I think I'll call it the "Odyssey Duet." Trust me, it'll make sense later.

Hey, I could have called it "an attempt to put a crazy amalgamation of everything a stickist would want in one box...plus a hi-fi headphone amp..." but this title sounds better.

With precious few true dual-channel heads out there (and the ones that are cost quite a pretty penny), instead of hackneying together a collection of various boxes, I fancied perhaps it's time for a diy design that could put all the things I'm looking for into one box. I'm not a good enough engineer to design a whole circuit board from scratch, but there's plenty of component diy designs and kits out there to be able to pull something off that's reasonably flexible and (hopefully) sounds pretty decent.

Of course, I could put all this together and it could sound like crap--that's the risk of DIY.

The Design Brief:

Tube preamp stage built off of the PAIA Tubehead: http://www.paia.com/proddetail.asp?prod=9305HSR

I always liked the idea of mix-and-matching tone between solid-state and tube preamps, like on the Hartke LH series. This circuit lets you do that, and run clean, or dial in a little or a lot of overdrive, as well.

I think it'll run with active pickups as-is. Might have to tweak it a bit for passive pickups, though.

Optimized 4-band EQ (lo/hi shelving and 2 mid parametric bands) - The center two bands per channel could be derived from this PAIA kit: http://www.paia.com/proddetail.asp?prod=6760K&cat=27 - Or this part I could scratch-build. It seems like an awfully decent price for all those parts, though. Bass and Treble shelving controls to be manually added. The EQ points for the shelving controls on each channel would be optimized for a high (guitar) range on one side and a low (bass) range on another.

Transformer interstage before signal select, to allow use with balanced or unbalanced inputs.

This may seem odd at first but most of my headphones are balanced, and running the inside balanced from this point makes it easier to implement the following part of the design:

4-board JISBOS line-driver (by AMB) running to balanced headphone/line outs, power amp input, and/or a combination of unbalanced line outputs/power amp input.

I just happen to have the JISBOS kits from Glass Jar Audio. They were originally for a headphone amp I wanted to build but they are reportedly a decent line stage as well. With the right transformer implementation, just inverting one half the signals from each side this way enables greater output flexibility. Or, I can just route them to a jack for my balanced HD580's and have one heckuva practice amp.

Just a side note. The "Odyssey" part of this name, incidently, comes from the fact that I sold my beloved old Odyssey2 game system to finance the purchase of the JISBOS boards. Just in case you're wondering.

Premium Penny & Giles 3000 series stereo faders for the mains level control. Just because I bought them really cheap on eBay, and they're cool. They're also easier to manage if I happen to want to run my eMu1212m through the system and listen to some tunes on aforementioned balanced headphones.

High-Power Class D Stereo Audio amplifier: Someone mentioned amp kits from this vendor - http://myworld.ebay.com/onlineusa/

I already have all the tone-shaping I want up to this point, now I just need good clean, efficient power. Using a stereo amp gives a couple of different output options as well:

a.) Channel output summed (I'm thinking either in the amp, have to research, or via transformer) to bridged mono output for one cabinet or stack.

b.) Separate channel outputs each driving a different side of the amp, allowing for a separate cabinet for each side of the instrument.

c.) Bonus: Channel output summed, active crossover circuit (surprisingly simple when I found it) to dual-mono output for bi-amping. MUWAHAHAHA!

My first cab. to pair this with will be a feaRful design - probably a 12/6: http://www.talkbass.com/wiki/index.php/Fearful Haven't decided if I want to add a tweeter or not.

Now the fun part. The reason I'm posting all this is I'd like the design (or at least parts of it) to be useful to others, and it really can't be unless I know what really would be useful. So let me know. What features would you like to see if this were your amp? Any comments or constructive criticisms will be greatly appreciated. I'd also like to hear the seasoned diy'rs input on ways to cut costs, make the build better, add useful features, etc.

I'll post a block diagram when you've helped me refine the design brief a bit.

Thanks for listening! What'dya think?

This looks very interesting and I love talking DIY, electronics, and Chapman Stick in the same conversation! Have you made any progress on this yet? Thanks for the tip on the digital amps, I've been looking for a source like that for a while. I'm going to make something that will be smaller. I also don't need the tube pres, I like my Mosfet one very much. I'm using a small mixer after my preamp and that works pretty well for now.

Keep us posted, I feel another build coming on this summer.

-Eric

Great thing. Splendid thinking. Just one thing. You might want to add separate fx loops/fx send/returns on both channels. Could be a good thing perhaps.

Sincerely//Jan

I *just* now got started on the concept, though I already have the JISBOS boards and could start stuffing them any time.

Don't those digital amps look cool? It was BasilBadger that turned me on to them. It looks like they just started a new line that accepts either single-ended or differential inputs.

I'm not sure how much I'll use the tube stage on the Bass side, but I need the Guitar side to go anywhere from clean to bluesy. The documentation says the circuit can almost act as a limiter--I'll be interested to see how it responds.

Along these lines, I'm wondering how desirable a separate optical compressor for each channel would be.



Thanks! An effects loop on each channel is a great idea. I'll start reading up on plausible implementations.

EDIT: I also thought about adding a tuner out, as well. When you switch a Stick/NS to mono mode, how does that work with a stereo cable? Does it output both pickups to the tip and ring, or just one of these conductors? reason I ask is, I'm thinking a tuner out should really be tapped before distortion/eq, and it would be a lot easier to tap it from just one side then tapping both and summing them.

Tip only.

Sincerely//J

O.K. - After the first three block diagrams I realized some prioritization is in order. The level of complexity/parts count for every little feature is through the roof - which puts the cost really up there as well.

Here's a listing of the various features - loosely placed in three categories, from mission-critical to "over-the-top."

Mission Critical (or Critically Practical):
1. Dual Channels summing (after processing) to a single speaker output.
2. Decent power output.
3. Separate tone shaping for each channel (3-band, either static or semi-parametric).
4. Separate effects loops for each channel.
5. Tuner out
6. Headphone out w/mute switch

Nice-to-Haves:
7. Bypass switch for eq/effects
8. Active crossover in-unit - for biamped output (would be "over-the-top," but actually costs no more to build than a passive crossover).
9. Balanced Direct Out
10. Switchable modes for biamp out/full-range stereo speaker out/mono speaker out.
11. Input transformer coupling for multiple input types (balanced, unbalanced).

"Over-the-top" (or, raises build cost signifigantly).
12. Hybrid tube/solid-state preamp.
13. P&G Stereo Faders (properly implementing them raises the complexity considerably).
14. Transformers for mode switching/signal summing/signal splitting.
15. Balanced headphone out.

Any ideas/suggestions on things you think might be important -vs- things that you wouldn't really use all that often?

O.K. The design went through a couple of changes...

Here's a link to the block diagram (originally I had it in-line but it's too big--I'll re-size it later):
http://img827.imageshack.us/img827/8665 ... agramb.jpg

Some design changes:

-2 discrete instrument input channels with EQ and an Aux/FX send.

-2 Line-in inputs for FX return or other input sources.

-Stereo bus input stage architecture for maximum flexibility.

-Switchable to balanced direct inputs for use as a balanced line-driver/dedicated headphone amplifier.

-Neutrik Combo output jacks to drive XLR-terminated balanced headphones or other balanced line equipment. 1/4" outputs can be used for the same purpose, or, when the mode switch is engaged, can provide two separately buffered, single-ended stereo outputs for driving two pairs of headphones

With this design, you could run a stereo instrument into a stereo P.A. with complete balance and level control, run a stage amp/monitor and send a separate signal to FOH. Mix in other instruments, two completely discrete (and separately pannable) effects loops....there's almost too many applications to list.

It's a bit of a "trick" using the negative phase signal from the transformers to provide two separate single-ended signals. The cost is that the second stereo signal is out of phase with the first. Obviously I have to work some sort of phase-reversal in order for any other on-stage applications to work.

My biggest dilemna is what to use for the instrument preamp/EQ. I was originally thinking of using the PAIA tubehead preamp:

http://www.paia.com/proddetail.asp?prod=9305HSR

...it seems like a very tonally flexible circuit, and the JISBOS were actually designed as a follower to the SOHA tube headphone amp, so perhaps there'd be good synergy there, but there's several other options out there, like these by Rod Elliot:

http://sound.westhost.com/project94.htm
http://sound.westhost.com/project27.htm

Any thoughts, suggestions?

I found this great Nelson Pass article: http://www.passdiy.com/pdf/diyopamp.pdf

For the input/EQ stages, I'm thinking of doing a bread board mockup of this ESP parametric:




Only substituting using a discrete jfet opamp (along the lines of fig. 16 in the article) in place of the input buffer, the make-up amp after the parametric, and possibly the simulated inductors as well.

Catch is, the 2SK389 and LSK389 dual jfets are pretty hard to find. I might have to get a batch of singles and do some serious matching.

Any news about this project?

Well, I got distracted trying to figure out what I was going to implement for a distortion circuit, but I think I finally landed on one. I'm going to try a starved-plate variation of this one:

http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3097

...maybe with tubescreamer clipping on one of the opamps.

Got some 12ax7 and 12au7 tubes to start playing around with today. The guy sold me the wrong octal sockets, though.

Need to sell my TubeMP to finance more of this project. More to come!