Bearfoot - Pale Green Compressor  [traced]

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Post by Liquids »

Seiche wrote:^good idea but is response time actually something that is specified when buying LDRs? I know musikding doesn't (they only give the light and darkness resistance).
It's not specified, likely because typically LDR response time is abysmal...and hence potentially something that the manufacturers don't want to sort, spec, and publish (= guarantee) min/typical/max for. That would also raise the cost of the component...and we're back to packaged optocouplers.

It's pretty common with the more expensive optocouplers to see those specs...which is, after all, the component that an LDR+LED is trying to make sub for, but done cheaper. Or maybe in some selected cases preffered over for spec reasons, not just price... :o

There is of course limitations to what we can find out...but starting with max dark and min bright resistance is #1...narrows it down...then to make the 'it responds pretty quickly' less subjective and slightly more measurable (feed it a 3v peak signal, then cut it, see how fast it takes for a (slow?) DMM to go from reading that min bright to max dark resistance after 1 second, 5 seconds, 10 seconds....is a help. Look at the Vactrol / perkins-elmer pdf datasheets to see what kind of things they spec for. In an ideal world, it would be cool to do the same exact test with whatever optocoupler one might have on hand with those particular specs published, functioning in the same (clone) circuit, as a comparative, to reduce the DMM's impact or skew of such measurements.

Probably few other people care about this, but as I've said elsewhere, the optical specs have exponentially more impact on the resulting sound of this kind of circuit than probably any other section of the circuit.

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Post by Liquids »

And again, here is a good example...no idea where Steve from Smallbear got his rise/fall time info, but it's indicative of something...I've always suspected these are the same or exact parts also available on futurlec...but it's info, and info is good (even if only general)

http://www.smallbearelec.com/Detail.bok?no=711

Photocells CdS 5mm Diameter
SKU: 2506A

Type Light Resistance Min-Max Min. Dark Resistance
8002 8 - 24K 500K
8005 40 - 120K 1 Meg
8006 80 - 240K 5 Meg
8102 9 - 20K 300K
8104 27 - 60K 2 Meg
8105 50 - 94K 2.5 Meg
9200 10 - 50K 5 Meg
9203 5 - 20K 10 Meg

"Rise times are 55-70 msec. Fall times average 25 msec. All will work in the Easyvibe with adjustment of the "Drive" trimpot. In my tests, #9200 and #9203 gave best range of depth."

Not that the RNC compressor is at all optical, but it's a well-regarded compressor....look at the range knob on attack speed:
http://www.fmraudio.com/rnc.htm
Note that 60msec (in the zone Steve gives for the 'rise time' of these LDRs) is like '8' of 10 on the RNCs attack knob

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Post by Donnerbox »

... sorry for miscommunication - you CAN run the PGC at 18v ... the ODs are limited to 15v as the CA tops out at 16v and some even less because of the voltage manipulation.

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Post by Seiche »

I wonder if the "P50067" that's marked just over the LDR/LED arrangement is any indication or clue as to what type of LDR should be used :hmmm:
my google kung-fu wasn't good enough though.

edit: this and this suggest that the Hamamatsu photocells have part numbers that are "p" + 5 digits. But I couldn't find the p50067. Possibly a custom job? The shape of the LDR looks about right.
Last edited by Seiche on 22 Mar 2012, 14:33, edited 1 time in total.

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Post by ezugaru »

The only thing I found, related to this, using light dependat resistor and 50067, was a PDF filer of a Patent Document which reference a device that use a "Matrix Display" that varies i's resistence depending in tje light emitted by a device on each pixel. Funny... :slap:
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Post by RnFR »

Liquids wrote:And again, here is a good example...no idea where Steve from Smallbear got his rise/fall time info, but it's indicative of something...I've always suspected these are the same or exact parts also available on futurlec...but it's info, and info is good (even if only general)

http://www.smallbearelec.com/Detail.bok?no=711

Photocells CdS 5mm Diameter
SKU: 2506A

Type Light Resistance Min-Max Min. Dark Resistance
8002 8 - 24K 500K
8005 40 - 120K 1 Meg
8006 80 - 240K 5 Meg
8102 9 - 20K 300K
8104 27 - 60K 2 Meg
8105 50 - 94K 2.5 Meg
9200 10 - 50K 5 Meg
9203 5 - 20K 10 Meg

"Rise times are 55-70 msec. Fall times average 25 msec. All will work in the Easyvibe with adjustment of the "Drive" trimpot. In my tests, #9200 and #9203 gave best range of depth."

Not that the RNC compressor is at all optical, but it's a well-regarded compressor....look at the range knob on attack speed:
http://www.fmraudio.com/rnc.htm
Note that 60msec (in the zone Steve gives for the 'rise time' of these LDRs) is like '8' of 10 on the RNCs attack knob
my guess is that the reason that the LDRs with the higher dark resistance will work better, is because the rise and fall are moving further in terms of resistance, hence making it "faster". in other words the ratio is higher. kind of like putting a car into a higher gear. so, one would guess that the higher the "off" resistance, the faster the LDR would seem to operate.

just an aside- in regards to the FMR(which I love, btw), or any other pro audio comp, the attack and release times of guitar pedal compressors are extremely slow. although, I haven't used the FMR on guitar, but I'll rarely use any comp that far into its attack range- except an already squishy guitar that for some reason has wide dynamic range jumps(usually a boost dialed up too high). keep in mind that guitar amps are usually already compressed, and this is on a mic after the amp and a speaker, so it's much different than a pedal which would go before a rig. using a pro audio comp before an amp, you may want attack times that high, but I tend to find pedal comps much too slow in attack and release. not to mention that these are rarely adjustable. adjustable attack would be a great asset to tailor to ones playing style, guitar, or any other tones before the comp.
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Post by merlinb »

RnFR wrote: just an aside- in regards to the FMR(which I love, btw), or any other pro audio comp, the attack and release times of guitar pedal compressors are extremely slow.
It is probably worth clarifying that the attack time on optical guitar compressors is usually slow as you point out, because of the limitation of LDRs. For OTA and FET compressors it is usually very fast. Most are Dynacomp clones, which has almost zero attack time (which is what you want for a legato sound).
I'm currently working on a design for a five-knob OTA compressor that will do pretty much everything, and will be posted here eventually.

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Post by RnFR »

yes, thanks for clarifying that. we were speaking on LDRs, so it didn't cross my mind. looking forward to seeing your compressor design!
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Post by Liquids »

merlinb wrote:
RnFR wrote: just an aside- in regards to the FMR(which I love, btw), or any other pro audio comp, the attack and release times of guitar pedal compressors are extremely slow.
It is probably worth clarifying that the attack time on optical guitar compressors is usually slow as you point out, because of the limitation of LDRs. For OTA and FET compressors it is usually very fast. Most are Dynacomp clones, which has almost zero attack time (which is what you want for a legato sound).
I'm currently working on a design for a five-knob OTA compressor that will do pretty much everything, and will be posted here eventually.

Do everything is ambitious! Sounds wondeful though.

I've heard it said that despite the limitations of opto-based components, they're used successfully to the ears since their tendency is to come across as exponential in response; that is as compared to an OTA, which by default responds linearly? (The question mark means I am playing telephone from what I think I've heard here. :D) If it's true, is this something you noticed, compensate for, don't notice, etc?

I always love a design by a man with true electronics know-how. I'm looking forward to see it, it Merlin!

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Post by Seiche »

any updates? Are all the other parts values like on the CB version of the FGC?

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Post by Seiche »

mmolteratx wrote:Quick reverse is that it's the same as the FGC, minus Q1, Q5, C1/12/15/16, R4/5/6/24/25/26, jumper R7, move the volume control to where the out pad is, R22 is 22k. Will draw up a real scheme in a bit.
mmolteratx wrote:Left most film cap is 47n, the others are 100n. Small electros are 1uF, 50V, large electros are 22uF, 35V. Ceramic cap is 100p. Diodes on the right are 1n914, diode in the middle is 3V1 zener. Transistors on the right are BC550, one on top left is 78L05.
attempt of a preliminary schematic with mmolteratx' comments (I also left off C3 = 47p, as there is only one ceramic cap on the pcb, and the FGC_CB seems to have a 100p cap between pin 5 and 8 as per the schematic, this might be wrong though and C3 simply 100p and no cap between 5 and 8 ):
PaleGreenCompressorSchematic_Beta.png

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Post by coldcraft »

seems like the input is shorted to the output by that stretch of wire coming off R3 to A.
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Post by Seiche »

coldcraft wrote:seems like the input is shorted to the output by that stretch of wire coming off R3 to A.
you're right :?
wasn't sure how leaving off all the components mmolteratx stated would change the topology in this region. separating the circuit with the "A point" is possibly redundant and that wire is not there.

might simply look like this. Maybe mmolteratx can elaborate.
PaleGreenCompressorSchematic_Beta2.png

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Post by coldcraft »

looks good now.
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Post by mmolteratx »

Seiche wrote:
coldcraft wrote:seems like the input is shorted to the output by that stretch of wire coming off R3 to A.
you're right :?
wasn't sure how leaving off all the components mmolteratx stated would change the topology in this region. separating the circuit with the "A point" is possibly redundant and that wire is not there.

might simply look like this. Maybe mmolteratx can elaborate.
PaleGreenCompressorSchematic_Beta2.png
Yup. This is about it. I'll open it up in a bit and make any necessary corrections, if any.

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Post by pedalgrinder »

so what the hell makes this worth more than the fgc? :slap: the pedal world of electronics never ceases to amaze what bullshit they come up with. If pcbs were made better with thicker copper point to point would be a joke also.
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Post by Seiche »

pedalgrinder wrote:so what the hell makes this worth more than the fgc? :slap:
well it's not is it? at least the handwired version is not (FGC is more than $ 300,-). And the PGC is handwired too. the pcb version of the FGC is of course cheaper because cheaper to make. or did i miss something?
pedalgrinder wrote:If pcbs were made better with thicker copper point to point would be a joke also.
what do you mean by that? :scratch:

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Post by pedalgrinder »

ok i was looking at the pcb versions thanks for clarifying. I just mean i don't reckon you get what they claim out of point to point wiring. thicker copper in a pcb would achieve the same affect for little or no resistance. Thats all i can see the point to point achieves in audio. High frequency well then your talking a different story on how you put it together
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Post by Seiche »

mmolteratx wrote:Yup. This is about it. I'll open it up in a bit and make any necessary corrections, if any.
bump. That'd be great, I'd like to order some parts in a few days to verify this. :thumbsup

Would be awesome if you could measure the LDR (in circuit). Otherwise I'll go with a vactrol or something, maybe a VTL5C3?

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Post by Seiche »

hmm, donner said on facebook there will be a revision soon with more compression (apparently people were asking for this).

how can this be accomplished? different LDR?

edit: apparently it's a pot change.

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