[BUILD] 1176LN Rev D DIY

[Bobby Baird]

Audiophreak I had the same problem after I finished my build with 1k@0db signal into my input xlr. My input gain was all the way up and 2k Q bias pot maxed out. I was only able to get 7db of gain reduction. Bummed I noticed as you turn up the 2k Q bias the input gain would raise. I then thought to myself if I had a higher value pot I could increase input gain simultaneously lowering the internal threshold of the compressor. Sure enough my hypothesis was correct. I replaced 2k bias pot with a 5k bias pot and was able to dial in the right input setting to achieve the desired threshold of the 1176 for 10 db gain reduction calibration. As a side note once I recalibrated for 10 db gain reduction in vu mode. The 2k pot for tracking adjustment was maxed out and I could not get it to show 10 db of gain reduction for proper meter tracking. I then replaced it with a 5k pot, and I was able to fully calibrate gain reduction for 10 db standard. I assume with the wide range of fet transistor values a 2k pot will not work in all circuits. Squish 8)

 

[audiophreak]

Hi Bobby Baird,
              not sure thats the fix for my situation , but reading you post gave me an idea .... I was able to do the Q Bias calibration and my 2k pot was not maxed out, but I did have 18 turns on a 24 turn trimmer to drop the 1db .. hmmmmmm 
  In mnats calibration video it states that " you may have to adjust the out put to get it right " for the Q Bias , and not JUST using the Input pot to set VU at +1 

  That might increase the total input level from input pot and shift or move the thresh hold point

  I'm gonna try it tomorrow and I'll post back

 

[peartplus]

Hi all,

Is there any reason that I would occasionally blow fuses when I press the off button on the meter switch? I'm on my 3rd fuse. The +30 and - 10 test points are fine. I'm in the US, and using 250ma slo blow fuses. Any thoughts?

 

[Echo North]

Hi all,

Is there any reason that I would occasionally blow fuses when I press the off button on the meter switch? I'm on my 3rd fuse. The +30 and - 10 test points are fine. I'm in the US, and using 250ma slo blow fuses. Any thoughts?

High in-rush current, maybe the slow-blow isn't slow enough.  You can up your fuse a little.  350mA, 500MmA at the absolute most and make sure you use slow-blow.

 

[Echo North]

Imagine your toroidal power transformer is a sponge.  Even though your compressor may only draw say 100mA, the toroid needs to saturate and will pull a lot of current very quickly on start up.  Say 400 or even 500mA.  We use a slow-blow to allow for that quick in-rush current but maintain a lower value of fuse for safe operation.

So it's that in rush that popping your fuse. Maybe it higher for what ever reason or like I said, the brand of slow-blow is too quick.  Probably the later.

 

[Echo North]

Just to clarify ,  with 1k@0db , input almost all the way up and output almost all the way down I get 4dbGR@20:1 , 5dbGR@12:1 , 6dbGR@8:1 , and 7dbGR@4:1    in VU mode shorting pad 22 to ground.

    Q Bias  and Discrete Meter circuit adjustments went fine and voltages from schematic are all good .... any Ideas ??  suggestions ??

Did you HFE match the transistors in the meter section?  The two that are not the FET.  Q12 and 13 I think?

 

[peartplus]

Imagine your toroidal power transformer is a sponge.  Even though your compressor may only draw say 100mA, the toroid needs to saturate and will pull a lot of current very quickly on start up.  Say 400 or even 500mA.  We use a slow-blow to allow for that quick in-rush current but maintain a lower value of fuse for safe operation.

So it's that in rush that popping your fuse. Maybe it higher for what ever reason or like I said, the brand of slow-blow is too quick.  Probably the later.

Thanks for this info!  Would the current also spike when turning off the unit?

 

[audiophreak]

well that didn't change anything -

    Thanks Bobby Baird, that's great that's how you fixed yours, but if it were a known issue I'm sure mnats would have documented it or chimed in by now.
   
      Time to walk away for a little bit and contemplate ... if anyone has any other suggestions I'm all ears

just to recap :  Q Bias calibration done - no problems
                        Discrete Meter circuit calibration - no problems
                        Q12 and Q13, matching hFE
                        Q1 and Q11 are matched
                        All voltages are spot on or very close to those indicated on schematic on mnats 1176 Rev.D page

unit passes audio cleanly and I hear compression happening
   

 

[Echo North]

well that didn't change anything -

    Thanks Bobby Baird, that's great that's how you fixed yours, but if it were a known issue I'm sure mnats would have documented it or chimed in by now.
   
      Time to walk away for a little bit and contemplate ... if anyone has any other suggestions I'm all ears

just to recap :  Q Bias calibration done - no problems
                        Discrete Meter circuit calibration - no problems
                        Q12 and Q13, matching hFE
                        Q1 and Q11 are matched
                        All voltages are spot on or very close to those indicated on schematic on mnats 1176 Rev.D page

unit passes audio cleanly and I hear compression happening
 

What is the issue?  You can't get GR metering calibrated?

-make sure you have the jumper in the right place (default position R in circuit).
-Make sure you did the null adjust with the jumper putting the R out of circuit.

 

[audiophreak]

What is the issue?  You can't get GR metering calibrated?

-make sure you have the jumper in the right place (default position R in circuit).
-Make sure you did the null adjust with the jumper putting the R out of circuit.

 
  Yes, that is correct Hairball Audio , with 1k@0db xlr input , input  control almost all the way up and output control almost all the way down I can only get 4dbGR@20:1 , 5dbGR@12:1 , 6dbGR@8:1 , and 7dbGR@4:1  while still getting the meter to "0" when GR shorted  in VU mode,  shorting pad 22 to ground.

  I was able to do the Q Bias calibration and Discrete Meter circuit calibration with no problems just like mnats video's - I'm starting to think maybe I have a problem with the ratio's ie:  20:1 isn't realy 20:1  ??? 

    ...... so close ... so close

 

[jplebre]

something is definetly not right. 0dBm with ip all the way up would slam the signal so hard it wouldn't know what it's source was anymore.
the o/p control does not affect GR.

with 1k@0db xlr input , input  control almost all the way up and output control almost all the way down I can only get 4dbGR@20:1 , 5dbGR@12:1 , 6dbGR@8:1 , and 7dbGR@4:1  while still getting the meter to "0" when GR shorted  in VU mode,  shorting pad 22 to ground.

I'm confused by the above. With the meter in VU mode, you see VU/dB relationship, not gr.
If you have a 0VU reading at +4 that means you have +4dBU o/p of the unit.

That is possible with input up to get -4VU (0dBm) with input really high, o/p very low, with or without GR for a 0dBm(assuming 0dBm=0dBu)

Not sure what the problem is though. I may be misunderstanding

 

[audiophreak]

Hi jplebre,
                Its part of the calibration procedures, with unit in VU mode you send  1k@0db in and adjust op to 0vu with GR turned off by shorting pad 22 to ground , then lift ground from pad 22 and observe how much GR is being applied to the ip, going back and forth adjusting ip and op controls engaging and disengaging GR until you get 0vu with GR disengaged and -10db when engaged. at lest thats the way I see it in mnats awesome calibration videos.  No matter how I do this I can only seem to get -4/5 db of =GR .
  How ever the first calibration Q Bias went flawlessly and second Discrete Meter circuit calibration also went flawlessly and after those calibrations I checked all the voltages listed on mnats schematic and all are spot on or real close.

      Do to mnats awesome PCB layout , I was able to swap out main PCBs with the second main PCB which I had not installed yet and went through the calibration steps again unfortunately with the exact same result  :'(

  So ... not a conclusion , but educated guess ...
1.)  either I made the same mistake on both main PCBs - ( not sure about this as all voltages check out )
2.) wiring to Ratio and meter boards is wrong - ( not sure about this as I've checked several times and its pretty strait forward)

  One thought ,  the VU meters I'm using are vintage Simpson 1507's edgewise "A" scale , and remembered there was a 5k trimmer in the bag with the hardware .  Inside the meter I see a small four pin devise , probably the rectifier and another small structure , thinking a resistor , both wound with fine white thread, the meter ohms at about 19k , I know the 3k6 resistor on the meter board is in line with the meter .... what would be the result of too much or not enough resistance on the meter ??

  On my way out to work a couple shows over the weekend , cant get back to this till Monday or Tues.
  any thoughts or suggestions or information I can check and post back ,  much appreciated   

 

[jplebre]

ahh gotcha

  One thought ,  the VU meters I'm using are vintage Simpson 1507's edgewise "A" scale , and remembered there was a 5k trimmer in the bag with the hardware .  Inside the meter I see a small four pin devise , probably the rectifier and another small structure , thinking a resistor , both wound with fine white thread, the meter ohms at about 19k , I know the 3k6 resistor on the meter board is in line with the meter .... what would be the result of too much or not enough resistance on the meter ??

Yes it could be. the internal resistance will affect the deflection of the needle. Not sure I'm saying it right.

Since I don't really fully understand the issue or have an idea what it could be, I would start by taking the meter out of the equation
Are you using a DAW? You can emulate the o/p meter on your DAW.
depending on your soundcard 0dBm could be anything like -20dBfs, -18, -16, or lower than -20. refer to manual

Then watch your daw meters for the drop. If it reflects the same as meters then it's a reason to look into GR reduction circuit. You might find out it's just a meter thing

 

[Echo North]

The 3.6K resistor (and 8.2K) is on used in +4 and +8 respectively.  They are out of circuit in GR mode.  In GR mode the meter is just comparing DC.

I would guess there is a meter issue though which should be a standard VU which is basically a 100-200uA DC meter with a bridge rectifier.

Mike

 

[berkleystudios]

any one ever have feed back issues with high input an out puts? seems if i turn my unit up too much  i get a weird feedback and delay, recalibrated and it became better but still happens

 

[audiophreak]

The 3.6K resistor (and 8.2K) is on used in +4 and +8 respectively.  They are out of circuit in GR mode.  In GR mode the meter is just comparing DC.

I would guess there is a meter issue though which should be a standard VU which is basically a 100-200uA DC meter with a bridge rectifier.

Mike

Thanks Mike,
                    I'll try a new meter tomorrow .

 

[jplebre]

any one ever have feed back issues with high input an out puts? seems if i turn my unit up too much  i get a weird feedback and delay, recalibrated and it became better but still happens

Delay, no.
Feedback, yes. I forgot to put the C25 to earth connection in one of the units and it went crazy in the last 10% of the pot.

Also, if you have some things unshielded is very easy to get that happening. But follow mnats wiring guides (where to put shielded cable and how to connect the shield to avoid ground loops) and I'd say is pretty impossible to get oscillation

 

[audiophreak]

Hi All ,
          Well ... its not the meter ,,, so ,,,,  time to do some checking and testing    will report findings.

 

[audiophreak]

Hi All,
        Checked and recorded voltages from mnats schematic , there are a few variances and some readings are different depending on if the unit is in GR or VU , hopefully a more skilled member might spot something and suggest a direction  ;D
          B                          C                  E
Q2 =  1.06                      1.8              0.55
Q3 =  1.8                  11.62 (12,59)    1.18
Q14= 11.62 (12.59)      30.14        11.01 (12.00 )
Q4 =  1.01                  4.95( 4.2 )        0.42
Q5 =  3.32                25.16( 26.18 )    2.77
Q6 =  2.77                27.54( 28.93 )    2.22

Q11 = G= -1.26            S= -0.91            D= 11.16  in GR    and12.01 in VU

            B                                        C                                E
Q12 = -0.91                    10.35 inGR & 10.87 inVU         -1.5
Q13 =-0.94 inGR&-0.99inVU (1.04) 10.52( 9.75 )              1.52
Q7 =    4.55                            14.93( 15.25 )                  4.0
Q8 =14.93( 15.25 )                      30.14                        14.3 
Q9 = 3.49                                      17.19                          2.91
Q10=17.2                                      30.13                        16.61

Q1 =  G=  -1.44            S= 0.0                  D= 0.0


  tested with no input signal , the readings in red are slightly off from the ones listed on Schematic .  QBias adjusted and Discrete Meter circuit adjusted as per videos, but the Gain Reduction Meter Tracking Adjustment ... I set the controls as described , Input pot a little past half way up and out put pot about 1/3 , I get about 2db drop in meter when GR pad 22 UN-shorted to ground , no matter how much I turn input up to increase GR on incoming signal , I only get 4-5db GR ( after adjusting output to 0 ) it ends up with input almost all the way up and output almost all the way down and cant seem to get more than 4-5db GR.

  any  Ideas  ???    ....before I start destroying the PCB de-soldering

any and all help, suggestions appreciated 

   

 

[Echo North]

Hi All,
        Checked and recorded voltages from mnats schematic , there are a few variances and some readings are different depending on if the unit is in GR or VU , hopefully a more skilled member might spot something and suggest a direction  ;D
          B                          C                  E
Q2 =  1.06                      1.8              0.55
Q3 =  1.8                  11.62 (12,59)    1.18
Q14= 11.62 (12.59)      30.14        11.01 (12.00 )
Q4 =  1.01                  4.95( 4.2 )        0.42
Q5 =  3.32                25.16( 26.18 )    2.77
Q6 =  2.77                27.54( 28.93 )    2.22

Q11 = G= -1.26            S= -0.91            D= 11.16  in GR    and12.01 in VU

            B                                        C                                E
Q12 = -0.91                    10.35 inGR & 10.87 inVU         -1.5
Q13 =-0.94 inGR&-0.99inVU (1.04) 10.52( 9.75 )              1.52
Q7 =    4.55                            14.93( 15.25 )                  4.0
Q8 =14.93( 15.25 )                      30.14                        14.3 
Q9 = 3.49                                      17.19                          2.91
Q10=17.2                                      30.13                        16.61

Q1 =  G=  -1.44            S= 0.0                  D= 0.0


  tested with no input signal , the readings in red are slightly off from the ones listed on Schematic .  QBias adjusted and Discrete Meter circuit adjusted as per videos, but the Gain Reduction Meter Tracking Adjustment ... I set the controls as described , Input pot a little past half way up and out put pot about 1/3 , I get about 2db drop in meter when GR pad 22 UN-shorted to ground , no matter how much I turn input up to increase GR on incoming signal , I only get 4-5db GR ( after adjusting output to 0 ) it ends up with input almost all the way up and output almost all the way down and cant seem to get more than 4-5db GR.

  any  Ideas  ???    ....before I start destroying the PCB de-soldering

any and all help, suggestions appreciated 

 

Are you measuring the audio at the output?  Can you only get 4/5dB at the output?  I'm trying to figure if your issue is that you can only get 4-5dB of actual GR  or it's a meter not displaying GR right?

Sorry if I have asked this already.  Been away for a few days.

Mike