Switchable Preamps using a Bass head?

[ccouch7]

I searched around a little but couldn't find a whole lot pertaining to this:
Has anyone tried the A/B box method of hooking up a Sansamp RBI (or any other preamp) to their bass head for switchable preamps?
How did it work out and what kind or brand of A/B box did you use?

I want to build an A/B selector with a toggle switch instead of a foot switch into a 1u rack space so I can switch from
the bass head pre to the RBI pre for a little versitality.
This diagram is from the RBI owners manual.


Here's the rig I'm using with the RBI effected output going straight to the amp input right now, and I can bypass the RBI
via the front panel (active) switch, but I'd rather be able to switch from one to the other.

Just looking for some opinions before I take the plunge of building or buying and modifying a A/B switch box.

Here's one quick idea I had as far as front panel layout.

[ccouch7]

Bump for one more try.

[RickenBoogie]

There are many AB options, but for best results, I'd get a buffered AB box/switcher. Look into what Lehle has to offer, not cheap, but solid gear.

[ccouch7]

Thanks, I wasn't sure about buffered since it's going thru the amps effects loop?, even though I was checking out a Lehle not realizing it was buffered.

I was also looking at a Wobo which has relay switching I think. I'm not sure if Wobo is buffered or not, I'll have to look into that.

Thanks for the reply, this is one of the things (whether I should use a buffered box or circuit or not) I was not sure about, mostly because it will be in the effects loop of the amp head. And I'm not sure how that will effect the signal especially when using the amps preamp.

I just dug out an old dead ashly 1u rackmount crossover that I think I'm gonna gut and make a new aluminum faceplate for and buy a good A/B box like the Lehle or something and put it's guts in the rackmount.

Any more opinions? I'm all ears.

[LiquidMidnight]

I don't know about functionality, but I think it would look awesome in your rack.

[ccouch7]

Thanks man, I am going to build it, I'm just stuck on what kind of a/b box to get right now. I emailed WoBo, but no response yet.

[ccouch7]

Well I picked up a Boss AB-2 just to test the switchable preamps with my bass head and it worked pretty darn good.

The M6 has a bypass switch on the effects loop to take it out of the signal chain, so it was easy just to turn it (the effects loop) on/off to see how the AB-2 was affecting the signal, I couldn't hear much of a difference at all.

I tested it with my SansAmp BDDIdeluxe (as the RBI is in my other rack right now).I plugged the bass into the main input of the deluxe and the parallel out of the deluxe into the front of the head, then main out of the deluxe into the B input of the AB-2, the amp FX loop send into the A input of the AB-2 and the output of the AB-2 going to the amps FX return, Just like the Tech21 diagram in my original post really. Pretty cool to completely switch from one pre to the other with an A/B box.

I'm going to test it with the RBI next.
I'm not going to use the Boss AB-2 in the rack as it's only battery powered and I looked inside and there is no way to mount those guts in a rack enclosure.
But I know it will work without any unwanted noise or anything. After testing the RBI I'm going to look around at other A/B box's and use the AB-2 in my guitar rig.

[IvanMike]

another thing you can do is this.

if the mesa has an effects loop that is switchable to on/off or has a blend knob, and the mesa has a tuner out you do the following.

• plug into the mesa front.
• plug from the tuner out of the mesa into the sans amp front.
• Plug from the sans amp line out into the mesa effects return (you don't need to do anything with the mesa effects send, nor the sans amp effects loop)
• now you have inputs going into both preamps (sans amp and internal mesa) that are isolated from each other.
• The switch becomes the on/of switch for the mesa effects loop and/or the blend control. All the way off or "dry" is the mesa preamp, all the way on or "effects" is the sans amp preamp.
• If the mesa has an effects blend knob, then you can actually blend the two as well.
• no a/b switch needed.

[ccouch7]

Thanks for the reply man, I tried the method you described as soon as I got home and read it.
I was hoping the tuner out method was going to work as it's simpler, uses only two extra short cables and the switch is already right
there on the back of the head.
But I guess the Mesa preamp is before the tuner out or there's something funny about the effects loop on the Mesa?
So I was getting sound from both preamps when I turned the loop on, it didn't sound bad at all and there was no noise or anything.

I'm sure this would work with a different amp head.
I appreciate your jumping in here and helping, thanks.

EDIT: This method does work if I pull out the mute switch on the Mesa.

[ccouch7]

Originally Posted by IvanMike

another thing you can do is this.
no a/b switch needed.

My apologies to IvanMike, this method does work when I turn on the mute (got to thinking about it today) on the Mesa's preamp.
And I can have the choice of blending both preamps if I want by turning the mute off. Pretty nifty!

I think I still may go with the A/B box way in the end mostly just for the single switch on the front of the rack
with indicator lights to show witch preamp is on, but thanks for your input and ideas, much appreciated.

Sorry about this roller coaster ride of a thread, just doing some preamp experimenting right now.
Now I'm thinking of just getting all the parts and building my own A/B switcher.

Also I feel I should say I really love the preamp in the mesa, it definitely does the job.
I'm doing this just for some extra variety for recording and my band plays a pretty wide range of music.

[ccouch7]

Well just to update this thread, I've tried four different A/B switchers, Boss AB-2, Wobo looper, Loopmaster A/B and finally
one of my Boss LS-2's. The LS-2 works the best for me as an A/B preamp selector in the FX loop.
The A+B mix setting on the LS-2 is a nice bonus for the option of blending the two preamps together, and some of the
other settings and possible routings could be useful in the future.

The Wobo and Loopmaster both had a loud pop the first time I would push the switch after powering on, only the first time
and were fairly silent after that. The pop was bad enough that I was worried about harming my speakers, but I'm sure it was
the way that I was trying to use them in the FX loop that was the problem as the Wobo works excellent on my guitar recording
pedalboard as an FX looper and the Loopmaster works good in my guitar recording rig as a A/B amp selector.

But the LS-2 works really well and is totally silent and transparent for me. I also took the bottom off the pedal to peek inside
and it looks like I could rehouse it a shallow 1u rack enclosure.

I made a couple of mockups to try and visualize how I would put it together below.

[RickenBoogie]

looks awesome, bet it sounds pretty good too.

[ccouch7]

Thanks again Rickenboogie.

I finished putting my preamp selector together. It's working good.

Here's a link Rack PreAmp Selector Rehouse/Build
to the rehouse/build thread (warning: with lots of pics) for anyone
who is interested in this type of thing.

[makohund]

That thing is awesome. Great work.

Out of curiosity... did the other "non A/B switch" method not work?

Bass-->RBI front input
Sansamp Out-->Effects Return (to amp power, then speakers)
Uneffected Out-->Amp input

The way I understand this setup:

-With the effects loop of the amp switched off, the RBI sends your "uneffected" signal to the head, effectively using that preamp and power as if there were no RBI.

- With the effects loop of the amp switched on, the RBI is bypassing the pre and feeding the power amp.


I ask because I just found this out (from Tech21, while troubleshooting an unrelated problem), and was going to try it out tonight. Of course it requires the loop to be switchable, but it looks like the Mesa is.

Looking at it I see the switch is on the back, I guess that'd be a pain to flip at a gig without a footswitch. (Mine is on the front, with a status light.)


I did think about an A/B switch setup of some kind to switch between using the RBI as a switchable pre, vs entirely in the loop as an effects unit, without having to swap any cables around. Haven't taken any time to think through how to do it yet, though.

[tech21nyc]

Yes, if you have a switchable efx loop there is usally no need for the A/B box. It works quite well.

[ccouch7]

I appreciate it, yes the non A/B box method can work as will the way IvanMike describes above. Over the four or five months that I experimented with this trying to find what would work best for me I tried a bunch of different ways and this is what I landed on.

The main problems with the non A/B box method for me and my rig was 1) a loud pop (probably not true for all amps) when switching the FX loop in and out on the amp without powering down first, and 2) I didn't want to have to fumble around behind the amp and the switch is tiny.

I guess I knew this project would look funny to some with the LS-2 and all, but I like it and thought others out there might be looking for ideas on how to do something like this.

[tech21nyc]

I doubt Mesa designed that efx loop switch for that purpose. Even with a relay you can sometimes get "pops" and "clicks." For silent channel switching the circuit has to be designed to suppress the noise.

Anyway I'm glad your project worked out for you. Very nice professional looking work.

[ccouch7]

Thanks!