Fender Frontman 65R

[J M Fahey]

The whole power amp could do about the same with half the components They obviously hadn't heard of the KISS principle. How could you start with a blank sheet and end up with a circuit that complicated? Design by committee? How does the component count compare with a Fahey amp Juan?

Compare it yourself

This is the basic core, designed to drive a speaker, period.

As you know, I know itīs useless to do tricks inside the NFB loop, because by definition it smashes them, so instead any trick I like is on its own mini board between preamp and power amp, and is triggered by the 8 mA signal which appears as soon as I reach clipping.

I mentioned in an earlier post that I often design around current as much as I design around voltage.

The mini boards do all kinds of wonderful stuff, ranging from a simple optocoupler compressor/limiter to nonlinear and "fast" FET compressors which introduce lots of bluesy intermodulation to narrow band filters to imitate cheap OTs to 50>100 Hz "autowah" which gives bass deep growl to sag in a preclipping stage which adds .... sag .... to variable duty cycle clipping to ......
The sky is the limit, I find great fun in designing silly little boards which flavour otherwise boring "Big Op Amp" amplifiers.

[Dave H]

They didn't start with a blank sheet and come up with this. They started with a conventional amp, and tweaked and tweaked and tweaked.

Right, I see. It starts to make sense when you look at it like that.

[Dave H]

Compare it yourself

Great! thanks I didn't like to ask to see the schematic in case it was a trade secret.

It has about 35 components. The Fender amp has well over 100 (I lost count)! Leo didn't waste a single resistor if he didn't have to. He must be turning in his grave.

[Enzo]

http://bmamps.com/Schematics/fender/..._Schematic.pdf

Same power amp, but the manual has a brief theory of operation. No surprises.

Juan saved parts count by eliminating current limiters and using darlingtons instead of discretes on the output/driver stage.

[J M Fahey]

I am a "minimalist", "simpler is better" if at all`possible, but also a "functionalist" , "necessary things must be done if necessary", sorry for he redundance, so I strip everything to the bone but at the same time try to keep features.

In this case the current limiters are there but "hidden in plain sight" [tape]

To begin with, being a mixed feedback circuit, besides the current not going through the roof is output is shorted, because ifyou lose the voltage part of the feedback, the current part stays there.

So in case of a short output current does not try to become infinite but rise by some 50%, which is manageable.

Besides that, Darlington base is fed only "so much current" which is again limited :
* top one which is where I first noticed it, is fed from a constant current net: the bootstrapped voltage stage resistors: R6 and R7, you have 42V/4k4: 9.5mA , call it 10mA for simpler mental calculations.

TIP14x darlingtons are rated current gain Hfe=1000 so these are limited to 10A tops ... same as Fender (and most others) do with a dedicated current limiter.
So I am saving at least 4 parts (one transistor, 2 resistors, 1 diode) without losing the functionality. [surprised]

* bottom one needs some extra parts [sad] because otherwise in a short Q4 will *try* to feed Q6 tons of current to *try* to keep output voltage constant ... killing it and then comitting suicide; but D1 D2 R14 limit that; even in a short and with input differential pair ordering Q4 to send extra current to Q6, current through Q4 is clamped to "one diode drop"/R14 .

Proper calculated value of R14 is 33 ohms because:
650mV/33r=19.7mA , call it 20 mA peak
since 10 mA is sucked by R6+R7 (remember: "bootstrapped/constant current") only 10mA tops is available for lower darlington Q6 ... it is again current limited in case of a short [tape] , in fact as current limited as the Frontman.

I donīt seem to be saving much: 3 parts (2 diodes and 1 resistor) instead of Fenderīs 4, but there is a twist: conventional short protection by clamping bottom Darlington base tends to kill Vas transistor Q4 because it shorts its load , while limiting Q4 itself tends to save both Q6 and Q4.

In fact, most of my amps in faraway provinces or neighbouring Countries can be repaired by owner or friend "who can solder" just replacing both Darlingtons and wirewound emitter resistors, in general no catastrophic chain destruction.

Calculated value 33 ohms works well but with some Darlinton batches amp clips slightly earlier on the bottom peak, proof they are slightly unded rated current gain, so I lowered value to 22 ohms and it works fine.

Math does not lie, but sometimes parts are not exactly as datasheet says.

Distortion detector Q7 is also chopped to the bone but still functional, in fact it works too well; if you simulate (or build) it, and connect something from ground to free end of R19, say a Led with anode to ground and cathode to resistor end, which has its own dedicated connector for that, youīll see that Led lights *only* (with 8mA pulses through it) when amp starts to visibly clip.
Iīll let you analyze why [wink]

That only-when-clipping current can be used to turn a clipping indicator Led ON, which can light a photocell or two to cleanly limit signal or change frequency response (kill bass so amp does not fart when overdriven) or trigger some kind of autowah (bass players using their fingers love that, it provides a raunchy growl ... literally) or discharge a cap to trigger a FET limiter (or a 3080 and similar) or activate a clamping diode so "amp clips unsymmetrical" or .........

And yes, those miniboards are a trade secret and in fact are gooped.

Absolute worst case, if one fails , user can bypass them and have a conventional amp.

If I need no tricks, just a simple plate amp which lives inside a powered cabinet and just amplfies what is fed to it, then I build this .... which is very common and found all over the place for the last 40 or 50 years:

unless I counted wrong, 27 parts in total.

To be more precise and considering assembly speed or PCB design,itīs 60 holes in that PCB.

This compares favorably to the same functionality TDA7294 amplifier

which uses 41 holes [tape] and is supposedly the "much simplified" solution.

Besides, I trust way more a couple TO247 packages than a 15 pin "double TO220" when fed +/-42V and loaded with 4 ohms.

And scaling up this simple 100W power amp is easy by adding extra TIP14x and ballast resistors, while doing so with chipamps means doubling *everything*

Thatīs why I find zero logic in multiple chip amp configurations: doubled or tripled in parallel, bridged ones, the doubled bridged (so 4 in total) monstrosity found in Marshall MF450, etc .... itīs WAY simpler (and more robust) doing it discrete !!!!!!!

Chipamps are great if you use them as God intended: *simple* solutions ... once you want them to do what they werenīt meant to .... you end up in a complex unreliable mess ... Marshall seems to have learned it the hard way and are back to discrete [smile]