All things G1176 - the new "repost" thread.

[Dave-B-Brown]

Hello 1176 world!!

I have been learning from these posts for over a year, I have now reached the stage of asking questions.

O.k, everything is done, calibrating underway, my first problem is that when I turn the output pot, the level on the VU rises slowly and at a certain point jumps up a lot and when turned up full the signal drops to zero.
When I turn the input pot the VU rises and falls accurately and smoothly.
Just wondering if I have a faulty pot or a if it is something else.

 

[gswan]

Sounds like a faulty pot, probably a crack in the track. Measure the signal on the wiper with a scope and turn it to see if the pot itself is causing the levels to jump around at a certain point.

 

[Dave-B-Brown]

i dont have a scope, only a cheap digital multimeter.
I have been reading all your recent posts, I am also using MNATS Rev J,
after the last couple of posts I have become confused about the GR on/off
situation. I think I have understood that it doesnt bypass the unit, it only turns the meter on/off, is this correct?

 

[gswan]

The GR disable simply does that. it disables the GR amplifier. The signal still goes through the preamp and line amplifiers. To truly bypass the unit you would need to add some switching/relays to re-route the input and output connectors.

The metering is different again. There are three metering settings. Two monitor the output signal (one for +4dBm and one for +8dBm) and the other setting monitors the gain reduction. The gain reduction metering needs to be calibrated to track what is actually happening at the signal FET.

Some switch wiring configurations make it hard to disable GR and still meter the GR for calibration purposes.

 

[Dave-B-Brown]

O.k, thats what I thought before I became confused.

As is stands I have good level in and out I have set the Q bias and the GR meter to zero, I have not yet completed the GR meter Trck, where do I measure 50mV on the output? and when In GR I do see the meter move ever so slightly showing reduction, is this what it should be like before being calibrated or should it be a lot more?

 

[gswan]

I've heard about a calibration procedure that measures or uses a 50mV signal. i don't know anything about that one. I use the one that switches the GR in and out and calibrates fro a 10dB drop when GR is switched in.

The VU meter is almost useless at giving any kind of accurate reading of gain reduction, particularly on fast transients (drum hits etc) since the ballistics are too slow. It wobbles about when you use it really giving an indication that the compressor FET is being activated and the gain is being altered in some way.

Since the input control is kind of used as the compressor threshold adjustment, the VU meter lets you know when compression is starting to happen, but don't rely on its accuracy to tell you how much.

 

[Dave-B-Brown]

Is that the method on the AXT sytems site(is that you?), I just found that site and it is a lot easier to follow than some of the others.
I'll have a shot at doing it that way.
I am a first time DIY'er, I didnt know what a resistor or even what a PCB was a few months ago, so sorry if I am a little slow.

 

[kenrinc]

Confirmed. Don't know how I missed this since it's right there on the first page of Jakob's G1176 site:

"You could use the procedure from the original .pdf manual, but that could be a bit tricky because we changed the bypass to be on the meter switch instead of on the attack switch. So when they say "turn attack fully counterclockwise to bypass", it could translate into something like "bypass the unit with the meter switch".

This is also where he details calibrating with a 1khz, 50mV signal

Ken-

 

[lofi]

how important is it to calibrate GR? does it make a diference to the unit, or just to a visual indication of whats happening?

today i scoped the input from my signa generatorl to 2.2Vp-p, then attack ccw realease cw ratio +20 vu selected output cw and input ccw, set my scope onto the output and raised the input pot til i got 2.5Vp-p, moved the vr til i got 2.2Vp-p

so now it is just starting to compress.

now i cant set gr i supose ( actually i wanted to know if it was working so i skipped the rest)

bass track out of the daw, through the 1176 and back into the daw, and start playing with the settings

use the daws meters to see if alls well.

hey guess what ...

i've made me a compressor!!! its transparent but heavy, can get dirty yet stays clean, quick yet slow (insert here a couple of hours of meaningless drivel soliloquizing over how nice this is)

long n short - I AM VERY HAPPY!!!! :green: :green: :green:

 

[gswan]

The GR calibration is just a metering function, it does not affect the performance of the unit or its audio functionality. The Qbias is the critical setting for the compressor functionality to work correctly.

You should also realise that the attack and release settings are at their shortest when turned fully CW.

 

[lofi]

thanks for all your help, and a great site you have too!!! a wee bit of wiering to finish and its onto chan two!!!

so happy!!! :green: :green: :green:

gonna have a cup of tea to celebrate

Iain

 

[gswan]

[quote author="Dave-B-Brown"]Is that the method on the AXT sytems site(is that you?), I just found that site and it is a lot easier to follow than some of the others.[/quote]
Yes, that's my site. I decided to put it together as another reference for constructors. I also added some details about circuit modifications and some troubleshooting and measurement tips. Hope you find it useful.

 

[Dave-B-Brown]

I have tried the calibration, all the trim pots do have an effect on the levels but ratio, attack and release seem to do nothing, just wondering what I should do as the next troubleshooting step?

 

[gswan]

[quote author="Dave-B-Brown"]I have tried the calibration, all the trim pots do have an effect on the levels but ratio, attack and release seem to do nothing[/quote]

1. Have you calibrated the Qbias for the 1dB drop in level (ie FET just into conduction)?

What kind of signal are you feeding in to the unit? You won't get much change if you use a steady-state sine wave. The attack, release and ratio settings will appear to do nothing.

If you have calibrated everything, I'd suggest measuring the ratio transfer curves to show that the unit is compressing approximately to the values marked on the panel for each ratio.

 

[Dave-B-Brown]

I used the sine signal for calibration, and then used a drum track to test if the ratio, attack and release were working.
As i only have DMM , how do I measure the output to test the ration as per your site?

 

[Dave-B-Brown]

Somehow when I was adjusting the Q pot I got a spark, it's now making the meter needle wobble around, off to jaycar I think!

 

[gswan]

Is your DMM capable of reading RMS AC voltage at audio frequencies?

If it is then you can easily measure the AC transfer characteristics of your compressor at different settings.

Measure the signal levels and convert them to dB and plot them (there's a speadsheet on my site) for each ratio setting.

 

[Dave-B-Brown]

No It doesnt, I may need to make a purchase.
Also, I notice that my compressor(not yet) brings out the highs on audio that I put through it, is it supposed to do this? i am using the Lundahl on the input and not the op-amp

 

[gswan]

You are supposed to check and calibrate the unit before putting audio through it. Who knows what it might be doing.

1. Check your DC steady state voltages around the circuit. The usually finds wrong value resistors, transistors incorrectly inserted etc.

2. With GR turned off, using a 1kHz sine wave input, check the input amplifer and the line driver amplifier sections and calculate the gain of each to verify that they are working correctly.

3. Perform the Qbias calibration procedure.

4. Perform GR meter calibration and setup.

5. Measure the unit transfer characteristics for the different ratios to verify how much compression you are getting and where the knee is.

6. Use the unit on real audio signals and be amazed.

 

[Dave-B-Brown]

YES, SIR!

I will not ask another question until I have all that done,
Much appreciated