Unique way of muting for thump prevention, outside audio path.

[gmphadte]

My amp, the Low TIM, assembled with the Lightspeed attenuator was making a loud thump when the changeover relay for input selector was operating. The relay has 4 changeover contacts. I used it for selecting two inputs. I have many other relays but nothing to compete with this one's contact quality. It is siemens make and the contacts are still shining, after so many years.

So, I designed a logic that would cycle through mute-relay operation-unmute on input select signal and connected this to the lightspeed attenuator.

Now there is no thump issue. The process is slow and amp goes silent during its period.

I also designed the speaker relay contact spark inhibition as an additional feature.

The block diagram goes like this

edge detector---multiple pulse generator---driver---relay

Gajanan Phadte

[stratus46]

Wouldn't it make more sense fix the actual problem (thumps) rather than putting an electronic Band-Aid on it? Thumps come from DC shifts but these can be controlled rather easily. I guess I'm being bratty but you haven't fixed the thumps, just made an elaborate muting system.

G²

[gmphadte]

You r right. The advantage here is that it is outside the audio path and due to the slow behavior of the LDRs, it cannot be done faster, (won't work).

The thump I was getting was from the relay coil field inducing into the contacts.

Gajanan Phadte

[MrNudge]

could you use a solid state relay?

[stratus46]

Originally Posted by gmphadte

You r right. The advantage here is that it is outside the audio path and due to the slow behavior of the LDRs, it cannot be done faster, (won't work).

The thump I was getting was from the relay coil field inducing into the contacts.

Gajanan Phadte

Any chance the coil circuit is part of a ground loop getting into the audio?

G²

[gmphadte]

No, it is having its own dc supply and a separate ground. The thump is evident on relay energizing.

Gajanan Phadte

[gmphadte]

Originally Posted by MrNudge

could you use a solid state relay?

No, the contacts pass the input signal.

Gajanan Phadte

[sakis]

electronics cannot be treated with cortisone..... IE there is no reason on trying to suppress the problem .... idea is to cure it once for ever..

small 1N 4148 diode will make miracles

[gmphadte]

1N4007, it is already there, but it does no miracles with this relay.
It is supposed to act when dc is removed.

Gajanan Phadte

[sakis]

then ...its simple as that you need to find out if the thumb is relay related or source related ....

simple as that remove the plastic cover of the relay in order to be able to push the contacts to change source by hand with no actual power .... if you don't get any thumb means that the thumb is related to the power supply or configuration of the coil /relay

if you do get a thumb it means that excessive DC exists in your sources coming from any reason

check that and get back to us

kind regards sakis