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Old 13th October 2012, 10:53 PM   #1
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Default STK 4038 II mono amplifier

Hello

I have built a small mono amplifier based on the STK 4038 II amplifier module.

I have built a prototype and while switching the power ON, there is some DC voltage at the amplifier output and the cone of the connected speaker moves noticeably outwards and also makes a subtle noise. After the power has been ON for about 2..5 seconds, the speaker cone slowly returns back into its 0 position.

Is this kind of behavior a sign of a problem in the amp design? The amplifier circuit & PCB are both probably from some old Elektor design, but unfortunately I don't have the schematic at hand at this time. The power supply has a 2x22 VAC/100 VA transformer, 35 A diode bridge and two 10'000 F capacitors.

After this brief start-up issue this prototype amplifier appears to function very well. Even though the wiring in general is not very tidy and the signal wires are not of shielded type, there aren't any extra noises or other issues. I'm planning to add an active lowpass circuit and drive a multimedia subwoofer (2 x 6" woofers) with it. I just used the 12" to play some tunes from my laptop.
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Old 13th October 2012, 11:21 PM   #2
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Sounds like capacitors taking time to charge up.
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Old 14th October 2012, 09:55 AM   #3
AndrewT is online now AndrewT  Scotland
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you have crippled the heatsink by lying it on it's side.

The output offset is typical of many amplifiers during start up.
Some get it all done in the first fews tens of milliseconds. Others take much longer. Your amp seems to be in the "longer" category.

I'd suggest you use a speaker isolating relay with a time delay that is longer than the time the offset takes to reduce to near zero mVdc.
I'd also suggest you use this relay to isolate the speaker if the amp ever go faulty and sends an output offset that can destroy your speaker.

Ah, a thought. Is your amplifier AC coupled and using a single polarity supply?
That topology uses an output capacitor. That capacitor has to charge up during start up. That could be what your are seeing as aht speaker movement. The DC current running through the speaker to charge up the output capacitor to (roughly) half the supply voltage.
Again an output relay to isolate the speaker. But you must add a resistor to allow the capacitor to charge up before connecting the speaker.
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Last edited by AndrewT; 14th October 2012 at 10:06 AM.
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Old 15th October 2012, 10:06 PM   #4
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Thanks for suggestions.

I might re-locate the heat sink into the back side wall of the enclosure, at least it would be in free air although it would still be horizontal. It would also free up a little bit of space inside the enclosure for the lowpass and protection circuits.

The power supply is a dual polarity type with +/- 33 VDC no-load voltages and the speaker output does not have a large capacitor in series... Attachment shows a page from the amplifier module datasheet. The circuit in my amplifier is not much unlike it, except that in my amp there is some kind of a pre-amplifier circuit using LF353 op-amp.

I need to look for a suitable DC protection circuit for use with this amplifier.
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Old 15th October 2012, 10:40 PM   #5
Einric is offline Einric  United States
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My tube preamp makes my LM4780 amp do the same thing if I don't wait for the tubes to fully warm up before switching on the amp.
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Old 15th October 2012, 10:50 PM   #6
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If the two PSU caps are different the rails wont start up equally.
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