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bob4 1st December 2012 08:35 PM

Need opinion on Simple mods to Trace Elliot 715 Bass combo
 
1 Attachment(s)
Hi,

my bass combo (pdf with schematic on page 2 attached) has two annoying properties:

1. no mute switch to practice silently with headphones

2. power switch on the back

There is some free space on the front panel, which I would like to use for a mute switch and a power switch.

1. The signal for the headphone jack is derived from the power amp output with a voltage divider. In order to play silently through headphones only, the speaker plug on the back of the amp has to be unplugged. Not very practical! I want to put an appropriately rated SPST switch between R18 and the speaker fuse. Should do the same trick as unplugging, right? Or should I use a DPST and interrupt the sleeve connection as well to be safe?

2. I thought about adding a second power switch (DPST) between the original switch on the back and the primary of the transformer. I'm a little worried about running the mains wires back and forth through the amp - I hope I don't get hum problems.

Any opinions/suggestions?

best regards,

Bob

Enzo 1st December 2012 09:10 PM

Safe? No, just opening the speaker lead will do it. Or like a zillion practice amps you could even have the phones jack interrupt the ground return of the speaker jack. No panel switch needed.

As to the second switch, either way you will have the mains going to the front panel and back as long as there is a power switch up front. Me, I expect power switches on the backs of amps, but if I wanted one on the front, I'd just go with the one. A series switch means the rear one still has to be on all the time. I'd leave the rear one as a dummy and mount your new one out front. Or move it and cover the hole out back. Or you could wire a pair of DPDT switches in series so that either one could turn the amp on or off, like the switches at the top and bottom of the stairs in your home. Of course then off and on would change depending on the position of the other switch.

Twist the wires and run them along the corner/side of the chassis and you should be OK.

AN alternative might be to mount a large triac in the rear by the mains wiring and run a line from its gate put to the front switch. Seems overkill to me though.

bob4 1st December 2012 09:48 PM

Thank you for your reply and input Enzo!

Quote:

Originally Posted by Enzo (Post 3266129)
like a zillion practice amps you could even have the phones jack interrupt the ground return of the speaker jack.

I also expected the amp to do that, but for some weird reason they didn't design it that way.....


Quote:

Originally Posted by Enzo (Post 3266129)
Me, I expect power switches on the backs of amps

It's my home practice amp, placed under a table. Switching it on currently requires quite some gymnastic effort which I want to avoid in the future. :wchair:

Same motivation with the mute switch.....

Quote:

Originally Posted by Enzo (Post 3266129)
I'd leave the rear one as a dummy and mount your new one out front. Or move it and cover the hole out back.

The original switch is square. I don't have appropriate tools to make a rectangular opening for it. So I'll just get a new switch which can be mounted into a drilled hole.

Enzo 3rd December 2012 11:07 PM

Or buy a small outlet strip with a power switch on it, and leave than handy. PLug the amp into it and leave the amp switch always on. The power strip acts as your power switch. Hey, there you go, wire the amp power for always on, using an external mains switched strip, then repurpose the existing power switch as your new speaker switch. NO new holes at all.


Just a small nitpick. I'd call it a speaker switch, rather than a "mute" switch. To me, mute means the whole thing is silenced. But that is just my own feeling. And what you would call it would only matter if discussing it with others.


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