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Frontman 25R mods

Started by Freddie, February 11, 2010, 06:37:11 PM

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Freddie

Trying to get a better drive sound from the 25R. Looking at the schematic of the 15R and the 25r the difference in the two circuits seems to only be the switching and two diodes. What would be the purpose of D1 and D2 in the 25R drive circuit?

J M Fahey

As a general rule, these little beginner amps are already optimized for what they are.
I don't think there's much to be made to "improve" the sound, at least speaking of amps made by the "Big Ones", say Marshall, Fender, Peavey, Laney, Crate, remember they *know* how to make a good amp.
They are very good for what they are, and there is *NO* mod that will turn them into Mesas, Soldanos, Diezels, Engls or top line Peaveys.
That said, the area where cost-cutting causes most damage, is on the speaker (and attending cabinet).
If you can wire an external speaker jack that cuts the internal speaker when you plug a cable there, and you hook it to the 4 ohms jack of a Marshall 4x12" or equivalent box, you will be *blown* by it, no kidding.
Power, sound, chuggy lows, good non buzzy highs, the works.
I assume you are playing LOUD.
Try to do it at least once, get somebody to let you plug your modded amp there (I mean the new jack).
You'll be amazed.

Freddie

#2
Being poor and not able to afford anything that glows in the dark I have to work with what I have. NO I don't play loud I hardly ever turn it above 3. I play at home for my own pleasure not giging too old at 56 for that. I would like to replace the nickle and dime opamps with some OPA2228's. Just wondering what the hell Fendr put the stupid diodes in there for. If they are not needed for the JFET switching circuit it would sound better without them. I don't want to start changing things without a plan. Would also like to get a Ramrod speaker in there.

phatt

Just re-read the post above your reply,,, JM Fahey is no mug 0:)

Heck replacing opamps in such an amplifier is just thowing money out the window.
Phil.

Enzo

Why are the diodes stupid?  And if you don't like D1,D2, I see no reason you wouldn;t hate D3,D4 and their LED buddies as well.

Those are clipping diodes, when the signal reaches a certain amplitude, they start clipping it - it adds distortion.  They also serve to limit signal excursion into the later stages.


I would agree that replacing the op amps won't accomplish much.  Nothing at all wrong with TL072s.  Keep in mind that this amp is not a hifi amp, no guitar amp is a hifi amp.  The amp is not designed to faithfully reproduce the input signal, in fact it is not designed to RE-produce anything.  The amplifier is a primary producer of sound, it is part of your instrument.   Guitar amps are of limited bandwidth.  A guitar speaker will be rolling off the highs over 3kHz to 5kHz, and the amp circuits generally are not much interested in anything over 5kHz.   SO "improving" the circuit to have video bandwidths and lighting fast slew rates won't do anything for you.

J M Fahey

Hi Freddie.
Fully agree about your Ramrod idea.
If you can build a somewhat larger box for it, even better; you said you won't be carrying it around much.
When I said "Loud" I did not mean "upgrading" to a 50 or 100W amp, but to let loose the one you have now.
I'm sure you are very annoyed at the buzzy thin sound you have now, and most "solutions" consist in heavily cutting the top end, which will kill the buzz, but also sparkle, definition, the works, and not adding needed bass.
Your Ramrod or equivalent "serious" guitar speaker will give you a much better tone, and using the amp "loud" as , say, above 6 or 7, will clip the annoying peaks *without* cutting definition.
To lower power somewhat, you can buy an 8 ohm speaker which will lower your power to about 15W.
Anyway, good tone requires certain minimum levels, the best 1W amp in the world may *record* excellent but your ears will think otherwise.
As far as OpAmps, diodes, etc., the maximum has already been squeezed from them.

Freddie

I was asked why d1 d2 were stupid. Well they are whats known as hard clipping diodes. The problem with the distortion is it is too hard. Getting rid of the hard clipping stage would leave only soft clipping like a tube screamer pedal sounds so much nicer.

bobhill

Looking at the schematic from Fender.com for this amp, it already uses a form of soft clipping. If you look at the clipping stage (looks to be the first half of the second tl072 in the signal path, kinda hard to tell with the blur filter fender seems to have for this pdf), that is the method of clipping used in the Timmy pedal, with a pair of leds substituted for the second pair of diodes. The resister before the junction between the diodes and leds increases the soft knee characteristic turn on of the leds. All the pair of diodes are doing is to raise the clipping threshold by .6vdc, giving you a clip point of approximately 4.4vdc p/p. Having scoped more than a couple of these circuits, the waveshape is a soft turn on. I will leave it to those who are busier arguing about it than playing as to if this style of clipping is more "valve/tube" like, or not. Be that as it may, the only thing that would be accomplished by removing D1&2 is to lower the clipping threshold, pushing things to the square wave distortion point that much earlier. And make your output signal 33% quieter.

The caveat in all this is how much gain previous preamp stages have given the signal, and how close the signal peaks come to the power rails. Again, thanks to the Fender blur filter, this is rather hard to calculate.

The main point to take from this, is that the amp is doing the job it is designed for. It's problems come from inadequate cabinet size for the speaker. The speaker itself is not the worst I have ever seen, but it is crippled by the cabinet. I have seen some folks kind of tame the ice pick from these by using a Jensen Mod 10 to replace the stock speaker. Your mileage may vary, and like clipping diodes, most of what you hear on the internet about speakers needs to be taken with a very large grain of salt.

And by the way? Those "hard" clipping diodes? The tubescreamer uses three of them in pretty much the same configuration to produce it's sound. Only difference is the crappy op-amp used in the TS. If your fender was using the same JRC4558 that the TS used, I could see changing them out. The magic in the 4558? They cost Maxon and Ibanez $0.012 each. The Fender uses TL072's which are just fine. And depending on your soldering skills, none of the op-amps are socketed, and these traces lift up awful easily.

The folks above gave you some very good advise, and they do know what they are doing and have been doing it for many decades. That is what is neat about this place, no BS and folks that have been there.

Roly

Quote from: J M FaheyIf you can wire an external speaker jack that cuts the internal speaker when you plug a cable there, and you hook it to the 4 ohms jack of a Marshall 4x12" or equivalent box, you will be *blown* by it, no kidding.

Totally Agree.  Speaker upgrades generally give you the biggest bang for your buck, and the smaller (and cheaper) the amp, normally the bigger the improvement.  Cone area can do wonderful things for a small "economical" amp.


Thing is, "nickle and dime opamps" such as the TL07x-series are these days so good (compared to crud like 741's or even 4885's), and so cheap for what they are, it's just stupid.

For a while I had a love affair with the LM833 Dual (now apparently discontinued) and used them in several retro-fits and referbs, and also in the preamp upgrade for my stage Twin-50 keyboard amp.  In 50 years in electronics they are the closest thing I've ever seen to a "perfect" amplifier, dead silent with oodles of gain to boggling frequencies.

Originally the preamp gain came from "low noise" BC109C's, but still had quite a significant and audible noise floor.

The pre I built to replace it was nominally the same except it used LM833's (at a buck-and-a-half a pair) for the gain.

The first shock was the bandwidth - it initially oscillated like crazy at 455kHz (!) and I had to specifically limit the bandwidth to 33kHz, down from 5MHz, to ensure stability.

The second shock is that it is now so quiet you can't tell if it's turned on without looking at the pilot light.  {It can be a little unnerving on stage actually because now there is no audible clue that you have left the gain really cranked and are just about to give everyone an unintended blow-wave.}


Quote from: FreddieI was asked why d1 d2 were stupid. Well they are whats known as hard clipping diodes. The problem with the distortion is it is too hard. Getting rid of the hard clipping stage would leave only soft clipping like a tube screamer pedal sounds so much nicer.

How "hard" a diode clips depends mainly on the conditions it is operating under.

An anti-parallel silicon diode clipper has a nominal cut-in or knee at 0.65V ... or 0.5, or 0.7 depending on how you measure it, who you have been talking to, the web sites you have been visiting, wind direction, humidity, the Dow-Jones, and the state of your ulcer.

Because it's actually a temperature-dependent curve.
(below, top-right "Forward" quadrant)



And a real diode;

Forward voltage depends on the current.
(both images thanks to: https://learn.sparkfun.com/tutorials/diodes/real-diode-characteristics)

If you drive this diode pair (via a resistor) with a signal of say 10Vpk it will hard limit at around 0.65V, but if you drive it with a signal of around 0.65Vpk it will only slightly depress the extremes, very soft clipping.  Now if you increase the dive to say 1V pk you will get heavier compression of peaks (but still "soft") and some compression of louder parts.  Increase the drive again to say 2Vpk and the peaks will be hard limited and all the rest progressively soft compressed according to its amplitude.  Back up to 10V drive and you've got rock-hard shred.


What gets interesting with diode clippers is that if they are asymmetric, one voltage is greater than the other (the classic being mixing a silicon and germanium diodes or different coloured LED's) then the spectrum of the output suddenly contains a whole lot more second harmonic to go with the third which is already there.

These days it is so easy to make the electronics good cheaply that cost-cutting hits other part of the design such as speaker choice, cab size, material and thickness (and you can't fix wot ain't broke).
If you say theory and practice don't agree you haven't applied enough theory.

teemuk

#9
QuoteI was asked why d1 d2 were stupid. Well they are whats known as hard clipping diodes

Why is everyone overlooking the simple fact that these diodes are practically connected to the inverting input, which practically sits in 0V under normal operating conditions. These diodes do next to nothing when the amp operates normally but I suppose Fender has found out they offer somekind of transient/surge protection either for the switching FET or the opamp itself since they widely seem to be using this scheme in their amps.

But ordinary signal clippers they aren't. More akin to those diode clamps that hook up to supply voltages; operation only when things would go south otherwise.

The most funny part is that I've seen people copy these Fender circuits right down to these protection diodes but sans the FET switching, which effectively means wasting two diodes for nothing.

gbono

I'm wondering if the design, using the GP diode clamps, is for ESD suppression. :-\

Roly

That will teach me not to check the circuit before commenting.   :-[

You are quite right, with the FET Q2 on they will have no practical effect.

So what are they for?  My guess is that without these diodes when the FET is off it would see the full output swing of the previous op-amp, and that this might be enough to bring it into conduction on peaks.  Maybe.

Quote from: gbonoI'm wondering if the design, using the GP diode clamps, is for ESD suppression. :-\

Doubt that; not connected directly to the outside world, TL072's are pretty robust, and in the middle of the amp but nowhere else.


In fact this already has a pretty fancy clipping network around U2-B (D3, 4, 25, 26 and R22).  My comments above should be related to these diodes, not D1 and D2.  Sorry.  :'(
If you say theory and practice don't agree you haven't applied enough theory.

bobhill

Can I look properly abashed? I will blame it on my laziness on not searching for a readable copy of the schematic. I had noticed the protection diodes, but not having things legible, I assumed (there is THAT word again) that we were not talking about these, as they are not clippers. My response was from seeing the circuit, but not values, around U2b, the "timmy" clipping arrangement. I think that when the OP talked about clippers, we all automatically zoomed into the clipping stage of the pre amp. I did. Sorry. Grumble grumble. No cookies (biscuits) and milk tonite... No Arnott's for me.

Roly

Yes, suitably abashed  :-[ , and I don't even have the excuse of a fuzzy circuit - I just assumed; incorrectly.  xP  {'tho I think the "Arnott's" reference will be lost on most - they are an Aussie biscuit maker.}

Perhaps this one is more readable;
http://elektrotanya.com/fender_frontman-25b_sch.pdf/download.html
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Can anybody please provide Frontman 25 Rschematic?

The one linked above is for Frontman 25 B:duh

Thanks.

By the way, Frontman 15R (or not R) would be interesting, since it has been also mentioned .