Preventing source selector 'thump' using transformer?

[redal]

I'm using a simple source selector switch in my system and since adding a turntable + Rega mini preamp I'm experiencing 'thump' when changing input.
The systems is as follows:

PC sound Card - Switch
Turntable - Rega phono amp - Switch
Switch - Amplifier (LM3875) - Speakers

I've tried running a 1M ohm resistor from + and - to ground for each input, but this hasn't made a difference.

I'm wondering if adding an audio transformer, maybe Edcor tppc 15-15 (15k to 15k) to the switch output might prevent this? I would really appreciate any advice. Also I will be changing the system in the near future to be bi-amped (adding a tpa3116) using an active low pass filter and pllxo (~150 hz) and also a simple preamp (rod elliot p88). Therefore there may be re jig things a bit in the future and the switch would then be before all this 'stuff'

Any help gratefully received.

Andy

 

[richie00boy]

There is probably an output capacitor in the preamp that is charging, causing the thump. I think what you did with resistors was not quite correct, try a 47k resistor from the preamp output phono socket centre pin to body/tag. Do both left and right channels.

 

[richie00boy]

On the other hand, it could be the soundcard that is missing the resistor and the preamp already has one. So try the same mod on the soundcard and see if that helps.

Also, are you using a break-before-make, or make-before-break switch? You need the first type.

 

[DF96]

Resistors alone won't solve the problem if there are any missing coupling capacitors. Check that all outputs and inputs have coupling caps, with ground leak resistors on the outboard side. Where necessary, add these (caps and resistors) to your selector switch box.

 

[redal]

Riche00boy & DF96,

Thanks for the reply's they are really helpful.

Regarding the soundcard & preamp output caps/resistors I'm not sure of the situation.
The soundcard is a Xonar d2x, which I do expect to have an output cap and from what I've read has an output impedance of 100 ohms (if this is of any use).

Regarding using a 47K resistor on the phono & souncard input to switch, is the value chosen because it's the same as the phono pre's input loading (47k?).

Switch type BBM or MBB is unknown, I bought a fairly cheap audio in selector a few years ago and took the switch and rehoused in a fancier case, is there any way to test the switch? Maybe wiring input A + B to one side of a continuity meter and the other side to the ground and slowly switch to see if there is any point where both A+B are in contact.

Apologies for all the questions,
Andy

 

[DF96]

I don't know why a 47k resistor was suggested. 1M should be fine, unless the caps are very high values or electrolytics. The output impedance is not an issue.

Switch BBM or MBB should not make much difference. If all the points being switched are DC grounded then no thumps.

 

[bear]

Is there a resistor to ground on the wiper side of the switch?

This is likely a "break before make" switch, so for part of the time the wiper is floating...

 

[redal]

There isn't currently a resistor to ground on the switch wiper side, this just connects directly to + and - out (to amp) and ground connected to switch body and all other grounds.
I think I understand what your saying about the wiper floating, do you mean that its suddenly 'hitting' a signal causing a thump? The phono preamp/cartridge combo AT95 does seem to have a lot of gain (I've got to set the volume on amp really low). Perhaps I have always had a quiet 'thump', but comparatively low signal voltage on other sources maybe meant this was a lot less audiable.
I will try adding a resistor to ground on this also (+ and -) and see what happens.

 

[redal]

Solved! Thanks for all the help, you guys are the best!

I added a 1M ohm resistor from switch wiper to ground as suggested by bear (+ and -) and now switching is totally silent (apart from the music).
I can't really hear any negative effects from the resistors to ground (on all inputs to ground) so I'm guessing I just leave them all in place.

Just for my curiosity the reasoning for the resistors is to offer a path to ground for any output capacitors on sources (so that they can discharge) which is sufficiently high resistance as not to short the signal to ground. The reason for the switch wiper to ground is to give it a path to ground in the same way.

Many thanks for all the great advice!