Early model Behringer BX4500H noisy; manufacturer issue

[cambie]

I'm a technician, not a bass player. I post this only in the spirit of trying to help those who do play, and may need the help.

The noise-floor issue with (presumably early) versions of the BX4500H is detailed at thread 664518 "FS: Behringer BX4500H, 450W amp" from January 2008, and it's only from there that I was able to fix the unit I worked on. Most posts relating to it date from 2006-2007 it seems. So I'm guessing that the problem and remedy is already well known, and has long since been addressed in manufacture, and my own input is thus redundant actually!

Still, the issue was a real headache for the owner of the unit. And he's in the "here and now". He ran the amp with volume at max, and during breaks between sets, he preferred to leave it that way and just turn the volume knob on the guitar itself to minimum. It's in this state, or between songs, that the buzz noise was VERY apparent, unusually loud and clearly objectionable.

Unplugging the bass at the amp input itslf, left the amp totally silent. Unplugging at the guitar end, left the noise still there. This aspect initially seemed odd.

The amp was some years old, but had gravitated to being locked in a cupboard, basically as an outcome of this issue. It wasn't right to sell it like that and give someone alse the headache, and he wasn't happy to continue using it either.

So I figure a few units with this manufacturing defect might still be out there.

The owner certainly wasn't aware that this was a known manufacturing defect. Is there a Behringer technical bulletin about it out there? I don't know. Behringer don't seem to publish technical stuff much.

I never found a schematic for model BX4500H, but there is one for a BX1200 that's pretty easy to find with Google. The input and small-signal stages seem pretty close to the BX4500H.

As the above-mention post leads to, there is a hum loop between passive/active inputs ground, and the ground for the footswitch socket. These three sockets mount on the one small daughter-PCB. The former two are insulated type sockets, and the later is not. The former two ground to the main small-signal PCB by a connector wire. The footswitch socket (also) grounds via it's sleeve. And thus the ground loop is formed.

Unusually, the passive/active input sockets, which are TRS (tip-ring-sleeve) sockets, not just TS, use the switch connected to the ring terminal. This switch controls the "Enable" pin of a 4053 analogue switch IC. This analogue switch disconnects signal from passing to the power amp when nothing is plugged in to either passive or active input. Hence the "all quiet" with no plug plugged in, but noise when anything was, even with input signal seemingly "grounded" by turning the volume right down on the guitar.

The previous thread information suggested that cutting the ground wire leading from the daughter-PCB, and letting the inputs ground through the footswitch socket sleeve and the chassis, was the solution that Behringer has applied as service counter-measure, and presumably then in production. Although quick and easy to do, this seems a poor solution to me. I guess it likely does basically work, but would be likely to still give hum and noise, due to the still very roundabout input ground signal path.

The previous thread also then indicated an alternate; to replace the footswitch socket with an insulated type. This would be a much better solution.

The TRS socket has an unusually small diameter, and none of my stock of generic sockets would fit through the hole in the front panel. Enlarging the hole risks damaging the front panel, and "butchery" is generally to be avoided anyway. I don't have a ready source of Behringer parts, nor access to the Peavy part that apparently also would fit and work.

I had success with removing the footswitch TRS socket from the daughter PCB, bending the ground (sleeve) pin out of the way, so it no longer connected to the daughter-PCB, and refitting the socket (now with the ground (sleeve) pin disconnected).

The Behringer sockets are a different construction to the generic types I'm more familiar with. One corner is angled. The ground terminal is the one on this angled/flanged face.

The daughter-PCB is double-sided board, so unsoldering components is more difficult than for single-sided PCB's. Removing all solder (solder pump and/or desoldering braid) can still work, but soldering all terminals and heating all at once can sometimes work better. I found, in this case, that desoldering all five terminals very fully and carefully, and then carefully teasing out each of them while applying heat to each in succession, eventually got there. Much care and patience needed, as the solder track is easily damaged!

Anyhow, the result is a very obvious and quite impresive reduction in the noise floor from this amp, which otherwise I thought was really well designed and made. 450W into 4 ohms seemed perhaps a little over-stated, but Behringer tend to do that. Layout and cooling system looked pretty good, I thought.

And I thought the 51134 IC based frequency-halving to get the "Ultra bass" was really clever - I've not seen that before and it seemed to work quite well, and mixed back into the original signal gives a "chorus" like effect wich I thought quite nice tonally.

Anyhow, I hope the above helps someone, sometime, and of course my thanks to those who did the hard yards and worked out the actual cause in the first place!

Cheers,
Paul