1987 Polytone Minibrute IV speaker pops on shutoff

[hofmoan]

We recently got a 1987 Minibrute IV. The power supply looks clean on the scope, It checks out great with signal tracing from input, through the pre amp and through the power amp, DC offset is 14mV. The only annoying problem is when you turn it on it makes a snap/pop sound and when you turn it off it makes a duller pop that causes the speaker to move and then 5 seconds later the same thing. Anyway to clear this up?

[Enzo]

SOunds like a good portion of solid state amps around the world. Unless yours is quite exaggerated, I don;t look upon this as a problem.


If you must get rid of it, engineer in a speaker delay relay. A relay completes the circuit to the speaker after a moment of delay. That will eliminate the turn on thump. And make the relay so that it will drop out instantly at power off, and hopefully it will open before the power down thump.

[hofmoan]

Thank you!

[Metro]

Seems common to Polytone amps

I have an 87' Minibrute II with the same issue. The power on/off thump is quite a lot louder than most solid state amps. Seems to be something specific to the power supply in these amps.

Enzo, would a relay thats switching the speaker outputs possibly harm the output transistors? As the energy spike is not being dissipated by the speaker.

[Enzo]

There are zillions of amps that have speaker relays. WOn;t harm a solid state amp to have the load removed. There is no energy spike, there is only a voltage transient. Without a load there is no path for current, so no spike.

Besides, the voltages in these amps are limited to the power supply. The loudest pop will be no larger than a maximum signal excursion. SO if your speaker can handle the full power of the amp, it can therefore handle the thumps the amp can produce.

[drewl]

Couldn't you use a cap to couple the AC out to the speaker?
The cheap fix?

[km6xz]

Almost all Polytones did that, for years without any damage to the speaker. I had one jazz player who was concerned enough to hire me to mod his to eliminate the thump. i built a DC servo amp for the feedback path that pretty much eliminated the problem but the relay would be easier and just as effective.

[Metro]

I have seen many amps with relays, Im just used to thinking that removing a speaker from a power amp without substituting a load or cutting bias or HT/equiv is detrimental, as it induces overvoltages in OPT's, output transistors and feedback circuits. Obviously more of an issue in valve amps than SS, but still not best practice, am I wrong? There are many relay switched amps out there, and they are working well, but am just interested in the theory. Im always learning and appreciate your words greatly, thank you. M

[Steve Conner]

It's absolutely not a problem for SS amps. The output transistors can never see more than the sum of the rail voltages, no matter what you do, and there is no OPT.

I once built a hi-fi amp that produced an almighty crack on power-up, in spite of having a DC servo. Eventually I got fed up with it and installed a speaker relay, and it got rid of the noise with no other problems or circuit changes.

[Metro]

I mentioned OPT in reference to the issue of valve amps not being shown a load, excuse my vagueness. Sorry it was off topic.

[Enzo]

drewl, a series cap at the output wouldn;t work. A cap blocks DC, but that is steady DC. CHanging DC voltage is no different from AC voltage to a series cap. A large DC spike is exactly the same as half a large signal spike.

[J M Fahey]

Although it will break nothing, turn on/off pops can be quite unacceptable in certain settings, be it quiet Jazz sessions, Theater musicians, certain Church settings, etc.
I sometimes made PA systems for Buddhist temples, Yoga places, etc. , where a mouse whisper is too much.
I often solve it by adding a "Standby switch" which cuts speakers off, and glue a prominent label close by which says something like: "Turn on/off procedure: make sure standby swith is off, turn "line power" on, count to 10, turn standby on - To turn off, reverse procedure - Doing otherwise will void your guarantee"
Works like a charm.

[loudthud]

I found a schematic for the Mega Brute power amp. It's very similar to the first solid state amp I ever built that worked. It needs an active current source in the tail of the input transistor pair and it will be nearly thump free. The current source needs to be about 3mA. It replaces the two 4.7K resistors in the tail to the negative rail.

[jrfrond]

Power supply rails will almost NEVER discharge equally upon turnoff, because they are usually loaded differently. When one rail discharges much faster than the other, you get a "pop", which is just a momentary DC imbalance until both rails hit zero. Then you might see the speaker cone move slowly as the last of the discharge occurs. It is completely harmless and really just a sonic annoyance. Many manufacturers mask the issue with a speaker relay or an electronic mute circuit (most power amp IC's have a "mute" pin to facilitate this), just so that the DC spike never hits the speakers, but it is more for customer satisfaction than anything else.

[loudthud]

Power supply rails will almost NEVER discharge equally upon turnoff, because they are usually loaded differently. When one rail discharges much faster than the other, you get a "pop", which is just a momentary DC imbalance until both rails hit zero.

Even if the rails discharged equally, you would still get a pop. See attached diagram. On the left is a simplified diagram of the Polytone power amp. As the negative rail discharges, the current I1 in the Diff Pair (Q1 and Q2) is reduced. When the rail gets to about half the normal value, there is not enough current left to make the 0.7V across R1. When this happends, Q3 is cut off and the output slams the negative rail. When an active current source is employed as in the right diagram, the current I1 remains almost constant and the diff pair can maintain control of the output until the rail falls to about 2V. The reverse happends at power-up.