O.K., this went in a completely different direction then I thought it would...just got done reworking my concept into much more of a dual-channel instrument mixer/preamp with great headphone out capability.
The (re-worked) block diagram (
larger version):
Changes to 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.
-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. As mentioned earlier in the thread, 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.
One dilemna I have is what exacty to use for the instrument preamp/EQ. I posted a query on TalkBass asking this question (I know, I'm such a forum-whore!
) , but if there's any diy guitarists/bassists around here who'd like to chime in I'd be glad to hear from you!
Some current options:
PAIA tubehead preamp:
http://www.paia.com/proddetail.asp?prod=9305HSR
..or a couple Rod Elliot designs:
http://sound.westhost.com/project94.htm
http://sound.westhost.com/project27.htm
I am slightly concerned about retaining a quality input signal, what with all the routing on the instrument input stage. I guess that's why I included a source switch that couples just the balanced inputs directly to the transformers. If there's a more elegant way to do this I'd be glad to hear of it.
Any thoughts, suggestions?Designing a DIY Multi-Channel Preamp/mixer for Stereo Output instruments.
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 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:
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 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:
http://sound.westhost.com/project94.htm
http://sound.westhost.com/project27.htm
Any thoughts, suggestions?
