• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

2 Preamps out of phase?

S

SquashedMan

Jul 19, 2008
15
0
#1
Hey I have been experimenting with "bi-preamping", and have a question I hope some of you guys can answer. I've been using a splitter to send a signal to a Sansamp RBI and Alembic F-1X, and mixing the signals at the other end, just 1 volume knob for each.

The THEORY is that the Sansamp can add some grunt, while the Alembic just brings some warm bottom end and a SLIGHTLY smoother midrange, with the trebles backed way off. Both sound GREAT on their own, nice full tones, doing exactly what I aimed to do with them as described above. When I mix them together I end up with a really thin trebly tone. Am I right in assuming this is a phase interaction here? Also, what is available for me to correct this issue? I want it to sound huge out of the cabs first, recording comes a bit later :P

Thanks in advance everyone!
 
#3
+1, if you are using a passive mixer then that's probably your whole problem right there. If you're using an active mixer then yes, phase may be an issue. Little Labs makes a few products for phase correction. A cheaper option is to run each pre through its own channel of graphic EQ, and shape dramatically so that each has its own strong frequency range(s) that do not interfere with the strong range(s) of the other.
 
#5
bongomania said:
+1, if you are using a passive mixer then that's probably your whole problem right there. If you're using an active mixer then yes, phase may be an issue. Little Labs makes a few products for phase correction. A cheaper option is to run each pre through its own channel of graphic EQ, and shape dramatically so that each has its own strong frequency range(s) that do not interfere with the strong range(s) of the other.
Click to expand...

I agree...

bi - "preamping" should be approached the same way as bi-amping...

take the signal, split it through an active crossover...then do what you want with it...but if you are remixing them, phase correction is still recommended.
 
#6
How are you mixing them? passively? Many effects boxes, preamps included, also do not like to have their outputs fighting against other outputs (its an impedance thing).

Anyway just another thing that might go wrong there.
Click to expand...
I'm currently using an SM Pro Audio IN5 I've borrowed off a friend till I can decide whether I'm going to go through with this. Not sure whether it's active or passive, but I assume it's active since SM's other summing box draws a lot of attention to it being passive.

+1, if you are using a passive mixer then that's probably your whole problem right there. If you're using an active mixer then yes, phase may be an issue. Little Labs makes a few products for phase correction. A cheaper option is to run each pre through its own channel of graphic EQ, and shape dramatically so that each has its own strong frequency range(s) that do not interfere with the strong range(s) of the other.
Click to expand...
Interesting point, it's what I may have to do. What I was hoping to do is get a rounded bottom end out of the F-1X and bring some grit to the lower notes with the sansamp, and mix them accordingly. Just doing a few quick adjustments now to the 3 band EQs on each now, if I roll off the lows on the sansamp, what I end up getting is a nice smooth F-1X on all notes played off the E string and a Sansamp everywhere else!
 
#7
PilbaraBass said:
it's quite possible that one preamp is inverting the signal, too...

My suggestion is to build a simple opamp mixer with the ability to invert one of the channels...

otherwise the only other way to tell is by using a scope...
Click to expand...
Hey would you be able to expand on this stuff? I'm kinda a newbie when it comes to signal manipulation, until now it's been head>cab>sound :P
 
#8
It's common for certain types of overdrive effect (for example) to invert the phase of the signal- it's just a byproduct of the type of transistor used to make the effect, and as long as the signal stays in series (instrument --> pedal --> amp) then it doesn't matter at all. But as soon as you split the signal and combine it with itself, then suddenly the signal is (partly) canceling itself out due to the opposite phase of the two split lines. Any sort of preamp circuit may have this "usually harmless" type of inversion in it, and an OD like the Sansamp is particularly suspect.

Using an oscilloscope, you'd run the output of each pre into two channels of the scope, and overlay the waves to see where they do or don't mirror each other. You can also sum the two inputs to see where major peaks or valleys of signal level occur.

There are a lot of DIY electronics sites that have simple recipes for opamp devices; if you have a hankering to do some soldering, you can Google "diy opamp mixer" or visit diystompboxes, diyaudio, geofex, ampage, etc.
 
#9
Squashed, the SM Pro Audio looks like a powered rack mixer. I don't think you're facing an impedance issue then, probably a phase one.

If you're not up to building your own toys, some blenders come with a phase switch option.

Barge Concepts VB-jr comes to mind. The phase switch is a $15 upcharge, but the whole thing's pretty cheap. Basically it takes two imputs and blends them, and has the option of switching the phase of one.
 
#13
PilbaraBass said:
I agree...

bi - "preamping" should be approached the same way as bi-amping...

take the signal, split it through an active crossover...then do what you want with it...but if you are remixing them, phase correction is still recommended.
Click to expand...

+1


That's what I used to do. I had a Peavey chorus pedal that had a crossover in it, and output the straight low and effected high channels. Then I ran the highs to a TubeWorks BlueTube pedal for some grind, and the lows to a EH Microsynth mainly for octave dividing. Then the outs from those went to a small battery powered mixer I made, and then back to the effects loop on my GK-800RB. And of course the 800RB was in biamp mode with an 18" and 2X12".

:D
 

Latest posts

Top Bottom
Back