Genz Benz M Line 200 - Clipping after extended use

spforster · #1 08-23-2011, 05:05 PM

I've got a client's amp here that has a problem after extended use. According to the client, after about 1-1.5 hours of reasonably spirited playing (at moderate volume levels), the clipping light starts to light more and more as he plays.

I'll be trying to replicate the problem with a signal gen and a dummy load, but it would really help me out to have a schematic of this amp. Anyone have a copy they can pass along? Thanks in advance!

agedhorse · #2 08-23-2011, 05:43 PM

You sure he's not playing louder and turning up as he plays longer? Common scenario.

When testing, be sure not to use a ground referenced scope or load in bridge mode.

First thing is to verify that this symptom actually happens with a known static signal (one that does not turn up as the night goes on). Once you can verify the symptom as really happening, I will walk you through things. The clip LED is a simple threshold meter referenced to 2 seperate points (pre and post eq) so if he brings up say the LF eq, that will increase the level that the LED meter sees. This is to guard against accidental overloading of the eq stage.

billfitzmaurice · #3 08-23-2011, 05:54 PM

Quote:

Originally Posted by agedhorse

You sure he's not playing louder and turning up as he plays longer? Common scenario.

+1. Ears become desensitized with long periods of high volume, and players will turn up to compensate. Speakers become less efficient with long term heat build up, also prompting the player to turn up.

spforster · #4 08-25-2011, 09:01 AM

He swears up and down that he sets his gain and volume at the beginning of the set and doesn't touch the amp the whole time and that he always plays with his volume all the way up and tone controls in the same position on the guitar. Of course I can't say I know this is the case for sure- I wasn't there, and I understand what you mean about ear fatigue, etc. But, he does use the amp strictly as a personal stage monitor, so he can watch the front panel the whole time.

In any case, yesterday I ran a 1khz signal through it with the gain set so the clip light was just barely beginning to glow. Load was on amp a (the channel he uses), not bridged, 8 ohm load with volume set so it's pushing around 50W. I let it run like that for about 2.5 hours. I had he scope on it the whole time and got nothing less than a perfect sine wave on the output the whole time with no change in the clip light.

Now some of the differences of my test are that the chassis was on my bench, not in the cabinet (possibly less heat build up around the preamp board), and that it is possible that he pushes the amp a little harder than 1khz@50W does. I could try a pink noise signal next and get a higher rated load plate so I can up the output, also put it back into the cabinet.

agedhorse · #5 08-25-2011, 09:50 PM

There's no difference in heat build-up. That's barking up the wrong tree.

Try playing some pink noise, into an 8 ohm load and set it so that the peak LED just barely flashes and you are not clipping the output. Look at it in 3 hours and see if the LED is just barely flashing and you have ~ the same output level.

You can also do the same playing a bass loop and using an SPL meter. Just use common sense when driving the speaker very hard this way.

spforster · #6 08-26-2011, 09:08 AM

Yesterday I tested the amp as described- pink noise at just below clipping (light was just glowing), into 4 ohm resistive load (I upped the power handling on my load plate to 100w, was running at roughly 80w) for 3 hours. No change in clip light or output level.

I'm starting to think we may need to explore possible issues with his bass guitar, which has an onboard preamp. It seems unlikely to me that output level from an active bass would go up over time, or that eq would drift, but I suppose it's possible.

agedhorse · #7 09-04-2011, 01:14 PM

Let us know what additional testing discovers.

cheapbasslovin · #8 09-06-2011, 02:21 AM

Same thing happened to me with my ML200, but it could be directly traced to me thwacking my strings harder to keep up with the rest of the band. Didn't change my settings or anything, but layed into the bass like the strings needed a good spanking for their bad behavior.

Until the clipping started, then I told the band to back off with me.