always
Hello, a good friend wants me to build him a pedal to make crazy noises with. He wants to have up to 4 separate effect loops to blend into a single output. This is what I have in mind:

-Input essentially would go into two ROG split n' blend circuits (see below). In other words, a single input would be split to up to four send and return lines (I guess I'd use a TL074 or two 072s for input and output opamps). Instead of a single blend pot, there could be a level pot for each line--so replace the blend pot with 4 level pots that would each go after the 10UF capacitor (i.e., one lug goes to ground, other end goes to output jack).

-If lines 2-4 have phase switches like the one for U2A in the schematic, would that help the user to deal with any weird phase reversal issues before combining the signals?

-My friend doesn't need this, but I thought I'd maybe add a momentary feedback switch, kinda like the Beavis feedback looper, to line 1.

-He also wants the send and return to go out to a a single TRS cable. So I'm thinking this should be fine with a stereo jack, with, say, tip as send and ring as return, right?

Does that sound reasonable? It seems too simple. Is there anything I'm missing?

Thanks in advance!

p.s. Sorry if anyone has seen this already. I also posted this query on a FB group, but I only got a couple of responses, and I wanted to also check here to make sure I've not missed anything.
Digital & DSP / Re: Detecting Presence of Expression pedal
« Last post by Blackaddr on Today at 01:00:51 PM »
It's actually linear because the pot is linear, your midpoint calculation of 2.75 V seems off? Here's how I calculated it.

For a 10K pot, and a 1K pullup, at the pedal midpoint the pot will be at 5K.

3.3V* [ 5K * (10K + 1K)] = 3.3 * (5/11) = 1.5V

So when the pedal is at 0 (heel down), your ADC see's 0V. When the pedal is at midpoint, ADC sees 1.5V. When the pedal is toe down, pedal sees 3.0V.
Building your own stompbox / Re: dunlop univibe woes
« Last post by pinkjimiphoton on Today at 12:49:36 PM »
there isn't one, apparently. dunlop is blowing me off about it, and this particular incarnation is unlike any other i have seen... not that i haven't seen this version a couple times, but usually its either more based on an actual univibe or like the rev a schem that lolbou and dirk came up with.

other than a couple semis, the one i have is different from all the others pretty much. it has weird switching, but much more standard parts.

i wish i could see it well enough to trace the dang thing, but the glaucoma makes it real hard to focus on stuff like this one.. its gotta fiberglass or epoxy board, but you can't follow the traces on it on the underside, it is covered with like, a pattern or something with a mess of solder pads on it.

way above my humble fuzzmonger paygrade!

that said tho, voltage tests were what saved my ass. check all the voltages, and where ever its wrong, suspect the passives associated. if they're ok, try replacing the semiconductor thats reading weird and odds are you'll nail it.

just don't scratch yer nards too long second guessing yourself ;)

rock on peeps.... LLAP
Building your own stompbox / Re: Help With Op-Amps
« Last post by antonis on Today at 12:40:32 PM »

I'll bet you guys a bottle of cider (especially for Stephen) in case of problem doesn't concern output current limiting resistors..  :icon_wink:
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