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thanks, les paul, but I checked and they are good. I've got two of the channels hooked up and they are both running this way. That's what makes me think it is something to do with components being in the wrong spot.

regarding C6, C7, R4, R5 what did you do?
 
Something is wrong.
Do you still have sound if potentiometer disconnected?
If turning the pot up or down doesnt affect signal at all and gain remains the same chances are you got a short somewhere.

Its important to follow the BOM to stuff components on board, doesnt matter what schematics says.
Now C6 or c7 wont affect the gain .
Whats important is that R5 is 100R and R4 10k...if they were wrong simply no sound would come out.
 
3nity: shouldn't R4 be 100R??? According to your previous posts, this would be right... Am I simply not awake yet? Coffee, anyone ;)
 
This is my first 312 build and i was wondering what the C8 and r6-r7 values are used for the cinemag XFRMS ...any help would be appreciated..cheers))
 
LesPaul said:
3nity: shouldn't R4 be 100R??? According to your previous posts, this would be right... Am I simply not awake yet? Coffee, anyone ;)

it depends on which BOM you look at the one at the top of this thread says R5(10k) and R4(100r). The updated BOM found further down the thread says R5(100r) and R4(10k). I have mine set up like the updated BOM.

3nity, thanks for the clarification, I'll continue to dig around. You've been a great help
 
WROOOOOOOOOOOOOOOOONGGGG oh my bad!
R4 should be 100R and R5 is 10K.....fix that and tell me.

I will update the bom.
 
tfletchii said:
Well still no luck. I switched the resistor and still no change. I disconnected the pot from the preamp and there was no change. I'm getting a great sound and good level. Some how I'm bypassing the gain pot. Any new ideas would be appreciated.
You already confirmed, parts value of R4 is 100R and R5 is 10K.
If your disconnected pot is giving you the same condition of (unity)gain operation with a known working opamp, there are only two reasons left why it doesn't work.

Double check your disconnected pot is still a pot by measuring ohms between the two lead ends that connect to pcb. Readout should give you something in range close to zero with the pot in its CW and about 10K with the pot in its CCW position. Getting an infinite resistance readout shows the pot being broken (DOA or died from overheating a pots contact pin) or the more unlikely case of a broken wire inside your cable. (With your pot still connected to pcb, shorting out both wire lead ends connecting to the pot by a maybe croko clip will give you the max.gain setting position of the pot in its CW position as well, but that might/should get loud...)

Other reason is a maybe broken pcb trace between opamps inverting input and 0V reference voltage or a parts lead end of C6, R4 or the pot within this shunt arm trace section not/poorly soldered, so current flow gets interrupted.
 
Thanks guys. I'll post some pictures later today. This is happening with both preamps. I wouldn't think it was the OP amp because everything sounds right but I can check it as well.
 
tfletchii said:
I'm still unable to get my gain knob to respond. The preamp sounds great and is sending plenty of signal to my mixer...
Could I have an issue with R4 (10k) or R5 (100r)...
I've also changed my jumpers to go 1 into 2, 3 into 4...
You already fixed the swapped value resistors of R4 (should be 100R) and R5 (should be 10K).
Without knowing your definition of 'plenty of signal' from whatever source level,
Jorges version of a fault fix assumes a low output level source (maybe ribbon mic) that gets boosted by +63.6dB (17.5dB from input transformer with jumpers set for paralleled windings, 40dB from opamp gain stage, 6dB from output transformer) because of a short between the pot connections.
My version of fix assumes just the opposite (opamp gain stage 0dB instead of 40dB because of an interrupted path within the shunt arm, so opamp is working as non-inverting unity-gain buffer), resulting in your preamp only boosting signal by +23.6dB.
 
tfletchii said:
Here are the pictures. Hope this is helpful.
::) You still have not fixed the swapped value resistors of R4 (should be 100R instead of your fitted 10K) and R5 (should be 10K instead of your fitted 100R) as repeatedly said.
 
Ok, just coming back to this project and want to clarify some things.

I have two preamps.

Cinemag input and output
RC - 150k
C8 - jumper
R7-R6 - 150k
JP2 - Jumper pins 2-3

Classic API input and output
RC - 150k
C8 - 220pF
R7-R6 - 10k
JP2 - Jump pins 1-2 & 3-4

This correct?

Thanks
 
Leroux just one 150K is good on the cinemag.
you can jumper as you like!

Buildafriend have you tried another op-amp?
Thanks
 
Its happening in all channels. The boat motoring is new, and I still get the white noise ish oscillations past unity gain on all channels. The issue gets worse as the units approach the toroid but I cant afford that cool toroid shield that you suggested. When I moved the XLR jack around the boat motoring went away.. thats a big clue. Im about to crack it open now and see if anything has changed. Ill drop a photo if I can. Thanks 3nity you are mega helpful.
 
Did you joined all XLRs pin 1 to a star ground??

did you used a ground buss??
one copper wire from side to side joining all xlr pins 1 and then to ground.
then connect one wire from GND at preamp to the same star ground.
This will make you sure you have a nice grounding.
 

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