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60W Power amplifier

Started by teemuk, July 04, 2007, 09:40:09 PM

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Quote from: joecool85 on August 14, 2007, 07:02:50 AM
The tone in SS amps is all in the preamp really anyway, the poweramp should produce a non-colored, amplified version of what came out of the preamp.  The second most important tone quality for an amp is the speaker.

Basically, I couldn't agree with you more - however, I do feel that the concept of a "transparent" SS power amp is nowadays a bit "clichéy" and not always sought after either: There are many examples of different approaches, namely various integrated soft clipping (or VAS saturation limiting) schemes, various "tube emulators", which are closely implemented to power amp itself, and - of course - the current/mixed mode feedback configurations, which make the frequency response very unlinear and quite often introduce some tone controls (ie. presence, "damping" etc.) as well. Nowadays, it seems to be more common for a guitar amp to have a current feedback path than not to have it.

As is, the 60W amp design, which started this thread, is a very good example of an unlinear, untransparent power amplifier. However, remove the current feedback loop and the opamp stage it connects to and it becomes quite linear. This is the approach I like: You can put an On/Off button to it. ...And it is not even a very unique design! Anyway, I hope this made my point clear: I couldn't call many (modern) guitar power amps linear and transparent when compared to, say, PA or HiFi amps.

I'm all for protection circuits so long as you can make some good sounding music come out of the amp, and with an LM3886 it is totally possible.

And those chip amps are a very good example how even extremely complex circuits can have a very nice tone - sometimes due to a complete lack of "tone", if you know what I mean.

Also, LJ King, when you mention that the current limiter/protection scheme makes the tone go bad is this something you have deduced by measuring? Have you scoped the circuit and pinpointed that the bad tone is indeed caused by the limiter - or have you just assumed it is?

Anyway, speaking about those protection circuits (namely SOA); I definitely agree that most of them are capable of causing very nasty noises and side effects - and that has also been proven and thus is a point I do not wish to argue. Sometimes these side effects can even blow tweeters. A badly designed VI limiter circuit may even do as much damage than omitting that type of protection completely. Those circuits are a very good reason to equip power amps with "catch" diodes.

However, the main reason for this (I assume) is that tracking the SOA curve is a very complex process and (as far as I know) can't be done with a simple circuit. Proper reading considering the topic is United States Patent 7,061,740. The circuit in my 60W design is extremely simple BUT also POOR - and if you know how it works, or should work, you will note this yourself as well. However, It is very typical to amps from the 70´s and included because I feel that it's better to have even a simple protection than none at all. There are few simple mods to improve it (ie. connect resistors to either ground or rail from shunt transistors' bases). Basically, that would turn the tracking from a straight line to a "sloped" straight line, which tracks the sloped SOA curve a bit better. To get a more "curved" slope ("multislope") you need to add these resistor connections to various voltage references. It gets more and more complex the more accurate you make it. How many manufacturers actually bother to do something like that? No wonder some of them interfere with normal operation. If that explanation went over anyone's head I suggest to read the aforementioned patent.

Nice to see that other people join this discussion too; it was getting a bit too heated for my taste. :)


And for my part in the heat, I apologize. I just can't stand to see things that are technically untrue bandied about as truth.