DOD250-Based Telephone/Microphone Circuit

[beansell]

Hey all, long-time lurker, first time poster. I've been building PPCB circuits for a few years now, and thought it was about time that I try my hand at designing a circuit start to finish.

The circuit in question is based off the incredible DOD 250/MXR Dist+. I intend to build this into an old landline telephone, specifically this style model which was distributed throughout Australia by Telstra in the olden times (1980s, probably before and after too). I'm still working on how best to implement it, but it'll include a handset with a working microphone, phase flip switch, and new instrument cable (those old coiled cables are NASTY).



I've been breadboarding it for a few months and ended up with a big mid push, heavy filtering, and lots of gain. I'd also like it to be guitar friendly, so it can be used with the handset mic or as a standalone pedal circuit.

There are just a couple things I'm unsure of:

1. Have I implemented the bipolar supply right? Pinched the power layout right from the Plumes schematic, but I'm not totally convinced I've achieved the headroom I'm after. I've tested with 9vPP vs 18vPP and can audibly hear less op amp clipping, but perhaps there's more clean output to be had?
2. Is my filtering in the first gain stage R5&6/C5&6 working like it should? It sounds pretty good, but I found it somewhat inaccurate when plugging the values into a basic RC filter calculator. I'm also rolling off LOTS of low end with C7/R7 to help get the EQ curve I'm after.
3. Is there anything else I've overlooked?

Also if you read this far, thanks! I'm a little nervous to post my work on the big, scary internet for the first time, so any feedback is greatly appreciated.

 

[Feral Feline]

Welcome to the forum.

Mic-level and Guitar-level are are not the same as far as I understand, perhaps you've already taken it into consideration — do you have a way to switch between the two?

Will the phone-buttons be used to activate relays triggering functions?

 

[beansell]

Hey thanks! Great forum, very welcoming.

Mic-level and Guitar-level are are not the same as far as I understand, perhaps you've already taken it into consideration — do you have a way to switch between the two?

I hadn't thought about accommodating it for both signal types, great suggestion! I was hoping I'd be able to create a 'one size fits all' kind of circuit, perhaps that's a little optimistic. Testing both signal sources shows a decent (and expected) level difference. Should I try reduce the input impedance to cope with a mic signal a little better? The mic measures at 25 ohm output impedance.

Will the phone-buttons be used to activate relays triggering functions?

You'd be the third person to suggest such a thing. The numpad has a multi-pin connection, so it's certainly possible, just not sure what I'd hook them up to.