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

[baadc0de]

Also, this is the shot after the output transformer..

A tech friend of mine thinks I might have the OT wired wrong and/or am presenting wrong impedance to the output section.
I have a board marked V1.1 31.01.2009 and the OT from hairball's shop. Am I correct in assuming that:

- the colour codes printed on the board are the same as the ones on the OT
- output transformer orange and yellow wires must be joined together
- the red wire from the OT goes to the pin 2 of the XLR out and to the X pad of the meter PCB
- the blue wire from the OT goes to the pin 3 of the XLR out and to the Y pad of the meter PCB

are these assumptions all correct?

 

[Biasrocks]

A tech friend of mine thinks I might have the OT wired wrong and/or am presenting wrong impedance to the output section.

The output should be terminated into 600 ohms,
but I don't think that will solve you're problem.

Mark

 

[Echo North]

- the red wire from the OT goes to the pin 2 of the XLR out and to the X pad of the meter PCB
- the blue wire from the OT goes to the pin 3 of the XLR out and to the Y pad of the meter PCB

are these assumptions all correct?

Blue goes to pin 2 and red to pin 3.

 

[Echo North]

You need to set your Q Bias.  This is not something you can do by ear and it crucial that it is correct.

Do you have a decent DMM?  If you have a decent DMM it should be able to read 1K signals.  Put it on pin 2 and 3 on the input and do your Q bias.  2.75VAC is +11 and 2.45VAC for +10.

Mike

 

[baadc0de]

You need to set your Q Bias.  This is not something you can do by ear and it crucial that it is correct.

Do you have a decent DMM?  If you have a decent DMM it should be able to read 1K signals.  Put it on pin 2 and 3 on the input and do your Q bias.  2.75VAC is +11 and 2.45VAC for +10.

Mike

Just checking... I should probably set the DMM on output XLR pins, not input?

 

[Pieter001]

Finnished my 1176. Did something wrong with the calibration first.
Is it normal that the output of the 1176 is realy hot when you turn it in bypass mode?

I use it on vocals to compress it 20 db. Then i use the output to gain the signal up to go back in protools. When i turn it in bypass mode the output is realy high.
I think he bypasses the compression but not the output gain level. Is this normal?

 

[Echo North]

You need to set your Q Bias.  This is not something you can do by ear and it crucial that it is correct.

Do you have a decent DMM?  If you have a decent DMM it should be able to read 1K signals.  Put it on pin 2 and 3 on the input and do your Q bias.  2.75VAC is +11 and 2.45VAC for +10.

Mike

Just checking... I should probably set the DMM on output XLR pins, not input?

Correct sorry.

Start on the input to confirm you are feeding a .775VAC signal (0db).  Then switch to the output to measure the Qbias 1V drop.

Mike

 

[baadc0de]

Tried to adjust and..

- a 1k .776VAC signal corresponds to -17dBFS of my interface.
- with the Q bias fully to ground, I get 0.037-0.039VAC, making it impossible to have the q-bias procedure to work.. this is with an output cable (back to the A/D) attached or not

I'm using unbalanced interface cables, if that's of any concern (though I unplugged it to see if that was any difference).

What do you suggest?

Also, does this look OK? I have .775V on input with limiter set for q-bias adj regarding attack/release/meters/ratio. This is the "D" pin of the Q4 J309..

- I have switched Q4 with Q2 to see if Q4 would be faulty, but it's the same
- I changed Q5 and Q6 for new ones, with no change

 

[Pieter001]

Is it normal that the output of the 1176 is realy hot when you turn it in bypass mode?
I use it on vocals to compress it 20 db. Then i use the output to gain the signal up to go back in protools.
When i turn it in bypass mode the output is realy high. I think the unit bypasses the compression but not the output gain level.

Is this normal?

 

[Rob Flinn]

Baadcode,

SOme digital meters won't read a.c voltages acurately at much above 400Hz, it may be worth checking your meters spec, to see if you're getting a true reading

 

[Greg Dixon]

Is it normal that the output of the 1176 is realy hot when you turn it in bypass mode?
I use it on vocals to compress it 20 db. Then i use the output to gain the signal up to go back in protools.
When i turn it in bypass mode the output is realy high. I think the unit bypasses the compression but not the output gain level.

Is this normal?

Yes that's normal. When in bypass, only the side chain compression circuit is bypassed. The audio always goes through the amp circuit. If you're compressing the audio 20 db and then bypass it, the signal will be 20 db higher.

 

[Pieter001]

Thanks for your reply. Haven't used the real 1176. Is this bypass gain thing the same as with the original 1176's?

 

[dmp]

I think a true bypass is a nice addition to an 1176. The originals did not have this. Turning off gain reduction is not the same as bypass.
I've found (in my revD, still building a revA) that the preamp clips fairly easily with the gain reduction off, so I don't think it is very useful.

 

[Pieter001]

I like to compress my vocals a lot like 15 or 20 db's. The bypass function is not usefull at all for me. I think a truebypass function is a good idea.

 

[dmp]

Yup, and if you do not need a power switch you can use the 'off' pushbutton to control in/out bypass...

 

[baadc0de]

Baadcode,

SOme digital meters won't read a.c voltages acurately at much above 400Hz, it may be worth checking your meters spec, to see if you're getting a true reading

Sample rate: up to 1MHz
Input voltage: -20 - +20 V (0-3V on +5 inputs)
ADC resolution: 12 bits

This should suffice, no? Also, if it read the signal correctly throughout all of the other stages, then it should have read the output stage correctly too, or am I misunderstanding something?

 

[Rob Flinn]

Sample rate: up to 1MHz
Input voltage: -20 - +20 V (0-3V on +5 inputs)
ADC resolution: 12 bits

This should suffice, no? Also, if it read the signal correctly throughout all of the other stages, then it should have read the output stage correctly too, or am I misunderstanding something?

If it's reading the right levels elsewhere in the circuit then it is probably fine, however I think the sample rate in this instant is not quite as directly related to the frequency response as when you're dealing with audio interfaces.

 

[baadc0de]

If it's reading the right levels elsewhere in the circuit then it is probably fine, however I think the sample rate in this instant is not quite as directly related to the frequency response as when you're dealing with audio interfaces.

I can imagine that a relatively high sample rate is not necessarily correlated to a phase correct, linear frequency response. But it is indeed reading the levels appropriately in other parts of the circuit. I did re-run the tests at 300hz just to be sure and there's no difference to speak of.

 

[baadc0de]

Some time ago, I desoldered all resistors in the input section and checked them, there were 2 wrong (and I fixed that).
Also, C6 was a straight wire instead of a capacitor. I fixed that.

I desoldered all resistors in the output amp today and checked them, all are ok.
I am now desoldering all resistors from the GR driver section and found out R41 was 270k instead of 270r.
I'll desolder the rest and the meter driver section for good measure. Maybe there's more gremlins there..

I bought this board populated from a seller here on prodigy-pro. Thus far this is the 4th mistake in values... Oh well. Live and learn. I'm probably never buying a populated project again.

 

[Biasrocks]

I bought this board populated from a seller here on prodigy-pro. Thus far this is the 4th mistake in values... Oh well. Live and learn.

Dealt with something similar here yesterday.

1K stuffed in the Q-Bias section (R82 iirc) of a REV J instead of 10K, luckily I noticed it, the resistor had turned a nice charcoal color.

Mark