The Steel Guitar Forum

I don't know if this is a common problem or not, but I'm posting this on the chance someone else is experiencing the same problem.
Some time ago I purchased my first amplifier, a Session 500, Mk. IV. I like the amplifier, but about ten seconds after switching it off, a very loud POP came from the speaker. Over time, the POP seemed to become worse, to the point where I was concerned that the speaker itself might be damaged. This is symptomatic of instability in feedback amplifiers that sometimes occurs if things are not carefully controlled in the design for the entire range of supply voltages.
The problem persisted when I isolated the output amplifier by inserting a plug in the "pwr. amp in" jack. I studied the circuit (obtainable from Peavey) of the 400BH amplifier module and noticed the designers had selected bleeder resistors to discharge the +52 volt supply faster than the -52 volt supply, and reasoned that was done to avoid instability during power-down. When I added another resistor across the positive supply bleeder, the POP would not occur. I checked the filter capacitors and they were both within specification.
Not wanting to decrease the positive bleeder resistance permanently, I looked further, and found that by replacing the quad op-amp chip (TL074) on the 400BH module board, the POP was eliminated. The original chip was made by TI, while the replacement was made by Motorola and obtained from a local ACTIVE Electronics store. Aside from the POP, the old chip worked perfectly. I have been using the amplifier for some time since this substitution, and it is working fine. Bear in mind that this is a sample of one, and the same substitution might not work is another instance of the same symptom.
-Dave Kocher

Thanks for explaining your experiences with the Peavey smp. I'm sure that this info helped a lot of field technicians.

Interesting Dave. It is a common practice to use bleeder resistors to get the charge off of capacitors. I do this with my infrared pedal, where there won't be a pop when a person plugs into my pedal. Sounds to me like increasing the resistor on the plus 52 side would bleed off the power slower. I'll tell you something else; It is difficult to keep the negative power supply exactly equal to the positive supply. It may be that when the amp was new, the supplies were closer to being the same value. Since values on parts change over time, something may have changed enough to alter the value of the plus or minus supply slightly. When this happened, the resistor value would naturally change in relation to the change in voltage.
Session 500 amps have been out of production for years. Like old T.V. sets, I suppose nothing lasts forever, so I don't see how Peavey would be to blame for this. It appears to me that the real danger is the speaker getting DC. DC can burn up a speaker real quick. It is probably a small enough surge of DC to not burn up the speaker.

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<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Keith Hilton on 25 April 2000 at 10:04 PM.]</p></FONT>

Keith: As my post says, I placed another resistor (1.8K ohms) across the positive bleeder (1K), thus decreasing the total resistance, and discharging the positive supply faster. I determined the relative rates of discharge by connecting two 100K resistors in series across the + and - supplies, and measuring the voltage of the midpoint to ground during discharge. This went a few volts positive before I changed the bleeder value, and the POP occurred at about 25 volts supply voltage. There never was appreciable DC on the output. All the test voltages in the amp agreed with the values shown in the circuit diagram.
I think the message is that if a unit with a 400BH output module has this problem, and all the voltages check out ok, try changing the chip. It probably is related to the direction the op-amp outputs go as the supply voltages decay, and that may vary between op-amp manufacturers, and maybe even production runs.
-Dave Kocher

Dave when you say, "You placed a resistor across", does "ACROSS" mean it was hooked parallel, or in series? That was my question from the start, even though I did not mention it. Since you say you reduced the resistance value, I figured you had to be hooking them up parallel, because that's the only way I know to lower resistance, when a 2nd resistor is added.
I think you have brought up a good solution to this particular problem. Keep up the good work!

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Yes, poor terminology. In my last post, it should be "connecting two 100K resistors in series between the +52 and -52 volt supplies"
-Dave Kocher

Sounds to me like changing the 5k uf in the 52V supply and the 1k uf in the 15v would be what's called for. If the caps in the 52v supply are NOT the newer 'doorknob' style caps, change them first.

I had a S500 doing the same thing. I changed the 52v caps first, still had the problem, then put 5K uf 63 volt caps in the 15v supply, and problem gone. As a bonus, the amp is quieter than it's ever been.