Can someone help me understand the Fender Sidekick tonestack?

[vintagelove]

Hello and thanks for looking. I have a sidekick 35 reverb, and while the amp has a number of redeeming features, the voicing of the amp just doesn't suit me 9not enough bass, too much mids). So I grabbed the schematic, and hmmm...

Could someone help me understand what's going on in the circuit or point me to a good resourc?


Lastly a few quick questions.

1 - Where did all the bass go? Is it the tonestack itself or the feedback loop of the first gain stage that cuts the bass? Could I change a component and get it back?

2 - I'm thinking about bypassing the tonestack all together and potentially wiring in Robs/framus mid cut. I could use the following "presence" stage to act as a treble control. What would I change to shift the boost higher in frequency?


Thanks so much for your time.

 

[Digital Larry]

The two op-amps that are pointing down appear to be gyrator sections which are often used in graphic EQs.

10-Band Graphic Equalizer Using Gyrator Circuit – Electronic Circuit Diagram

Generally speaking, to move the frequency range higher, make the capacitors smaller value.

 

[Digital Larry]

To answer your question about bass, each capacitor in series in the signal flow acts as a high pass filter. Without doing a full analysis on the circuit it's going to be hard to tell which one is dominant.

 

[vintagelove]

To answer your question about bass, each capacitor in series in the signal flow acts as a high pass filter. Without doing a full analysis on the circuit it's going to be hard to tell which one is dominant.

Definitely, but glancing at the circuit, the coupling caps at least look decent sized (.1 for the most part). Unfortunately the feedback loop in the 1st gain stage, and the tone circuit are beyond my current understanding.


At a quick glance, can you see any major tone shaping in the 1st stage? I say major, and I suspect the active tone circuit, because the stock tone is very mid heavy, and low bass light.


I doubt anything is wrong with the amp, as I've read others who heard the same. What's there in the bass is strong, it's just shelved high, and altogether different from the traditional blackface voicing. Also, from a read of the manual, the mid is boost and cut, so 6 on the mids for this amp is diffe than what you'd expect. I'm normally on like 1.


Thanks again btw.

 

[Digital Larry]

Unfortunately the feedback loop in the 1st gain stage, and the tone circuit are beyond my current understanding.

Well, me too, without plugging it into SPICE or something. The tone circuit however is just two bands of gyrator boost/cut plus another simple cap only for a high shelving boost/cut (I think). I had designed a gyrator based EQ many moons ago which is why I recognized it.

 

[SRBMusic]

Well, me too, without plugging it into SPICE or something. The tone circuit however is just two bands of gyrator boost/cut plus another simple cap only for a high shelving boost/cut (I think). I had designed a gyrator based EQ many moons ago which is why I recognized it.

This. The bass and mid are active circuits where the gain and center frequencies are controlled by the feedback loop around their respective op amps. Oddly, the treble looks like a passive cut-only circuit, the same as that in any classic passive tone stack.
I’d first try bypassing the tone section to see how you like it. Changing the frequency response of the bass and mid controls involves understanding the specs of the op amp used, and a bit of math (or SPICE) to make a difference.
Lessee, I think my OpAmp Design Basics textbook is up in the attic somewhere…

 

[printer2]

Gyrator Filters

ESP - Gyrator Based Active Filters - Characteristics and Examples.

sound-au.com

 

[63 vibroverb]

Does your Sidekick have the stock speaker in it? The stock speaker sounds lovely (great mids and sparkle) but the bass is lacking. Small voice coil and small magnet will do that. And the size of the cabinet doesn't really help much either.

However, I upgraded the speaker to something bigger/louder (101 db, 2" voice coil, 100 watts, much heavier magnet) and the amp came alive. The bass sounds and feels huge on "3". Just changing the speaker would be less invasive than trying to mod the tonestack.

Something like a G12H30 or clone may suit you if the amp is overly middy to you. It's a scooped speaker with big bass and extended highs.

 

[Wally]

An Eminence GB128 will bring some low end out there. I would do a speaker change and/or add a graphic EQ to the input to shape your EQ. The Soeaker and the EQ pedal could last Beyond your use of this amp. Imho, that amp is not anything I would spend time on inside the circuit if it was working as designed.