Hi, I have a Pre amp that runs from a 15VDC SMPS. At turn on it gives a loud thump when turned on. If I add a capacitor in the positive line will that help to prevent the thud. If so what size, the SMPS is rated at 1 amp.
Cheers for any replies.
Cheers for any replies.
Not likely. You could mute the preamp output for a few seconds upon startup.
If that's not possible, you can add a small enough output coupling capacitor to tame the thump.
If that's not possible, you can add a small enough output coupling capacitor to tame the thump.
I'd start with the largest reasonable size nonpolar or bipolar capacitor,
which would make -3dB at 20Hz with your amplifier input impedance load.
So if the amp is 100k, use 0.01uF.
If the amp is 50k, use 0.022uF.
If the amp is 20k, use 0.047uF.
If the amp is 10k, use 0.1uF.
If the thump is still objectionable with the capacitor, instead you'll need a muting circuit at the preamp output.
I assume the thump happens with the volume turned down? Is the thump still there when turning the power off?
which would make -3dB at 20Hz with your amplifier input impedance load.
So if the amp is 100k, use 0.01uF.
If the amp is 50k, use 0.022uF.
If the amp is 20k, use 0.047uF.
If the amp is 10k, use 0.1uF.
If the thump is still objectionable with the capacitor, instead you'll need a muting circuit at the preamp output.
I assume the thump happens with the volume turned down? Is the thump still there when turning the power off?
Last edited:
Not sure if this applies to the OP but I found when testing LM1875 single-supply that thumps are caused by race conditions on the input pins. The solution is to make the "-" drive the output low while the capacitor on the "+" side is charging. It was a matter of adding a cap across the existing one.
If remember right with non inverting
output is low and when switched on output goes high...pop
think trick was with virtual ground or 1/2 supply reference
make cap large so it comes up slow.
output is low and when switched on output goes high...pop
think trick was with virtual ground or 1/2 supply reference
make cap large so it comes up slow.
For a capacitor-based slow start, one may have a current source (e.g. LM317-based) feeding a capacitor. The voltage across this capacitor then needs to be regulated by the series or shunt method. As the capacitor charges at a constant current, the delay value is also easy to calculate. For example, with a 100mA current source and a 100uF capacitor, the timing (for 15V) would be:
100uF x 15V (say) / 100mA = 15ms.
I've used this method (with a shunt regulator) for opamps / DAC chips etc.
100uF x 15V (say) / 100mA = 15ms.
I've used this method (with a shunt regulator) for opamps / DAC chips etc.
I used a VN10KM MOSFET to drive the relay, so I could use a big resistor and small capacitor for the timing components.
A diode discharges the capacitor to the positive rail at switch-off, and a Zener limits the gate voltage to below the maximum of 15v.
A diode discharges the capacitor to the positive rail at switch-off, and a Zener limits the gate voltage to below the maximum of 15v.
Hi again & thanks for the input. I will try Rayma's recommendation's 1st as its the easiest. I have also (since asking the question seen this method recommended) with a grounding resistor after the capacitor, Is that important. The pot is 100K.
Will report back cheers
Will report back cheers