1987 Polytone Minibrute IV speaker pops on shutoff

[J M Fahey]

Loudthud, you are the guy.
I had never looked at it that way, yet it's quite logical.
Thanks.

[loudthud]

Loudthud, you are the guy.
I had never looked at it that way, yet it's quite logical.
Thanks.

Yah, it's amps like the Polytone that when you power them up slowly with a variac, the output sticks to one rail until about half way up, it suddenly pops down near zero. Kinda rough on any speaker that's connected. The amp I built was so quiet, you could barely tell it was working when you turned it on. I wonder what ever happened to that amp... I think I still have it somewhere. It was built in the days before I had a scope or a variac, just a Radio Shack VOM and a light bulb limiter.

[Enzo]

Yeah, cool, I never really thought it through thoroughly like that. But have often run into the forbidden zone on the variac. Something about the zen of amp repair makes me decide case by case if the amp is just waking up that way or if there is indeed a current problem about to blossom.

[gbono]

I like your explanation but I'm wondering why I have seen two identical brand/model amps with and without thump issues? Is it a difference in the differential pair transistors - Vbe(on)?

[loudthud]

I like your explanation but I'm wondering why I have seen two identical brand/model amps with and without thump issues? Is it a difference in the differential pair transistors - Vbe(on)?

That's a good question. The standard fix (that you see in other designs) was to insert a diode and capacitor in the nagative rail that would keep the current alive in the diff pair for a little while longer as the rails collapsed. Maybe something like 470uF. The schematic I have of the Mega Brute shows C5, a 1uF non-polar cap connected to a center tap on the rail resistor. I wouldn't think that would be big enough, but it could be a mistake on the schematic or was changed in other versions.

http://www.mif.pg.gda.pl/homepages/t.../polytone2.jpg

Can someone post a link to a more accurate schematic?

[km6xz]

As long as the drive is controlled, or servo amp(or if not separate, the diff amp) are operational the thump should be under control. A lot of techs assume the main rails for the current amps being unbalanced in dissipating the filter charge causes the thump. Most run in a combination of AB and class B, using drivers in AB and the high current amplifiers running straight class B so unless "told" to by base current, they are not going to cause the speaker to deflect hard to one rail. So look in the diff amp, servo or delayed driver supply(like the old DC300 which had simple voltage doubler charging a cap through a resistor driving the emitter of a PNP, with the main rail positive line connected to the base, and the collector having the whole driver and predriver as load. The rails came up immediately, balanced or not, and the doubler came up to a higher voltage slower, 1/2-1 second later, after the supply rails were stable, which turned -on the driver and predriver.)
So look for too fast of drop out of the servo amp or diff amp when tracking down thump in amps, commercial or home made. Diff amps can be suppled through an isolation diode, or even just a resistor, with a decent sized filter cap on the lower potential side can keep the positive and negative supplies to the front end active until the bias network drops low enough to prevent conduction in the output current amps.
There are also options to use a fast drop out rectifier/filter arrangement to remove bias fully in a Class AB amp, as soon the power switch is opened to mute the output, adding a diode can also mute delay turn on.

Not many amps suffer from this problem anymore because the cause is well understood. The Poly amps were pretty bad for it because of a crude adaptation of the original RCA circuit, with few refinements. It is possible to make a mini-brute stable and reliable without much difficulty.

[loudthud]

As long as the drive is controlled, or servo amp(or if not separate, the diff amp) are operational the thump should be under control. A lot of techs assume the main rails for the current amps being unbalanced in dissipating the filter charge causes the thump. Most run in a combination of AB and class B, using drivers in AB and the high current amplifiers running straight class B so unless "told" to by base current, they are not going to cause the speaker to deflect hard to one rail.

When the tail current for the diff amp falls to about half the nominal value, as it does when the negative rail discharges to about half the nominal rail Voltage, there is not enough current available to make .7V across R3 and Q3 stops conducting. When this happens, the output swings to the negative rail because R10 and R11 pull the Base of Q5 to the negative rail. The same thing happens when you bring the Voltage up slowly with a Variac. Until you get to about half the normal rail Voltage, there is not enough tail current in the Q1, Q2 diff pair to turn on Q3. With a constant current source in the tail of the diff pair, this problem is avoided because there is always enough tail current to keep Q3 turned on and the diff pair can maintain control of the output.

I couldn't find my original attachment to replace the one that was lost. I took a Polytone schematic for a 55 Watt amp I found and modified it to add a current source.