Relay channel switch - help needed

[txstrat]

I'm currently using a print relay for channel switching and it works fine so far. I'm using a DPDT footswitch to open or close the ground connection (I know a SPST would do) for the relay coil.
What I'm thinking about is to change to a bistable relay to elilminate the slight hum of the relay coil while being "on".
Is there a way of using the same switch to give an impulse to the bistable relay to make it change it's state? Even without having the 12V at the relay all the time?

Thanks

Matt

[txstrat]

Oh, I forgot something very important.
With the relay closing I also switch an LED on to indicate the state of the relay. Would be nice if I could manage it like switching the relay by impulse and at the same time feed the LED with permanent voltage to make it shine.

Matt

[Robert M. Martinelli]

Hi Matt,
why don' t you use a DC coil relay ? that way you should not worry about the relay coil humming when on.....just remember to add a diode connected in parallel to the coil to shunt the current coming back from the coil when switching off.

Otherwise, you could build a toggle relay using flip-flops and LDRs, that way you could switch channels by applying pulses at the f-f inputs, and connect an LED in series with the LDR command signal to have them on when driving the relevant LDR.

Hope this helps

Best regards

Bob

[txstrat]

Hi Bob,

thanks for your reply. How ya doin' anyway?
I'm using a DC coil relay with a diode. Maybe the DC is not filtered enough. What would you recommend for a filter cap in front and after an 7812 IC?
By testing the crunch channel wihth higher gain settings I found out the hum is also there (not as loud but noticeable). Perhaps it's simply because of the higher gain settings.

Matt

[Robert M. Martinelli]

Hi Matt,
I'm ok, I could use some vacation, though....if I only could remember what this word means....

Sorry, hearing you talking about hum I thought you were using an AC relay....my mistake...

To properly filter ripple you' ll need to put caps as close as possible to the 7812, I would use at least a 1000 microfarad cap at the input, paralleled with a 100nF, and a 10 to 22 microfarad at the output, paralleled with another 100nF. The two 100 nF caps serve as a supplementary filter for high-speed transients which may pass through the big filter capacitor due to its slower time constant.

Keep also in mind that a 7812 can handle up to 1 Amp, but it needs at least a 30 mAmps load to function properly, so if your relay is drawing less than that you'd better use a 78M12 ( 0,5 Amps max ) or a 78L12 ( 0,1 Amps ).

It seems strange to me that the relay coil is causing all that hum, ripple should be at a nasty percentage to pass from the coil to the contacts..... maybe to figure out if the hum is coming indeed from the relay coil you could try to operate the relay coil with a 9VDC battery, removing the 12 VDC, and, if the hum disappears, then try the 7812 with the filtering array I suggested above.

Hope this helps

Take care

Bob

[Enzo]

I suspect the problem is not one of filtration. The relay supply itself doesn't need to be all that well filtered at all. The problem is the return current is sharing a ground path with the signal stages - I';d bet my lunch money. The ripple current in the relay circuit is affecting the ground of a sensitive stage.

Does your +12v serve any audio circuits or is it strictly for relays? Are you switching the hot side or the ground side? Most of us wire the relay to hot and then ground the bottom to turn it on. Where is the ground connection for the relay?

In other words, assume the hot side of the relay coil is wired to +12VDC, then the cold side runs down to the FS jack tip. The FS jack sleeve then connect to... where? If you just picked the closest ground point... BUZZ... nope.

Clip a wire to the central ground point of the amp - the main filter cap ground right at the cap. Disconnect the wire to the FS jack tip contact, and clip it to that ground point. Still hum?

[Robert M. Martinelli]

Enzo' s got a good point,
the risk of ground loops is always "just around the corner" when modifying an amp, and yes, relays indeed don' t need too accurate a filter to work properly, I was just worried about the "AC component" ( ripple ) flowing through the relay coil, if there is some "sensitive" point nearby ( high impedance ) the noise could use it as an "enter point". The "Battery" test should help you to state whether the hum is coming from the relay coil or not; fixing the ground issue as per Enzo' s advice should eliminate the hum; should some hum still be there, taking down ripple with a better filter could help.
Best regards
Bob

[txstrat]

Does your +12v serve any audio circuits or is it strictly for relays?

Strictly for the relays.

Are you switching the hot side or the ground side? Most of us wire the relay to hot and then ground the bottom to turn it on. Where is the ground connection for the relay?

I switch the ground side.

In other words, assume the hot side of the relay coil is wired to +12VDC, then the cold side runs down to the FS jack tip. The FS jack sleeve then connect to... where? If you just picked the closest ground point... BUZZ... nope.

The ground of the footswitch is connected to the ground of the speakers. But I've already tried a separate path to ground for the relays alone and it made no difference.
Could it be the footswitch itself? It has a metal housing that connects to the chassis. Maybe there is the "bad" ground.

Clip a wire to the central ground point of the amp - the main filter cap ground right at the cap. Disconnect the wire to the FS jack tip contact, and clip it to that ground point. Still hum?

That's what I'll try next.

[txstrat]

should some hum still be there, taking down ripple with a better filter could help.
Bob

I started with 25uF (had no others at hand) on either side of the 7812 but there was a hum like in a bees hive even with the relays off. I am currently filtering the 12v DC with two 100uF / 25V caps before and after the 7812 (no 100nF caps so far). The relays have a nominal current rating of 12.5 ma but they work fine so I assume they draw not too less current for the 7812 to work properly.

[Robert M. Martinelli]

I started with 25uF (had no others at hand) on either side of the 7812 but there was a hum like in a bees hive even with the relays off. I am currently filtering the 12v DC with two 100uF / 25V caps before and after the 7812 (no 100nF caps so far). The relays have a nominal current rating of 12.5 ma but they work fine so I assume they draw not too less current for the 7812 to work properly.

Hi again Matt,
If the relays are working properly and the voltage out of the 7812 is stable, then the current is enough to let the 7812 work, the only thing I would do is to put the bigger capacitors before the 7812 and only a small one after it; the reason for this being that if you put a big capacitor after the 7812 the initial current demand coming from the cap could damage the 7812, I would advise to use only 10 to 22 microfarad at the 7812 output, just to kill whatever residual ripple you could have there ( but if you use a 1000 microfarad or more at the input that should be very unlikely ). The two 100 nF capacitors are not a must, they only help with fast transients and help to guard against oscillation if the filter capacitors are not close to the IC.
Regards
Bob

[Enzo]

Where are the 25v caps grounded?

Does the transformer winding for this 12v supply share with anything? In other words does it also serve to make bias or something? If that winding exists only for the relays, then the 12v supply need not share ground at all. Common its filters together and connect them to ground only at the FS jack. if the jack is plastic, then don't ground it at all, just tie a .1 or .047 cap or something from its common to the chassis.

The return for filter caps is a high source of ripple current.

[txstrat]

Enzo,

from your asking I assume ground of the filter caps of the 12V supply should not be shared if possible. Unfortunately thats what I do with the preamp ground (see attachment).
What could I do with the supply and the relay? I'm not shure if I understand what you mean with "no ground at all".
I believe both, the ground for the filter caps AND the ground for the footswitch should not be shared, right.
BTW the hum is not that loud but audible and from all of your advice I would tend to say I could eliminate it with the right wiring, right?

Matt

[Enzo]

I don't KNOW that, but it is my gut feeling still.

But right away, get those filter cap grounds AWAY from the preamp grounds, that is exactly the way to introduce ripple currents into the preamp ground. Move them to the star point and see what happens. Just lift them and use a clip lead.

The hum may not be loud, but you wish it were not there, right?

By no ground at all, I mean exactly that. Imagine the 0 point on the transformer was not grounded, and the filters did not go to preamp ground. The 0 point was wired to the filter commons. No connection to the star point nor the preamp ground, nor the speaker ground. The left end of the switch also not to ground. WOuld not closing the switch still power the relay? In other words, what if I wired up a transformer, rectifier, filter and Vreg on a wooden plank. They'd still make 12v. There would be a + and - end to the 12v, but no ground connections. Plop a relay coil across that 12v and it energizes. Now in fact, at this point we could then ground the circuit at ONE spot - the FS jack ground. That would reference the circuit to the chassis, but no current would glow through the chassis, it would all be in the wires. And if you used a plastic jack, there would be no connection to ground at all, but the 12v would still get switched onto the relay.

Just moving those filter caps to the star point should help. DOn't let the FS ground share the preamp ground either.

[txstrat]

Alright, that's an explanation that even I understand
I'll do exactly what you advised. And yes, the hum ain't loud but I wish it was not there.
Just give me some time to fix that... I'll be telling you what happened.

Thanks alot

[olddawg]

On a lot of audio equipment that have switching relays a completely separate and isolated supply is used; a completely separate small 12v transformer, bridge, filter and regulator with the B+ and the ground isolated from the chassis (usually on a separate small board) and the wires twisted to the relay. If, like mentioned before, there is no hum when operating the relay with a battery or bench supply, that is the way I would do it. Small 12v transformers are cheap.