Bass Switcher with Discreet 'Gain'

[Swedish Chef]

So I've been away, occupied with a little DIY:-



- No it is not posed, she is just very rude already -

...but now I am clawing back sleep and have been commisioned to make up a switcher box for a bass player friend who want's to do the following:

Play, let's say 4, different basses during the course of a set, probably changing between most songs.

Have individual 'Gain' controls for each bass with which he can even out the differences in output level between basses.

Have his tech ready to handover between songs which may mean having 2 switchable(?) i/p jacks to toggle between. (Although up to this point he has always just unplugged before swapping and plugged himself back in again with the new bass)

So I have scratched out some ideas based on 4 i/p stages, using a single FET as a buffer, perhaps like the Fetzer Valve, or with a BJT current mirror on the drain of the FET with a pot between the mirrored BJT collector and the Drain (I will get some schems up...!!!). With the Fetzer type I was thinking of shunting the o/p with a 1M pot (Gain) and taking the wiper as the o/p, which could then be shunted to ground with a logic triggered BJT, then mix together the o/ps from all the stages into another unity buffer.

The logic side of things, I think, is reasonably straightforward...
He says...

Schem fragments up later!

Cheers

chef :sam:

 

[skipwave]

Hey Chef,

I have exactly the same need. Producer friend needs a switcher for his live bass rig, which includes several electrics and an upright. He unplugs constantly during a set.

I had the same thought on the JFET frontend. Possibly tailoring one input specifically for the upright. Was thinking about an overkill IC line driver after summing node, but simple source follower is probably fine.

Thanks for reminding me, he's probably wondering if I'd forgotten (and I had)! :green:

 

[clintrubber]

I assume the level-differences are not too drastic, right ? Say within some 12 dB of each other.

Then some discrete Sziklai/CFP-stages might do nicely: N-type JFET & a PNP-BJT. Four pots that set the gain for each stage and a one-out-of-four selector after them.

Doing it with some JFET-input opamps will give a more compact & ready to go circuit, but is less s*xy imho. And you already wanted to go discrete.


A CFP-topology without gain is here:

http://www.jensentransformers.com/as/as004.pdf
(ignore right side of circuit)
Inserting resistance between the source of Q1 & the collector of Q2 will add gain.

But there's a better way when wanting variable gain
This circuit shows a variation of the how-to-add-gain:

http://sound.westhost.com/project93.htm

Note it's mirrored w.r.t. Jensen-circuit: Q2 is now an NPN (here it's called Q4) etc.
But look at R5 & R8, these are what it's about.
If you make R5 variable it'll need to be a revlog I assume.

The advantage of this second topology-topology w.r.t. the Jensen is that the DC operating point is unchanged when the pot-setting is changed. And the pot won't be scratchy since no DC-current flows thru it.


Last but not least #1:
For a related example do a search for the nice FET+PNP-schematic DK posted here a long while ago. As is it has just a little gain, but that could be changed. Topology is in the same vein as the Jensen-circuit. As I understood it, the DK/GR-circuit is intentionally not superclean (at least not when loaded), dunno if you would actually prefer that or not.

Last but not least #2:
The Douglas Self-site has some nice Sziklai-variations:
http://www.dself.dsl.pipex.com/ampins/discrete/cfp.htm
http://www.dself.dsl.pipex.com/ampins/discrete/twoq.htm (fig 27)
'Going FET' for the first transistor won't be hard here.


Enjoy,

Peter

 

[Swedish Chef]

Cheers Peter!
That's some interesting reading, so I'll go and do it! ;-)
Here is the switching idea which I prototyped earlier, and works...! :green:


chef

p.s. I'll post more fragments when I get the chance to draw them up acceptably, ha ha ha

 

[clintrubber]

[quote author="Swedish Chef"]Cheers Peter!
That's some interesting reading, so I'll go and do it! ;-)[/quote]
Enjoy!

Be sure to let us know what amplifier-circuit you finally end up with. Obviously the stuff above needs to be pasted together (a bit from here, a bit taken from there) since none of them in present form do already what you need right away.
But as long as you don't have too extreme differences in level from those basses it'll be fine.
IIRIC Self says 10..12 dB of gain from the CFP-circuit is about max.
But then again, adding just one NPN can drastically change that (up to 45dB now, as per the modded Altec 1588B-recipe).

Conratulations with the switching-circuit :thumb:

Hmm, and come to think of it, I'd skip'n'forget all the stuff above, make a 4-in 1-out tree with three BOSS LS-2 pedals and call it a day

Bye,

Peter

 

[clintrubber]

duplicate post removed

 

[clintrubber]

Hmm #2, why won't he sell just one bass ?

It'll be less DIY-fun for you, but I'm about sure
you can do a 1-out-of-3 selection already with the stock LS-2 pedal.
And that's with level-setting: one straight, and the level for the two other selections by the red & green knob.

The basses #2 & #3 would enter the LS-2 at the returns (check for Hi-Z).

Set the white knob of the LS-2 in the fourth position. On each pedal-press there'll be one step in the circle:
bypass (#1) -> A(#2) -> B(#3) -> bypass (#1)...

Cheers,

Peter

 

[jrmintz]

[quote author="clintrubber"]Hmm #2, why won't he sell just one bass ? [/quote]

That's easy:

Four string

Five String

Fretless

Flatwound

Maybe upright

I could use one of these things...

:green: :thumb: :green: :sam:

 

[clintrubber]

[quote author="jrmintz"][quote author="clintrubber"]Hmm #2, why won't he sell just one bass ? [/quote]

That's easy:

Four string

Five String

Fretless

Flatwound

Maybe upright

I could use one of these things...

:green: :thumb: :green: :sam:[/quote]

I see ! :thumb:

Let's add a Bass-VI

But, I don't expect just a level-matching will be satisfactorily. Doesn't the 4-bass-player want to tweak his EQ a bit when switching instruments ?
Using a separate pre-amp for each and switching after the preamps will be better then (but you'll need to synchronize an eventual DI-feed as well).
And that's a lot of gear to haul; maybe 4 buffer stages into a small mixer with simple tone-control can do the trick. Selection then by hand (solo).

Just some thoughts.


BTW, a 2-bass setup is of course a lot easier. I'm using the Ampeg SVT-IIp preamp myself and don't need the graphic for the main bass. So when switching to #2, I kick in the graphic to get the desired change in tone/level.

Bye,

Peter

 

[Swedish Chef]

[quote author="clintrubber"][quote author="jrmintz"][quote author="clintrubber"]Hmm #2, why won't he sell just one bass ? [/quote]

That's easy:

Four string

Five String

Fretless

Flatwound

Maybe upright

I could use one of these things...

:green: :thumb: :green: :sam:[/quote]

I see ! :thumb:

Let's add

...[/quote]

Oh man, if you knew...
The guitarist has around 8 changes, yes different guitars!!!!!!, and they have enough money to lug a HECK of a lot of stuff around the World with them :shock:
On the previous album the guitar tech had 10 amps including redundant spares running per gig, and how did he keep tabs on all the levels/ switching? Yep, he had about 10 of those switchers hooked up :shock:
Glutton for punishment!
This switcher idea was just to have something that would be a single unit and less visually confusing than loads of those switchers.
I've scribbled out a few more fragments but dont have a camera with me so will post them later when at home.


Yes, the tech has enquired about an 8 way one for guitars if the bass one is a go-er.

Seth: the bass player doesn't have an upright (yet!) but the guitarist does use an acoustic so an i/p for one of those would do the trick, eh?

More later :green: :thumb:

chef

 

[clintrubber]

Hmm, time to hear about who's lugging around all this stuff :grin:
Care to unveil the name of the act ?

Cheers,

Peter

 

[Swedish Chef]

Shhhhhhhh.

www.athlete.mu

:green:

...and they have just stolen my drummer to be their second live guitarist :evil:

Ah well, :thumb:

chef

 

[clintrubber]

[quote author="Swedish Chef"]...and they have just stolen my drummer to be their second live guitarist :evil: [/quote]
Did you really think we hadn't spotted that 'special functionality*' in the design you posted above ? :twisted:


*: certain source-selections result in putting the mains voltage on the bass-PU's...

 

[Swedish Chef]

[quote author="clintrubber"]
*: certain source-selections result in putting the mains voltage on the bass-PU's...[/quote]

Heh, heh, if only I didn't love 'em all so much! :green:

"Press selector 1 for sparks, 2 for smoke, 3 for sharks..."
:twisted:

But I do... :wink:

chef

 

[jrmintz]

Doesn't the 4-bass-player want to tweak his EQ a bit when switching instruments ?

Not me, at any rate. I have my instruments set up so they all sound intrinsically the way I want them to and any tone modification is done with my hands. It's taken me 25 years to collect the right instruments and tweak them, BTW. I want amplification that stays out of my way, but I do need to level-match the instruments.

 

[Swedish Chef]

[quote author="jrmintz"]

Doesn't the 4-bass-player want to tweak his EQ a bit when switching instruments ?

I have my instruments set up so they all sound intrinsically the way I want them to and any tone modification is done with my hands. ...but I do need to level-match the instruments.[/quote]

That's pretty much the deal here too.

Just a level matching thing. As transparent as possible...

Fingers crossed.

It was my brother-in-law's birthday party tonight so no time to post schems, but I promise to do it tomorrow

chef

 

[SonsOfThunder]

chef!

Congrats on the little DIY! She's a cutie!

Don't worry my 2 yr old princess does the same kind of rude thing still. :grin:

Enjoy!
Charlie

 

[clintrubber]

[quote author="jrmintz"]

Doesn't the 4-bass-player want to tweak his EQ a bit when switching instruments ?

Not me, at any rate. I have my instruments set up so they all sound intrinsically the way I want them to and any tone modification is done with my hands. It's taken me 25 years to collect the right instruments and tweak them, BTW. I want amplification that stays out of my way, but I do need to level-match the instruments.[/quote]

I see, yes, then no EQ-twists needed :thumb: That's good, less hassle.

My case was about wanting to have a backup-bass sounding like the main bass. Which wasn't the case without some EQ-twists, despite they both being Jazzes. Hence the kick-in-the-graphic.

Have a good weekend,

Peter

 

[Swedish Chef]

Better late than never:

Input stage ideas:



Current Steer LED Driver:


Shunt:
- Tough! Will have to wait until tomorrow: Imageshack is playing silly beggars -

Charlie:
Cheers mate!:grin:
She is already great fun!!!
Looking forward with serious apprehension to her crawling though!!!:shock:

chef

EDIT:

 

[clintrubber]

[quote author="Swedish Chef"]Better late than never:

Input stage ideas:[/quote]

As drawn, those two PNPs will be doing random things. Want a current mirror there ?