Peavey xr600b clipping

[Kip H]

I just found this forum and it is a wealth of knowledge! We are getting the band back together after a short pause to raise families, careers you know the drill...... About 25 years later. Anyway I bought a xr600b with sb3 speakers. I don't know if i have a problem or am just expecting too much from this system. I tried running a portable mp3 player into it and couldn't get very much volume. No louder than your TV(normal volume). Peavey told me it wasn't designed to imput this player and I was overdriving it. But even with a mic I can't turn it up any louder than mic level 5 and volume 3 or vice versa without it clipping the sound. I used to be a electronics tech in the military so where do I start to look to see if it's working properly. Thanks for any help. Kip

[Enzo]

You can contact the parts department at Peavey and ask for the schematics. The XR600B is the mixer, and I think they will include hte power amp too, but it is best to ask for it separate. Is the power amp - the rear panel - a 400BH maybe? Or a 400B/G? Should say on the rear panel, if not, it says on the main power amp board on the foil side.

Peavey told me it wasn't designed to imput this player and I was overdriving it.

Baloney. if you overdrive the thing, it distorts, it doesn't get all quiet.

First check the patch jacks - the row of jacks lower right. Plug a cord into each one at a time, and on the free end of the cord, measure for DC voltage. There should be none. But all those jacks are protected by clamping diodes, and if one shorts, 15v wil appear there.

By the way, after doing this test, does it now work? Because the next test is this: plug a cord into the MAIN out jack, and the other end into the POWER AMP IN jack. They are amaong those patch jacks. This bypasses not only the graphic, but also the cutout copntacts on the power amp in jack. if the sound is restored with this connection, then the jacks need service, specifically either the power amp in jack or the graphic in jack.

No help?

Isolate the problem. Preamp or power amp. Plug a line level signal into the power amp in jack - it connects straight into the power amp, completely bypasses the mixer. If it is still weak, then the power amp has isues. If it is strong, then the mixer has issues.

If the powr amp - PA - is OK, then plug that signal into the graphic in jack. Now it runs through the graphic on the way to the PA. If it is OK, then the mixer has issues, but the graphic is OK.

If there are power supply issues, then the graphic will be affected same as the mixer.


The power amp will operate alone, so if it is bad, we can apply signal right to it and work that way. the mixer though needs the power map, since all operating voltages come from the power amp. At this point make sure both power rails are present in thepower amp, and also the low voltage rails - +/-15VDC. Those need to make it to the front panel for the mixer.

If the power amp is bad, tell us and we will work through it. One thing, there is a small 8 leg IC on the PA. Only DIP8 on there. It is the limiter. REmove it and see if sound is restored.

If the mixer is bad, same, but for now, if all the input channels are affected th same way, then likely they are OK, and the master section is bad. Make sure to try all channels, and be aware the XLR inputs go through an extra stage with the 1/4" jack being a cutout type.

[Kip H]

Wow you ask a question and you better be able to handle the answer! OK I checked the front inputs and didn't have any voltage..good. It's been awhile so work with me. I'm not sure if I understand how to properly mix a signal. If the input has a level adjustment where do I set that and how does that relate to the main (volume?) level.the volume control on the mp3 effects this too. Also when you state a line level signal do I have to use one from a signal generator (which I don't have) or can I use the portable mp3 player? When I use the power amp input, with the mp3, with the player volume around midpoint it clips badly. When I use the same setup going into the 1/4 plug at the mic input I can only get mid level (and not that loud) using both the mic level and main level adjustment. If one is set at 1 then the other can't go above 4 without clipping. I didn't get a chance to look inside the unit at the IC yet. thanks

[Steve Conner]

Does the "Compression" light on the front panel come on? If so, the power amp is getting driven hard enough to limit, and it should be deafeningly loud.

Try unplugging all the speakers, and then try each speaker and cable individually. There might be a short in a speaker cable somewhere.

If the compression light does NOT come on, then the signal is getting lost before the power amp. You probably just forgot to turn up some level knob somewhere. Or maybe the contacts in the power amp input jack are dirty like Enzo mentioned. Try poking a jackplug in and out a couple of times.

A third possibility is that you have a blown speaker, like one of the woofer cones is ripped or a voice coil is scraping somewhere. This results in really awful sounding distortion. Again, trying the speakers individually will let you figure this out. If they're both blown, it may be time to have words with the guy you bought it from!

If all else fails, the innards of the XR600B might be bad somewhere. Enzo has mentioned that the filter capacitors in the power amps on those things sometimes go bad or come loose. I guess that would cause it to light its compression light and clip badly, but probably hum a lot too.

[52 Bill]

Think of the Main volume control as a Master volume, start with it about half way up. Then keep the individual channel volumes down. If you set the channel volume up too high with the Main volume down it will cause distortion as the pre-amp will overdrive the mixer stage.

[Kip H]

The compression light does come on. I know that the speaker wires are good as they are new. I think the problem is in the power amp. I haven't looked at the speaker cones but I'll check that out. But even routing the moniter out to a guitar speaker still doesn't provide any volume. The system was bought from a guy that got it from his brother when he passed away and didn't know anything about it..the price was right..CHEAP. I think that it sat unused for years. Thanks for the info on the volume settings. With the mic input volume set to one I can't turn the main up past 4 maybe 5 without clipping and even then it's not very loud.

[Enzo]

Some thoughts.

New is not the same thing as good. New things can fail, so never assume, check the cords.

The monitor out is a line level signal;, it will NOT drive a speaker. it is a line level to be sent to an external amplifier for monitors so they may be independently controlled.

A line level signal just means something like a tape deck, CD player, whatever. Nothing cosmic. I don't know what outputs your mp3 pplayer has. If you are using headphones output, then it might indeed overdrive the preamp, but that would still not result in low power output. It would be loud but real distorted.

If the compression light comes on, you could be overdriving the power amp, but it should then be VERY loud - full power in fact. The compressor itself could be faulty.

Plug a signal source into that powr amp in jack - lower right corner. If the power amp is OK, it should come out pretty loud.

[Kip H]

Only DIP8 on there. It is the limiter. REmove it and see if sound is restored.

I finally got a chance to really go over the steps you guys had me do,with no real luck. I just couldn't get any real volume out of it. So I decided to look inside and see if i could see anything obvious...nothing. Then I remembered the note about the DIP8. I removed it and WOW the volume tripled at least. So is this the answer replace it..... and where do I get one?
RCA 519 708778 on the chip. The schedmatic shows it as a 87478 U2.
Thanks for all your great help.
KIP

[Enzo]

The chip is PV part number 70487478. Generically it is a CA3094.

I believe in some applications an LM3080 would work, but I'd have to look at the schematics first.

This part is a limiter, it prevents clipping. If you simply leave it out, the amp will run fine, you just will no longer have a brute force anti-clip.

[52 Bill]

The CA3094 is a LM3080 with an added darlington transistor pair. Electro Harmonix used this chip alot in their FX boxes. They have been out of production for a while now, but I believe that Steve at Small Bear Electronics still has some in stock.

Hope this helps.

[Steve Conner]

The Peavey DDT circuit will activate the limiter if it senses any clipping in the power amp. There are a whole lot of different faults that could make the power amp clip prematurely and activate the DDT. Taking the limiter chip out will disable it, but it doesn't necessarily cure the actual problem that made the power amp clip prematurely in the first place.

[Enzo]

I don't think the amp was clipping prematurely, I think his chip itself failed. With the chip removed, the amp works fine, I think he reported. That would not be an uncommon failure.

Here is the test for that. Put the 478 IC back in the socket. On the power amp, right next to the 14 pin TL074, there is a four pin Molex connector. The cable connected there trails off to the front panel. Unplug this cable from the connector. This disables the limiter circuit, but leaves the 478 in the circuit. If the 478 is bad, it will still load the signal way down. If the signal wakes back up, the 478 was doing its job and something else is broke.

And even in that case, I would suspect a bad TL074 before I'd suspect the power amp itself. Assuming the PA sounds good without the IC.

[Kip H]

The amp seems to work fine with the chip out. With the volume almost all the way up and the mic input low, the sound will clip as I increase the input level. I guess this is how it's suppose to work. (but I'm just begining to figure out how a amp works) I called Peavey with their part number. It cross referenced to another number and they are sending me one. I'll try Enzo's test before putting the new chip in and see what happens.

[Enzo]

On that four wire connector, two of the pins are for the clip LED on the front panel, and the other two complete the circuit to the limiter. On some model amps that use that power amp board, there is a switch on the front panel that connects those two pins together or not. It turns the limiter off or on. On other amps without the switch, there is just a jumper wire between the two pins on the connector, leaving the limiter on all the time. So unplugging it, opens the circuit, and disconnects the LED.

Measure the power rails on the power amp. 45- 50 volts, whatever they are. Now bring a sine wave up into the amp and watch the output. The wave should clip at about a volt or so shy of the rail voltage. SO if there are 50v rails, then clipping should occur at peak voltage of about 49. Watch on your scope and see.

[Kip H]

I got the replacement IC from Peavey today. They crossed it to another number (I don't have it with me...I'm at work) I put the chip in and turned the amp on with all inputs and volume at zero. As soon as I turned it on, there is a loud humming noise coming out of the speakers. I don't have the equipment to generate or look for the output signals. So I'm guessing there is something wrong with that limiting circuit. The amp works for my needs without that limiter so I guess I'll just leave it out.