Where does the bass get cut off in this amp circuit?

[vintagelove]

Hello and thanks for looking. I have an old Fender sidekick reverb 35. It’s an interesting and somewhat popular little amp that is actually quite loud, great size and weight, but… I’ve just never gotten along with the sound. Two issues, it’s too midrange’y, and it’s definitely got the lowest bass range shaved off. It’s the same through a large cabinet, and it’s not that mine is broken, they all kind of sound like this. So my question,


Where did the bass go?

Unfortunately, both the feedback network of the first gain stage, and the gyrator EQ section are beyond my current knowledge. So I come seeking help. Is there a simple change I can make to get that low end back?


My original thought was to just bypass the tonestack completely, and perhaps replace it with a mid cut control (like Framus). However, if the bass is cut is in the feedback of the first gain stage, that doesn’t really do me any good. Anyway, thanks again for looking. Any help is much appreciated.

 

[rackmonkey]

At a glance, I'd suspect the tone stack hanging off the second opamp. If you can do it without cutting PCB traces, try taking it out of the circuit to confirm. It's the ham fisted way to do it, but messing with the calculations on those gyrators is trickier in my opinion than snip & test here.

 

[vintagelove]

At a glance, I'd suspect the tone stack hanging off the second opamp. If you can do it without cutting PCB traces, try taking it out of the circuit to confirm. It's the ham fisted way to do it, but messing with the calculations on those gyrators is trickier in my opinion than snip & test here.

Hi, thanks for replying.


That was my thought as well, as it should be easy to lift a resistor and jumper it for testing.


Do you see anything in the first stage that is shaping the low end? Thanks

 

[rackmonkey]

Nothing obvious

 

[gyraf]

C13/R22 looks wrong - 100nF into 10kOhms = 159Hz

 

[JohnRoberts]

I see closer to 73Hz for that one pole, but every other pole in the path is cumulative.

The RC formed by C13 (0.1uf) is loaded by 15k (R222) effectively in series with 10k (R21) in parallel with 22k (R23) or 0.1uf x 21.7k at least for signals below the diode clamp voltage.

70Hz is a little high but not crazy.. (caveat I didn't look at every HPF pole) .

JR

 

[vintagelove]

Thanks guys, I’ll start with bypassing the tone section and see if that improves things substantially.

 

[EthanSpaulding]

You should be able to set the tone controls at midpoint (halfway between cut and boost)to bypass their functionality.

Nothing else jumps out at me as unreasonable. It does have clipping diodes after the 100nF C13 and that has some interesting implications to the corner frequency that I have a tough time thinking through how to analyze. Especially when the op-amp output impedance is isolated by the 1.5k (R21).

Possibly switching the position of C13 and R21 could fatten that tone up. Though, it's not clear if overdriven tones are the tones that are too bright.

 

[vintagelove]

You should be able to set the tone controls at midpoint (halfway between cut and boost)to bypass their functionality.

Nothing else jumps out at me as unreasonable. It does have clipping diodes after the 100nF C13 and that has some interesting implications to the corner frequency that I have a tough time thinking through how to analyze. Especially when the op-amp output impedance is isolated by the 1.5k (R21).

Possibly switching the position of C13 and R21 could fatten that tone up. Though, it's not clear if overdriven tones are the tones that are too bright.

Hello, thanks for taking a look.

I am only thinking about the clean sound. Oddly enough, it’s not too bright at all, if anything, it’s borderline dull (which is a surprise for a fender). It is all midrange. The only vaguely Fenderish sound on it is with the mids at 0-1 or 2. I could live with that, but it’s still lacking in bottom end even with the bass at 10.

You know, I never even noticed those clipping diodes down there as at a glance it just looked like it was sending signal to the reverb. I might just lift those and see if it opens up a bit.

Thanks again.

 

[EthanSpaulding]

Oh interesting. I think we were all looking at corner frequencies for the high pass sections that might cut excessive bass. You could evaluate the capacitors that are in the feedback loop of all the stages for anything below 8kHz, let's say. Clean tones can really deal with a lot of top end before it is too bright and scooping midranges by 10dB or more simply sounds like a flat response.

This amplifier has a ton of eq shaping. One might really need to go through and PSpice each stage to find anything crazy. The presence control for instance is a bandpass circuit that should be very flat at minimum boost, but even at minimum will cut top end because of the filters interaction with the mix resistor from the reverb... Now that I mention it, that may be the offending stage. Great chance to practice Spice!

 

[Trouble Report]

Hello and thanks for looking. I have an old Fender sidekick reverb 35. It’s an interesting and somewhat popular little amp that is actually quite loud, great size and weight, but… I’ve just never gotten along with the sound. Two issues, it’s too midrange’y, and it’s definitely got the lowest bass range shaved off. It’s the same through a large cabinet, and it’s not that mine is broken, they all kind of sound like this. So my question,


Where did the bass go?

Unfortunately, both the feedback network of the first gain stage, and the gyrator EQ section are beyond my current knowledge. So I come seeking help. Is there a simple change I can make to get that low end back?


My original thought was to just bypass the tonestack completely, and perhaps replace it with a mid cut control (like Framus). However, if the bass is cut is in the feedback of the first gain stage, that doesn’t really do me any good. Anyway, thanks again for looking. Any help is much appreciated.


View attachment 99728

I'll jump in with some of the other responders to condemn C13. Try increasing it by a factor of 10, like 1uF.

 

[iampoor1]

Make sure the coupling caps are the right values too. Never know someone could if changed them in the past.

Also anyone know what's up with that volume and gain control arrangement? Very odd circuit!

 

[vintagelove]

Make sure the coupling caps are the right values too. Never know someone could if changed them in the past.

Also anyone know what's up with that volume and gain control arrangement? Very odd circuit!

I’m certain everything in the preamp is stock (I have it out on the bench now). This is just how they were voiced. Important to note on that regard, this is from the Fender Riviera period. Considering the time, I think they were attempting to make this something that could compete with other brands and do a rock sound, rather than the classic fender sound. The clean voice was definitely compromised so you could get breakup without it being harsh or farting out on the lows.



Btw, I’d love to see an explanation for that first gain stage feedback network as well. Seems a lot of engineering went into what was a basically a first band worthy practice amp.

My guess from listening is that they are voiced slightly different, and the volume knob brings it up to just below where the clipping occurs, adding the gain pushes it into those clipping diodes.

 

[Chandra]

Before touching anything inside,, try feeding the preamp to another power amp and also from another preamp to this power amp.
This will rule out which side the problem.
Regards.

 

[Mr. Moscode]

More than likely the amp is bass limited to not waste power from noise below an open E string assuming this is for guitar and not keyboard. And it probably doesn't have a very large power transformer or a speaker that can take much abuse, BUT, that said:

Here are my suggestions assuming you can't measure the frequency response:

1. Remove and short out C17. Not needed.
2. Change C18 to 4.7 or 10 mfd
3 Change C21 to 47 mfd or larger - 100, or 220 mfd. Start with 47mfd
4. If necessary change C13 to 10 mfd

Test out after each change to hear the result.

There are tone generator phone apps out there that are pretty good to use to measure frequency response. A scope or an ACVM will let you see it but your ears will let you hear it. A decent DVM might be able to measure.

The input wiring drawing has to be an error. Obviously if the amp works the drawing is a mistake. The jumper from input 1 tip to input 2 switch contact probably doesn't belong there as it shorts Input 1 to ground with nothing plugged into input 2.

The input clipping diodes are there to protect the input op amp from static or a broken ground shield. If you're clipping because of those diodes you are putting an input that exceeds the +/- 15 volts power supply voltage of the input op amp. I assume +/- 15 is the op amp supply voltage as the schematic doesn't have voltages listed.

Good luck and enjoy.

Mr.M

 

[vintagelove]

More than likely the amp is bass limited to not waste power from noise below an open E string assuming this is for guitar and not keyboard. And it probably doesn't have a very large power transformer or a speaker that can take much abuse, BUT, that said:

Here are my suggestions assuming you can't measure the frequency response:

1. Remove and short out C17. Not needed.
2. Change C18 to 4.7 or 10 mfd
3 Change C21 to 47 mfd or larger - 100, or 220 mfd. Start with 47mfd
4. If necessary change C13 to 10 mfd

Test out after each change to hear the result.

There are tone generator phone apps out there that are pretty good to use to measure frequency response. A scope or an ACVM will let you see it but your ears will let you hear it. A decent DVM might be able to measure.

The input wiring drawing has to be an error. Obviously if the amp works the drawing is a mistake. The jumper from input 1 tip to input 2 switch contact probably doesn't belong there as it shorts Input 1 to ground with nothing plugged into input 2.

The input clipping diodes are there to protect the input op amp from static or a broken ground shield. If you're clipping because of those diodes you are putting an input that exceeds the +/- 15 volts power supply voltage of the input op amp. I assume +/- 15 is the op amp supply voltage as the schematic doesn't have voltages listed.

Good luck and enjoy.

Mr.M

Hello, thanks a bunch for the detailed answer.

FWIW, I do have the tools to measure it. I’ll be sure to do so as I make the changes. I’m just not experienced enough to really measure that complicated of a circuit reliably.

Surprisingly, the amp actually has a pretty hefty power supply/section for what it is. It easily gets over a drummer believe it or not. The thing is, if you check out 1:40 below, they voiced it so you could do this (which makes a lot of sense considering when it came out, and it’s competition).



The problem is, it leaves you with a clean sound like this below. Which isn’t "bad", but it’s not really the typical fender’y sound. Midrange’y and kind of dark, without that big fender bottom that’s so nice for clean playing.

 

[JohnRoberts]

I used to share lab space with guitar amp design engineers, my sense is that they didn't select poles in the frequency response casually.

JR

 

[vintagelove]

I used to share lab space with guitar amp design engineers, my sense is that they didn't select poles in the frequency response casually.

JR

Certainly. There‘s not much worse than too much low end going into clipping. If you check the first video I posted above, it actually has a decent high’ish gain sound. Unfortunately I’m not interested in that.

I am just trying to modify the voicing to my preferences for clean sounds only.

 

[Mr. Moscode]

I assume the "complex" circuit is the tone controls. It's not any harder to measure than the power amp. If you have an oscillator with a dial you can find the middle frequency of each control by centering all of them and then sweeping through the frequencies with oscillator knob until you find a reasonably flat response, which may or may not correspond to the pot's centers. Once you have that then turn up one of the pots and find the f where that control peaks. Then turn that control all the way down and find the f where that knob dips. Should be the same in that circuit. Rinse and repeat.

The gyrator circuit was commonly used in graphic EQ's. It's a resonant circuit that you dial into the feedback loop. It could be and was also done with coils and caps, just op amps are cheaper.

Be that as it may, you're seasoning to taste, anyway. All guitar amps are tone filters, not good for hifi. Just part of the guitar. Also, the stock speaker is also part of the guitar. However there is bass to be found in that circuit.

If you are looking for that "tube" sound - well, I've never heard a solid state amp that has it that's why they still make amps with tubes. I've lived in tube and hybrid hi end hifi in my career.

Please let us know how it turns out.

Mr. M.

 

[JohnRoberts]

If you are looking for that "tube" sound - well, I've never heard a solid state amp that has it that's why they still make amps with tubes. I've lived in tube and hybrid hi end hifi in my career.

IMO the peavey transtube was a passable solid state mimic of "tube" sound characteristics for a small fraction of the price.

JR