Behringer GDI-21 mods?

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Behringer GDI-21 mods?

Postby screamersusa » 28 Apr 2009, 02:47

Has anyone modded the GDI-21?
I think I'd like to add a mid control to it, True bypass, and fix the darn squealies the thing makes with EMG pickups.

I'm also considering using them as building blocks to build a 4 channel preamp.
I have a Tech 21 tri-ac that I love, and the Behringer GMX amp as a backup.
The gmx looks like two GDI-21 with a different input configuration on the jack.
The GDI just sounds like crap against the other two.
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Re: Behringer GDI-21 mods?

Postby BAMF » 03 Dec 2009, 10:25

I've been able to convert my first GDI21 to True Bypass.

It involved installing two new jacks (new output and input), tapping a 3PDT those to the existing jacks, then plugging the old input jack with a stub PL plug to turn it on.

I wrecked it in use so I don't have it but if I remember right, I wired the original switch to a small pushbutton. Just in case the pedal doesn't turn on on its own. Then when engaged that's the time I can switch it in and out of my signal path using the 3PDT switch.

Yeah that was really inelegant. I should have at least tried shorting out its switching FETs instead.
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Re: Behringer GDI-21 mods?

Postby fransvanhelvoort » 02 Mar 2017, 11:19

I just bought the Behringer and dit some mods in it. First of all the stock behringer gdi21 sounds like shit, But the tech21 sound like shit too. The behringer is a copy of the tech21 sansamp! They differ in ic's but that is not essential for the bad sounds. The badness in the sound are the high frequencies in the output signal. Look on YT and you will hear several demo's with this terrible earbleeding sounds. And that is exactly the sound of these devices The most demo's are heavy filtered! The same for the tech21 OD or the AC!
So my Behr had that earpiercing high too and I did filter that on the second drive opamp. C30 (Sansamp C14) is 220pF and I changed that to 680 pF. That killed the high frequenties. Then the next mod was the Marshall british sound. Way to much mid/highrange imo. Solved that by adding another 2n2F cap in parallel with C36 (C25 on sansamp). That makes for me the british sound very usefull. Even with single coils instand ZZtop! The clean mode is like a ballsy JTM45 and the other modes it sounds like a JCM800. Very nice. Then there is a bad volume jump from Fender mode to Marschall/MesaBoogie mode. to solve that I removed R13 (R28 sansamp gt2) and placed a jumper. That makes the volume jump less. I did an AB test with my strat and Fender frontman amp between a full Fender Super Reverb tube preamp (with high voltage) en de modded Behringer. Who needs tube's?? This little cheap Behr beats the real tube pre-amp!!! So the Behr is going to replace my tube preamp and my Vemuram jan ray. The only thing left is my modded Svex Superhardon.
The Cal. (MesaBoogie) mode is very useful now for the high saturated tube sound. When you lower the volume on your guitar it is going to sound like Fender. These boxes are very sensitive for playing with your guitar volume pot and the tone pots on it. So you have to tweak a bit to get the wanted sound. But it is in this bad boy!
Screamersusa wanted to build a mid control but this tone stack is a Baxandall type. When you put the low and high pot on about 9 o clock, you will have a mid sound. That is the advance of a Baxandall tonestack.
I like the Behr so much that I ordered a PCB of the tech sansamp at Smallbear electronics. That is a pcb for normal (no smd) parts and I can put some other IC's (BurrBrown Opa's) in it an listen if there is a difference.
Greetings from Holland and Rock'on!
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Re: Behringer GDI-21 mods?

Postby fransvanhelvoort » 11 Mar 2017, 15:10

Hi there, after a few days of playing around with my BehrVtGDI21 I realize I was not quite satisfied. So I opened the box and turn my previous mods back to the original schematic. I missed the sparkling high freq of the Fender sounds, I missed the turbo gain in the Marshall and Mesa sounds. So I opened the box and turn my previous mods back to the original schematic. The next stap was looking at the low pass filter in the schematic. By changing C39 into 2n2F the earpiercing high frequency's where gone. The second stap was changing the C36 into 4n7F.
This 2 mods are very effective and very useful for this fantastic tonemonster
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