DIY A-B switch box (w/pics)

[fordgtlover]

I'd love to see what you have.

I have no real intention to make this statistically valid. I am primarily doing it for my own interest - but your warnings are accepted gatefully.

 

[fordgtlover]

I have finished my A-B tester. I actually finished a few days ago, but the push-on/push-off button was very loud and somewhat defeated the ability to switch (relatively) silently between devices. So, I wanted use a momentary switch, but finding a circuit to latch the relays was bit of a pain. You can see the latching circuit on top of the board. After a failed attempt, I ended up using the 'doing it discretely' circuit found here

As you can see, the wiring is certainly not the neatest job in the world, but the casework is reasonably neat. As I said previously, it was done primarily as a proof-of-concept. I intend to make a smaller and much neater version at some stage with better connectors and cable. The relay are quite large (29mm x 12mm x 25mm), but I couldn't find any neat little Omron ones close by. I'll track some down for the next version.

The long leads are connected to the Normally Open (NO) and Normally Closed (NC) terminals. The RCA & 1/4inch sockets are connected to the Common (C) terminals. The LED is powered when the relays are triggered. The toggle switch on the side turns the LED on and off to allow blind testing. It runs of a cheap 12V wallwart.

Anyway, here are the pics of this ugly beast.

The dimensions of the box are:
158mm x 95mm x 53mm

 

[stevenkelby]

That's the stuff, looks good.

How long will burn in take?

Keep us informed of results!

 

[J.D.N]

Hey, i'm not sure i understand 100% what you are using this for, but im possibly trying to do the same thing.

I am looking to make something so i can switch the pre-out signal from my pre-amp to either my headamp or my power amps. I was just going to use an input selector type twist knob and wire phono sockets directly on to it. Is this the wrong way to do it? Should i be looking at something more like what you have made?

Cheers

 

[fordgtlover]

^

I made this primarily to allow comparison between two devices. The idea is that you can plug two amps or DACs into it and switch between them. There is no external indication of which amp the listener is hearing.

I think for what you need your idea sounds like a more reasonable solution.

 

[fordgtlover]

Quote:

Originally Posted by stevenkelby /img/forum/go_quote.gif That's the stuff, looks good. How long will burn in take? Keep us informed of results!

I have been listening to it all night and I am pretty happy with it.

I am going to do some testing tomorrow.

Early indications suggest that I may not actually want to post the results

 

[fordgtlover]

The only problem I am having with this box is that occasionally there is an audible click in the headphones when I press the momentary switch.


I'd be grateful for any advice on how to fix this.

 

[threepointone]

I'm not sure exactly how your relay thing is wired, but it could either be a bit of DC offset in one of the amps (sometimes you can hear this, even at safe levels). Or, if your relay is toggled on and off simply using a switch, it's probably because you haven't pressed it long enough or because of switch bounce or something like that--usually you have to apply current for ~2ms or so depending on your relay and what voltage you're running at. In my experience, the relay switching delay isn't really audible.

btw, i wouldn't bother testing cables--someone is almost certainly going to argue that the wiring in your a-b box isn't made of pure gold, the relays have to be made of pure teflon, etc. . .

 

[fordgtlover]

Quote:

Originally Posted by threepointone /img/forum/go_quote.gif I'm not sure exactly how your relay thing is wired,

Thanks for your response.

It is wired as described in the diagram in post 4

Quote:

but it could either be a bit of DC offset in one of the amps (sometimes you can hear this, even at safe levels).

I have checked the DC offset at the switchbox output and it is less than 1mV on both amps.

Quote:

Or, if your relay is toggled on and off simply using a switch, it's probably because you haven't pressed it long enough or because of switch bounce or something like that--usually you have to apply current for ~2ms or so depending on your relay and what voltage you're running at.

I'm using a momentary switch with this 'Doing it discretely' latching circuit. While the author of that website doesn't mention debounce for this circuit, I thought that was really the point of using the latching circuit; that the momentary switch triggers a voltage drop in the transistor and allows voltage to flow through the mosfet, and the second press applies the full voltage back to the transistor, which turns off the volatge to the mosfet. So, the momentary switch is only used to initiate the voltage flows, and debounce shouldn't be an issue.

Is this correct?

Quote:

In my experience, the relay switching delay isn't really audible.

When using the push-on/push-off switch that I initially had installed, the click of the switch was so loud that it was difficult to detect whether the relay switching was audible, but I understand that the relays should switch in 10 to 20mS and so should not be audible.

Quote:

btw, i wouldn't bother testing cables--someone is almost certainly going to argue that the wiring in your a-b box isn't made of pure gold, the relays have to be made of pure teflon, etc. . .

Agree - I have no desire to claim this as any sort of absolute proof of anything really. I primarily did out of curiosity.