blackmer compressor

[Thomas P.]

http://www.spinsemi.com/appnotes/spn1001/AN-0003.pdf

what do we think output-wise? would it need a buffer or just a cap and a pot?

I just completed a layout for it and have still space on it for that kind of thing....

[Thomas P.]

bumpidibump

[markm]

Have you tried this circuit yet?
Look interesting. 

[Doug_H]

From a cursory glance I would say if you are using the +-12v supply specified, a pot alone would do it. You may not even need the pot. The TL084 should have plenty of current drive.

That's an interesting looking circuit. I'm glad to see someone looking at app notes.

[puretube]

a make-up-gain never hurts for guitar...

[Doug_H]

a make-up-gain never hurts for guitar...

Unless it's already designed for guitar and that is already anticipated...

[Thomas P.]

Have you tried this circuit yet?
Look interesting. 

I haven't tried it yet because I wasn't quite sure because of the output. I'm willing to share the layout, though.
But with the restrictions in the article about certain short traces I had to make it double-sided. I guess this makes it kinda hard for PnP user....

here are some previews (blue is bottom, red is top)

[puretube]

a make-up-gain never hurts for guitar...

Unless it's already designed for guitar and that is already anticipated...

that was an "embedded" statement, not for: "standalone" 

(but then again, yes, one could increase R17, like mentioned...)

[analogguru]

Very intersting:

I had a look at the application note.  As Keith Barr writes the Blackmer-VCA is a standard since the 70´s - nothing really new.

but:

Anybody knows something, what Keith Barr (the co-founder of MXR and founder of Alesis) has to do with Spin-semiconductors ?

Is Ketih Barr no longer with Alesis ?  Or is Spin-Semiconductors the new brand-name for Alesis-chips ?

analogguru

[Ronsonic]

I don't know the history well, but I know Alesis has been through some ownership changes in the last few years.

Was astonished to find a brand new SR-16 on the shelf at Guitar Sinner last week. Didn't check to see if it still had the big-hair / big-snare revberded drum sounds.

Ron

[Harry Palms]

I breadboarded it today. Pretty straight forward sound. Nice. A little like an OS. I haven't read the app notes and only played it for about 10 minutes. It's late and I'll mess with it later. Seems like a winner though and something worth building.

I went with the spare parts I had available.
4558
TL082 (X 2)
5089/5087

[Thomas P.]

OK, that's it! I'm buying a breadboard this minute

[Doug_H]

Ahhh... The power of breadboarding. Another convert...

[Harry Palms]

Spent more time with it this morning. Now I've got to head out to work.

Forget the OS observation. That was late last night at low amp settings. When you open it up it'll sing and sustain nicely. It has the same issue that I've run into before with building this kind of thing. Sustains great up to a certain point, a certain string and fret point. Good all the way from your low E to your G. All the way up the neck. Starting at the B string and climbing chromatically good sustain up until the the A note, second string (B) at the 10th fret. It snuffs out here and up the rest of the neck and all up the neck on the high E chromatically. Not completely snuffed. Just not the same response. You have to run a higher level signal through it, crank the amp, to get it to sing and then it just gets too loud and feedback starts to become an issue.

Sounds nice as an OD with slight distortion too. This one will get perfed and used eventually.

+1 on the breadboard. By all means get one and let the fun begin!

[goosonique]

feedback starts to become an issue.

.....in a good or bad way ?? 

[Doug_H]

Forget the OS observation. That was late last night at low amp settings. When you open it up it'll sing and sustain nicely. It has the same issue that I've run into before with building this kind of thing. Sustains great up to a certain point, a certain string and fret point. Good all the way from your low E to your G. All the way up the neck. Starting at the B string and climbing chromatically good sustain up until the the A note, second string (B) at the 10th fret. It snuffs out here and up the rest of the neck and all up the neck on the high E chromatically. Not completely snuffed. Just not the same response. You have to run a higher level signal through it, crank the amp, to get it to sing and then it just gets too loud and feedback starts to become an issue.

I wonder if this is some sort of resonance issue that could be tuned? I've noticed weird fingerboard-position-dependent responses before with overdrives and distortions. I know where my guitar's sweet spots are and the dead spots aren't always in the same place. Seems to be dependent on the pedal circuit.

[johngreene]

Forget the OS observation. That was late last night at low amp settings. When you open it up it'll sing and sustain nicely. It has the same issue that I've run into before with building this kind of thing. Sustains great up to a certain point, a certain string and fret point. Good all the way from your low E to your G. All the way up the neck. Starting at the B string and climbing chromatically good sustain up until the the A note, second string (B) at the 10th fret. It snuffs out here and up the rest of the neck and all up the neck on the high E chromatically. Not completely snuffed. Just not the same response. You have to run a higher level signal through it, crank the amp, to get it to sing and then it just gets too loud and feedback starts to become an issue.

I wonder if this is some sort of resonance issue that could be tuned? I've noticed weird fingerboard-position-dependent responses before with overdrives and distortions. I know where my guitar's sweet spots are and the dead spots aren't always in the same place. Seems to be dependent on the pedal circuit.

It's probably having a rolloff of gain in the side chain as the frequency increases. Not sure why there would be an overlap between the B and E strings but maybe the waveform from those strings is different (which makes me think side chain rather than VCA). They do sound different for the same note after all. Anyway, possibly a change to add some high frequency emphasis to the the side chain might do the trick. Or a LP filter (yes, low pass) that has a corner frequency at the top note on the fretboard to help the higher notes be more sinusoid and therefore more uniformly rectified. It would be interesting to plot the side chain voltage level vs. input frequency....

--john

[Harry Palms]

Just too much gain goose where random string vibrations get amplified, sustained and out of hand.

Doug, this one seems to decay more rapidly at that particular A frequency and above. I found that it did it also at the G string 14th fret A note too. I went and changed the strings for new ones and the same. You know how the weather gets here. Change your strings, leave your guitar in the garage and the humidity ruins them in a week.

I've noticed weird fingerboard-position-dependent responses before with overdrives and distortions.

Exactly.


John, for my first shot at it I went with a passive bandpass with the low pass target of 3khz and the hi pass 1k6hz. Narrow I know and probably not the right target center frequency but it was just to get started. Before this though I had run a 100k volume control at the output to control it. Just stock, as is, I'd also recommend upping the attack resistor for a sharper, more clear attack. I went with a 25k pot with the 1k at the bottom of the turn to get back to the reference point. If I don't do anything else I'll at least do this.

From there I went with the filter/gain network. Not what you had in mind but for this shot I came off the emitters with the filter and fed this into a gain stage. This then into the entry leg of the attack pot. I upped the release cap to 100uf. Gain started at 2:1, then 6:1, then 11:1 and then I stopped. I liked it. The targeted notes weren't decaying as rapidly as before and it was a sharp, tight and crisp driving tone. I liked it so much that I started to doubt my memory of what it sounded like stock. I decided to wire up a dpdt switch so that I could toggle back and for between the two to better judge and, well, it was downhill from there. I got a little too aggressive with changing things around. I tagged on a prewired baxandall stack before the volume control, ran the wires back and forth from the dpdt, hooked the volume control back up, wired a pot for the bleeder resistor. A jumble of wires - the next thing I know I'm lost (and in need of sleep). Out to the shop again for a quick test... noise then nothing. Back to the drawing board. Tomorrow's a new day. Wait, tomorrow's here already. Jeez.

Thank you for your help John.
And thank you too Thomas P for digging up the circuit.

[Thomas P.]

here's another visual (~1.1MB)
http://www.rzuser.uni-heidelberg.de/~tpfluege/comp.png

on the lower left you can see the VCA while on the lower right is the detector.
I also made one draft with all those transistors as SMD. I believe I'm going to build this one since it looks really cool...