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Fredde79 7th December 2012 12:28 PM

Bass preamp: non-inverting an inverting tone stack (opamp)
 
Hi!

I'm building a bass preamp (not my first). I'm using a three band active baxandall tonestack found in the LF353 datasheet (I'm using OPA2134 however).

If I'm not completely lost, this tonestack produces an inverted signal, and would need to be re-inverted. Right? So I plan to put an inverting gain stage after it, and then use the remaining opamp section as an output buffer. Does this seem at all sensible to you?

http://subsite.tilaa.eu/share/projec..._tonestack.png

pinkmouse 7th December 2012 12:43 PM

Why bother?

Fredde79 7th December 2012 12:49 PM

Quote:

Originally Posted by pinkmouse (Post 3274534)
Why bother?

I don't know, maybe it isn't necessary? What if I want to split the signal with a DI box before the preamp, wouldn't I have both an inverted an non-iverted signal on stage?

JMFahey 7th December 2012 05:09 PM

The signal will be inverted, no doubt, but it usually does not matter.
Even if you use a direct box, it will go to the PA, away from the stage.
In fact many amps invert at random, sometimes even in the same amp (as in Fender Twins) where if you jumper both channels, you lose signal big way, because they go to the same power amp and speaker (not your case).
And if you join many amps on stage, you'll typically chain drive them from your main preamp out to all other power amps, which *usually* (95% of them), are non inverting.
But anyway we are talking about a single op amp stage, so go ahead if you wish, and add a "phase" switch, to select taking signal before or after it.
Most PA and recording mixers have that available.
It might be useful in some situation if, say, you drive 2 amps, straight at their inputs, from, say, some Bass effects processor.

Fredde79 7th December 2012 05:30 PM

Thanks for your replies! So basically it doesn't make any difference if I make the gain stage inverting or non-inverting? But if I have a separate gain stage, there's no harm in making the first section inverting like in my mockup, right?

Loudthud 7th December 2012 06:17 PM

You have to be carefull in musical instrument amps when using non-inverting stages like the first stage in your schematic. If the input goes below about -12V, the output of that stage will slam the positive rail because the input common mode range has been exceeded. It's better to use an inverting stage there (noise may be higher), even better to add anti-parallel diodes at the summing junction to ground. This is why many opamp circuits have a reputation of sounding bad.

Fredde79 7th December 2012 08:42 PM

Ok, thanks. Is this what you have in mind?

http://subsite.tilaa.eu/share/projec...ion_diodes.png

Loudthud 7th December 2012 09:22 PM

Yes, that would work but I would suggest that you make the input buffer inverting. Many modern bass amps use a variable gain stage as an input buffer between the bass guitar and any tone shaping circuits.

Fredde79 7th December 2012 10:44 PM

Quote:

Originally Posted by Loudthud (Post 3275165)
Yes, that would work but I would suggest that you make the input buffer inverting. Many modern bass amps use a variable gain stage as an input buffer between the bass guitar and any tone shaping circuits.

Ok, thanks I'll try that!

Fredde79 7th December 2012 11:52 PM

Did I understand you correctly, something like this? (I'm thinking the gain pot should be reverse log in this case.)

This would be a neat solution (if it's plausible), I'm a bit worried about input impedance though.

http://subsite.tilaa.eu/share/projec...ing_buffer.png

BTW: feel free to stop answering when you grow tired of my questions :)


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