The official G9 help thread

[andow]

I recently tried running mixdowns through the line input of my G9 and really liked what it does to my mixes. One thing that bothers me however is the lack of low end. I know that it wouldn't be perfectly flat down to 20Hz, especially after reading this thread:
https://groupdiy.com/index.php?topic=9813.0
But just for reference I measured the frequency response and accidentally discovered something strange: the lower cut off frequency rises if the unit is on for some time!
In the attached image you can see measurements I made over a period of three hours, the corner frequency has gone up one octave!
Any idea what might be causing this?

Ok, new insights! 

I changed the output tubes, swapped the input tubes, the effect is still the same on both channels so it has nothing to do with the tubes themselves.
I opened the unit to measure voltages, everything worked fine as long as the lid is off. As soon as the unit was closed and in the rack again the cutoff frequency started to rise, so I suspected it has something to do with heat. When putting a fan behind the rack the cutoff frequency slowly dropped again, so it's definitely one of the regulators getting too hot.

I can't measure voltages with the G9 closed in the rack but I guess it's the 78S12. The heatsink already is pretty big, but maybe it has to be even bigger...

Is this some kind of behaviour one would expect when the heater voltage regulator is getting too hot?  ???

 

[gyraf]

yes, if it lowers voltage when safe-limiting to prevent overheat, this will happen. Try with a different brand 78S12?

 

[andow]

yes, if it lowers voltage when safe-limiting to prevent overheat, this will happen. Try with a different brand 78S12?

Thanks, I'll try that! I never thought the brand could make a difference...
I think I will also get a bigger heatsink anyway - just to be sure... 

 

[gyraf]

If heat sink is not over 80degrees C, you should rather look at thermal coupling between regulator and sink. Heat transfer paste and/or silicone-type isolation..

Jakob E.

 

[andow]

If heat sink is not over 80degrees C, you should rather look at thermal coupling between regulator and sink. Heat transfer paste and/or silicone-type isolation..

Jakob E.

I already have silicone isolation between the regulator and the heat sink and heat transfer paste between the heat sink and the case. BUT the heat sink is inside the case, which doesn't have too much ventilation....
So I guess I will try to mount the regulator directly to the case and then the heat sink on the outside of the case.

 

[gyraf]

..seems to be a good plan..

 

[bakahed]

Hey,
I'm getting a lot of hum on my G9 and I was wondering could it be because of faulty capacitors?
I almost blew 3 25v capacitors on the power supply because I connected the diode bridge the other way around. They were very hot and the tops are bulged now. Once I resoldered the diode bridge I get the correct voltage and everything is working but I'm getting a lot of hum.
My brother suggested that it could be the reason so I wanted to ask if I should order new ones.
Thanks,

baka

 

[dasnevestheo]

you should definitely replace the caps that got hot, it's not that expensive, a good PSU is very important. What transformers are you using ? Did you used shielded wires for the ins and outs ?

 

[bakahed]

you should definitely replace the caps that got hot, it's not that expensive, a good PSU is very important. What transformers are you using ? Did you used shielded wires for the ins and outs ?

alright, I will order new ones. I'm using two 2x115 primary 2x15 secondary transformers from don-audio. yes, I have a shielded wire for in and out on both channels. I also did the oscillation mod to get rid of oscillations.
I tried placing transformers in different areas like in the corner and on the back and the hum level is the same at both places so I'm not sure if they're the reason.

 

[dasnevestheo]

yeah your PSU looks messy.. maybe clean that up !

 

[gyraf]

..reverse-wiring the bridge may also have upset the 78S12 (IC2) and the 10u (c19) at its output(if it was the heater voltage bridge)..

Jakob E.

 

[bakahed]

..reverse-wiring the bridge may also have upset the 78S12 (IC2) and the 10u (c19) at its output(if it was the heater voltage bridge)..

Jakob E.

Oh yeah, it did... It actually burned it. With the smoke and a loud pop. C19 looks fine though.
I noticed something odd and maybe it is a clue. When I plug in through instrument input I don't get any hum whatsoever. Could it be that I grounded it improperly? I connected all of the ground pins on XLR's to the chassis and IEC ground to chassis as well.
Also when I don't have anything plugged into instrument and XLR inputs the hum is the worst. As soon as I plug in a microphone it gets better but like I said earlier, the hum disappears completely if I use intrument input instead.
It isn't really audible at all when I mix down but I do hear it on my headphones when it's quiet.

Was looking to build this preamp for years but couldn't afford it and now I finally got it. This is for sure a great sounding preamp.  Thanks Jakob.
Let me drop a sample
https://soundcloud.com/bakahed/blue-thunder

EDIT:
I soldered the xlr out ground tab to the xlr signal ground and it made a significant difference. Still some hum but only one channel 1. Channel 2 has no hum.

 

[served]

Hello.

I have built the G9.

1 BIG issue:

The GAIN switch is acting strange. On the last 2 positions it makes a loud POP and makes the sound lower and very dark.
I can also see a change in voltage measured on V1B cathode (R25) to ground. Its rising dramatically. From 80 to 120VDC.
Any suggestions? I cant seem to find the cause.

Also C7 has a bit too much voltage on it around 70VDC.

2 Issue:

Why is R11 and R12 differently layd out on the PCB and Schematics?
Is this okay like that?

Schematic: http://www.gyraf.dk/gy_pd/g9/g9_sch.gif

 

[gyraf]

pcb is known working

several times earlier in this thread it's mentioned that instability can occur at the highest gains - fixed by cutting two (4) traces and wiring the front-panel jacks directly in stead of through pcb+interconnect. Maybe this is your issue?

If not this, then check wiring and control pcb for shorts

Jakob E.

 

[served]

Hi.

Thanks!
I shortcircuited the inputjack on the main board to remove it from the formula.
Strange is that the sound goes lower on the Hi-Gain positions. And the Switch suddenly starts to POP.

 

[gyraf]

Did you cut the traces as recommended? Otherwise you'll still possibly have the oscillation possibility.

Jakob E.

 

[LolloTheMad]

Hello, i just finished building the first channel of my G9. I tested it and unfortunately something isn't working properly. There is a noticeable loss of low frequencies even if the HPF is switched off. I tested line in, mic in and instrument in and all have the same loss. I attach 3 spectrum to show you the problem. The first one is the signal entering at the Line Input (a white noise). The second one is the signal after C10 and the last is the signal at the line output. I tried measuring the signal before the output transformer and it has the exact same low frequencies loss. What's wrong with my G9? Thanks in advance for any help.

 

[gyraf]

Impossible to tell from this.

Wrong  capacitor?
Wrong tube?
Wrong voltages?
Wrong output transformer?
Wrong wireing?

Is low loss the same for both channels?

Jakob E.

 

[LolloTheMad]

Impossible to tell from this.

Wrong  capacitor?
Wrong tube?
Wrong voltages?
Wrong output transformer?
Wrong wireing?

Is low loss the same for both channels?

Jakob E.

I built mine with a custom pcb and the two channels are independently connected to the power supply. I finished only the first channel, as soon as i finish the second one i'll test it too.
I double checked all components and those seems ok. Tubes are EH ecc82, tried swapping with other ones and same result. About voltages, could you provide me some test points where i can compare voltages with a working unit to check for issues? I actually know only the power supply voltages and those are ok, 246V for the high voltage and 12,08V for the heaters.
Thanks, Lorenzo

 

[gyraf]

What output transformer?