[BUILD] 1176 Rev A - Back to the beginning...

[0dbfs]

Looks like there should also be a negative DC voltage between pad 21 and ground that offsets the sidechain audio. If the sidechain audio is not high enough, the diodes will not conduct and make the CV...

-j

 

[Randyman...]

Did you use 2n3707's or other in there? I guess if there wasn't enough signal in that section for some reason then that may affect something.

I used 2n3708's as per the Hairball Mouser Cart.  I didn't test for Hfe, but supposedly the 3708's are much less likely to be below 250 from what I gather.  Is that an issue?

Interesting about touching the attack pot. Is something in that section shorted to GND? How do you have the ratio board grounded? Is there only one path to ground? Can you change the grounding of that part around to see how it changes things?

The Ratio Board grounds to the GR Disable DPST Switch.  That switch is grounded at the PCB (next to Pad 22) - and that also serves as my PCB Chassis Ground connection.  Star Grounded with the IEC, Stereo Link Board, and Input/Output XLR Shields.  The Attack/Release aren't connected to chassis ground on the tabs.

I eventually ended up wiring that section point to point n the back of some switches instead of using the board.

I'm hoping to avoid that  I'll see if the board/switches are bad first.  When I switch ratios I do get meter action (quickly pegs in either direction), but it always settles back to 0.

No audio is expected after the diodes since it is only a positive going waveform at that point. What are your compressor input/output settings? Can you turn input up and output down with no changes (for more compression)?

For the Q Bias adjustment, I had the output full open and the input just barely open to ensure I was well below the threshold.  I did the Null Adjust w/o any audio and the jumper pulled.  I'm testing for Gain Reduction with the input around 12 O'clock (right before clipping) and the output down to get +4 on the meter.  Bypassing the GR does nothing to the levels.  Adjusting the Q Bias affects the GR meter and output levels.

Does the GR meter show reduction (like the audio does) when you adjust the Q bias pot with the sidechain bypassed (GR-OFF or pad-22 to gnd)?

Yes.

Looks like there should also be a negative DC voltage between pad 21 and ground that offsets the sidechain audio.

-1.6v @ 4:1;  -2.5v @ 8:1;  -3.3v @ 12:1; -8.95v @ 20:1.  So it does appear like the Ratio Board is doing something on the S4(B) side - correct?  (That's basically the threshold it's adjusting - correct?)


I'll try bypassing the Ratio Board and Attack/Release after I go to the store on Monday.  I might probe around Pad 19 and the pots in a little while.


I can't thank you enough for your continued help!  I just know I'll nail this shortly with your (and this board's) help!  :-*
8)

 

[0dbfs]

Yes s4b is the threshold and s4a is the ratio.

I wonder if the diodes are getting enough audio signal to conduct? I think they need +.7V to conduct but not sure right now. If you send a tone through you could set your meter to ac to measure that at the input to the GR amp and output.

I was also thinking that you could just bypass s4b and connect pad 15 straight into the GR amp (so it's full level) and then set the ratio switch to 4:1 for minimal threshold.

If that causes compression then maybe something is up with those 4 transistors in the GR amp. Maybe the s4a is dropping too much signal to the sidechain. Are the resistor values there correct? It's recommended to measure before stuffing/soldering each component just to confirm if nothing else.
Getting close now but no smoking gun. At least R32 hasn't burned up .

Cheers,
J

 

[Randyman...]

Yes s4b is the threshold and s4a is the ratio.

I wonder if the diodes are getting enough audio signal to conduct? I think they need +.7V to conduct but not sure right now. If you send a tone through you could set your meter to ac to measure that at the input to the GR amp and output.

If I measure from Ground to CR2 and CR3, I barely get .01VAC @ 4:1 and .1VAC @ 20:1 with a full-tilt 120Hz and 1KHz tone running into it (it does drop to 0v w/o any input signal).  If I measure DC from ground to CR2 and CR3, I get from -5.95v @ 20:1 to -1.7v @ 4:1 (all measured at CR2/3's anode).  When I probe CR2/3 with my audio probe, the level is approx 12dB lower than at the C17/Pad22 Sidechain Input.  Say -10dBfs at Pad 15, -20dBfs @ PAD22 (20:1), and -43dB @ CR2/3 (also @ 20:1).  The interface I'm using is +13dBu = 0dBfs.  Do those CR2/3 levels seem low?

I was also thinking that you could just bypass s4b and connect pad 15 straight into the GR amp (so it's full level) and then set the ratio switch to 4:1 for minimal threshold.

If that causes compression then maybe something is up with those 4 transistors in the GR amp. Maybe the s4a is dropping too much signal to the sidechain. Are the resistor values there correct? It's recommended to measure before stuffing/soldering each component just to confirm if nothing else.
Getting close now but no smoking gun. At least R32 hasn't burned up .

Cheers,
J

I gave that a shot - no go.  I got around 4V AC at Pad 22, but CR2 and CR3 were still getting pretty small AC Voltages (below 1VAC).

I also tried to hardwire bypass the Ratio Board and Attack/Release - but nothing changed.  I checked the Ratio Board while it was disconnected, and the resistance followed the 4:1 diagram you drew me (verified against the Schematic).  It seems good as does my Attack/Release setup.

I measured every resistor twice before soldering them in, so I'm thinking the issue might be in the Transistors or Caps somewhere?  Everything seems fine with the exception of the sidechain audio making it back into the Gain Control amp at appreciable levels.

I'll keep hacking away at it.  Thanks for sticking with me!  

 

[0dbfs]

Randy, I am following your efforts here with great enthusiasm and must commend you for your determination and persistence in moving forward with this. Thanks for your inspiration!

I would expect that the GR amp has to put out enough signal at Q8 & Q10 to offset the -DC at Pad 21 such that the signal is in excess of 0.7V to get past the diodes. Looks like you have proven that this is not the case at the moment. Good news as far as I am concerned because instead of an unknown it appears that some absolutes have been added to the pool of knowledge.

I don't know the specifics of troubleshooting the GR amp but looking into the configuration of each of the two sets of transistors Q7/8 & Q9/10, they are each set up in a common-emitter-stage -> emitter-follower-stage. Looks like Q7 & 10 (common-emitter) should provide gain while Q8/9 are unity gain and impedance conversion to drive the 47k output of each amp. ie; That combo of stages appears to provide gain, polarity inversion, and a low impedance for the following stage.

I believe that the R39 + R41 to GND (~5k) set the gain of Q7 (common emitter). Q10 has R48 + R50 to GND (almost 8k). Q9 also has a voltage divider at it's input which would attenuate the signal from the previous stage so that may have been part of the design to get each end of the differential output at the same level. I'm guessing that audio at the collectors of Q7 and Q9 is supposed to be higher than the audio at pad22 and the audio at the emitters of Q8/10 should be slightly lower than that at the base of Q8/10 (since the emitter-follower-stage is supposed to be slightly less than unity). Also, the 22k connection from the junction of Q10E/C19+/R49 back to the junction of Q7e/R39 looks like an NFB(?) loop which would both reduce overall gain and provide stability/linearity.

DC voltages around those parts would be important but I can't say right now what they should be although I would like to understand/know that based on the 30V supply, the various resistors in those circuits, and the properties of the Q's. Looks like Q8/10 should have 30V at the collectors. From there, the dividers get more complicated and may involve internal impedances of the Q's based on their particular operating points.

In any case, looks like the signal at the input of the GR amp is much lower than the output and this may be where the problem lies.

I think I have read around here that some use 5088's (?) instead of 3707's or 3708's.

Cheers,
j

 

[Randyman...]

Randy, I am following your efforts here with great enthusiasm and must commend you for your determination and persistence in moving forward with this. Thanks for your inspiration!

Lol!  You're doing me the favor!  I'm grateful that you and others have taken time to help me get these awesome boxes in working order.  The inspiration that people like you, mnats, Mike/Hairball, Ruckus and ptownkid give me is the reason I'm here in the first place!  ;D  I would not have taken this project on w/o their awesome kits, and people like you offering their expert help.  Just 4 or 5 years ago, kits of this caliber were few and far between.  It really seems like the DIY segment has taken off in 2010!  Kits out the whazoo!!!

I would expect that the GR amp has to put out enough signal at Q8 & Q10 to offset the -DC at Pad 21 such that the signal is in excess of 0.7V to get past the diodes. Looks like you have proven that this is not the case at the moment. Good news as far as I am concerned because instead of an unknown it appears that some absolutes have been added to the pool of knowledge.

I don't know the specifics of troubleshooting the GR amp but looking into the configuration of each of the two sets of transistors Q7/8 & Q9/10, they are each set up in a common-emitter-stage -> emitter-follower-stage. Looks like Q7 & 10 (common-emitter) should provide gain while Q8/9 are unity gain and impedance conversion to drive the 47k output of each amp. ie; That combo of stages appears to provide gain, polarity inversion, and a low impedance for the following stage.

I believe that the R39 + R41 to GND (~5k) set the gain of Q7 (common emitter). Q10 has R48 + R50 to GND (almost 8k). Q9 also has a voltage divider at it's input which would attenuate the signal from the previous stage so that may have been part of the design to get each end of the differential output at the same level. I'm guessing that audio at the collectors of Q7 and Q9 is supposed to be higher than the audio at pad22 and the audio at the emitters of Q8/10 should be slightly lower than that at the base of Q8/10 (since the emitter-follower-stage is supposed to be slightly less than unity). Also, the 22k connection from the junction of Q10E/C19+/R49 back to the junction of Q7e/R39 looks like an NFB(?) loop which would both reduce overall gain and provide stability/linearity.

DC voltages around those parts would be important but I can't say right now what they should be although I would like to understand/know that based on the 30V supply, the various resistors in those circuits, and the properties of the Q's. Looks like Q8/10 should have 30V at the collectors. From there, the dividers get more complicated and may involve internal impedances of the Q's based on their particular operating points.

In any case, looks like the signal at the input of the GR amp is much lower than the output and this may be where the problem lies.

I think I have read around here that some use 5088's (?) instead of 3707's or 3708's.

Cheers,
j

I have a bunch of SIP sockets on the way and 40 more 3708's along with an Hfe tester - so I'll be pulling, crudely testing/matching and socketing all TO92's.  I really think the issue might be somewhere in there since I'm fairly confident in the rest of my build.  I might give 5088's a shot if I can't find (16x) 3708's above 250 Hfe.  I do tend to get my solder joints pretty hot - so I'm either thinking I damaged some of the transistors with heat, or they just weren't up to snuff in the first place (low Hfe?).

I need to immerse myself in Transistors, bias, NFB, common-emitter-stage -> emitter-follower-stage, etc etc.  I know the basics enough to get myself into trouble - then I come crying here for a "bail out" when I get over my head  :'(  I do have to say this project is a good deal more involved than the Seventh Circle Preamps (built 14 kits thus far).  The wiring was a bear - but with some time, patience, and good diagrams is it pretty easy to figure out (in contrast to comprehending all of the Transistors and their functions in the GR section).  I was really hoping for a working unit on the first power-up (I was seriously confident until I didn't get any GR).

I'm anxious to learn the nitty gritty of complex circuit operation, but the way I learn is a bit unorthodox.  Little by little until it clicks.  You've been "clicking" my cog wheel in the right direction!  ;D


I think I'm in a holding pattern until the new 3708's and Hfe tester arrive unless you have any more jewels of knowledge to provide?


You are one awesome guy, j!  ;D


8)

 

[0dbfs]

Randy, Thanks!

Wish I could help more with the final troubleshooting steps before slinging your silicon-shotgun towards that box and spraying it with hot solder and new components. Yes, it sounds like your attention to detail in the procurement, assembly & testing stages have also been proven due to the extensive tests all coming back with expected results. And leading into the suspect GR amp circuit.

I'll read up and poke around on those GR amp circuits a bit in the meantime and see if I can come up with anything more definitive as far as expected readings, etc.

Looking forward to it!

Cheers,
jonathan

 

[dustbro]

Just powered up my 1176. The unit works and sounds great!
I'm noticing quite a bit of physical "buzz" noise coming from the Hairball toroid when its powered up. Is this normal? Again, everything works, and the audio has no buzz... just the toroid.

 

[Echo North]

Just powered up my 1176. The unit works and sounds great!
I'm noticing quite a bit of physical "buzz" noise coming from the Hairball toroid when its powered up. Is this normal? Again, everything works, and the audio has no buzz... just the toroid.

No, not normal.  You have it connected right?

May be a bad one.  Shoot me an email.

Mike

 

[Stagefright13]

Randyman you get that working yet? I suspect it is just a wiring error. 3708's always work. I had 2 borderline ones out of 75.

I had a problem with my 2-1176 when I first built it. Both sides no less. So I shelved it for a couple weeks cause I was busy. Then I was like ok let's fix this sucker. Literally took me 15 minutes. Had 2 wires switched.

It's usually something obvious but looking at it too much...

Cheering you on!
John

 

[Randyman...]

Thanks for the inquiry, John!

I went ahead and ordered some more 3708's and a HFE tester - they arrived yesterday - but it seems like that might not be the issue.

I'm getting audio at PAD22 and into C20/CR3 and C19/CR2 - BUT I'm not getting any DC voltage that varies with the audio back into Pad 7.  This means the GR Disable switch is wired correctly (audio goes away at PAD22 when "GR = OFF" as it should), and the issue *appears* to my un-trained eye to be somewhere in the full wave audio rectification in the GR Control Amp - or the transistors surrounding it.

I've bypassed the Ratio Switch with a hardwired circuit (0dBfs's schematic a few pages back - Thanks 0dBfs!!!).  I also tested the Ratio Board while it was disconnected, and it gave the proper resistances per the Schematic (at least at the 4:1 setting).

I also bypassed the Attack/Release pots with hardwired resistors and a cap - same results (passes audio just fine, but no gain reduction).

I know the Q1/Q11 work as when I touch PAD7 & Ground, I get constant gain reduction on the meter and at the output.  It just seems like the Sidechain Audio is not being converted into proper DC Bias Voltages or the DC Bias Voltages are too high - so I'm kind of stuck there with my limited knowledge.  

My DMM only shows like 0.1VAC at CR2/CR3's anodes with a full-tilt 1K (and 120Hz) tone, but PAD15 is closer to 4V AC.  If I state this in dBfs levels: -10dBfs at Pad 15, -20dBfs @ PAD22 (20:1), and -43dB @ CR2/3 (also @ 20:1).  Does that level at CR2/CR3 seem low?


I do get approx -1.5VDC at Pad 7 with Release full CW, and about -1VDC with Release full CCW.

If I measure DC Volts from ground to CR2 and CR3's anodes, I get from -5.95v @ 20:1 to -1.7v @ 4:1).


I also tried feeding PAD15 (from output pot) directly to PAD22 with a 4:1 setting and still didn't get any GR (had 4V AC at Pad 22).


A few last ditch questions:

Are any of the mnats rev A 1.2.5 "Transistor Orientation" silkscreens printed incorrectly?  I notice Q11 specifically lists "G/S/D" (recalling from memory) when none of the others do.  I installed it as per the silkscreened part outline had it oriented (I could only see that affecting the meter - not the actual GR Circuit and bias voltage back to Pad 7, correct?).  Same for the CR2/CR3 diodes.

Here is a re-post of one of the PCB's for inspection (Click for larger image):
PS - Pads 28, 29, 7 & Ground connect from the bottom  




Thanks again for you guy's awesome FREE support!  ;D  Hoping to nail this over my Sunday/Monday days off  8)

 

[Rob Flinn]

It's labelled GSD because it's a fet,  Gate Source & Drain as opposed to a bipolar transistor which is Collector Base & Emiter.  Q1 & Q11 are the same type of fet & should be lablled the same

I still think your problem is likely to be a hook one.  One thing you could do is check that you have conneted to the ratio board correctly, I remember doing these connections the wrong way round, becuase there was something about the hook info that didn't click for me.

 

[0dbfs]

Check R40 and R41 against schematic/bom/meter.

Cheers,
j

 

[0dbfs]

Those (R40/R41) appear to set the gain of the first Q7/8 pair in the GR Control amp which affect the gain of the second stage as well. That could well make the output of the GR CTRL amp not "loud" enough to offset the -DC threshold voltage at pad 21.

I hope that's it! I would also check and confirm all the other resistors in that section just to be sure.

Best,
jonathan

 

[Randyman...]

I think that's it!  I double checked R41, and it was .270 Meg (270K).  I must have initially thought the meter read .270K during assembly and went with it.  Of course, both units have this mistake since they were built side by side.  I was also missing the 270K release pot resistor - so that makes complete sense.   :-X

Off to the store to pick up some 270 Ohm resistors.  I'll report back in a few hours.

Confidence is high.  Repeat - Confidence is high  


Thanks guys - Detective work at its finest IMNSHO

8)

 

[Randyman...]

One down - Works perfectly after the R41 swap!  I'm about to swap R41 in the second unit, re-calibrate them including the tracking adjust (now that I can complete that part) - and see what I can destroy with them

I quickly tried the first unit with my snare subgroup and it has major mojo!  Exactly what I was hoping for.

I was fairly confident my problem would be this simplistic in nature.  I knew I scrutinized every aspect of the build, but it was my one slip of mistaking a .270Meg DMM reading for a .270K reading that "did me in".  I hate my cheap-ass DMM's auto-scaling!  Who in the hell represents 270 Ohms as ".270K" anyways????  Just display "270 Ohms" :-\


Quick Q - Is it normal for the Release Pot to have such a profound effect on the gain reduction?  It's mainly that first 10 degrees of travel from full CW that has the greatest effect from little GR to massive GR along with a longer release (with the same Input gain setting).  I knew the attack/release would interact with the GR to a degree - but I can't recall if the few "real" 1176's I've encountered behaved this way (and I certainly never had the luxury of using a real Blue Stripe).

Would a Log Taper 5K pot work better to spread that small low-z interaction into a larger physical range of motion?

Again - This place, and more correctly the people here, are fabulous.  I especially appreciate 0dBfs' and John/Stagefright's help.  I owe you some karma  :-*

8)

 

[Rob Flinn]

Randy

great to hear you made some progress.  I bet everybody here has made the meter range mistake... I know I have !!

 

[Randyman...]

Randy

great to hear you made some progress.  I bet everybody here has made the meter range mistake... I know I have !!

Thanks, Rob.  I also figured out that I actually swapped the Release Pot's 270K resistor and R41 270R !  That's why my Release pot is so "touchy" (RE my comments above).  I'm handling that now.  So I actually made the meter range mistake TWICE in one build (well, two builds at once). DOH!

I've built 14 Seventh Circle Preamp modules over the last 4-5 years and never missed a beat until now!  :'(  I don't like this new DMM at all (I'll blame it on the meter's stupid auto-ranging ballistics  )

Back to work - Almost done...

 

[0dbfs]

Awesome! Congrats on the acquisition of two great brand new
handbuilt reduction machines! What's next on your list now?

Best,
Jonathan

 

[Randyman...]

Awesome! Congrats on the acquisition of two great brand new
handbuilt reduction machines! What's next on your list now?

Best,
Jonathan

Thanks in no small part to your handy detective work - you stud!

The SA-3A and SB4000 are paid for - just waiting on the parts.  And, of course, a 2-1176 Rev D once Hairball has everything lined up.

After all that, I'll likely attempt one of the lovely 2-LA2A models and get my feet wet with tubes.

I'm learning more with each build.  I think about how I almost dropped $2500 on a one channel UBK Fatso as my entry into owning my own "boutique" compressors!  I'll have 6 channels for that same outlay (plus some time and experience under my belt)    I knew I could DIY some nice compressors if I gave it a shot with these sweet kits.  The Seventh Circle preamps sound great - and I have high aspirations for all of my future DIY projects (the Neve EQ is looking tasty if not expensive).  I'm getting to love this place more everyday.

Almost done warming up / calibrating them now.  About to try the pair linked on the drum bus  ;D