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

[Tousana]

I'm looking for a little help here, everything seemed to have gone off without a hitch, with the build, that is until the testing began. This testing basically came about because in the first use of the unit, it compressed very quickly, I could hardly crack the input to achieve a moderate amount of compression on a bass track.  All tests and measurements have be preformed with an A/P unit, by a tech with 40 years experience. I know my limitations.

The unit is experiencing excessive distortion in the signal when feeding a 1k tone @ +4dbu.  The input pot is set to yield -3db of GR, which occurs around -40 on the input knob. When the GR is disabled and the output is adjusted to read unity (+4dbu)at the output that knob sits at about -36, the funny thing is, it's showing about 0.326% THD+n and that seems excessive. Comparing a stock 1176 Blackface under the same conditions showed only 0.01% THD+n.

A frequency sweep test was also preformed with the output terminated into 600ohms which showed another interesting but strange reading, which was a rising response above 10k, that eventually reached +1.5db above unity (+5.5dbu) at 100khz.

Seeing the results of previous tests led us to measure the distortion of the unit further. With the test signal back at 1k and raising the input knob to give +16dbu at the output the  distortion rises to +12%. Looking at an oscilloscope clipping can be seen, indicating that this distortion is occurring in the preamp stage and the input knob is sitting at 12 o'clock.

The verdict seemed to indicate a lack of local feedback in the preamp stage, excessive distortion, to much gain, and a rising frequency response.

With this being said, all voltages measure within 5% of the values shown on the schematic. I've been told that it seems that the circuit was built correctly and components Q2 and Q3 where even swapped with replacements and no change in the performance of the unit was noted.

The calibration procedure has been preformed three times, and the GR display on the meter agrees perfectly to the amount of GR shown at the output, so that I think indicates that the calibration is correct.

Anyone have any suggestions as to where to look next, to get this unit up and running? This is unlike any experience I have ever had with an 1176 in my time in the studio, which ha been a while. I would really like to use this thing, and any help or suggestions would be welcome.

Thanks much.

 

[django32]

Have you got the output loaded?

 

[Echo North]

Are you sure it's distortion and not noise floor? (THD+N)

 

[Tousana]

It is definitely distortion and not noise, as it is showing on the oscilloscope as distortion.

Does the Rev. A have inherently have more gain in the preamp stage than than Rev. D?

The reason I ask is that, as I stated before, I'm achieving 3db of GR at around -44 on the input knob at a ratio of 4:1, and on the other blackface 1176's that I'm familiar with this same amount of GR is usually achieved at a much higher setting of around - 30 at the input knob.

 

[Echo North]

It is definitely distortion and not noise, as it is showing on the oscilloscope as distortion.

Does the Rev. A have inherently have more gain in the preamp stage than than Rev. D?

The reason I ask is that, as I stated before, I'm achieving 3db of GR at around -44 on the input knob at a ratio of 4:1, and on the other blackface 1176's that I'm familiar with this same amount of GR is usually achieved at a much higher setting of around - 30 at the input knob.

The Rev A has 5dB more gain than the D, which is almost double the gain (6dB being double) and it is a far dirtier box. The A lacks the drain resistor on Q1 causing it to saturate and distort MUCH more than the D. That is what the A does.

You can add that drain R and see if that makes it more familiar.

Mike

 

[Spencerleehorton]

Got my Rev A pcb today! Going to start to put it together!

Spence.

 

[django32]

Still having issues with mine. the signal dies at q1 I've tried replacing the transistor but to no avail does anyone have any ideas?

 

[Echo North]

Still having issues with mine. the signal dies at q1 I've tried replacing the transistor but to no avail does anyone have any ideas?

What does the signal do with no transistor?

Mike

 

[Tousana]

It is definitely distortion and not noise, as it is showing on the oscilloscope as distortion.

Does the Rev. A have inherently have more gain in the preamp stage than than Rev. D?

The reason I ask is that, as I stated before, I'm achieving 3db of GR at around -44 on the input knob at a ratio of 4:1, and on the other blackface 1176's that I'm familiar with this same amount of GR is usually achieved at a much higher setting of around - 30 at the input knob.

The Rev A has 5dB more gain than the D, which is almost double the gain (6dB being double) and it is a far dirtier box. The A lacks the drain resistor on Q1 causing it to saturate and distort MUCH more than the D. That is what the A does.

You can add that drain R and see if that makes it more familiar.

Mike

Mike,

Thank you for your answer to my question, we where sort of chasing our tails, but with your answer on the gain of the Rev A, we have been able to close the unit up, and start using it. It seems to be working flawlessly, again thanks!

 

[django32]

Still having issues with mine. the signal dies at q1 I've tried replacing the transistor but to no avail does anyone have any ideas?

What does the signal do with no transistor?

Mike

sorted that all out, had dodgy batch of transistors, now my new problem is when i try to set up the bias my meter goes up rather than down I.e i go from +1 upwards rather than being able to turn it down to 0

 

[Echo North]

Still having issues with mine. the signal dies at q1 I've tried replacing the transistor but to no avail does anyone have any ideas?

What does the signal do with no transistor?

Mike

Pot may be in backwards? No worries if it is, just the the rotations in reverse to the guide/video.

sorted that all out, had dodgy batch of transistors, now my new problem is when i try to set up the bias my meter goes up rather than down I.e i go from +1 upwards rather than being able to turn it down to 0

 

[django32]

would that do it? my procedure is to ground tab 22, have the bias all the way wound to ground and wind it up when i've got the meter set to 1db except it goes up to 2db rather than back down to 0. It's blowing my mind!

another edit, I wired up the other side of it as its a stereo unit and it's doing exactly the same aswell as it being impossible to 0 in gr mode on the meter

 

[Echo North]

would that do it? my procedure is to ground tab 22, have the bias all the way wound to ground and wind it up when i've got the meter set to 1db except it goes up to 2db rather than back down to 0. It's blowing my mind!

another edit, I wired up the other side of it as its a stereo unit and it's doing exactly the same aswell as it being impossible to 0 in gr mode on the meter

You want the bias most neg to start. So if the Bias is a say 0VDC to -2VDC, start at -2VDC and wind towards 0.  Closer to 0 the output will start to drop.

Is that the issue?
Mike

 

[Shattersignal]

I just dropped by to mention that I built my mnats rev A with Orange Drops at first.  Then I auditioned Mundorf Supremes in the coupling positions--huge improvement to clarity and dimension from the unit.  Then I installed Mundorf Supreme SGO and EVO oil SGOs in the coupling positions.  With these caps, this compressor is to die for.  My lust for a CL1b has diminished significantly--the 1176 now has all the clarity and depth of a $3000+ all tube compressor, but with all the nasty punch and grind that you can only get from a classic 1176.

The cheap Orange Drops do have their place, though.  They lend a placticy, trashy sound that can be perfect for that classic 1176 smack--but I find that those caps always tend to place the instrument in the same depth, every time. 

I use my rev A 1176 on vocals with my modified Manley Ref C or C12 clone, into an 1073s or Redd 47 all the time and I love the added dimension that I get with the $300 upgrade to capacitors in this compressor.  I retain way more character from my mics and mic preamps after upgrading the caps in the comp.  If you haven't tried great caps in the unit, you might not realize how high-fidelity a rev A 1176 can be, and still retain the flavor that makes a vocal (or anything else) sit perfectly in the mix.

 

[django32]

would that do it? my procedure is to ground tab 22, have the bias all the way wound to ground and wind it up when i've got the meter set to 1db except it goes up to 2db rather than back down to 0. It's blowing my mind!

another edit, I wired up the other side of it as its a stereo unit and it's doing exactly the same aswell as it being impossible to 0 in gr mode on the meter

You want the bias most neg to start. So if the Bias is a say 0VDC to -2VDC, start at -2VDC and wind towards 0.  Closer to 0 the output will start to drop.

Is that the issue?
Mike

yep that was the issue, It's bias'd up nicely now and i've done all the other adjustments, I'm just not getting any gain reduction now haha. Was wondering what the trim on the gain reduction board was for as well

thank you for the help

edit; not getting any signal after the two fdh333, could that be the cause, I'm not getting any compression on either unit as well so its got to be another stupid mistake i've made somewhere

another edit, grounding the input to the gr makes no difference to the output but obviously kills the signal through it. All the voltages are within spec in the amp. When i turn the ratio it reduces the output at the end of the gr circuit but i also get a pop on the output of the unit and the gain reduction meter swings up or down depending which way i turn it in relation to the dc pop

 

[eqp1a]

Having issues calibrating the gain reduction meter driver. Upon further inspection, I'm reading ~3.5VDC at the drain of Q11 (as opposed to the ~10.5VDC listed in the expected voltages), and ~2.9VDC (as opposed to ~9.5VDC expected) at test point 10. R79 checks out at 15k. Anyone encountered this before? Thoughts?

Cheers

Ian

 

[Echo North]

would that do it? my procedure is to ground tab 22, have the bias all the way wound to ground and wind it up when i've got the meter set to 1db except it goes up to 2db rather than back down to 0. It's blowing my mind!

another edit, I wired up the other side of it as its a stereo unit and it's doing exactly the same aswell as it being impossible to 0 in gr mode on the meter

You want the bias most neg to start. So if the Bias is a say 0VDC to -2VDC, start at -2VDC and wind towards 0.  Closer to 0 the output will start to drop.

Is that the issue?
Mike

yep that was the issue, It's bias'd up nicely now and i've done all the other adjustments, I'm just not getting any gain reduction now haha. Was wondering what the trim on the gain reduction board was for as well

thank you for the help

edit; not getting any signal after the two fdh333, could that be the cause, I'm not getting any compression on either unit as well so its got to be another stupid mistake i've made somewhere

another edit, grounding the input to the gr makes no difference to the output but obviously kills the signal through it. All the voltages are within spec in the amp. When i turn the ratio it reduces the output at the end of the gr circuit but i also get a pop on the output of the unit and the gain reduction meter swings up or down depending which way i turn it in relation to the dc pop

You should be seeing DC after the fdh333 diodes. That's where you AC side chain signal gets converted to DC.

If you're not getting correct compression check:

With in/out knobs mid way apply a 1khz 0dB signal and adjust the output to 0dB on the output
- that you see ac at pad 22 that starts larger and get's smaller as you move from ratio 20 to 4.
- that you have a negative DC voltage at pad 21 that starts larger and get's smaller as you move from ratio 20 to 4.

Post the values at each pad at each compression setting.

Mike

 

[Echo North]

Having issues calibrating the gain reduction meter driver. Upon further inspection, I'm reading ~3.5VDC at the drain of Q11 (as opposed to the ~10.5VDC listed in the expected voltages), and ~2.9VDC (as opposed to ~9.5VDC expected) at test point 10. R79 checks out at 15k. Anyone encountered this before? Thoughts?

Cheers

Ian

Might consider replacing that Q12 transistor, checking for bad joints, confirm wiring in that section, confirm trimmers are not damaged, double check all R values by color code.

Mike

 

[eqp1a]

Having issues calibrating the gain reduction meter driver. Upon further inspection, I'm reading ~3.5VDC at the drain of Q11 (as opposed to the ~10.5VDC listed in the expected voltages), and ~2.9VDC (as opposed to ~9.5VDC expected) at test point 10. R79 checks out at 15k. Anyone encountered this before? Thoughts?

Cheers

Ian

Might consider replacing that Q12 transistor, checking for bad joints, confirm wiring in that section, confirm trimmers are not damaged, double check all R values by color code.

Mike

Thanks Mike; I've done all that now, still can't get any movement on the GR meter. When you said Q12, did you mean Q11? Cause I bought a couple matched pairs of the 2N5457s from you guys back in October thinking it might be an issue with the GR meter fet (Q11), but they never arrived (I chose the cheap international shipping option). Plenty of 2N3708s lying around though.

 

[Echo North]

Having issues calibrating the gain reduction meter driver. Upon further inspection, I'm reading ~3.5VDC at the drain of Q11 (as opposed to the ~10.5VDC listed in the expected voltages), and ~2.9VDC (as opposed to ~9.5VDC expected) at test point 10. R79 checks out at 15k. Anyone encountered this before? Thoughts?

Cheers

Ian

Might consider replacing that Q12 transistor, checking for bad joints, confirm wiring in that section, confirm trimmers are not damaged, double check all R values by color code.

Mike

Thanks Mike; I've done all that now, still can't get any movement on the GR meter. When you said Q12, did you mean Q11? Cause I bought a couple matched pairs of the 2N5457s from you guys back in October thinking it might be an issue with the GR meter fet (Q11), but they never arrived (I chose the cheap international shipping option). Plenty of 2N3708s lying around though.

Maybe.  Send us an email and we'll get a pair out to you.