So I'm dealing with unwanted distortion, it sounds like a classic fuzz if I play the open E or low B. I have the following setup: P Bass with DiMarzio Split P (their signal is very hot and the range is extended) into a Yamaha PB1 preamp (from what I've heard also a hot signal) with a flat EQ and QSC PLX 1804 poweramp, which goes into a Marshall DBS 7412. To avoid the unwanted distortion I put a Keeley Bassist compressor limiter before the preamp, but when I turn up the gain (the pedal has lots of gains on tap) to match up the gain loss through compression the fuzz unwanted distortion comes up again and no matter if I turn up the preamp gain or the poweramp it fuzzes out before it gets loud enough. I really think the cabinet is the problem. The poweramp shows no signs of clipping (it has a light) nor its internal compressor goes up, so the poweramp is not actually clipping. What do you fellas think it could be the problem?
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Unwanted distortion
- Thread starter Dmadmadma
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Sounds like too much gain. Less gain, more master.
Start with the simplest signal path and add links to the chain one at a time.
Thia includes any contour, compression or eq. Start with as close to bass>amp (with as "flat" as possible eq) >cab as possible. Are your pickups in need of a new battery? A low battery can present itself as a distorted sound. Make small changes towards your final setup, listening for the distortion at each change. If it is the cab, you might find just playing at a decent level makes the distortion present.
Gain staging is important here. Like Jimmy said above if you dont want overdrive; more master less gain.
Thia includes any contour, compression or eq. Start with as close to bass>amp (with as "flat" as possible eq) >cab as possible. Are your pickups in need of a new battery? A low battery can present itself as a distorted sound. Make small changes towards your final setup, listening for the distortion at each change. If it is the cab, you might find just playing at a decent level makes the distortion present.
Gain staging is important here. Like Jimmy said above if you dont want overdrive; more master less gain.
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the same happens, it fuzzes out again quickly... is speaker headroom a thing? the cabinet is supposed to push 800watt, so I don't know what is going on... I'm sure if use an suboctaver I would break it.
- Feb 12, 2006
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- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
There are a whole lot of possible causes... here are some likely ones:
1. That amp certainly has enough power ~900 watts RMS at 4 ohms) to damage that cabinet if you aren't careful.
2. Gain structure is a likely cause of what's happening, specifically on that preamp, you will want to use the XLR output into one XLR input of the power amp, turn the input level control for that channel fully up on the power amp, turn the level control for the XLY input to the 3:00 position and now use the input volume control on the preamp to set your volume. Do not use the pedal (as in eliminate it from the signal path) until you resolve the root cause.
1. That amp certainly has enough power ~900 watts RMS at 4 ohms) to damage that cabinet if you aren't careful.
2. Gain structure is a likely cause of what's happening, specifically on that preamp, you will want to use the XLR output into one XLR input of the power amp, turn the input level control for that channel fully up on the power amp, turn the level control for the XLY input to the 3:00 position and now use the input volume control on the preamp to set your volume. Do not use the pedal (as in eliminate it from the signal path) until you resolve the root cause.
What is the cab impedance, what is the QSC output for that impedance, and how is the QSC configured? Am I assuming correctly your cab is a 4x12?
Keep in mind as well that if you are setting things up (especially cascading pre amps) with much boosted low end, the driver's mechanical limits could be half or even less than half of the cabinet's electrical (watt) rating. Maybe you are just expecting too much beef at high volume from a cab meant for good midrange at volume. Marshall bass cabs arent particularly known for being low-low beasts. My limited experience (dbs era 1x15 of some kind) with their cabs has me thinking they're voiced more like old Peaveys, not Ampegs.
Keep in mind as well that if you are setting things up (especially cascading pre amps) with much boosted low end, the driver's mechanical limits could be half or even less than half of the cabinet's electrical (watt) rating. Maybe you are just expecting too much beef at high volume from a cab meant for good midrange at volume. Marshall bass cabs arent particularly known for being low-low beasts. My limited experience (dbs era 1x15 of some kind) with their cabs has me thinking they're voiced more like old Peaveys, not Ampegs.
1. I haven't turn it up so high. But how can I make sure that the cab is or isn't damaged?
2. I will try that! To be honest, I've never used the XLR output of the preamp to plug it into the power amp, but the normal jack plug with a normal patch cable.
The cab's impedance is 4Ohm and for that impedance the QSC can deliver up to 900, but of course I'm not pushing it that far. The cab is 4x12. What do you mean how is the QSC configured?
I am only using one preamp, I am not really boosting anything in particular, everything is flat. I might be expecting too much beef, you might be right, but the unwanted fuzz is happening at already low volumes, as in proper bedroom volume.
I think the amp has stereo, dual mono and as well bridging capability. Just wanted to be sure you hadn't been running, say, the amp configured to throw 1800 watts through the cab.
Are you attenuating the power amp too much? Those controls aren't really "volume" knobs. For the most part they're kind of set-and-forget limiting control to protect the cabs. Your volume should be dialed in by something upstream, and the power amp should be set kind of high if not wide open.
Are you attenuating the power amp too much? Those controls aren't really "volume" knobs. For the most part they're kind of set-and-forget limiting control to protect the cabs. Your volume should be dialed in by something upstream, and the power amp should be set kind of high if not wide open.
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oh right, it has dual mono without bridging capability, so it is 900watts pro channel each at 4ohm.
Oh perhaps is that, I do attenuate them quite a bit. shag.
- Feb 12, 2006
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- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
When you attenuate the power amp a lot and then drive it hard, either you run out of drive capability or you overdrive the input stage before the attenuator. The Yamaha preamp can swing roughly +26dBu out of the XLR output which is plenty to distort the input stage of the power amp if the attenuators are way down. There is a caution about this in some of the QSC manuals IIRC.
so regarding the knobs it says "Gain controls are recessed and 21-step detented meaning" and then it shows this. I think I'm not getting it right. Before I turn up the volume knob of the preamp where should the poweramp knobs be dialed? (ignore the lower knobs in the pic, I have the 1804, which is non-bridgeable)
The NOTE at the bottom is an important point. The gain controls adjust the sensitivity to input signals. An amps’s sensitivity specification is how much voltage at the input it takes to drive the power amp to full power.
Another important point is that the lower notes have the most energy in them. If the amp is being overdriven by the preamp, it would be these lower notes that distort first.
On the power amp, set the gain so that when you are playing, the clip light is not on. Use the preamp to set the volume level. You will have to find the right balance between the preamp level and the power amp level and overall volume. This is what they call gain staging.
Another important point is that the lower notes have the most energy in them. If the amp is being overdriven by the preamp, it would be these lower notes that distort first.
On the power amp, set the gain so that when you are playing, the clip light is not on. Use the preamp to set the volume level. You will have to find the right balance between the preamp level and the power amp level and overall volume. This is what they call gain staging.
- Feb 12, 2006
- 61,024
- 5
- 220,886
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Turn the power amp all the way up, that will be the rated sensitivity position for the power amp.
Once you test it out this way and solve your distortion problem, if there is excessive noise you can turn the power amp down a few clicks but if you turn it down too much, the distortion may return.
This is standard gain stage work, entirely normal. Note QSC’s comments about only using the top 14 clicks for normal operation, this is what I was warning you about earlier.
Once you test it out this way and solve your distortion problem, if there is excessive noise you can turn the power amp down a few clicks but if you turn it down too much, the distortion may return.
This is standard gain stage work, entirely normal. Note QSC’s comments about only using the top 14 clicks for normal operation, this is what I was warning you about earlier.
Have you tried a different speaker cab yet? I find a 15" is least likely to break up. I use a tuner pedal with built in buffer between my bass and amp for a better matching input. If you have another bass that doesn't break up maybe try lowering the pickup height.
I did it and it worked! Thank you so much! The fuzz is gone! I also tried it with and XLR output, which has it's own output knob on the back panel and with the volume knob on the front I can use them as pre and post gain (right?). Is it okay if I keep the XLR output knob at max and only use the volume knob to regulate it?
The only thing that is still bothering me is the cab, I don't know if it's broken or I'm expecting too much beef from it or it cannot handle the beef of (passiv) the extended range pickups. Compression is not really working for me because if I turn up the gain of the compressort to compensate the same happens again but with a tone I don't really care for. Or if I EQ the lows down it kind of starts to weaken out. Without anyf these measures everything rattles in the cab and the room as well. Perhaps I need a new cab.
I have not but currently the other cab I have access to is a 410. My other bass has a less hot but still hot output, will try it though, lowering the pickups might be an idea as well but I think at this point the main issue is the cab being overwhelmed by low notes.
- Feb 12, 2006
- 61,024
- 5
- 220,886
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Yes, you can use the XLR out with the knob fully up. This configuration exactly mimics the 1/4" output but has 6db greater drive capability.
The amp has a lot of power for that particular 412, the mechanical limitations are very real. I would expect problems like what you are experiencing at around 500-600 watts.
I have posted a lot of information over the years about what happens when you exceed the real world power handling of a speaker, and how speaker ratings are often confusing and misleading (both intentionally and unintentionally). You might search for some of the past posts where this has been discussed in detail.
Thank so much for your feedback! In your opinion, what should I look into in a cabinet if I don't want to deal with this sort issues again?
