Simple Gain Recovery after tonestack?

[Ripthorn]

Hi guys, I have a preamp that I built where the tone control can be bypassed.  The issue I have is when I switch in the tonestack (passive baxandall), the volume drop is quite dramatic.  I want to put in a gain recovery after the tonestack to bring the level closer to what it is when the TS is bypassed.  I put together a simple opamp gain stage as shown here (http://www.muzique.com/lab/boost.htm) on the very bottom right (resistor values changed to what I had on hand).  The issue is that when I start to bring the gain up from unity, the signal starts to clip before very much gain is recovered.  Up to this point in the signal path, there have only been two amplification stages and then the tonestack, so I don't think my input signal is too hot to be boosted cleanly.

My question is whether this topology looks suitable for the application and whether there are any other suggestions out there.  The output of this gain recovery would go straight to volume, then power amp.  Thanks.

[tonyharker]

What OpAmp are you using and with what supply voltages?

Tony

[phatt]

Hi Ripthorn,
                Try turning the whole idea around the other way,, you will have much more success that way.

Have a looksee at my tone stack as it does exactly what you are asking,,
ie, When the tone is *bypassed* there is NO Gain.  All the gain is engaged only when the tone stack is ON.
http://www.ssguitar.com/index.php?topic=1136.0

The volume is really just a gain trim so you can attain a good balance between the on and bypass levels.
BTW you will have a darn hard time trying to get distortion out of my circuit,, it runs super clean.
Have fun, Phil.

[Ripthorn]

Thanks for the replies, guys.  I am using a TL072 (one half of it) with 12V supply.  Your circuit looks good, Phil, I might have to give that a go.  I just hope I'm not slamming the opamp input.

[phatt]

Thanks for the replies, guys.  I am using a TL072 (one half of it) with 12V supply.  Your circuit looks good, Phil, I might have to give that a go.  I just hope I'm not slamming the opamp input.

Not sure if you are you referring to *Your Opamp*or *My Opamp* ?

The tone stack (like most of them) sucks up about 60 to 70% of the signal swing,,,
ie, 1 Volt input comes out about 300mV on the treble wiper,Then through the Active buffer and gain section which pulls it back up.
If you are running this from a 9 volt cell then the output can only get to about 6-7volts no matter how big the input swing.
One not so obvious advantage of having a *passive input* tone stack is the input signal can be larger than the supply rail for the opamp and still come out clean,, (as long as the incoming signal is not already distorted of course)
Contry to common practice I actually put my tone box *FIRST* in the signal chain but it will work after other gear as well,, depends on what gear you already have.
One chap who uses one of my tone boxes has a maze of floor pedals and he also has found it better at the front end.
Oh and don't make the mistake of trying to up the gain,, use dedicated pedals for that.
Have fun,, Phil.

[Ripthorn]

Thanks for the advice Phil.  I was just starting to think I should just do the boost in my effects loop.  I do have my circuit running at +/-15V now, so I do get a little bit of a boost there, but still a good several dB lower than the bypassed signal.  In the effects loop it is.

[phatt]

Not sure what the problem is?

Here is how it would work in my mind,
Your preamp> Tone stack *(with it's own dedicated gain block)* then a true bypass switching arangement so the tone circuit With it's OWN gain block is either *In or Out* of circuit.
>> then onto the rest of the circuit.

Though my schematic does not show the true bypass switch it is used to do exactly as I've just explained.
I'm quite sure there are other ways to do what you're trying to do but I've not found one that is as straight forward and as simple to impliment as the mechanical bypass.

I just set the gain trim on my tone circuit so that the *on state and bypass state* are at the same level.
Hope you sort it out,, Phil.