Site Links
Howdy, Stranger!
It looks like you're new here. If you want to get involved, click one of these buttons!
Quick Links
Categories
Cheap, simple, line level A/B switch box (2in, 4out)
In searching the web, I found several DIY style A/B switch boxes for loudspeakers, but none for line level preamp or power amp switching. Some use relays and remote controls. Others use a couple of DPDT switches and banana jacks. For my project, I decided to go with two DPDT switches and a bunch of cheap RCA jacks. Total cost: ~$30.
The problem is this: When trying to compare the sound of two preamps (or power amps), the differences are very subtle. It is therefore extremely important, at least for me, to be able to switch back and forth quickly before my brain forgets what the other preamp sounded like. Even the 5 to 10 seconds that it takes to swap out cables is much too long.
I plan to test this switcher on my three 6J1 tube preamps, to see if I can hear a difference in sound quality.
Comments
If you are interested Bill, I can ship you a cool looking cigar box to house all that in. I don't smoke cigars but I think some of the boxes look really cool and they are only 3 boxes for $5 at a local high end cigar/wine shop. They have all kinds of shapes and sizes. You just have to spray them with some Febreze and let them sit out in the sun for a day to remove the cigar smell.
PM me an address and I'll send you a 4 in 1 out video switch. It has 4 pairs of audio input jacks and a pair of output jacks.
Ron
Thanks for the offer, Craig. A cigar box would probably look really cool, but it is unshielded and would pick up too much noise. I selected these thicker steel electrical boxes because they were cheap and will provide me with the necessary RFI/EMI shielding.
Thanks, Ron. Do those use plastic or metal cabinets?
They're plastic.
From what?
Often audio cables have some shielding on the low level side. Shielding might not be too big of a concern in most environments but at a workbench there are a lot of devices that are inhearently noisy. I have constant noise in my desktop system caused by multiple computers, power supplies LED lights, function generators scopes, etcetera. This noise persists even though I have a very nice dac, sheilelded cables and amplifier.
Bill. You should check those switches for matching resistance on the terminals. I have seen significat resistance in "high" current switches. Recently had a failed switch with 40 ohms on one side of the switch. I have also had it where switches resistance changes after multiple actuations. This might be do to wetting of the contacts or corrosion.
If you have some extra CAT5 cable laying around it makes great wire for projects like this.
From electromagnetic power supply noise floating in the Aether (60Hz, 180Hz, etc). I can easily measure these noise levels and watch them spike up and down based on whether or not I have small areas of exposed, unshielded interconnect cable. Happens on my work bench as well as my speaker building set up area. The noise is everywhere. Are you saying that shielding doesn't make a difference?
I picked up two new switches and the contacts test good on my meter. For cable, I have several rolls of Belden 83006 #22 awg silver plated copper hookup wire that I like to use (teflon insulated).
Very nice.
Wiring it up:
Project completed. I added some labels with double stick carpet tape.
Really well done!
Nice! To bad ya didn't have a quad pole switch.
How did you do that? White tape, run through a printer with black toner leaving the text white?
I used Photoshop layers and a standard Epson Ink Jet printer. I started by creating pure black background layers of the correct size and then typed pure white text layers to overlay the pure black background layers. I used Tahoma fonts for this project. (I also have Marantz fonts, which I used for the 6J1 preamp labels). After finishing the template in photoshop, I printed it onto a single sheet of 8.5x11" high gloss photo paper. Then I applied several 2" wide strips of double sided carpet tape to the underside of the photo paper. I used enough double sided tape to cover all the printed surface areas. Then, I carefully cut the switch holes out with an exacto knife. Finally, I cut out the various labels on my paper cutter, removed the backing, and stuck them onto the project.
I use double sided carpet tape because it creates a very strong bond. Regular double sided tape does not work very well, it always seems to peel off along the edges.
I thought about that, but local hardware stores only had the double pole type. Decided to use what they had, as I wanted to get it put together quickly and start some listening tests. I now have it hooked up to two 6J1 preamps and the 2 switch arrangement works well to quickly swap both channels back and forth. This project is becoming a real eye opener for me. Was not expecting the results that I am getting. One thing that I have found so far is that it is extremely critical to get the preamp levels matched up properly to make a valid comparison. Even a very small difference makes one preamp sound better than the other.
Thanks, Steve. Works like a charm so far. I'll add some comments about 6J1 preamp version sound quality differences to the other thread.
Adding some 50k potentiometers to both boxes for line level matching might be helpful, and keep A and B circuits very close to identical.
Maybe go with a pair of precision, 10 turn - 50k pots, instead of single turn types. Right now, I am level matching using the preamp's internal 50K pots. I turn the lower gain preamp all the way up. Then I match the other preamp by feeding both preamps a 1kHz sine wave and watching the relative output on my scope. But this method is very "touchy" due to the limited adjustment range of the single turn 50k pots. When playing music, I find that I need to "fine tune" the levels slightly to get a perfect subjective match in SPL.
https://www.mouser.com/ProductDetail/Bourns/3540S-1-503L?qs=v8eK7PNYWrvZdWZgRONwJg==