G1176 Vu and Gain Reduction tracking WAY off

I was looking at some other posts where people seem to be having similar issues that I am. I just finished an 1176 using mnats boards, and I went through the calibration process outlined on gyraf's site. After setting the 0 VU mark when no signal is passed in with it in GR mode, I then switched over to calibrate a signale at 1:20 ratio. After I set the gain tracking to show -6 db, the 0 mark now reads close to -3. When I set it back up to 0, the GR setting now only shows about -3 for the -6 db of gain. I've tried tweaking both R54 and R55 pots to no avail. The gain reduction always seems to be off by 3 db or more.

Is it possible the pots need to be different values than what are listed in the schematic now that some of the other parts have changed? I am using TrueRTA along with Adobe Audition and a voltmeter for the calibration process. This same setup worked like a charm on the SSL box I built.

On a side note, I notice that the toroid transformer I have hums pretty loudly when it's powered on. It doesn't seem to affect the audio signal at all, but it is audibly noisy when you're standing close to it. I verified that I'm getting the proper +30 and -10 voltages at the right spots on the board, so I don't think there's any issue with the power supply section itself. Is it possible the transformer is too small for the circuit, and the excessive power draw is causing the buzzing? The transformer does not heat up like I would expect if that were the case. It is the Digikey Amveco toroid 62042 that provides 417 ma at 24 volts.

Are your FET's matched ?
The Urie 1176 has a FET for Gain Reduction and a FET for metering.
In order to give a reasonably accurate indication they need to be matched.
I believe I raised this matter before when asking what FET people were using for this project.

Yep. Absolutely right.

If your FETs are a long way off, the metering will NEVER track the actual signal GR.

basically, here's how it works: the limiter reduces the gain with a control voltage fed to a FET. -In order to try and display the GR, it sends a constant DC voltage to a second FET which it hopes will act just like the first, when the same control voltage is used to reduce the constant DC voltage. Small variances between FETs can be trimmed for. However, if the onset (the "pinch-off" point) is significantly different between the two FETs, you can never trim it out satisfactorily.

To build an 1176, I would buy 10 FETs. Pick the closest two with the controls at mid-travel, or build a FET tester. -Search for posts by Peter Purpose including the words "FET", "Match" and perhaps "PRR".

Keith

Try setting the meter trim so that the meter reads +3vu, then calibrate for for -6db gain reduction. The meter should fall close to 0VU at rest.

Worked for my 3 1176's and I didn't use matched FETs.

Also, what is your process for calibrating the gain reduction?

For the toroidal, try very carefully rotating the transformer when the power is on and listen to how this affects the hum. Neat wiring and proper shielded cables (as outlined on the Mnats site) are really necessary for this project.

(edit: I see you meant the transformer itself hums ...sorry, I don't know why this is. )

Hello, i've just finished my 2-1176, but not really completely...
I'm trying to adjust the VU/GR meter.It seems to be good for the GR but i've a question : the GR meter must indicate zero when the ratio is 1:4?
on VU meter position I have all the time the same value. I've made before the calibrating procedure.which value on the VU must I have when I have no input source ?
The VU meter come from surplus sales of nebraska : http://www.surplussales.com/Meters/MtrVU.html
the VU meter is at the top left of the page.
By reading another thread about the 1176 calibrating, I've shown that the 2 BF245 have to be same Hfe, is it an very important feature?

Thank you by advance :wink:

Hello!
I had the same problem in my copy of 1176

I was looking at some other posts where people seem to be having similar issues that I am. I just finished an 1176 using mnats boards, and I went through the calibration process outlined on gyraf's site. After setting the 0 VU mark when no signal is passed in with it in GR mode, I then switched over to calibrate a signale at 1:20 ratio. After I set the gain tracking to show -6 db, the 0 mark now reads close to -3.

Click to expand...

But I solve this problem next way - I repeat calibration procedure a fey times - after adjusting gain traking I rotate "input" to zero and trim VU to read 0db, then turn it to 12 o'clock, and adjust tracking, and repeat this procedure untill VU became work correct.

I had thought that the FET matching was only necessary for matching two units for stereo operation, but now I see (and having checked the schematic again as well) that this all makes perfect sense now. Looks like I'll be picking up a few more FETs and trying to match a pair to get more accuarate tracking. It seems like the way to go.

Thanks!

Yes indeed. Two parallel FET circuits in each unit, which it is hoped behave the same in terms of attenuation. Any error/difference between FETs shows up as a GR meter tracking error.

Try to match them as a 'quad', and you'll also have enough for a stereo pair of 1176s if you have two, or just go for the best 'pair' if you have only one 1176.

Oh and yes indeed, also; the controls interact with each other, so you have to repeat each adjustment after you make the other... set the meter zero, the 'minus-six', then re-do the meter zero, the 'minus-six'... re-do the meter zero AGAIN... the 'minus-six' AGAIN... -You should notice that each time the error which you're tryong to 'correct-out' gets smaller and smaller. You might try "overshooting" the correction early on in this sequence of 'succesive approximation', which can speed up the process somewhat.

:thumb:

Keith

I also have a problem which bothers me...


I was calibrating my two Purples tonight and 1st one went through very smoothly
but I can't set the second one to show the VU and GR correctly. My DAW
tells me always the true GR but Sifam meter goes off. Also the VU doesn't
work correct when I do the Bias calibration according to the book.

I guess I should match the fets also

Other than that this compressor is rocking :thumb:

BTW, HERE is a nice info how to do it

Actually Jacob gave us the smartest option how to test the FETs :thumb: :thumb: :thumb:

I would test them in-circuit, looking at actual GR vs. control-voltage when Q-bias has been adjusted. Seems to be the simplest and easiest (if you remember to put in sockets for the fets)

Jakob E.

Click to expand...

OK, I tried many FETs but my VU is still off for 0.5db. GR is close enough but the VU level is off.
Can I do something about it or is my meter showing off VU levels by default?

It seems that the FETs don't effect the VU metering and neither any of the trimmers... right?

hello,

It seems that the FETs don't effect the VU metering and neither any of the trimmers... right?

Click to expand...

it's the same with my 2-1176!!!
Gain reduction correctly displayed but when I switch on VU position in any case I see only a really small movement ..
What is the function of the TL071 ? I have really no idea to obtain a correct VU reading...

Well mine was off for 0.5db only so I just bent the meter needle to get it right :green:

VU metering is taken from the output XLR and only has the +4 and +8 resistors in the chain.. maybe change the +4 resistor for another one!

[quote author="warpboy"]hello,

it's the same with my 2-1176!!!
Gain reduction correctly displayed but when I switch on VU position in any case I see only a really small movement ..
What is the function of the TL071 ? I have really no idea to obtain a correct VU reading...[/quote]

look at the schematic.

when in vu-mode, the meter is connected directly across the output.

this means that if you have a working vu-meter, it will show the right deflection at a given level, passively, without influence from any of the electronics.

if it dosen't, then it's not a vu-meter you have on hand - search; there's lots and lots of repeated questions around here about the right meter type for 1176.

Jakob E.

Just thought I'd post a followup to let everyone know that matching the FETs did the trick. The GR tracking is now 100% accurate along with the zero meter when no compressing is going on.

Yay!

Enjoy the crushing! :green:

Keith

Hi there.  I just finished my build (via hairball kit and mnats pcb) and I've got it up to the calibrations.  The Q bias and the null adj. calibrations all went fine, but the gain reduction calibration is giving me problems.  I set it up as per the video, and I receive a 4 db drop when I switch the gain control on, but when I turn the input up to increase gain reduction, instead, the VU meter goes up, and I hear a high whirring sound from the components (i think the output tranny...not sure if that's a big deal or not), so it seems that the gain reduction isn't fully kicking in.  I got the hairball kit, and I'm assuming the jfets were matched (I should have checked them, but spaced), but I'm also wondering if my signal generator is the problem, though I'm not sure why it would be (unless it affected the q bias adjustment so that gain reduction isn't kicking in the way it should be.)  I'm using my pro tools as a tone generator, but I have no way to know exactly what db the output is.  I've tried several different volumes on the tone generator and still the same problem.  Anybody able to help?  It'd be much appreciated.

Maybe I am misunderstanding you, but when you turn up the input, the VU should go up. You turn down the output dial to keep the VU reading at 0. Then when you switch on compression the deflection goes down from zero.
The whirling might be oscillation between the output and input stages. Check the shielding on the wiring to the output pot. The wire shield should be grounded to the chassis.