Reduce treble in Fender Frontman 212r clean channel with circuit mod

[elipsey]

Hi,

Yep, it's really bright, and I want to do what I said in the thread title. Here's a schematic of the tone stack, and first stage:


Here's a link to the whole schematic (pdf).

Can someone here explain the function of all the stuff that's connected to the inverting inputs of the first pair of opamps? Please notice R4,5,6,7,8 near U1A, as well as R15,16,18/C7,8,9,10 near U1B and the volume pot. This looks like negative feedback to me... is that right? Can I get rid of some treble here somehow?

Thanks

P.S.
I considered posting in 'Theory & Design' because I want to learn more about the circuit, but I posted here instead because I have a specific goal in mind as well. Hopefully, it is clear from the title that answers like "buy new speakers" or "try an EQ pedal" are outside the scope of the question

[Jazz P Bass]

I would not recommend messing with the opamp circuits.

If you want, download the Duncan Tone Stack Calculator.
Download

I ran your circuit through it & it is indeed a trebly circuit.
I really could not get it to do much with the existing control pots.

I did have it scooping at around 1000hz by changing capacitors.
The best change that I got was making that treble cap 1p and the bass & mid caps 200n.


Note: in the TSC they use N instead of uf. 1n = .001uf which = 1000pf.

[elipsey]

Yeah, I have used TSC. Good idea checking out the tone stack, the values impressed me as being somewhat weird. Actually, I figured if nobody here could explain the other parts of the circuit, reducing the treble cap might be a good place to start. I'm thinking of just clipping different parts into the circuit until I find an improvement. But then I won't learn anything

Maybe I should take try this question in theory and design. I got on this forum in the first place with questions about op-amp reverb and stomp box stuff, I know some of you guys know whats up

[olddawg]

First, is the amp working correctly? I picked up a 212 R for $35 (I had to fix it) and it is fine with a Strat and an LP. It actually surprised me how good that cheap amp sounds. Try a different guitar. Does the Treble control cut the treble on the amp? If it is still to bright you can always put a stomp box eq in front of it or in the fx loop.

[g1]

Hopefully, it is clear from the title that answers like "buy new speakers" or "try an EQ pedal" are outside the scope of the question

[olddawg]

Yeah, I was looking the op on my phone and thought I would say something because I own one. It's a "silk purse-sows ear" sort of thing. Personally, I would put it back stock. I had a kid bring me another 212R the other day. He had a set of new Eminence speakers he wanted me to put in it. I asked him why and he said, "It needs them because of the hum". I said let's check it out first. It turns out the large power supply filter caps had up soldered themselves. (a common problem) After resoldering the caps, amp sounded fine and he was happy. He even offered to give me the speakers in exchange for the repair. I didn't take his speakers and sent him on his way no charge. I have no idea who told him to replace the speakers for a loud 60 cycle hum, lol. I guess a muddier speaker could roll off some highs. But these amps aren't excessively bright IMHO when working properly. Of course it is all subjective and depends on what you are plugging into it.

[elipsey]

Thanks dawg. The controls work normally, and the amp is free from unusual noise. I have tried several guitars, with normal results.

I would like to learn more about op-amp audio. I am intellectually curious about the function of (what appear to me to be) op-amps that receive input and negative feedback on both of their complimentary inputs in the early stages of this circuit. If someone here could explain how those work, that would be cool. Maybe I need to read an engineering text, but I can't figure out which one to start with so I would appreciate recommendations.

Making my cheap amp sound better might be a side effect of learning, but is not my primary goal. Perhaps I should have posted in a different forum, e.g. Theory and Design.

[Enzo]

There is absolutely nothing cosmic about guitar amp op amp circuits. Plain old basic audio. read up on op amp basics from any of the thousand resources google will find for you. They do not work any different in a guitar amp than they do in the output of a tape deck or something.


There is likely a "op amp cookbook" somewhere, which would cover the basics. texas Instruments might have a tutorial on their web site.

[olddawg]

Thanks dawg. The controls work normally, and the amp is free from unusual noise. I have tried several guitars, with normal results.

I would like to learn more about op-amp audio. I am intellectually curious about the function of (what appear to me to be) op-amps that receive input and negative feedback on both of their complimentary inputs in the early stages of this circuit. If someone here could explain how those work, that would be cool. Maybe I need to read an engineering text, but I can't figure out which one to start with so I would appreciate recommendations.

Making my cheap amp sound better might be a side effect of learning, but is not my primary goal. Perhaps I should have posted in a different forum, e.g. Theory and Design.

Start with this: Operational Amplifier Basics - Op-amp tutorial I started in electronics building electronic crossovers and equalizers as a teenager. Like Enzo says, there is nothing cosmic about it. But you might be better off bread boarding a few things. It isn't easy to modify cheap real estate like what is in that amp.

[J M Fahey]

"Fahey Mods for the FM212":

For less plain brightness pull C8.

Or better drill the panel at some convenient place, install a micro toggle switch , the kind used in guitars to switch phase/coils/etc. , lift just one leg of C8 and wire it to the switch so you can go back and forth at will.
Of course you'll need to add an extra wire from thye switch to the empty hole.
Label the switch "Bright".

For more warmth lift one end of C6, add a .1uF cap to complete the circuit to the empty hole , wire another mini toggle so as to short the new .1uF cap or not.
Label that switch "warmth".

Personally I like it as-is, unmodified , but for what you ask the are the answer.

That's why I suggest adding switches, so you can easily go back to normal

[Steve A.]

"Fahey Mods for the FM212":
For less plain brightness pull C8.

Agreed! You suggested that to me last year for Champion and Princeton ss amps I've been modifying. You explained how C8 was like a bright cap with the most effect at the lower volume settings. I went back and forth between different value caps and added series resistors to mellow out the brightness.

I'm not familiar with the FM 212R but the earlier incarnations of the basic design were *way* too bright on the clean channel. (Before I mod them I would usually have tbe Treble control set to 0 when using the clean channel.)

Steve Ahola

P.S. Op amps with their inverting and non-inverting inputs are quite different from tubes and individual transistors. And I do think the tone stack is appropriately scaled for the lower impedance of the op amps...

[Paul Wegmann]

Hi guys! Greetings from Brazil!
I Always felt an excess in some bass frequencies in my Frontman 25r, and using a tone generator into the amp, and an oscilloscope, I can observe that we have an increase of intensity in the range of 100 - 90hz, this frequencies really scream and is so bad because you have to play light that notes, sound like a subwoofer, you know... my question is: Why this exact frequencies "scream", and what can I do to resolve the problem? Thank you very much!

[Toosour]

Hey gang, I found this thread while searching for a way to tame the treble in our Fender Frontman 212r amplifier. I'm digging up this thread to add some photos of the mod suggested by JM Fahey in post #10. If someone else wants to do the mod, these photos may help. I did the mod and think it is worthwhile. The amp runs much warmer and is not so shrill. BTW just removing the bright cap does not change much. A very subtle difference. The warmth cap is what made the big difference . Since the mod is fairly straightforward, I won't add anything to his instructions from #10. The only thing I did different is to put the capacitors onto the switches instead of the circuit board. Cheers, David

[Jazz P Bass]

Nice!

I like the 'Fahey Mods' sticker.

Give credit where credit is due.

[dazz]

I need to replace my 212R's pots, so while I'm at it I thought I'd give Mr. Fahey's mod a go.
I have 0.068uF caps, will one of those work for the warmth switch instead of the .1uF?

Gracias Fahey!

EDIT: nevermind, I'm stupid LOL. I can always combine 2 in series in parallel with 1 for .1uF
EDIT2: The ones I have here are 2a683j polyester caps. Do I maybe need metallized film caps instead?