Really Cheap Compressor

mictester · Post by **mictester** » 27 May 2010, 04:55

The order of the pot and the 10k doesn't matter - all you want is a resistance that can be varied from 10k to 100k. Remember, resistors are "commutative" which means they can be connected in any order in series. You're looking at the wrong 10k resistor - there are two in that part of the circuit,; one feeds the incoming signal to pin 6, and the other is in series with the pot. If I've got the pot connections bass ackwards, I'm sorry - it wouldn't be the first time. The resistance increases as the pot is turned clockwise.

C2 isn't connected where you've drawn it!

JOHNO · Post by **JOHNO** » 27 May 2010, 10:34

mictester wrote:The order of the pot and the 10k doesn't matter - all you want is a resistance that can be varied from 10k to 100k. Remember, resistors are "commutative" which means they can be connected in any order in series.

Understood.

mictester wrote:You're looking at the wrong 10k resisto

No, I'm looking at the right resistor.

mictester wrote:The resistance increases as the pot is turned clockwise.

So does this mean the compression effect actually decreases as the pot is turned clockwize?

mictester · Post by **mictester** » 27 May 2010, 12:43

JOHNO wrote:
So does this mean the compression effect actually decreases as the pot is turned clockwize?

No. The gain of the second op-amp is increased (more resistance, less negative feedback), and the LEDs are illuminated with less incoming signal, so the compression is increased.

Basic op-amp theory: an inverting amplifier (signal into the "-" input, "+" input just used for bias, and a resistor from output to "-") has the gain defined by Rfb/Rin, so in our current example, Rin is 10k, and Rfb is adjustable from 10k to 110k, so the gain goes from 1 to 11. You could legitimately calibrate the control up to 11 to satisfy the Spinal tap fans!

JOHNO · Post by **JOHNO** » 27 May 2010, 21:47

Thank you for clearing this up. So it seem's to me the sustain pot is working backwards. I redrew the Pcb to allow a little more room to get my LED/LDR set up in. Apart from that change i left every thing else on the Pcb the same. The volume control works correctly but the sustain works backward's. As I turn it up the sound becomes more open with less compression. Don't get me wrong the circuit it self work's fine. I'm getting quite alot of compression form it. In fact it may be that I have the pots wired up incorrectly and that in actual fact its the volume pot that is wired backwards. Because the way I have the pots mounted at the moment, I cannot mount the circuit in and enclosure, the pots need to be under the board but if I do that then the volume will work backwards. I have a new Pcb just about done but I hesitate to post it until I know it is working correctly. Here is a pic of how I have the pots mounted at the moment. Sorry to be such a pain in the arse.
Johno

Photo 17.jpg

mictester · Post by **mictester** » 27 May 2010, 22:05

Nice job. The correction to the PCB layout just means moving the link between the two pins on the sustain pot. Sorry - my mistake! I'll post a fixed version as soon as I've corrected it!

JOHNO · Post by **JOHNO** » 27 May 2010, 22:09

I just removed the pots and placed them on the other side of the board where they should be an now the volume is working backwards. So the problem is in the volume pot wiring not the sustain as first thought. I'll try to get my new Pcb done as soon as I can but I'm busy with other project's at the moment.
Johno

JOHNO · Post by **JOHNO** » 27 May 2010, 22:35

Ok here is the amended PCB. It probably could have been layed it out a bit better but I didnt have much time. It allows for the pots to be mounted on the trace side of the Pcb so as to allow easy mounting into an enclosure. And also the pots work correctly. This really is a great little compressor for the amount of parts used in it's design. Thank's again Mictester.
Johno.

RC Compressor.png

JOHNO · Post by **JOHNO** » 27 May 2010, 23:09

Yep, I know I said I had no time for this but I've made some time. Here is another amended Pcb this one has one jumper
on the ground trace. And has straightened up the Led/Lrd. There is plenty of room to fit the Led/Lrd. If I'm posting too many attachment's, can one of the moderators please kindly let me know and I will remove some of them.
Thank's Johno

RC Comp 1.png

mictester · Post by **mictester** » 27 May 2010, 23:11

Here's the fixed version!

: Really Cheap Compressor PCB fixed.png (11.16 KiB) Viewed 2375 times

As you can see, there's just one track moved. Sorry for the error!

One other thing - remember to test it either in the dark or with a black tunnel for the LDR and LEDs made from insulating tape!

soggybag · Post by **soggybag** » 27 May 2010, 23:56

Great looking build I like the board mounted pots.

JOHNO · Post by **JOHNO** » 28 May 2010, 00:11

soggybag wrote:Great looking build I like the board mounted pots.

Yes, but in the picture above the pots are mounted on the wrong side of the board, there is no way you can mount it in a small enclosure with the pots mounted like that. On my amended Pcb the pots can be mounted at a right angle on the trace side of the board. This way it fits easily into a small enclosure. But you still need to use 16mm pots for it to fit.

mictester · Post by **mictester** » 28 May 2010, 04:55

JOHNO wrote:
soggybag wrote:Great looking build I like the board mounted pots.
Yes, but in the picture above the pots are mounted on the wrong side of the board, there is no way you can mount it in a small enclosure with the pots mounted like that. On my amended Pcb the pots can be mounted at a right angle on the trace side of the board. This way it fits easily into a small enclosure. But you still need to use 16mm pots for it to fit.

I mounted my board above the pots, with short lengths of tinned copper wire between the pots and the PCB. I then used some nylon studding bolted through holes on the PCB, retained with nylon nuts, bolted in to nylon nuts that are glued into the case. It's a very compact layout that way. I made twelve of them last weekend for same Japanese guys I met in a local recording studio. They heard one in use and asked where it came from - I was happy to accommodate them, but didn't like using up most of my Saturday to build boxes!

JOHNO · Post by **JOHNO** » 28 May 2010, 07:05

Didn't they notice that one of the pots was backwards?

JOHNO · Post by **JOHNO** » 28 May 2010, 07:35

JOHNO wrote:I mounted my board above the pots, with short lengths of tinned copper wire between the pots and the PCB. I then used some nylon studding bolted through holes on the PCB, retained with nylon nuts, bolted in to nylon nuts that are glued into the case.

I'm going to go out on a limb here and say, I dont know how you could use the layout on the first page without one of the pot's not working backward's. I don't care if you mount the pot's on the top or on the bottom of the circuit board, one of the pot's will work in reverse.

JOHNO wrote:They heard one in use and asked where it came from

But isn't this one of those effect's that you don't know it's on? I wonder how these chinese folk's knew it was on?

JOHNO wrote:I was happy to accommodate them

Yes i bet you were.
PS I don't know why the computer say's these are my quote's?

mictester · Post by **mictester** » 28 May 2010, 14:01

JOHNO wrote:Didn't they notice that one of the pots was backwards?

No, because it was a slightly different board layout, and the pots were wired correctly!

mictester · Post by **mictester** » 28 May 2010, 14:07

[quote="JOHNOBut isn't this one of those effect's that you don't know it's on? [/quote]

No - it's not that subtle! This is meant as a "quick and cheap" sustainer - it works as described, but isn't subtle at all. It's really good for finger-picked playing, because it evens out the irregularities of volume due to picking technique! It has a really fast attack (limited by the speed of response of the LDR) which means that it does overshoot on big peaks, so its operation is very audible at times.

livingnote · Post by **livingnote** » 30 May 2010, 14:14

Oh this was real fun - I just bumped up the input MKS (methinks) red WIMA to the 150n
"mystery grey" one and here's what she does (one vactrol):

No Comp

Comp in

Amazing how much of the sound is in cap choice, you pay essentially the same but there's
these worlds of difference.

Hides-His-Eyes · Post by **Hides-His-Eyes** » 30 May 2010, 15:10

Much brighter, I like it.

Tomby · Post by **Tomby** » 02 Jun 2010, 08:22

Hi!

Just finished mine.

Interesting compressed sound but I have 2 questions:

-In the previous samples the compressed sound is brighter than the bypassed sound. On my version the bass are kind of "muddy".

Is there a way to change that? (maybe some resistor type change, I don't know)
(note: I used one yellow and one green LED. I thought it could sound interesting but maybe the "muddy" side comes from that choice)

-I have a stock of "old" 3mm LEDs and some are much brighetr than the others. I first used the darker ones but It seemed to have no effect (but the board wasn't in the enclosure). So I changed to the brighter ones and it compressed well. Anyway do you have recommendation or specs for the Leds to use?

mictester · Post by **mictester** » 02 Jun 2010, 12:35

Tomby wrote:Hi!

Just finished mine.

Interesting compressed sound but I have 2 questions:

-In the previous samples the compressed sound is brighter than the bypassed sound. On my version the bass are kind of "muddy". Is there a way to change that? (maybe some resistor type change, I don't know)
(note: I used one yellow and one green LED. I thought it could sound interesting but maybe the "muddy" side comes from that choice)

-I have a stock of "old" 3mm LEDs and some are much brighetr than the others. I first used the darker ones but It seemed to have no effect (but the board wasn't in the enclosure). So I changed to the brighter ones and it compressed well. Anyway do you have recommendation or specs for the Leds to use?

The original ones I used were high efficiency (supposedly) green rectangular ones. They were fully illuminated (they wouldn't get any brighter) with a current of about 8 mA. Using dissimilar LEDs won't affect the quality of the sound, but might change the compression characteristic a little. The only reason to use 3mm LEDs was to get them to fit closely together on the board - you can use any size you like, and any colour you like, as long as the LDR responds to the colour of the LEDs. I found that all the LDRs I tried responded best to green or yellow LEDs.

Note: The effect won't work at all if there is ambient light falling on to the LDR - it HAS to be shielded or inside a box!

Don't waste your time worrying about resistor types - as long as you have the right values, the circuit will work well. However, carbon composition types will add to the hiss from the circuit, so are best avoided, and we don't need their bogus "mojo" factor.

If you want it to sound "toppier", reduce the value of the input capacitor a bit. The values I've given are really just guidelines and don't need to be slavishly adhered to! In most cases, sensible component substitutions will still leave you with a working effect.

Really Cheap Compressor [documentation]