Frontman 25 Tone Stack - Understanding and Adjusting

[BadAmplitude]

Qualifier: I'm electronics light!

The Fender Frontman 25 tone stack seems to be quite a bit different than the usual Fender tone stack and any others I've seen.

The result is that the amp requires EQ settings of something like Treble 1, Mid 7+, Bass 3 to get a usable tone. Widely documented 1000 times on the web as an awakening moment of "Oh, that's what I needed to know about this amp, now it sounds great". The amp is clearly very way over mid-scooped beyond the usual "Fender Sound".

The practical effect is that there's no where to go on the tone. Want it warmer? "Sorry, we're already at "1" on treble.". Want more mids? "Eh, we have a little more for you, not much more.". Less bass? "Dude, you're almost at the bottom already!".

I'd like to tweak it to a flatter curve and get controls that actually are usable like a typical Fender. But, I barely understand standard tone stacks. I have no idea what's going on with this one.

Thoughts on tweaking?

Full Schematic, Click the graphic when you get there to zoom-in: https://i.imgur.com/GD3dVX9.jpg

Tone Stack Excerpt:

[pompeiisneaks]

Although it is a fender, this forum is mostly about tube amps, and this amp is solid state. Some of the solid state savvy types may chime in, but it's no guarantee.

I don't know if tone stacks in SS amps need a different type of tweaking than normal but this one definitely seems oddly overcomplicated from the normal tone stack style I'm used to in tube amps for sure.

I would guess you could try just swapping in exact values from a known stack for an amp you'd like and see how it sounded. It may take leaving some components out, or jumpering some areas to make it work, and on fender pcb's that can be pretty iffy at best due to their extremely cheap quality...

~Phil

[Stevem]

The first thing I would try / audition would be to change out C15 for a .022 cap.
This will bring up the mid’s and the bass, question then becomes will the low end be too much for the bass pot to cut back if it’s not to your liking.

Note that these circuit board type amps where not designed to be worked on and if you apply too much heat for too long to get a part out you will lift up circuit board traces .

I find it best to clip out the part I need to replace, bend back the leads on the new part so I can get a nice blob of solder at each lead end and then just tack solder that to where I cut the leads off of the original part.

Since you are auditioning changes you only need to lift up or cut off one end of C15 and tack in the new cap.

If you clip just one end of C15 be sure to clip it in such a way that you can solder it back together if you want to.

I need to ask , does your guitar(s) not have a tone pot on them to roll back unwanted high end?

Is it the actual ringing of your played notes that are too brite, or is it the initial hit of your pick attack on the strings, as these are two different frequencies that the determination of can help guide in the rework of that amps tone stack.

[imjonwain]

Neat to see they did something a little different. I'll play around with it a bit tonight in spice if I have time.

The mid's control is a bridged-T notch filter. If you remove C18 it's the basic big muff style tone control. You can use the Duncan tone stack calculator to play around with it a bit and there's lot of examples online. The parts of the EQ I would assume are fairly interactive.
Links:
http://www.muzique.com/lab/notch.htm
http://www.geofex.com/article_folders/eqs/paramet.htm

The treble/bass part I need to think about a bit.

[BadAmplitude]

@pompeiisneaks - Thanks... I thought it crossed lines.

@Stevem:

>The first thing I would try / audition would be to change out C15 for a .022 cap.

Just verifying that you mean .022uF for c15? The current cap is only 560pF.

> I need to ask , does your guitar(s) not have a tone pot on them to roll back unwanted high end?

Yes, but I'm thinking that when all the guitars need to be rolled way back, the amp is an issue (and this one is known, so I'd like to tweak it).

As for the sound, I probably should have mentioned as a backdrop that this is on the clean channel (no gain). It's a shared EQ, but just to be clear, it's not the clipping influencing anything.

The sound is just thin. All around. Void of mids. Tweak the controls as I did to give a huge bump to the mids and it sounds good. But with the practical limits mentioned above.

PS. Thanks for the board/soldering tips
PPS. I am planning a speaker swap to something more mid focused, but I think that's going to treat the symptom as opposed to the problem.

[BadAmplitude]

Neat to see they did something a little different. I'll play around with it a bit tonight in spice if I have time.

The mid's control is a bridged-T notch filter. If you remove C18 it's the basic big muff style tone control. You can use the Duncan tone stack calculator to play around with it a bit and there's lot of examples online. The parts of the EQ I would assume are fairly interactive.
Links:
http://www.muzique.com/lab/notch.htm
http://www.geofex.com/article_folders/eqs/paramet.htm

The treble/bass part I need to think about a bit.

I can see that Mid resemblance now that you point it out. but it would be beyond me to model the treble/bass in with the in Duncan's.

[Stevem]

Yes, I mean upping C15 to .022 uf.

[imjonwain]

You might not want to go all the way up to 22n on C15. At least according to spice that'll give you a somewhat flat response across the control sweep since the treble and bass controls have a pretty limited effect stock.

I haven't tested any of this in person and it's all based on simulation so you'll need to test this stuff out. Tone is pretty subjective. If you have a stock of caps and resistors it's not too bad or PM me what parts you have on hand and I can play around.

The graphs below show the freq response of the tone stack (@ the output at U3B) for all possible tone control settings.

Stock Tone stack freq sweep: Big mid scoop and not much treble or bass sweep relatively speaking


Only changing C15 to 22n



You'll end up with much more signal loss from the tone stack but you can try the below couple of changes to get a much bigger sweep from the T&B controls.

I was trying to change as few parts as possible, no cutting traces, or moving things around on the board. So this circuit isn't ideal but it's a simple change.

With this basically you have parallel High pass and Low Pass Filters that mix at the op amp.






You can double or just increase R9 some amount to make up for the signal loss a bit and increasing C8 will cut highs relative to the rest of your signal.

[BadAmplitude]

You might not want to go all the way up to 22n on C15. At least according to spice that'll give you a somewhat flat response across the control sweep since the treble and bass controls have a pretty limited effect stock.

I haven't tested any of this in person and it's all based on simulation so you'll need to test this stuff out. Tone is pretty subjective. If you have a stock of caps and resistors it's not too bad or PM me what parts you have on hand and I can play around.

The graphs below show the freq response of the tone stack (@ the output at U3B) for all possible tone control settings.

Stock Tone stack freq sweep: Big mid scoop and not much treble or bass sweep relatively speaking


Only changing C15 to 22n



You'll end up with much more signal loss from the tone stack but you can try the below couple of changes to get a much bigger sweep from the T&B controls.

I was trying to change as few parts as possible, no cutting traces, or moving things around on the board. So this circuit isn't ideal but it's a simple change.

With this basically you have parallel High pass and Low Pass Filters that mix at the op amp.






You can double or just increase R9 some amount to make up for the signal loss a bit and increasing C8 will cut highs relative to the rest of your signal.

Thanks a bunch... I will look over the circuit.

I'm OK with rigging jumpers and or swapping components. I don't mind stocking up a bunch of small caps to do some testing.

One preliminary question: " parallel High pass and Low Pass Filters that mix at the op amp". I understand the part about the filters... but what's the general sonic outcome with this? Is it cut of both highs and lows that will theoretically mix with the mids and get back to a flatter output?

[imjonwain]

I actually uploaded the schematic without the changes marked by accident. I just updated my initial post with the marked up schematic.


More or less. It's mostly just increasing the amount of adjustment you have for the highs and lows so that you should have a little more variety in the control adjustment. It's not ideal but might be better.

Currently based on the settings you described you're adjusting the mids control to minimize the scoop and then cutting the bass and treble. I'll do a model of your control settings tonight so you can see what it looks like.


The highs on the graphs are mostly for reference as the guitar isn't putting out much (non-harmonics) over the low kHz as far as I understand.

[BadAmplitude]

Slowed down a little on this... waiting to figure out a parts order for an unrelated project so I can have everything shipped at once. Meanwhile, I had a .022 cap in stock so I put that in for C15 to see what would happen.

As predicted the Mid & Bass controls lost a lot of their capacity to adjust.

But two things surprised me:
1. It actually got a whole lot more trebly and lost bass.
2. Volume increased... a rough guess would be that it doubled. Hard to judge, I have no instruments, but that's my best auditory measurement (using headphones).

I set up a switch to do back-to-back testing of new cap vs. old since 1 & 2 seemed non-intuitive. But, with the switch in place, I was able to confirm both changes to the sound.

So, on to my question: Do these results change the suggestions above as to what to try with the larger circuit?

[wpaulvogel]

Although it is a fender, this forum is mostly about tube amps, and this amp is solid state. Some of the solid state savvy types may chime in, but it's no guarantee.

I don't know if tone stacks in SS amps need a different type of tweaking than normal but this one definitely seems oddly overcomplicated from the normal tone stack style I'm used to in tube amps for sure.

I would guess you could try just swapping in exact values from a known stack for an amp you'd like and see how it sounded. It may take leaving some components out, or jumpering some areas to make it work, and on fender pcb's that can be pretty iffy at best due to their extremely cheap quality...

~Phil

Usually the resistor values are 1/10 that of tube and the capacitors are 10 x the value of tube. I just worked on a Yamaha SS G100 amp and it had almost the typical fender values but by the 1/10 and 10 x value. The slope resistor was 10k the treble was 2.5 nF/25kpot, the bass was 470nF/25k pot. Mid was 470nF/1k pot. I modded it to 3.3k slope resistor, 4.7nF/25k pot, 220nF/25k, 220nF/2.5k pot. It responded like a Marshall.

[imjonwain]

But two things surprised me:
1. It actually got a whole lot more trebly and lost bass.
2. Volume increased... a rough guess would be that it doubled. Hard to judge, I have no instruments, but that's my best auditory measurement (using headphones).

So, on to my question: Do these results change the suggestions above as to what to try with the larger circuit?

Not too surprising. If you look at the first two freq response graphs I posted it shows what you're describing for changing that 1 cap. The scales aren't exactly the same so you can't compare them 100% visually but if you look at the the dBv values on the Y axis you'll see it. The modified circuit has a lot more loss so it will be closer to the original circuit overall volume wise depending on the control settings ~ish. Our hearing sensitivity isn't linear either so that can be a factor.