Impedance selector ratings

[Gaz]

I'm looking for a 3-position impedance selector for a 200W amp, and am not sure how to interpret the ratings. For example, this typical switch is rated 6A @ 125VAC:

http://www.mouser.com/ProductDetail/NKK ... OhN2GCY%3d

Now a 200W amp will put out about 28VAC and 7A into a 4 Ohm load. I'm wondering if the the 6A amp rating goes up since the voltage is much lower than the max (125VAC)?[/code]

[gingertube]

Gaz,
That 7 Amps into a 4 Ohm Load is the critcial factor in switch selection and I would be very sus about relying on extra current capabilty at lower voltage. I see that they rate that switch at 6 Amps at 125V AC and 3A at 250V AC.

On my last amp build I ran into the same problem (choosing a switch to handle the speaker current). For that I eventually found a 3 position, 3 pole switch in my stray parts box and I wired the 3 poles in parallel so that the current was shared between 3 contacts. That scheme may increase the number of switch options that you can consider.

I would either go for the 12A @ 125V AC rated part or get a multi pole switch and wire a couple of poles in parallel as suggested above so as to not exceed the current rating listed for 125V AC.

To look after the switch when installed do not switch the impedance with the amp ON.

EDIT:
Why? Switch Current ratings are based upon the current that they can BREAK without welding themselves due to arcing. That is why they give you a higher current rating at lower voltage. So if you (or anyone else) never tries to switch that switch with the amp turned ON then you will probably get away with the 6A rated unit.

Cheers,
Ian

[katopan]

Voltage and current ratings of switches are usually taken as being independant of each other.

[Gaz]

Thanks guys, I appreciate the answers. That's a cool trick - wiring the poles in parallel - and does really open up the options. Thanks!

[roberto]

In old builds you are obliged to replace with something similar to original, but in new builds I always prefer to add 3 output jaks, rather than adding an impedance selector: 1 for 16ohm, 2 for 8ohm, 2 for 4ohm. It will cost even less than a selector, and you have OT wires directly soldered to jacks.

[Structo]

Three jacks or a multi-wafer rotary switch with two or more wafers wired in parallel.

Here is how Ultrahookedonphonix did his switch.

[Gaz]

Cool! Anyone know where I could find a suitable switch. Having trouble locating on Mouser. Thanks.

[Colossal]

If you don't have to have a 3-position switch but can live with just 4/8 or 8/16R switching, you could use a nice fat ON-None-ON double pole toggle and wire it in parallel. Of course, the downside to a toggle switch on the back of the amp is a) it sticking out and b) the probability that the user will flip it accidentally to the wrong impedance. Cost/hassle vs. risk.

Otherwise check out the other multi-pole/multi-position switches in the NKK HS series: http://www.nkkswitches.com/pdf/hs_ps_ts.pdf

[gingertube]

There is one other option - I wouldn't suggest it for a commercial product but we DIY'ers can please ourselves and that is to use relay switching. Relays with 16 Amp, 25 Amp and 35 Amp ratings are readily available and not too expensive.

A story: In the day job I recently had to find a switch for the output of an aircraft ground power unit. The power unit was capable of 2,000 Amps at 28V DC to do jet starts. I did find a switch which would do the job, it was bigger than most guitar amps, cost as much and was a 2 man lift. I eventually found a relay at 1/4 the cost, 1/10 the weight and 1/8 the size to do that job instead.

Cheers,
Ian