DIY fix for 800RB "POP" at shutdown and startup?

sunbeast · #1 10-21-2010, 12:34 AM

I'm in the process of replacing the filter capacitors in my new-to-me old 800RB, and was wondering if there exists a fairly simple modification that could bleed the caps to ground at shutdown like most amps these days seem to, rather than sending all that energy to the speakers everytime I turn the amp on or off? It is especially annoying that I can't run the amp speakerless (for recording) without worrying about that "pop" once a speaker is connected again (even with the amp shutdown, I can still get a "POP" if I had powered it up without a speaker connected and then connected the speaker later)...

I've never before owned an amp with a design that didn't eliminate this issue before, so I'm assuming the solution is fairly cut and dry. Anyone performed this kind of modification before and want to point me in the right direction?

Thanks,
Karl

fdeck · #2 10-21-2010, 06:24 AM

Is there DC on the output?

BassmanPaul · #3 10-21-2010, 08:45 AM

The 800RB is an old design and it pops as the voltage rails charge. You could add a speaker protection circuit to the amp but you'll need a place for it and that amp's pretty full as it is. While I've never liked the pops, they are supposed to be fine for the speakers. I suggest you just live with the amp.

sunbeast · #4 10-21-2010, 12:07 PM

Quote:

Originally Posted by BassmanPaul

The 800RB is an old design and it pops as the voltage rails charge. You could add a speaker protection circuit to the amp but you'll need a place for it and that amp's pretty full as it is. While I've never liked the pops, they are supposed to be fine for the speakers. I suggest you just live with the amp.

What causes the "pop" when you turn the amp off though? With the top off, if I run the amp without a speaker load, then turn it off and plug a speaker in, it will still send this built up charge to the speakers- and in this case create a small spark at the speaker terminal as I'm plugging it in. I have tried two other 800RBs at the local used shop that both also have the pop when turning on and off.

In a good design, what change eliminates this problem? Is that the purpose for the silent "warm up" period at powerup, as in the Mesa solid-state amps I've had?

I can live with it, but I figured if there were a circuit that were somewhat easy to implement, then I might as well try it out while I've got it apart! Sounds like it may be more involved than I had guessed- does it even have anything to do with the caps sending their built up charge through the rails, or am I totally off?

Thanks,
Karl

**BLDavis** · #5 10-21-2010, 12:52 PM

You should always turn your master volumes down before turning the power on or off with these amps. You should hear a little "poof" sound through the speakers, but no loud pops.
I've played 800RB's since about '86 and the "poof" has never caused any problems with the four different amps or dozens of speaker cabs that I have owned.
As far as a "cure" goes, try giving RAG or Brian at GK a shout and see what they can tell you. I'm curious as well.
Later, B.

BassmanPaul · #6 10-21-2010, 03:03 PM

On power on the pop is caused by the output stage stabilizing as the power rails charge. On power down the same thing happens as the voltages drains and the output stage destabilizes.

A simple test you can do is to measure the voltage present across you speaker cable with a multimeter on its DC volts setting. With the amp on, there should be very little voltage. That's the DC offset and should be close to zero volts.

sunbeast · #7 10-21-2010, 03:43 PM

I measured around 14 mV...Is this a normal measurement?

Thanks,
Karl

BassmanPaul · #8 10-21-2010, 04:16 PM

That should be fine.

will33 · #9 10-21-2010, 04:47 PM

I've often wondered the same thing with my SM400S. Pop during on and off when set in stereo mode but when set to bridge mode it's silent both when turning it on and off. Let's keep it on topic as far as an 800rb fix goes but I wonder if this could be some sort of "general" fix in different amps, ie: find the right caps and mod it.

FWIW, I've owned an 800rb before and haven't had this problem with them although I'm not sure if they've had the exact same circuitry since the beginning. I do know the newer ones sound just like the old ones. There was a change somewhere along the line when the power switch went from horizontal to vertical or something like that.

sunbeast · #10 10-21-2010, 06:39 PM

I've been fortunate to have a pair of 800RBs in a used shop by the bakery where I work, and so the employees have been cool about letting me compare. One of their amps is slightly older than mine, and one is about 10 years newer. All three amps have the "Pop" issue, though it is admittedly less apparent through the smaller cabs I used to test there vs. my 6x10 at home- through my 610, it is pretty dramatic. I have heard this issue in most if not all of the solid state amps from the 70s that I've played through- Sunn, Ampeg, Acoustic, etc, but somehow wrongly assumed it would have been dealt with by the late 80s era of 800RBs (and as I pointed out- it is STILL an issue on the 1999 800RB at the shop here). I've certainly never played through another amp from the late nineties or newer that did this.

I still love this amp- it does what it does really well, there just appear to be some sacrifices taken to keep it at "a working bassist's pricepoint", and I'm kinda surprised that they wouldn't at least upgrade something as seemingly basic as this on the newer models.

Another question- I just replaced the main power supply filter caps for the 300 watt amp in mine to get rid of a very loud idling hum. While it is certainly less than before, there is still a pretty noticeable hum through the speakers with all volumes on zero. I assumed that a hum that happened with all volumes down would have to be the main larger filter caps that feed the 300 watt power amp, but could the smaller filter caps that supply the rails to the preamp and the 100 watt amp also let through hum even with the volumes down? I should have just counted on replacing them all at once, but I was too antsy to get the issue taken care of!

Thanks,
Karl

will33 · #11 10-21-2010, 06:47 PM

Quote:

Originally Posted by sunbeast

somehow wrongly assumed it would have been dealt with by the late 80s era of 800RBs (and as I pointed out- it is STILL an issue on the 1999 800RB at the shop here). I've certainly never played through another amp from the late nineties or newer that did this.

I'd be curious to hear what info comes from those more "in the know" than us here. Maybe I had a freak quiet 800? As far as it being more prevalent through your 610 as compared to small cabs I would think that it's just because the 610 is just plain a bigger, louder cabinet so in addition to making louder notes it would also make louder pops. I always like using the 800rb in a studio recording/demo recording or whatever situation because IME they are dead quiet. No cooling fan but they don't overheat, have a great sound and the one I had was dead quiet whenever it was on, no hum, etc.

WingKL · #12 10-21-2010, 07:10 PM

Can you tell if it is a 60 hz hum or a 120 hum? If it is a 60 hz hum, it could be the power transformer inducing currents into the preamp circuit. You can try the humbucking wire trick that Thunderfunk uses but I don't know exactly how that would be implemented in a 800rb. The basic idea is to run a wire across the power transformer with one end to ground and another to an input point like perhaps the effects return and if the induced hum voltage and what comes from the wire are out of phase the hum gets cancelled. You can experiment with it by moving the wire around on the transformer and gluing it in place when you've found the point when the hum cancels out.

As for the pop, try a snubber across the power switch and see if it helps.

sunbeast · #13 10-21-2010, 10:09 PM

Thanks- I believe I am hearing mostly 120 HZ (though it seems possible that there is a 60 cycle base beneath it too), and it will audibly oscillate with B notes at low volumes or on the decay of the note. And if the transformer was inducing currents into the preamp, wouldn't this only be an issue with the preamp turned up vs. all the way down (I also plugged in to the effects return to bypass the pre and got the same hum).

That snubber idea looks interesting- does it basically slow the rise of voltage at startup? I kinda lost it at some point in that description!

Another question- In replacing the filter caps, I came across a loose/oxidized connection on one of the two large transisitors that attach to the heatsink plate on the power supply board. Jiggling this connection with a probe induced a fair amount of noise WITH the preamp volumes turned up- could an issue with this transistor or the pair possibly be related to my hum? How would I test the function of this transistor?

I plan to resolder my filter caps this weekend just to rule them out, and may attempt another set of them at some point if there aren't any other suggestions. The only place I could find decent axial caps with such large values was a somewhat odd Ebay seller, so I may have been sent caps that had been sitting around for some time- or maybe even been used before for all I know! The axial versions must be more-or-less obsolete in the larger values, as most online stores carry only the cheapest chinese caps and have all the rest on backorder.

Thanks for all the help so far!
Karl

pgk · #14 10-22-2010, 04:25 AM

turn the master all the way down when powering up/down

sunbeast · #15 10-22-2010, 05:33 AM

Quote:

Originally Posted by pgk

turn the master all the way down when powering up/down

The "POP" happens regardless of where the Master is set. The Master Volume just controls how much signal from the preamp is fed to the power amp, so any disturbances from the power amp power are heard regardless of where the Master is set.

WingKL · #16 10-22-2010, 09:26 AM

If the push pull operation of the amp is not balanced it could very well cause hum as the power supply ripple rejection is reduced. That loose connection could be the issue. Reflow it if it's a solder joint or use contact cleaner/deoxit if it's a connector. It may be time to step up and invest in an oscilloscope, a signal generator (software ones work too) and crack open that GK 800RB service manual and schematic. If you send in a sine wave, the output should look like a sine wave if the amp is working right.

You can test your caps with a capacitor tester to make sure that they're good.