DIY A-B switch box (w/pics)

[fordgtlover]

I have drawn up a layout for a simple A-B switch box. I have very little knowledge of electronics, will this work OK.

What I am trying to do is to connect the headphone outputs from two amps and have one or the other switched by the headphone relay based on whether SW1 is open or closed. At the same time take the output from my DAC and have it switch through the DAC relay to the selected amp.

SW 1 is a Push-On / Push-Off switch
SW 2 is a SPDT Toggle
Relays are 12V DPDT (Jaycar SY-4052)
Power is 12V DC

Cheers

 

[MASantos]

Check the darwin source selector at Twisted Pear Audio News

There is a schematic there and a link to the developent thread at diyaudio.com

you can find lots of info there.

 

[Pars]

Don't forget the diode across the relay coils for inductive kickback...

 

[fordgtlover]

How's this?

 

[threepointone]

Hmmm. . .sounds a lot like an automated abx setup I did last year. I might post the project here if I have time.

as far as i can tell, the schematic looks fine. put the diodes in anyway, but since it's not driven from logic and it's probably a pretty low current relay, it might not be necessary.

The setup I had in my switchbox also switched the grounds. Honestly I'm not entirely sure if that's necessary, but some amps had driven grounds so I thought that might make a difference if I left all of the grounds interconnected.

 

[fordgtlover]

I'd planned on just connecting the grounds of all the headphone connections together and the grounds from the DAC together.

I guess I'll find out if this works...

 

[MASantos]

Grounds should be connected all together. You can do this in the PCB as well.

 

[fordgtlover]

OK

 

[fordgtlover]

Given that the DAC, amps and A-B box are all run from seperate power connections, will connecting all the grounds together cause a ground loop? Shouldn't each of the components maintain their respective ground?

 

[fordgtlover]

I have built the box. No problems with grounds. I kept the grounds sperate for each device.

This is an ugly box, but the results thus far are very interesting.

 

[MASantos]

DId you etch a board yourself, or did it P2P?

Show us some pics!!!

 

[fordgtlover]

It's perfboard. I'll snap off some pics in the morning and post them.

It ain't too elegant, but it sure works fine

 

[stevenkelby]

Gotta respect this, not many go to the trouble. This is just what's needed to test cables, is it light enough to ship around to other members? Cans, Amps and DACs can sometimes sound different enough to make this a waste of time, but this would be perfect to settle debates about some components, such as cables or amps with minor differeces. What procedure do you plan to follow to collect the data?

 

[fordgtlover]

Thanks for the positive comment.

It's in one of the larger jiffy boxes from Jaycar. It certainly could be shipped around no problem, but it's kind of bulky. I'd be able to build a much smaller one after this. Certainly small enough to ship around easily. The challenge will always be the array of cables required to hook everything up.

Much of the size requirement is due to me cabling directly to the perfboard to save money on connectors.

I wanted to keep the cost to a minimum because this was partly a trial to see how well it works, and partly to allow me to compare my amps. It has single RCA inputs, dual RCA outputs to the amps, and dual headhone inputs from the amps and a single headphone out. I have used cheap connector and cable, but it seems to work fine.

I'm struggling with a design for the connectors that makes the box more flexible; for testing a range of devices. This one was built to test my amps. The combinations of cables and connectors required is doing my head in a little at the moment.

As for test procedure, I'm afraid it is simply me volume matching the amps using a digital SPL meter and then doing some listening. The box has a single LED indicator that I turn off, so it remains unlit during amp switching. When I make my decision about which amp I am listening to, I turn on the LED switch. Depending on whether the LED is lit or not, I know which amp I am listening to.

I haven't really thought about data collection. Other than a pad and pencil, it would require someone much better at electronics than me to include any sort of electronic data collection - although that would be very cool..

I'll post some pics of this ugly beast in the morning.

 

[threepointone]

I've already done an ABX box with electronic data collection. I'm going to warn you beforehand that there's still a lot of audiophiles who don't trust ABX results, though, and after a lot of thought and research I've found that one or two are actually somewhat valid. (for example, ABX "puts too much stress on the listeners"--it takes a lot of practice to get used to ABX tests; the paper which devised the ABX test had the listeners train using the equipment for a couple of months before collecting data)

Maybe if I have time this weekend, I'll post some schematics / pics / etc. I tested a couple of different amp designs and op-amp designs with no statistically significant difference at all (I know of a couple of problems with my data collection, though, so don't consider that to be a definitive statement). The system is basically a vb.net ABX app which interfaces with a foobar and communicates with a PIC through the serial port. The sources were also switched using a bunch of relays, with a lot of headroom--IIRC it was possible to ABX balanced amps using the setup I used if I had used the appropriate xlr connectors