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Guitar Splitter

PostPosted: Mon Jun 19, 2017 7:57 pm
by Text_Edifice
Ok so I have mocked up a schematic for the guitar splitter I've been talking about.

Basically I want a straight connection through to amp 1 (J2) plus a buffered, isolated connection to amp 2 (J3) with the option to ground lift + phase invert this (J3) (I know this is backwards and could do this with 1 extra output- the schematic software I was using is extremely gumby).

I also want to be able to vary the input impedance on the buffer.

I need some help from the brains trust:

- the jensen schematic I'm basing the splitter off uses a resister and cap between both legs of the transformer on the output side. I'm not sure what this is for. Do I need it?

- I have omitted the output cap on the buffer, do I need this?

- should I be buffering the whole thing?

- have I got anything obvious horribly wrong?



Re: Guitar Splitter

PostPosted: Wed Jun 21, 2017 4:09 pm
by rick
the lil labs pcp i have has a case earth life as well as a di ground lift as well as individual channel earth lift as well as polarity switch as well as a level pot for each channel , i have to tell you i had to press them all at some point to set up a triple amp run , that doesn't hmm , buzz spit or lose tone .

if you love building stuff don't let me stop you but if you want to have somebody else do the 10,000 hours of r&D and actually just make records
i think the price of the pcp is fine

Re: Guitar Splitter

PostPosted: Wed Jun 21, 2017 4:25 pm
by Text_Edifice
I enjoy this stuff - but you're right. It would be cheaper to have someone else do the work for me.

The PCP looks ridiculous and would solve all my problems plus a couple of problems I probably don't have yet.

Balanced against that is the cost vs all I really want at the moment is to chase down a sound. It's not urgent and I might never need that sound again.

If I wasn't at the end of the world I'd probably find borrow the thing I need and move on. But I live at the end of the world, so making do with what I have to hand is also part of it.

I am checking ebay for PCP's as I type this though.

Re: Guitar Splitter

PostPosted: Sun Jun 25, 2017 8:44 am
by rob
your schematic won't work very well

email me if you want me to draw what you need

Re: Guitar Splitter

PostPosted: Sun Jun 25, 2017 10:43 am
by Text_Edifice
Thanks Rob,

after a bit more thinking I abandoned the above version and rejigged the design to basically be the Jensen 4 way splitter (with 2 outputs).

I am going to try a slightly different buffer and that may see me come undone. But I kind of want to troubleshoot the design to see if I can get it working.

Re: Guitar Splitter

PostPosted: Sun Jun 25, 2017 12:38 pm
by rob
I wasn't planning on offering you a completely free lunch :)

The best device will be the one you learn something from in its design and construction

Re: Guitar Splitter

PostPosted: Sun Jun 25, 2017 12:40 pm
by Text_Edifice
So far so good...

I have built and tested the buffered transformer isolated bit this morning. It works good.

I used as my starting point for a buffer with a single tl072 and the awa trannie. No noticable level drop (bit more juice and low end actually) and, more importantly, no additional noise.

Next step is to wire in the phase and ground switching, then the second (unbuffered) output.

Re: Guitar Splitter

PostPosted: Tue Jul 04, 2017 5:26 pm
by Text_Edifice
Finally got back to this today. Boxed it up and it works great.

Re: Guitar Splitter

PostPosted: Tue Jul 04, 2017 5:45 pm
by Text_Edifice
Photos for anyone who's interested.

Evidence of a bit of trial and error on the board... ... SHWWSa?dl=

Re: Guitar Splitter

PostPosted: Tue Jul 04, 2017 8:30 pm
by Text_Edifice
And the schematic I used (excuse crude drawing). ... 0.jpg?dl=0

The pulldown resistors are probably overkill - added to stop the pooping I eventually figured out was caused by not decoupling the 9v.

I figured the ground lift on the isolated out was redundant but did add a chassis connection to earth as the jacks I used were plastic and so isolated.

It's quiet and the through seems transparent. The buffer obviously changes tone a little and thinking about adding something post transformer to roll back the highs if I ever build a second one.

I also discovered I need more drill bits. And chisels. The holes in the case are a touch diy.

Re: Guitar Splitter

PostPosted: Fri Jul 07, 2017 7:58 pm
by Text_Edifice
So - a week-ish later and the splitter works great with all my amps that run at 240v.

I tried it with an oz valve amp and a gibson 110v valve amp and suddenly there was buzz.

I had thought to leave the chassis earth connected via a screw - but that didn't do anything.

A switch between chassis ground and the xformer isolated ground solved the problem. If both amps (240v and 110v) connect to the chassis ground of the splitter, no hum. If one is xformer isolated, the hum comes back.

So I am wondering (other than the difference in voltages creating 130 cycle hum?) why? and is having the earth of both amps connected possibly dangerous?

I know most of the commercially available splitters have a ground lift function so I am confident it's all fine but I am also respectful of the voltages involved in tube amps...