All things G1176 - the new "repost" thread.

[gswan]

[quote author="kenrinc"]I went through the GR meter tracking process again tonight.

Although I get nice sounding signal through the amp sections I get no response using the Attack control. I apply the 1khz signal, switch to +4 mode on the meter and turn input and output to mid. Then use output to set meter at 0vu. Then it says "turn attack control on" I'm assuming since I don't have a switch on the attack control, I have to switch to GR mode. When I do this, turning the attack control does nothing on the meter.

Ken-[/quote]

Ken,

Forget the attack control, it won't do anything for a sine wave. Turning the attack control instruction is simply to engage or disengage the GR amp.
You need to do this with your switches instead.

When you turn GR on with the 20:1 ratio selected you need to see a drop in output (ie the compressor is working) which recovers when you turn the GR off. The meter calibration is to show the correct drop in gain and the recovery to 0 when the GR is bypassed.

 

[kenrinc]

I'm pretty sure I understand all that the problem is I don't see any drop in output. Once I've adjusted relative levels and the meter shows 0vu in the +4 mode I switch to GR mode and don't observe any movement. It just stays at 0vu.

Ken-

 

[gswan]

If you have 20:1 compression switched on and you've calibrated the FET Qbias correctly then you should see a drop when GR is switched in (that is sometimes called 'bypass' for some people).

Switching the meter to GR is not the same as switching GR on.

 

[kenrinc]

OK so I am totally confused. I have 6 controls: in, out, atk, rel, meter and ratio. I have no GR on switch. I assumed the meter switch turned on GR but from what your describing that is not the case. So the meter knob only displays whatever the meter switch is set to (No GR, GR or output).

If I inject some music via a CD player into the XLR input I can easily see the output dance on the meter when the meter is set to +4. When the meter is switch to GR the meter sits at 0vu. Turning any of the other controls does nothing.

ken-

 

[gswan]

What are the labels on the ratio switch? Is there a 'bypass' position anywhere?

 

[lofi]

if hes using the Mnats board the meter board is three way, bypass/gr/vu.

can i use a scope to measure the meter? its going to take a while for me to get a vu meter in and i am in the studio all thurs night. I want to convert the dbu amounts into p to p amounts and try a full calibration, but maybe the meter feed is more esoteric than the 'double the number in your head and multiply by the log of a small pie' or whatever it is :grin:

heres hoping

Iain

And Thank you gswan for all your help!!!!!

 

[gswan]

[quote author="lofi"]if hes using the Mnats board the meter board is three way, bypass/gr/vu.[/quote]

Then he's going to have to find a way to meter GR and be able to bypass/engage the GR amplifier at the same time.

[quote author="lofi"]can i use a scope to measure the meter? its going to take a while for me to get a vu meter in and i am in the studio all thurs night. I want to convert the dbu amounts into p to p amounts and try a full calibration, but maybe the meter feed is more esoteric than the 'double the number in your head and multiply by the log of a small pie' or whatever it is[/quote]

Yes, you can use a scope. If you measuer the p-p voltage, halve it and multiple by sqrt(2) to get the RMS value (for a sine wave only). To convert the dBu values to p-p values use the opposite.
For example,
0dBu = 0.7746Vrms = 1.095Vp = 2.191Vp-p
-10dBu = 0.245Vrms = 0.346Vp = 0.693Vp-p

 

[lofi]

[quote author="gswan"]

Yes, you can use a scope. If you measuer the p-p voltage, halve it and multiple by sqrt(2) to get the RMS value (for a sine wave only). To convert the dBu values to p-p values use the opposite.
For example,
0dBu = 0.7746Vrms = 1.095Vp = 2.191Vp-p
-10dBu = 0.245Vrms = 0.346Vp = 0.693Vp-p[/quote]

dont know how accurate i could get my scope but i shall stab that

0dBu = 2.2Vp-p
+1dBu = 2.5Vp-p
-10dBu = 0.7Vp-p

should be ok?

thanks again for your help!!!

Iain

 

[kenrinc]

It's mnats rev J board. I've built it using the instructions shown on his site using the 5 position Slam switch. Ratio switch is 5 pos: 4:1, 8:1, 12:1 20:1 and Slam. Meter switch is 3 pos with GR off, GR and +4. The unit may be working "slightly" as I can see a tiny bit of meter deflection in GR meter mode with input and output maxed but that seems a bit excessive. I'm assuming I'd be able to get well over threshold with a roughly -10db input signal.

I guess I'm still confused as to how to enable GR on the unit. I'm assuming it's always enabled unless the meter switch is set to GR off?

Ken-

 

[gswan]

[quote author="kenrinc"]
I guess I'm still confused as to how to enable GR on the unit. I'm assuming it's always enabled unless the meter switch is set to GR off?
[/quote]
It sounds like it. I didn't build mine like this since it made no sense. For calibration you need to meter GR with GR set to off. With the 3-position rotary switch you can't do this. You will need to find another way.

 

[lofi]

thanks sir!! that info must be in here somewhere, im of to hunt down where we have gone wrong

 

[kenrinc]

I saw the 6 pos ratio switch you built for your unit but you also mention a switch to turn on GR (luv your site by the way). I see the little board you made to act as GR disable. Looks like I'm going to have to go over the schematic tonight and see if I can make sense of how to either turn it on or off. My guess is it's off or I miss wired something. Jeesh, I can't wait to get this thing dialed in, I need it like right now! :wink: Thanks.

Ken-

 

[gswan]

I managed to get hold of a handful of pots with DPST switches on them (these pots used to be common on old TV's). I needed a SPDT switch though, so I made up the signal relay board to switch the GR amp in and out as well as having an LED on the front panel to let me know that GR was activated.

It also made more sense to use a separate switch for the GR and the original switch on the attack pot was the right way to do it.

 

[kenrinc]

On the original, does the pot just shunt to gnd when it's turned off? I'm looking at the schem and see the pot symbol with a cross going through the middle. I assume these are both switched pots?

I'm actually surprised this issue hasn't come up before since this unit was built no different than the G1176. I'll have to do more searching. :roll:

Ken

 

[gswan]

Yes, on the original the input to the GR amp is shorted to ground to prevent any GR activity. I've seen it wrongly called 'bypass', when in fact it is just a GR disable switch.

 

[gswan]

[quote author="kenrinc"]I'm looking at the schem and see the pot symbol with a cross going through the middle. I assume these are both switched pots?[/quote]

If the 'cross' is what I think you mean, then no, this is not a cross but an arrow which crosses the wiper terminal and points to the direction for clockwise rotation. This lets you know how to wire up the pots so they work as expected.

 

[kenrinc]

Cool, I gotcha. That answers my question so I guess the unit is always in GR mode so having the switch isn't necessarily needed accept to do this friggin calibration process! Maybe I can temporarily ground the pot? I've got a figure out a way to find out why it's still not working. I'm going to go back through it again tonight checking the attack and release pots to see if I may have missed something. I'm assuming that GR shows meter deflection to the left? The only Urei compressors I've worked with are the LA4 units and that's how they worked.

Ken

 

[gswan]

[quote author="kenrinc"]I guess the unit is always in GR mode so having the switch isn't necessarily needed accept to do this friggin calibration process![/quote]

It's also necessary if you want to use the unit without compression as a pre-amp or to drive it hard to introduce colouring to your signal through the transformers without the compression activated.

[quote author="kenrinc"]Maybe I can temporarily ground the pot?[/quote]

It's not the pot that needs grounding, it's the switch at the input of the GR amp (which is traditionally part of the attack pot assembly). The pot itself is used to vary the rate of change of charging voltage to the gate of the compression FET. The release pot varies the rate of change of discharge of the voltage on the compressor FET gate.

[quote author="kenrinc"]I've got a figure out a way to find out why it's still not working. I'm going to go back through it again tonight checking the attack and release pots to see if I may have missed something. I'm assuming that GR shows meter deflection to the left? The only Urei compressors I've worked with are the LA4 units and that's how they worked.[/quote]

Yes, the GR meter deflects to the left to show gain reduction.
It's probably nothing to do with the attack and release pots. Have you checked that your GR amp is working correctly? Make sure the DC bias voltages are correct and that when you feed signal through it you get the rectified signal at the output of the low-leakage diodes.

 

[kenrinc]

Checking out the GR amplifier and that may be where the problem lies. Feeding the 1khz signal in at the input and using the input knob to see signal at the base of Q12 (per your site), I have to max out the input knob to get any signal and even then have to set the scope down to .02v/div or lower to see anything. There is signal at Q13, Q14 and Q15 but it's low. Anyplace in particular I should start checking?

ken

 

[gswan]

Check that there's plenty of signal at the top of the output pot first. If so then you need to either check your ratio switch resistors or check that the input to the GR amp is correctly wired to the ration switch.