DIY Audio Source Switch Box: Avoiding a loud pop through speakers

Relays "chatter', "Bounce", so any DC that exists on a preamp input
gets converted into a transient. Some relays better than others. Same
is true for switches. I have had Push Button switches bounce for as long
as 400 mS. Eg. mechanical inertia and flex on contact arms causing
this. So you can always setup a simple test for relay quality to eval
this with your scope. Google "relay contact bounce test" for suggestions.
Scope set for single shot (if its a DSO).

MOSFETs (versus relays) do not bounce if their gate driven clean.......

Often we as designers have a propensity to throw caps at a problem
without really knowing the underlying source of a problem. Myself
included. At this point I would tend to switching just one path/load/input
and finding its nemesis, evaluating ground as well as rest of signal path.

Keep in mind preamps typically high Z so easily victims of even small
parasitic V's thru stray C coupling and currents.

Good luck, this can be a tedious process. Even mechanical contacts,
like your star ground and various screw terminals can be suspect.

Example of Z scaling effects to an edge input into stray C and Z....

Hi Les,

Les Jones said:

You have NOT answered the quesions in post #14

Click to expand...

I think I answered your questions in post #17, but if I didn't please let me know what I've missed. I apologize for any omissions.

Just saying that the signal grounds are connected to the relay is not the as as saying that they are switched by a set of relay contacts.. If you posted a schematic (Rather than a sort of block diagram in post #11.) then it would be clear to us exactly what you are doing. I suggest trying connecting all the signal grounds together. and NOT switching them. This can cause a problem if more than one of the devices (Signal sources and amplifiers.) has the signal ground connected to mains ground. This is because there can be small voltage differences between the ground on mains sockets.

Les.

Hi Dana,

You've given me a lot of information to digest. I truly appreciate your input so far.

danadak said:

I dont see what looks like a "stiff" ground in the system for starters. Are your amps
also dumping their power into that star ground ?

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Are you referring to the amps that will be selected and routed by relays 1-3a? If so, I haven't gotten that far yet. I won't be surprised if the amps complicate my current issue with the preamps.

danadak said:

"Normally" one creates, in an
analog system with small signals and power switching, a ground for the low level
signal circuits and one for the power, and join them where they come into the
chassis as a single point ground. Eg. connect the grounds together as close as
possible to the source, in your case the entry to chassis.

Click to expand...

I had supposed that the signals for both the preamp and (future) amp sections of the box would each be floating and not even share a ground with the power rails, but given what you are saying, and having read through the "small signal grounding" article that you recommended, this may be out of the question. Am I understanding correctly that I should try joining all of the ground leads for the RCA plugs (input and output) and then connect them to a star ground nearby on the chassis? That sounds similar to what I think Les was considering as well.

danadak said:

Your scope, where are you monitoring noise, for the 12V supply to the star ground ?
Curious, have you done that with scope set to single shot, edge trigger, say 12.5V,
sweep start at 1 uS, acquisition say 100 MSPS, and see what you get. Might be
significantly larger that you are aware.

Click to expand...

Because the noise manifests in the preamps common output to the amplifier I've been measuring between the signal and ground pins of the RCA (out) plugs on each channel. That's where I pick up the 190mV spike lasting under 300ms. I could be wrong, but that level of spike seems in the ballpark of what I'm hearing through my speakers after that noise has passed through my amps of various power ratings. I've been edge triggering at 30mV using single shot (or repeat).

Do you want me to measure from the star ground to somewhere on the relay block?

danadak said:

You could always consider a time delay relay that keeps a ground on inputs
until relays and power settles, then lift the input grounds.

Click to expand...

Someone on a different message board suggested using an opto-coupler and 7474 to overlap the signals between the two preamps as the relay engages. Is that the kind of time delay that you think would be helpful?

danadak said:

When switching small
signals one usually uses signal type relays, like the ones found in scopes.

Click to expand...

I couldn't find a signal type relay that could met these criteria: Latching, Socketable, 4PDT, Signal, 12V. Maybe such things are out there but I couldn't find anything. I'm open to suggestions.

danadak said:

What is the model # of your scope ?

Click to expand...

I've been using a Picoscope 4262 audio scope, but I have a faster scope if necessary.

This is a pretty long post so I'll stop now and try to address your second post separately.

Many Thanks --Jonathan

Les Jones said:

Just saying that the signal grounds are connected to the relay is not the as as saying that they are switched by a set of relay contacts.. If you posted a schematic (Rather than a sort of block diagram in post #11.) then it would be clear to us exactly what you are doing

Click to expand...

Not sure what the time frame would be for me to learn software and develop the skill to turn my design into a schematic. I'm willing to learn, but this won't be a non-trivial task. I don't know where to start but am always open to suggestions. I'm doing this and my other projects to learn...

Les Jones said:

I suggest trying connecting all the signal grounds together. and NOT switching them. This can cause a problem if more than one of the devices (Signal sources and amplifiers.) has the signal ground connected to mains ground. This is because there can be small voltage differences between the ground on mains sockets.

Click to expand...

I will try this and let you know. The amps aren't an issue yet so its a good time to try this.

Thanks again Les.

Someone on a different message board suggested using an opto-coupler and 7474 to overlap the signals between the two preamps as the relay engages. Is that the kind of time delay that you think would be helpful?

Click to expand...

The idea is

1) Ground both inputs
2) Switch the source
3) Lift ground on input of choice

Easy to do with a simple processor, or discrete logic.

Processor example :



Above is mBlock, code in blocks, it converts that to Arduino code, and you
program the Arduino with it once to run the application. I did it for a push
button to change the preamp selection.

By reading comments on code blocks you can see what it is doing. You can
do this with a Nano board or an ATTINY85 8 pin part, but that takes a Nano or
UNO board to program it.

Note it would be easy to use two buttons, one to select preamp desired, the other
to turn it on, also with indicator LEDs to show which is active. And code it so press
of both buttons turns off both preamps.

You see 3 states needed, so if using discrete logic a 7474 configed as a 2 bit counter
counter, and gates to decode state or a demux. Or use a counter and demux, like
a CD4017 so it goes thru 3 states and on 4th use that to reset it to initial state. Partsy
because of interface , eg. button or some form of switch or signal. And you would use
a button to clock it, but have to debounce button, and it would have multiple pushes
to go thru each state. Or add logic so it "walks" thru the states on its own after a
button push.

If you are new to logic/processors do it with a relay that pulses on creating ground first,
then allows other relays of latching type to handle the preamp switching. Time delay
relays to produce states -

https://www.digikey.com/en/products/filter/time-delay-relays/952?s=N4IgTCBcDaIE4FMA2BDAniAugXyA


Do you need to do this for more than 2 Preamps and 2 amps ?

Hi Danak. Just 2 preamps and 2 amps. I have some research to do...
Thank you! --Jonathan

Les, you solved my problem! I connected all of the signal grounds together and did not connect them through the relay and now the pop is gone even on the problematic TDA7293 amp. I’m pretty sure the cap and resistor helped as well. Everything is working really smoothly.

Thank you everyone for your suggestions. This was my first time posting to any forum and your responses were tremendously informative. Best wishes— Jonathan

P.S. I’m sure that I’ll need to enlist your help again before I attempt to connect the amplifiers.

I'm glad it is now working. If you ask questions on a forum again provide more information the enable people to help. If you has drawn a schematic to start with the problem would have been solved faster. You don't need to use software to draw schematics. Just paper and pencil is fine. When designing something I start using pencil and paper and only use a computer to produce a schematic at the final stage.

Les.

One last question about the coupling caps. I'd like to mount them on a crossover board on a side panel. Can they be 12" away from the relay terminals that they are connected to? I know that there are some use cases where capacitors must be kept close.

It should be OK to mount the capacitors 12" away from the relays. If I was doing it I would use the now unused pole of the relay just as a connecting point to the capacitor. The capacitor would be mounted between two screw terminals on the relay. You should be able to use lower value capacitors unless the output side of the relay is driving quite a low impedance load.

Les.

Hi Les,

As it turns out I was able to mount the capacitors quite close to the relay and outputs. I don't understand your suggestion about the unused terminal. For each channel I have the signal output of the relay connected to one side of the cap and the output side of the cap connected to the RCA jack and to the common signal ground via a resistor. Is that right? If so, I'm not sure how I could use an unused pole of the relay. Here's a photo.

You were originlly switching the signal and the signal ground so I assume you were using a double pole relay (I have to make assumptions as you do not provide the information.) As you are now not switching signal grounds you will now have these 2 unused connections on each relay. You could use one of these connections to connect a wire to the end of the capacitor and that wire could go to the RCA socket. the other end of that capacitor would be connected to the connection on that relay that was switching the signal path.
Les.

Got it. Clever idea! You can see the pole assignment for all relays in my original (first) post. Thanks again Les.