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DOD 280 compressor extremely low output

Started by arawn, August 18, 2011, 09:46:21 PM

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arawn

August 18, 2011, 09:46:21 PM
I built it with the tonepad layout, no millenium bypass, I have to dime the gain and volume on my amp to hear anything at all. No substitutions were made and I used a vtl5c2 for the led/ldr

voltages are below

IC1
1 4.37
2 4.37
3 4.17
4 0.0
5 4.34
6 4.35
7 4.32
8 8.70

q1
c 0.0
b 7.7mv
e .516

q2
c 0.0
b 0.0
e .516

resistance on the vactrol was 1m w/circuit off
thanks in advance 
"Consistency is the Hobgoblin of Small Minds!"

Gus Smalley clean boost, Whisker biscuit, Professor Tweed, Ruby w/bassman Mods, Dan Armstrong Orange Squeezer, Zvex SHO, ROG Mayqueen, Fetzer Valve, ROG UNO, LPB1, Blue Magic

soggybag

#1
August 19, 2011, 02:07:56 AM
Here's my amateurs perspective. All of the voltages around the op-amp look reasonable. I'll guess that that the transistor voltages look reasonable in a resting state for this circuit.

With no output or a low output you have 1M in parallel with the 3M9. Which creates about 800K. With approximately 505K as the other half of the feedback loop you get a gain of about 2.5. When the resistance on the LDR drops, let's say it goes down to 5K. you get about gain of 1. My math is not that great. If I'm anywhere close the compressor only produces about gain change of about 1.5.

The second op-amp is set up with a default gain of 1 in an inverting configuration. Looks like the output would be inverted. Obviously you'd want the thing to work normally, but you could always trade R7 for a 100K and get a gain of 10 from the second op-amp.

I guess the reasoning with the inverting op-amp stage is to apply the inverted signal to Q2 for a full wave rectified signal at the LED. TO smooth the compression.

Seems it might be possible to modify this by connecting C3 to pin 7 to get a noninverting output. Which makes think you might audio probe pin 7 and see if you are getting good signal here. If so then the problem is something with the second op-amp, probably R6 or R7. If the gain is really low at pin 7 it could be R4, 5 or 14.

Moving C3 to pin 7 starts to make this look like the Flatline compressor.

Fender3D

#2
August 19, 2011, 12:51:33 PM
I really don't know where soggybag got those numbers.
I can't find any refer to 3M9 resistor (on tonepad schem. you have 1M feedback resistor paralleled with the LDR, on VTL5C2 datasheet is stated: On Res 200-5k5, Off Res 1M).
There's no schematic annotation but for IC and transistors, so no R7, R6 or C3.
I'll without random numbers  :icon_smile:
This circuit is really simply when you lift the LDR.
Without it, it's a basic non-inverting preamp (gain = 1+1M/(500k+22k) or 1+1M/22k depending how you set the pot).
Without LDR you should have your signal amplified to output, unless you got shorts anywhere, or fried the IC or messed out parts values, or your battery is down  :icon_lol:
"NOT FLAMMABLE" is not a challenge

arawn

#3
August 19, 2011, 02:25:21 PM
the tranny voltages seemed kind of crazy to me, but beyond that its just verifying values for the gain stage on the op amp?
"Consistency is the Hobgoblin of Small Minds!"

Gus Smalley clean boost, Whisker biscuit, Professor Tweed, Ruby w/bassman Mods, Dan Armstrong Orange Squeezer, Zvex SHO, ROG Mayqueen, Fetzer Valve, ROG UNO, LPB1, Blue Magic

Fender3D

#4
August 19, 2011, 02:56:22 PM
You should have more than 0V at Q's collectors, unless either they or the 47u cap is shorted...
try lifting the LDR and check what happens
"NOT FLAMMABLE" is not a challenge

arawn

#5
August 19, 2011, 04:30:58 PM
lifted the ldr end of the vactrol and i get the exact same behavior.
"Consistency is the Hobgoblin of Small Minds!"

Gus Smalley clean boost, Whisker biscuit, Professor Tweed, Ruby w/bassman Mods, Dan Armstrong Orange Squeezer, Zvex SHO, ROG Mayqueen, Fetzer Valve, ROG UNO, LPB1, Blue Magic

Fender3D

#6
August 19, 2011, 04:42:42 PM
It should act as an MXR micro amp now....
check if 1Ms are really 1M, check your level pot, otherwise there's a short or bad solder somewhere
"NOT FLAMMABLE" is not a challenge

arawn

#7
August 19, 2011, 05:18:49 PM
I found the 47 uf cap to be shorted to itself (don't ask long story I'm a bonehead sometimes) and with the ldr lifted i now don't have to dime everything to get a signal but it's still way less than unity. any thoughts?
"Consistency is the Hobgoblin of Small Minds!"

Gus Smalley clean boost, Whisker biscuit, Professor Tweed, Ruby w/bassman Mods, Dan Armstrong Orange Squeezer, Zvex SHO, ROG Mayqueen, Fetzer Valve, ROG UNO, LPB1, Blue Magic

Fender3D

#8
August 19, 2011, 06:53:20 PM
If you lift the resistor going to pin 6 and the 0.05 cap going to Q2, you can exclude any fault from the rectifier part.
After that, if you can't get more than unity gain, then you misspelled some color on your resistors or your IC is bad.

BTW
watch out for bad soldering and/or shorts.
(you did put the jumper near the 1M res., didn't you?)
"NOT FLAMMABLE" is not a challenge

arawn

#9
August 19, 2011, 06:55:54 PM
yes i did, found that both of my 22k resistors (right colorcode) were half of expected value so i replaced them will continue checking and rechecking
"Consistency is the Hobgoblin of Small Minds!"

Gus Smalley clean boost, Whisker biscuit, Professor Tweed, Ruby w/bassman Mods, Dan Armstrong Orange Squeezer, Zvex SHO, ROG Mayqueen, Fetzer Valve, ROG UNO, LPB1, Blue Magic

arawn

#10
August 19, 2011, 07:08:52 PM
Bad Chip now I have a boost without the ldr. Still pretty quiet with it in place but i guess i'll live, kinda the nature of compressors
"Consistency is the Hobgoblin of Small Minds!"

Gus Smalley clean boost, Whisker biscuit, Professor Tweed, Ruby w/bassman Mods, Dan Armstrong Orange Squeezer, Zvex SHO, ROG Mayqueen, Fetzer Valve, ROG UNO, LPB1, Blue Magic

Fender3D

#11
August 19, 2011, 07:13:46 PM
lol
good to know
enjoy it
"NOT FLAMMABLE" is not a challenge

Fender3D

#13
August 19, 2011, 07:35:09 PM
I see, pretty different from notepad's ...  :icon_confused:
Sorry if I did look mean...  :icon_redface:
I was guessing whether this would have been another R28 joke  :icon_mrgreen: but this is another story...  :icon_lol:
"NOT FLAMMABLE" is not a challenge

arawn

#14
August 19, 2011, 07:52:40 PM
it's all good guys! besides you both helped and i am most appreciative and thankful. Go with the gods
"Consistency is the Hobgoblin of Small Minds!"

Gus Smalley clean boost, Whisker biscuit, Professor Tweed, Ruby w/bassman Mods, Dan Armstrong Orange Squeezer, Zvex SHO, ROG Mayqueen, Fetzer Valve, ROG UNO, LPB1, Blue Magic

soggybag

#15
August 19, 2011, 08:49:11 PM
No worries, I just googled for the scheme and looked at the first one I found. I should have looked at the Tonepad version.
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