Designing a DIY Multi-Channel Preamp/mixer for Stereo Output instruments.

[zaubertuba]

I finally settled on a basic design for my next build. Effectively a "swiss-army-knife" stereo instrument preamp designed with signal flexibility in mind.

This originally started as a balanced headphone amp concept based on Ti Kan's JISBOS (J-Fet input stage Bi-fet output stage) line driver. I already have the boards, and have had a desire to build a stereo preamp that would work with my NS/Stick. You know what they say about curiousity...

The Block Diagram ([Invalid or Expired Link Removed]):
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The Design Brief:

-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.

-Transformer interstage accepts various input sources, and provides the negative phase signal for balanced or dual-output.

-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. Of course, the cost of the whole build goes up in using these because I'll be inclined to match component quality with the P&G units throughout.

-4-board JISBOS line-drivers, configured with a little extra feedback to give a gain of 2 (unbalanced) or 4 (balanced).

-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:

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...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 the hifi preamp by Rod Elliot that PassinWind used for his preamp project, or Rod's instrument-specific preamp circuit from his guitar amp project:

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Any thoughts, suggestions?

 

[Passinwind]

...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 the hifi preamp by Rod Elliot that PassinWind used for his preamp project, or Rod's instrument-specific preamp circuit from his guitar amp project:

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Any thoughts, suggestions?

I actually have built separate preamps based on both those ESP boards, and there are links to my builds in the DIY FAQ at the top of the Amps Forum. You might also want to check out the threads on a DSP speaker controller solution I mess with from time to time, and my DIY tube line drive build.

Just got in from a really frustrating open mike scene, and gotta crash, but I'll dig deeper into all this tomorrow. I love stereo rigs!

 

[zaubertuba]

I actually have built separate preamps based on both those ESP boards, and there are links to my builds in the DIY FAQ at the top of the Amps Forum.

I know--at least one of those threads is where I got the idea of the possibility of those boards in the first place.

EDIT: What are your thoughts regarding the hi-fi board vs the guitar board?

You might also want to check out the threads on a DSP speaker controller solution I mess with from time to time...

Ooh...I've been wanting to dabble in something like this to triamp my fEarful.

...and my DIY tube line drive build.

Tempting...very tempting...

Just got in from a really frustrating open mike scene, and gotta crash, but I'll dig deeper into all this tomorrow. I love stereo rigs!

Can't wait for your comments, oh diy BassMeisterSan. <<bows--not worthy! not worthy!>> Thanks for stopping by!

 

[Passinwind]

I know--at least one of those threads is where I got the idea of the possibility of those boards in the first place.

EDIT: What are your thoughts regarding the hi-fi board vs the guitar board?

In general: in standalone usage the guitar board is better suited to passive instruments and the hi-fi one to active ones, although either can be readily adapted to both scenarios. The guitar board also required far less thought on my part to build up, if that matters to you. The hi-fi one is already in stereo format if you omit the midrange and EFX send/mix sections that I kludged into the second channel. I am currently using the guitar board as a post-EFX EQ in my slide bass rig, and the hi-fi one as my everyday gigging unit. So I am actually using both in my big rig, plus the Rane controller.

<Backstory, long and dweeby>
I gigged for around a dozen years with a Fender tone stack equipped DIY design, done on Radio Shack breadboard. It had a spiffy dual fuzz and lot of other features that came and went over the years. The basic idea was two outputs, one clean and one distorted, feeding separate amps and cabs. A buffer eventually blew up and I overreacted and was forced to start over. That was fine, since I was wanting a nice rack case and non-junkbox parts in my next build anyhow.

The project 27 board looked promising as an easy to build replacement, so I built one up and A/B'ed it vs. my Ashly BP-41 and both an Alembic F2B and F1X. I preferred mine, and I gigged with that for around 2 years quite happily. But the usual frustrations with the passive Fender stack started to bug me, especially with my EUB, which somehow became my gigging instrument 98% of the time.

So I built the Glass Audio Aikido tube line driver, figuring to use it inline with the Project 27 preamp, with maybe some Boogie-esque mid boost voicing built in. I also thought it could be useful as a no-EQ recording interface.

This was around the time the fEarful genesis was going on, which led to me wanting to try for something more truly my own in order to optimize for that style of cabinet. Rod's general purpose mixer/preamp board was languishing in my parts bin, so I decided to start there and see if it would give me a jump start. It was always my intention to do my own PCB eventually, once I figured out exactly what format I wanted.

As that build progressed I found something for a fixed midrange circuit that really suits me and the fEarful and nEarful cabs I had built. One of these days I guess I should reverse engineer it and figure out WTH I did! I remember using Ampeg and G-K topologies as a jump off, but there were quite a few forks in that road...

Taking that knowledge, I re-did all the other preamps I own, including the Ashly. The specs in the Project 27 thread are from the first iteration, and have improved by at least 6dB or so in its current state.

All this is a rather long winded way of saying that either board can perform very well if proper attention is given to power supply design and the other usual bugaboos of DIY builds.

I've played through many great commercial preamps at TB GTGs, but in all honesty I have always preferred my own stuff, often to my great surprise.

Thanks for stopping by!

I loves me some DIY build thread, so thank you! Enough about me though, how 'bout a little more info on what you are trying to accomplish?

 

[zaubertuba]

I have a NS/Stick that can output two discrete signals, one for the low strings on the instrument and one for the highs. The NS has active EMG pickups. I play it in a jazz fusion trio where the styles range from Dave Grusin to Gene Harris to Herbie Hancock. More and more, I've been picking up the guitar role in the trio (almost making us sound like a quartet), playing basic comping and lead lines on the high side of the instrument over tapped bass lines in my left hand. Basically, I have to be able to have two discrete tones, one for each side of the instrument, that outputs to my Crown XLS202 and my recently-finished fEarful 12/6/1.

The only multi-effects pedal that I've found I like that can handle the two sides of the instrument, has decent tones, and can output to mono is the Line 6 X3L. I really do like the pedal, but it's actually overkill for what I really need: Just a "clean" bass preamp for the bass side of the instrument (the stock NS Bass tone is really nice), and a guitar preamp with perhaps a decent overdrive and an effects loop so I can get my Pat Metheny and Lee Ritenour tones out of it.

The second half of the unit, after the transformers, still gives me the high-end headphone amp I've been getting ready to build. The dual/stereo mode options were originally intended for a single balanced headphone or a pair of unbalanced headphones, but seem to offer a really decent follower stage for driving an amplifier and simultaneously sending a line signal to FOH. The latter scenario being useful for our larger gigs.

I'm more inclined to go with the hi-fi boards. They seem to have the capability to integrate more options on the board. If I have an on-board overdrive I'd rather have that be all discrete or tube anyway, so I'm more likely to look at outboard FX options if I go more hifi with the internal preamp stages.

Come to think of it, I suppose I could use completely different preamps for the bass and guitar channels, maybe trying something more discrete or "tube-ish" on the guitar side?

The power supply I'm looking at using is Twisted Pear Audio's LCDPS: http://www.twistedpearaudio.com/power/lcdps.aspx. I'm kind of a stickler on power as I've seen the audible effects a sub-par power supply can have on even a "mid-fi" headphone driving circuit.

The JISBOS boards have been characterized as "sweet-sounding" in driving headphones, so while I have an inkling they may yield a pretty nice sound in this role as well.

I've thought about adding a compressor on-board, but the sustain on the NS is already quite amazing, so I'm really not sure how necessary this is.

What opamps do you prefer? I'm using Burr-Brown 2134's on my Balanced a47 (it's like a bridged and balanced Cmoy--the darn thing uses four dual opamps ), and rather like how they sound.

Do you have any ideas how to go about putting together a decent midrange filter? Seems that might be critical part of tone management.

 

[Passinwind]

Cool! We have pretty similar musical inspirations and I can see just where you're trying to go now. My nephew (Dirk Diggler on TB) is an 8 string Warr player, I'll ask him to take a look at this thread too.

I use the B-B 2132 and 2134 a lot. In some cases I prefer the LM4562, which also now goes by LME49720. Used in the ESP hi-fi board, I like it best with the B-B chip in the front end and the National one in the mixer section. Free engineering samples are your friend...

A lot of people have been bugging me about designing a nice, simple, variable midrange section -- it's actually the next thing I plan on getting to when the weather turns to wetter up here. The one in the Ashly BP-41 or one section from the old Moog Parametric EQ are both worth a bit of study time IMO. For someone like yourself who obviously want to go above and beyond, I'd look at switching rather than the usual dual pot you see in so many commercial preamps with variable mids. FWIW, I spent a lot of effort on the simple fixed mid control in my last preamp build. The interactivity with the other controls made for quite a challenge.

I sometimes use an Ashly 1RU channel strip which has a simplified feature set from the PQX series PEQ, and a really decent compressor in it. And then my Rane piece theoretically does up to 150 bands of PEQ...

 

[zaubertuba]

Cool! We have pretty similar musical inspirations and I can see just where you're trying to go now. My nephew (Dirk Diggler on TB) is an 8 string Warr player, I'll ask him to take a look at this thread too.

Well, I'm still quite the "newbie," as I hadn't seriously touched a bass for 20 years up until several months ago. I have a lot to learn, but the NS has really been a blast!

I use the B-B 2132 and 2134 a lot. In some cases I prefer the LM4562, which also now goes by LME49720. Used in the ESP hi-fi board, I like it best with the B-B chip in the front end and the National one in the mixer section. Free engineering samples are your friend...

That National chip has a real following in headphone circles--'sposed to be really resolving and detailed. The B-B 2134 is reportedly a "warm" sounding chip, though in my application the highs were shockingly detailed and airy compared to the discrete headphone stage in my brother's old NAD preamp (which was no slouch, either). That's one nice thing about opamps--being able to roll them to find the best "fit." Note to self--remember to install opamp sockets!

A lot of people have been bugging me about designing a nice, simple, variable midrange section -- it's actually the next thing I plan on getting to when the weather turns to wetter up here. The one in the Ashly BP-41 or one section from the old Moog Parametric EQ are both worth a bit of study time IMO. For someone like yourself who obviously want to go above and beyond, I'd look at switching rather than the usual dual pot you see in so many commercial preamps with variable mids. FWIW, I spent a lot of effort on the simple fixed mid control in my last preamp build. The interactivity with the other controls made for quite a challenge.

It's an old article, but I found this: [Invalid or Expired Link Removed]

...I'm sure there's a lot more out there, as well. I'll look up those designs you're recommending.

I was originally thinking of integrating the PAIA parametric ([Invalid or Expired Link Removed]) into my design, but I don't really need 4 bands of Parametric EQ, and doesn't look like it's been updated in a while, anyway.

I sometimes use an Ashly 1RU channel strip which has a simplified feature set from the PQX series PEQ, and a really decent compressor in it. And then my Rane piece theoretically does up to 150 bands of PEQ...

LOL...150 bands...only slight overkill there.

Maybe I need to play around a bit more turning effects *off* on my X3L and see if I really do need a compressor. If I end up ordering from ESP anyway, it looks like he has a nice simple optical compressor design.

Somehow I get the distinct impression that the build cost on this is going to stack up pretty quickly. That's O.K.--I wouldn't be doing this DIY if I was trying to save *that* much money, anyway.

 

[Passinwind]

Somehow I get the distinct impression that the build cost on this is going to stack up pretty quickly. That's O.K.--I wouldn't be doing this DIY if I was trying to save *that* much money, anyway.

Yep. Just make sure you build what you really want...

 

[zaubertuba]

Yep. Just make sure you build what you really want...

Yeah - Just played a rehearsal for a straight-ahead gig last night that really underscored how much I *don't* need (on the low side) for the less progressive stuff in our regular trio's book. Shut down all sorts of effects in the patch I was using--and all that was really left was tone management.

I'm wondering about my design, though--it accomplishes nearly everything I think is important to me, but I'll have to do *something* to invert the phase of the second stereo output channel if I want to utilize it in a live performance setting. I don't really want to introduce any more integrated circuitry after the input selector--the whole second half of this thing is more of a simple Class-A approach. Maybe I need to look at utilizing the transformers in a different way... Hmmm....

 

[zaubertuba]

Rod has a pretty intriguing looking para on his site, actually. I've been all over his site--don't know why I didn't see this before:

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It's actually a semi-parametric 4-band, with a bottom frequency switchable between a peak/notching and adjustable shelving, two sweepable mids, and a treble shelving switchable between two fixed frequencies. You could easily lose one of the midranges and adjust the range if 4 bands seems excessive. Of course, since Q is not adjustable, it may be better to leave as is.

Since it already has an input buffer, I'm tempted to just build this as an input stage. No boards available, but it's not so complicated you couldn't do it on perf. board. Or...maybe I'll break down, work up the rest of the circuit and etch my own board.

 

[Passinwind]

Rod has a pretty intriguing looking para on his site, actually.

No boards available, but it's not so complicated you couldn't do it on perf. board. Or...maybe I'll break down, work up the rest of the circuit and etch my own board.

Looks good, I'd buy one today if it were available as an unstuffed board.

It's really easy to contract out board fabrication these days. I have a backlog of designs in the queue, but hopefully by this winter I'll have my board design chops up enough to get started clearing 'em out...

 

[zaubertuba]

Looks good, I'd buy one today if it were available as an unstuffed board.

It's really easy to contract out board fabrication these days. I have a backlog of designs in the queue, but hopefully by this winter I'll have my board design chops up enough to get started clearing 'em out...

I think I'll mock the circuit up on perfboard first, since I want to try some other circuits along with it. I'd hate to order a board as-is and then find out later I'd have to kludge in a pre-post switch or something like that. Once I have it right, then I could probably order at least a couple boards with everything I needed.

Speaking of other circuits, I found this little circuit that might be nice for an effects send/return. Maybe I'll adapt it to two separate sends and change the blend control to a pan or something like that.

And that ESP opto-compressor is looking really tempting, now. Don't need it for fingerstyle, so much, but for the extremes of slapping and tapping I'm starting to see just how useful it could be. Am I right in assuming the ideal spot would be right after the input buffer, pre-eq?

 

[fdeck]

It's an old article, but I found this: [Invalid or Expired Link Removed]

That's a cool article. Note that Fig. 2A is how the midrange control works in the Ampeg SVT preamp, but of course the SVT does it with tubes.

 

[zaubertuba]

That's a cool article. Note that Fig. 2A is how the midrange control works in the Ampeg SVT preamp, but of course the SVT does it with tubes.

A tube based eq....now that would be something. If a bit overkill for my immediate purposes!

 

[fdeck]

A tube based eq....now that would be something. If a bit overkill for my immediate purposes!

I'm pretty sure that my JFET based vari-mid could be implemented with a couple of triodes:

This is a basic six-knob channel strip. Note that this isn't my latest design, but it's close. Here's my super-booteek point to point wiring:

 

[zaubertuba]

I'm pretty sure that my JFET based vari-mid could be implemented with a couple of triodes:

Heck, I'd be interested in just trying it with the JFETs. How d'ya like the sound?

Am I reading your schematic correctly that the eq is using dual-ganged pots throughout? (Sorry for the newbie question but that's the first schematic I've seen it presented in that way )

This is a basic six-knob channel strip. Note that this isn't my latest design, but it's close. Here's my super-booteek point to point wiring:

DUDE! That's a pretty fantastic P2P build you have there!

 

[fdeck]

Heck, I'd be interested in just trying it with the JFETs. How d'ya like the sound?

Am I reading your schematic correctly that the eq is using dual-ganged pots throughout? (Sorry for the newbie question but that's the first schematic I've seen it presented in that way )

My bad. I lifted this straight out of my LTSpice model, which doesn't support pots except by drawing a pot as two resistors. All of the pots are single ganged -- you can see three wires coming out of each dotted oval -- except for the frequency control in the midrange -- which is a ganged pot.[/quote]

DUDE! That's a pretty fantastic P2P build you have there!

I really just wanted to try out this circuit in a hurry without committing to a piece of circuit board. But it's not a disaster as construction techniques go. You can see the two pieces of copper wire -- those are from pulling the insulation off a piece of house wire. One wire is ground, the other is the power supply, isolated by plastic stand-offs.

 

[zaubertuba]

I really just wanted to try out this circuit in a hurry without committing to a piece of circuit board. But it's not a disaster as construction techniques go. You can see the two pieces of copper wire -- those are from pulling the insulation off a piece of house wire. One wire is ground, the other is the power supply, isolated by plastic stand-offs.

It's actually one of the cleanest P2P projects I've seen. A lot of them end up looking like an over-energetic gerbil got trapped in a box full of colored straw.

Your circuit got me to thinking though...anybody have a chance to compare a (mostly) discrete preamp like the Kruezer
to a more Hifi/opamp-based design? Passinwind--you did a tube driver; have you done anything with discrete transistors?

 

[Passinwind]

Your circuit got me to thinking though...anybody have a chance to compare a (mostly) discrete preamp like the Kruezer
to a more Hifi/opamp-based design? Passinwind--you did a tube driver; have you done anything with discrete transistors?

Not in about 25 years, as far as DIY stuff. It's on my list, but frankly, I doubt I can improve all that much on what I have already built. I love fdeck's build though!

 

[zaubertuba]

Check out this very concise article by Nelson Pass:

http://www.passdiy.com/pdf/diyopamp.pdf

I'm thinking I'll get myself some breadboards and play around a bit. Maybe see what a discrete-opamp JFET input stage would sound like on Rod Elliot's Parametric.