too much gain-volume

[Gio]

Anybody has an idea on how I can lower the gain of the normal channel of a Fender Frontman 25R combo amp? I've already tried to change the volume pot with one of a higher value but the little thing screams like hell after 1 on the dial is passed! It's playable only at 0.5 on the dial of the normal(clean) channel! The OD channel is far more versatile, although not just right either! Schematic can be found at Fender's Mr.Gearhed site.
URL is: http://www.mrgearhead.com/

[petemoore]

Stick a resistor in the signal path. Lessen resistance from a point in signal path to ground.
 Better Yet, put a 'volume control' [gain control] between stages, before the clipping stage.
 If serious gain cutting is done, tone change may also occur, re-voicing may re-change this.
 These will increase high end passing...
 A big R in S.P. may benefit by having a small cap across it [Sw'd?].
 Same goes for across a large R value gain pot's S.ignal P.ath connections.

[ezanker]

You could try replacing the linear pot with a log (audio) pot.  This will give you a more natural volume increase sound as you turn the knob.

I read somewhere that Fender puts linear pots in so that when you play the amp at the store it sounds really loud on 2 making you think:
"If it's this loud on 2, imagine what it's like on 10...".  :twisted:   Whether this is actually true or not, a log pot will let you turn it up beyond 0.5 before getting too loud.

Erik

[Mark Hammer]

You don't necessarily need to change the pot.  You may be able to get away with just tacking on a resistor to the pot to change its taper.  Try reading this article first: http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

The Normal Channel on the Frontman has a decidedly "left of center" approach to gain.  First off, the input stage (U1-A) is a little whack.  Some kinda differencing thing that I don't recognize.  From the input stage (whose gain I can't readily deduce) it goes through an attenuator formed by a 22k and 150k resistor, to a second noninverting gain stage.  If it was a "normal" stage, it would have a gain of about 30 (220k+7k5/7k5), a high-end rolloff of 72khz and low end rolloff of around 10hz.

Rather than have the typical inverting input pin connected to ground through a resistor and cap, it goes through a resistor and two caps in series.  The junction of those two caps is also tied to ground through on leg of the Normal Volume pot.

The other leg of the Normal Volume pot is also tied to ground (naturally, since the wiper is what's tied to ground) from the 22uf cap at the output of that stage (U1-B).  

The way it works is that as the wiper is rotated towards the 22uf cap side, it forms a low resistance path to ground for the stage output, making it quieter, and the other leg forms a higher resistance path to ground, effectively reducing the gain of the stage, but in a slightly offbeat frequency-dependent way.  Remember that caps function like frequency-dependent resistors, so the second cap (.0022uf) in series continues to provide a low impedance path to ground for high frequencies even when the pot provides a higher impedance path to ground for the rest of the spectrum.

In case that doesn't mean a lot to you, the gain in non-inverting amps is set by how close it can come to being open circuit in the feedback loop.  The feedback resistor and the path to ground from the inverting pin form a kind of volume control. When the feedback resistance is very high compared to the path to ground, it is as if most of the feedback signal is simply dumped and never makes it to the inverting pin.  If the path to ground is a LOWER impedance for some frequencies, then they get "dumped" even more, which means that the op-amp is functioning even more like open circuit for those frequencies.  If the high frequencies find it easier to get to ground through the cap than through the pot, you'll get more high frequency boost

In effect, what they've done is rolled a "bright" function, gain adjust, and master attenuation into one knob.  Offbeat, but actually pretty clever.  The Achilles heel would seem to be that making it work optimally may need a different choice of components or pot taper.

One way to address it is to stick a resistor in parallel with the leg of the pot tied to the junction of those two caps.  This will result in there being less change in gain over the range of the volume pot.  That doesn't mean you won't be able to turn the volume down as much, because there is still that path to ground on the output, which you will not be changing.

For now, I'd suggest sticking a 22k resistor in parallel with that one side of the pot.  If my take on this circuit is correct, that should yield a smoother change in volume.  If it tends to get you clipping when you pick hard, then consider changing the 7k5 resistor to 8k2 or 10k.

Let us know how it turns out.

[Ge_Whiz]

I don't see why you can't put a 1M pot in a box with a true bypass stompswitch to switch between gains. Personally, I never use the distortion on my Frontman - don't like the sound - so I always go via a ROG 'Peppermill' and control the volume with the level control.

The Frontman is seriously loud. I gigged with mine last night and sound checked with the volume turned up to 2.5. The sound guy asked me to turn it down as he was having trouble balancing it on the PA!

[It was a great gig and we got three encores, thanks for asking.]

[B Tremblay]

...so I always go via a ROG 'Peppermill' and control the volume with the level control...

[It was a great gig and we got three encores, thanks for asking.]

There you have it folks - conclusive evidence that runoffgroove.com circuits are a crowd pleaser!

[Gio]

First of all let me thank all you guys for your ideas and especially Mark Hammer for the circuit analysis.It's been a real lesson, thanks Mark!
So, here's how it went: I stuck a 20k resistor in parallel with the .0022μF cap, i.e. in parallel with the left side of the Normal volume pot.Mark's take on that one was right; there is a smoother change in volume! The overall volume is still too high though (seriously loud at all settings) and there's clipping at high settings (over 6 or 7). Maybe I should stick a resistor in the signal path, as petermoore implied (raise the value of R8 from 22k to let's say 50k) or lessen the value of R8 (signal to ground), or even change 'em both?  I' m gonna try now to lessen the value of the 20k resistor in order to get rid of clipping at high settings, but I' ll still need to lower the overall volume. I think a "hammer-moore" combination shall do the trick!!

[Gio]

By the way, do you know any other Greek guy in here? I can't believe I'm the only one in Greece with an interest in DIY effects!

[Mark Hammer]

I do believe Porhas is based in Greece.  You can take a peek at the Memberlist from the link at the top of the screen, and sort all the entries on the basis of location.  I'm not guaranteeing that everyone based in Greece will list their location as Greece, without any specific city, but you may find some other names of lurkers.

For reducing the overall volume, let's look a little earlier in the signal path.  If you look between the first two op-amps, you will see the attenuator network of R8/R9.  This drops the level down a bit for U1-B as well as tying the non-inverting input to ground.  This resistor pair can produce more attenuation of the signal by either decreasing R9 or increasing R8.  Since I don't know what the additional function of R9 is, the safest bet is to simply increase R8.  Try sticking in a 27k-30k resistor in place of R8.  It won't interfere with the functioning or rolloffs of the subsequent gain stage and should get you what you want.

[Gio]

Merry Christmas to you all! Keep on building!

So, Mark, unfortunatelly the approach of raising the value of R8, although it makes sense, had no effect on the overall volume! I even ended up  putting a 100K resistor in place; the overall volume remains the same!!!

I have an idea and I would like to have your opinion (and I mean everybody's opinion) on it before attempting it.
Looking at the schematic I think that there is a difference between the two channels in the way they feed the signal into the inverting input of op-amp U3-A.
First of all, from the OD Channel the signal is fed from the output of U2-B into U3-A through C49 and the 100K Drive Volume pot through the small cap C50 and then through R25(220K), that is through an overall resistance of about 320K when the Drive Volume pot is at its minimum setting.
On the other hand the output of U1-B (clean channel) is fed into U3-A through R12, then C6, then R14(91K) and R15(91K), that is through an overall resistance of about 185K when the Volume pot is at its minimum setting.
So, if my approach is right, the problem is that the input of the op-amp U3-A is fed with a stronger signal by the clean channel than by the OD channel since the resistance the signal has to go through has a difference of about 135K, which seems a large difference to me!
So I suggest that the right approach would be to try to supress the output of U1-B by adding some resistance in the signal path, in order to match the overall resistance of the OD channel after U2-B. I think it would be sufficient to raise the value of R15 to 220K.
My only concern is if the function of the JFET Q1 would be affected, and this is what I need some opinion about.

Once again, Merry Christmas to all you guys (at least the ones of either dogma of Christian religion).I hope you all have a good time and a clear mind to post me a word of knowledge!

[Gio]

So guys, the goal was achieved thanks to your suggestions!
Here's the trick: a 20K resistor parallel to the normal volume pot at the side where the junction of the two caps is connected to, as Mark Hammer suggested, normalised the volume increase rate and a 1M resistor in place of R15(91K) caused a signal volume loss of about 30-40%, making the Frontman 25R playable at bedroom volume settings. I should mention that the tone is not affected at all, the Fender bell-like tone is mostly emphasised at higher volume settings, over 3 on the dial.
Happy New Year to you all!