Topic: AN6291 based DBX Portable Encoder and Mic Preamp

[giovanpi]

Dear tapers, I have been a long time "lurker" in the forum as I have been taping concerts for a few years now.
I only record on cassettes, (for the fun of it) and my standard kit has been a Marantz 430 with Soundman OKM microphones and A3 preamplifier. But depending on the venue I'm sometimes a bit afraid to bring the rather large Marantz, so a couple of years ago I have bought a Sony D6C. Very nifty device if not for the fact that, IMO, DBX is so much better that Dolby for high dynamic range content (like a concert). So I've been wanting to build a portable DBX encoder for years so I could use this fantastic noise reduction with the little Sony D6C.
The Sony D6C has quite certainly the best cassette transport system ever developed for a portable recorder, almost a direct drive, but one that fits in your pocket.

This is why I designed this little box, with also includes a preamp for electrets, for a complete portable kit.

For reliability (aka avoid switches and be sure that it is always in a known state), this can only perform as an encoder, I have a DBX 224 to listen back to the recordings at home. It also doesn't have any setting for the gain of the microphones. But that can be customised changing a couple of resistors.

For anyone who wants to build one, all the schematics and drawings are available here:
https://github.com/pignoniG/AN6291-DBX-Portable-Encoder
Feel free to ask any questions about it.

I will be also making a few more, since I have a few extra boards and AN6291s, if anyone wants a prebuilt one send me a message, I will make a few available trough eBay.
https://www.ebay.it/itm/306882536342

[Gutbucket]

Welcome!

..and cool work, this is a blast from the past! Cool implementation of the first half of the DBX noise reduction compander system.  Potential users will of course need to implement the decoding side somehow as well.  Most likely method would be a playback deck that features DBX, or a stand alone decoder box like the DBX 224 unit you are using. I think I recall hearing about a DBX decoder plugin a few years back.. Aware of that? Is it a thing? If so is it available and does it work correctly?  I've got some DBX encoded master tapes from years ago I might someday get around to transferring.

[giovanpi]

Indeed decoding is a limitation... I thought hard about it, but since this was designed for myself I decided that not having to worry about a switch having poor contacts wile I'm at a concert wins over the possibility of having decoding in the same box...

A digital decoder is probably the best option for digital transfers, and this one works well: http://www.anaxwaves.com/DxIICodec he also offer a DBX I version

[Gutbucket]

^Thanks. Wise to keep it simple and robust, via minimal switches, external wires and connections.

[goodcooker]

This is a really neat homebrew idea and the finished device looks well made and professional. Did you design the circuit then send out to have the components installed on the boards or did you stuff those boards yourself?

[giovanpi]

Thanks! The first one I have made, the one that I used at a few concerts was assembled on boards that were complete empty. For this second run I get them with more or less half of the components already soldered on the boards.
I use http://jlcpcb.com and they offer very affordable pricing on assembly if you choose from a restricted list of components. So all the metal film resistors on the top come opre assembled, but the rest I do by hand.
For instance I could not use their catalogue of sad capacitors because X7R or X5R would have not been acceptable in the signal path. So wanting to use only C0G and tantalum I need to procure them myself.
Off course they will assemble custom components if you ask them to, but for the small number I may ever sell, that was not a reasonable approach.

[rocksuitcase]

We used to run with some GD folks who used a customized DBX 224 made into a portable device as an encoder. Then they told us all to buy DBX 224 units for home decoding/playback.
I patched out of them for a few shows and you need the DBX224 for playback. That was fine back in the day. Currently to use and locate a 224 is difficult enough, I still own one. But for it to be 100% usable as a decoder can be tough. I found a thread on tapeheads where some people are able to modify them now using some hacks and bring them back to spec.
https://www.tapeheads.net/threads/dbx-224-circuit-design-oddities.117489/