Solution - Peavey ProBass 1000 Preamp and clipping/farting problem

B

blanchae

Aug 3, 2018
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29
4,531
#1
I picked up a Peavey ProBass 1000 preamp for $20, it had a bad power switch which I just soldered to the on position and used the On/Off switched output on my power amp to turn it on.

I ran into a problem where the preamp would clip and then fart whenever the peak indicator lit. I would have to play with the Pre and Post Gain at minimum settings and push the power amp level to compensate. I was almost on the verge of dumping it until I found out why.

I was taking the ProBass' Low Impedance output to the power amp and it was an impedance mismatch. I switched to the High Impedance output which matched the power amp's input and everything worked perfect. Now I'm back to loving the combination and sound.

I've always associated bass farting with overdriving a speaker. Now I know better as impedance mismatch can cause a similar problem.
 
#2
Actually, it has nothing to do with the impedance, don't know where you got this idea.

The XLR output on that preamp is not line level (designed to drive power amps) but is actually a DI output with a nominal -10dBv level that's designed to drive the input of a console. There's an audio step-down transformer on this output that's responsible for the level shift.

The 1/4" full range unbalanced output is in fact low impedance and line level (0dBv) and is the output that is specifically intended to drive the input of a power amp.

The information is in the manual too. As a general suggestion guys, (not specifically to you), reading the manuals can often help you understand your gear better and may help you avoid problems rather than needing to solve problems. Impedance and level are two very different things.
 
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#4
After I had posted, I was thinking that it was the voltage level. I was originally taking it out of the Full Range output which is unbalanced out at 1Vrms (0 dbV at 100 ohms) not the XLR output (-10 dbV at 600 ohms impedance) which is balanced. I switched to the unbalanced line out (-10 dbV at 1000 ohms) and that's what worked.

The real point that I was trying to make is that "farting" is not always a speaker problem as I've always assumed.
 
#5
blanchae said:
After I had posted, I was thinking that it was the voltage level. I was originally taking it out of the Full Range output which is unbalanced out at 1Vrms (0 dbV at 100 ohms) not the XLR output (-10 dbV at 600 ohms impedance) which is balanced. I switched to the unbalanced line out (-10 dbV at 1000 ohms) and that's what worked.

The real point that I was trying to make is that "farting" is not always a speaker problem as I've always assumed.
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agedhorse said:
The information is in the manual too. As a general suggestion guys, (not specifically to you), reading the manuals can often help you understand your gear better and may help you avoid problems rather than needing to solve problems. Impedance and level are two very different things.
Click to expand...

BassmanPaul said:
RTFM is always the best plan! :)
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RTFM, would be a safe bet. However, I'm going to give you the benefit of the doubt because none of the archived manuals are saved in the Queen's English. There must be a disgruntled administrative employee getting their jolly's right about now.

The Probass 1000 is based one the preamp functions of the MegaBass. One of the functions of that pre is the ability run a crossover to two speakers using a dual channel power amp. You would be able to dial in highs and lows to the set of cabs you planned on using. See the crossover trim pot on the front???? They relate to the high and low preamp output on the rear. If you plan on using it plain Jane (preamp into amp) I would go FULL RANGE OUT into your power amp or amps preamp input.

MEGABASS REAR:
upload_2019-11-29_17-0-21.png


PROBASS REAR:
upload_2019-11-29_17-3-42.png



The clipping your experiencing is more than likely related to what the crossover setting is on. If you decide to dump your ProBass, I'll gladly and graciously take it off your hands...
 
#9
blanchae said:
Here's the manual. As I stated, using the Full Range Out causes clipping, the Hi-Z out doesn't.
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You need to have this pre checked by a tech. High and Low outputs are designed for crossover use. You should be able to use full range and controll the output using the post gain controll.

For $20, it had more problems than a broken power switch.

Attached is the Alphabass manual in english. This pre uses the same full range, high and low output functions.

I'm not just the president - I'm a member.
20191117_182747.jpg
 

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#10
The Hi Z output is a lower level output for driving high sensitivity amp inputs and even the instrument inputs of bass amps. When trying to drive a lower sensitivity input, there is not enough signal level available and the output will clip before the amp reaches rated power.

The full range output is a higher level output for driving the input of a standard power amp. Using this output to drive the input of a bass amp may overdrive the input.

I think you are misunderstanding how the preamp works and why.

In general m, the power amp's sensitivity control should be set to maximum
 
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