Fender: technical writer needed

[g1]

I can handle the odd error in schematics but it seems all the Fender one's I've needed to look at lately could have really used proof reading by a qualified technical writer. Is this a lost art? Are they just drawn out by computer programs that aren't smart enough to know if they will work or not?
Is this some kind of floating supply or what gives with the power amp output?
http://support.fender.com/schematics..._schematic.pdf

[J M Fahey]

Not sure about your doubt but maybe you refer to the "floating" drawing of the main bridge rectifier and filter caps, which supplies +/-45V.
If so, check that it's fed from P1/2/3 terminals, which come from the power transformer.
Same happens with low voltage supply fed from P4/5/6.
The transformer windings are drawn separately to account for the 3 different versions: USA (120V only) or Export 100V (Japan) or Europe (220/240).
They also show 2 power switch versions, to accomodate different Electrical Safety rules.
Take care.

[Jazz P Bass]

Isn't the circuit similar to some Peavey amps?
The output is grounded, yes.
But the the supply for the output comes from the transformer.

[Enzo]

Nothing wrong with this drawing. This is a common amp design. I usually call it "flying rails," but it has also been called "gounded emitter," and other names. This is the way most QSC amps are wired, and yes, many Peavey amps use this topography.

Nothing to do with the mains here, JM.

The "output" (or what would be the output on a more conventional amp) - the emitters of the final stage - are grounded. The center tap of the powr supply IS NOT GROUNDED. The speaker is connected between that center tap and ground. Another way to say that is that the center tap and the emitters are connected together through the speaker.

You are more used to seeing the emitters connected to the speaker, other side of speaker to ground, and the center tap of the power supply connecting to ground. Another way to say that is that the center tap and the emitters are connected together through the speaker.

SO both amps really work the same way.

But consider this. In the conventional arrangement, if the output stage fails and makes DC, there is then DC across your speaker. In the grounded emitter arrangement, if an output shorts, it pulls one of the power rails down towards ground. But the speaker sees voltage through the filter caps - no direct DC to them.

May I suggest that when you see something you don;'t understand on a drawing, a better first reaction might be, "what am I not getting here?" rather than, "They screwed this up totally and don;t know how to draw." Just a thought.

Want to apologize to Fender here, or just write them direct?

[Enzo]

Hmmm, in re-reading that, I guess my last remark looked a little more harsh than I intended. I meant it, but not in any hostile way.

[g1]

Not harsh at all Enzo and more generous than I deserve. Thanks for your explanation, especially the "why" part.
I would like to apologize here to Fender, rather than write them direct .
Your point about first reaction well taken. I will try to be less critical and remember I'm not up on the newer stuff being semi-retireded.
Thanks for your patience.

[log1982]

Enzo, good explanation.
May I ask you to reconsider your assertion that if any output is shorted no DC is on the speaker? I think that if an output was shorted, the common mode voltage of the transformer would be shifted, resulting in a net DC on the CT. One supply would go to zero, the other would go to something like 90 Volts, and the CT would be at 45V. The speaker would see dc through the CT. PSPICE seems to agree with me. Be gentle if I’m wrong…
..and the whole point is not to show somebody up, but this: Why would someone use this architecture instead of a traditional (as you call it) one?
Regards, Joe.

[jrfrond]

Yes, Fender's use of the flying rails system was popular for awhile.

The whole idea is to keep DC off of the speaker in the event of an amp failure, and if a rail shorts, there is STILL no DC on the center-tap, because it still sits at ground potential. If the speaker was connected in the more traditional manner, at the power transistor outputs, that transformer center-tap would still be at zero in the event of a short. QSC implements it in a slightly different manner. They use the filter cap ground instead of the transformer center tap, but the principle remains the same.

[log1982]

John F, if you wouldn't mind, would you look at my last post? I don't think the architecture protects the speaker against failure as advertised. If an output stage were shorted (or open) I believe DC is applied to the speaker through the CT. Respectfully, Joe.

[loudthud]

The advantage of this arrangement is that the output stage can be driven by an op-amp with 16V rails. KOC discusses this configuration in one of his books. It eliminates the usual diff-pair and voltage amplifying stage which the op-amp replaces as well as some low power high voltage transistors. You will still get DC on the output if one of the output transistors short.

[J M Fahey]

The whole idea is to keep DC off of the speaker in the event of an amp failure, and if a rail shorts, there is STILL no DC on the center-tap, because it still sits at ground potential.

That's the point.
It sits at ground potential only in a working amp.
There is no connection between transformer_center_tap/capacitors_common *except* through the speaker, and it "sits" at approximately chassis ground voltage only thanks to the DC servo action inherent in a working power amp of this architecture; as soon as one output device fails it can no longer keep this, literally, balancing act.

[log1982]

To me, loudthud's explanation appears to be the most likely advantage of the architecture. It's the only one I could come up with, and it seems that his familiarity with this technique confirms. Thanks. Always happy to learn something new.

[km6xz]

Yes, the amp is just as capable of melting a speaker if an output device shorts. This topography is simply another way of drawing the same output current path and a grounded center tap, just moving the load attachment point. As LoudThud pointed out, its chief advantage, and it is a big one, is a highly simplified driver is permitted with low swing voltages.
It is used in servo amps quite often for this reason. It is also used in large power amps in bridge configuration. Take a look at any of the Crown power amps, they use this in a Grounded Bridge arrangement that allows low supply rails and a lot of power from limited numbers of devices, which are inherently thermally balanced. Crown might not be the most hi-fi but they are very stable due to this sensible low voltage floating supply topography, they can get a lot of clean power from 40 volt rails in some of their amps.

[Enzo]

ANother potential advantage - thinking QSC here - is that they can reverse the transistors, and ground the collectors. In the TO3 days, this meant you could bolt the transistor case right directly to the heat sink.