Polykit Spring Reverb Driver

There were a few questions about my Spring Reverb Driver so I decided to create a public thread to have it all in one place.

It is a pretty simple module in Kosmo format which is capable of driving reverb tanks from 8ohms to 600ohms input impedance. It has a dry-wet and a resonance control. PCB and panel are the same so components stick out.

You need to adjust some resistor values according to your reverb tanks input impedance. The input coil must not be grounded to the tanks casing in order to make it work.

You can find the Kicad schematics, layout and gerber files at GitHub - polykit/kosmo-spring-reverb-driver: Schematics and PCB layout for a spring reverb driver in Kosmo modular format

This is a first version on the pictures. There were some flaws which are fixed in the actual version in the repository.

There is also a short demo here:

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@JUXTAPOZ74 here you can see the actual wiring of the sockets, ground comes from the panel itself.

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Are those inverted terminals on the potentiometers? I guess I mean reverse orientation.

I bended the terminals on the pots in the inverse direction if that’s what you mean.

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I am not sure if I understand the “input not grounded” part. I have a BELTON B SN2EB2E/T tank and I checkd this document. So, I translated the code to
small, new, 2 springs, input impedance 600ohm, output impedance 2250ohm, medium decay, no outer channel, with tray.

There is this table:

And for some of the other letters it says if the input is (should be?) grounded (A,B,C,D) but for my tank it says: “E: 10’’ Leads, No Outer Channel” what does that mean?

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I have no idea but you can test with a multimeter if it’s grounded to the casing.

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And if it is, then I should not connect that ground to the one on the pcb?

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No, you need to rewire the input coil so it’s not grounded anymore

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Hey, finally having time to build this! can I substitute the transistors? I don’t have BC640 and BC639 around…

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It should be no problem to replace them as long as they can drive some similar load.

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That would be maximum collector current in the data sheets? I guess for my 600 ohm tank I would not need as much current as for a 100 ohm one, right?

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You’re right, the BC6xx can drive 1A. Maybe you can just go with some standard transistors.

I am going with BC337 and BC327 now.

I found some mistake on your schematic, I think:

The BC640 is an NPN transistor, so the arrow should be pointing in the opposite direction, right?

That is also how it is in the sound-au.com schematic you reference to:

I made quite a bit of smoke! probably the transistors are gone :confused:

ECB CBE, what does that even mean?! Probably not important!! :joy:
I guess I found my mistake :wink:

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If it mattered which lead was which I’m sure they wouldn’t make them all look alike

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I got back to this! With a little more thinking (wasn’t hard at all, I was just slow as usual) I knew how to bend the transistors and now it works (they need to be upside down, but then the middle and right leg switched!)! sounds pretty nice! The feedback comes in very suddenly, even though I used a log pot, but maybe that is just how it is. Thanks @jkb again! :slight_smile:

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Cool, glad that it finally works!