input in guitar combo

[padamiecki]

Hi
Please help me with this circuit:
It is input in guitar combo Kustom 16.
The simulation shows that it slightly boosts the frequency 50Hz (~1db).
At first I thought that in this circuit is a kind of bootstrap for more high input impedance but why dumping resistor 220k at the input?
Why boost 50Hz?
Thank you in advance for the answers.

[padamiecki]

Nobody?
Dimitri please!

[wakibaki]

Circuit is not drawn correctly. R1 should connect to V+ not R2-R4.

w

[teemuk]

No, it's correct. It's basically a 2nd order Sallen-Key high-pass filter - but with gain. It's a valid circuit - used very often by Fender - but the approximation of gain and response is no longer as straightforward as in the basic Sallen-Key design. ...Nothing a SPICE sim couldn't crack in an eyblink, though.

For reference, basic Sallen-Key filter:


That kind of "hump" is characteristic for the response. It's effect of ~ 1dB boost is quite negligible compared to - 50 dB cut in low frequencies the filter also seems to be introducing.

[dimitri]

Hello Pawel,

Your simulation is correct. See below my simulation - gain in dB and input impedance in kOhm.
I have no idea why this circuit was used as the e-guitar combo input stage...

[wakibaki]

Originally Posted by dimitri

I have no idea why this circuit was used as the e-guitar combo input stage...

The circuit is incorrectly drawn, i.e it is in error, regardless of how the error ocurred.

It does not conform to a conventional Sallen-Key prototype, in all probability it is a conflation of 2 designs, one of which was single rail and used R1-R3 to set the operating point. Otherwise the gain-setting loop to the inverting input should not be connected to the filter loop which connects to the non-inverting input. You might be able to simulate it but how would you design it?

w

[padamiecki]

Thanks Folks!
Do you think that small redesign will improve the performance? (noise, low cut, etc.?)
What would you propose for do with this (best for the guitar)?

[LennyK]

Originally Posted by padamiecki

Thanks Folks!
Do you think that small redesign will improve the performance? (noise, low cut, etc.?)
What would you propose for do with this (best for the guitar)?

It depends what do you (amp owner) want. Tell us what type of sound, tone, you like. We will (or not ) tell you how to create it. There is no such a ting as:ultimate, one and only solid state guitar pre.

In terms of noise - is it to noisy amp ? If not - why even bother? If yes - problem may be hard to crack, since passive guitar pickups have lot of resistance. Every resistance is a source of noise. Best way to avoid it it would be using amplifier with low current noise.
But, I suspect - noise should not be a problem if your guitar amp is not hi-gain beast.

Is it clean or distorted amp ? If it's distorted - hi-pass filter is needed to avoid muddy, buzzy sound. That 1dB bump its not a problem. Characteristic of a guitar amp is not (and it should not be) flat.

Go to best guitar amp site and read, read, read:
Guitar Preamp
It's about tubes, but... The way of making freq. characteristic is the same in solid state.

Remember - most important in guitar amps is its feel, sound, tone, number of neighbours angry when you finish playing and number of devils you created. hahahha
Not one decibel this or other way. To make decent tone sometimes you have to add or remove 10 dB somewhere .

[padamiecki]

Originally Posted by LennyK

Guitar Preamp

Hi
I knew it before
My Kustom Arrow 16 became silent after decoupling power supplies on opamps into one star gnd point.
Of course
there is nothing more to do
but further experimenting...
thanks and

[teemuk]

Originally Posted by wakibaki

The circuit is incorrectly drawn, i.e it is in error, regardless of how the error ocurred.

No, it's correct. At one time Fender used it as an input stage in about every SS amp they produced, such as, for example, this one:

http://www.fender.com/support/amp_sc...ic_-_11x17.pdf

It does not conform to a conventional Sallen-Key prototype, in all probability it is a conflation of 2 designs ..

What I said already. The only drawback from that is that the filter's function and type depends on gain and you can't estimate the outcome with any reasonably simple equation. Nowadays we have SPICE so it doesn't matter.

Otherwise the gain-setting loop to the inverting input should not be connected to the filter loop which connects to the non-inverting input.

Why not? Many OpAmp filter designs exploit that idea.