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Old 19th January 2013, 06:03 PM   #1
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Talking How Solid State Amps Work

I have been trying to repair my Fender FM212R with little to no knowledge. Looking at the schematics for the first week was like looking at the Code in The Martix. So I went in search of info. I found more info (good and bad) about Tube Amps but Solid State seems to be an after though. I mean there are plenty of people offering their two cents about what's wrong and what to look for (and I appreciate all of you) but I was looking for info on how SS amps work. I finally came across an article that does just that. This fella goings from the beginning lays down the theory and the problems designers went through and how they overcame them. Its a great article if you are interested in stepping beyond guessing at what might be causing the problem into understanding what's going on within the circuits and opamps and so on.

I would like to have a discussion on this topic and maybe we can see what problems others have had and what they have tried to solve them. Maybe some of us can avoid making the same mistakes and such.
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File Type: pdf How Solid State Amps work.pdf (971.9 KB, 347 views)
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Last edited by badraven; 19th January 2013 at 06:14 PM. Reason: Forgot to attach the article
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Old 19th January 2013, 06:08 PM   #2
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Would You be so kind to post the article here, or perhaps give us a link to it?

This seems to be quite interresting.
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Old 19th January 2013, 06:21 PM   #3
Mooly is offline Mooly  United Kingdom
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Thanks for posting the pdf.

A lot to read in there but it seems to cover a lot of the basic groundwork.
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Old 19th January 2013, 06:32 PM   #4
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Thanks a lot.
Interresting reading, and very "self explaining".

Very useful when introducing people to what there is in amplifiers.

The closest document to "Amplifiers for dummies" I have seen in a while.
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Old 19th January 2013, 06:35 PM   #5
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Smile Keeping things cool

I have learned from this article why an SS amp has a + power rail, - rail and a ground terminal coming out of the transformer. AND with that explained, it goes on to explain that the motion of the speaker cone in and out is the result of transistors switching the circuit from the - rail to the + rail back and forth at a very high rate of speed. I always assumed the speaker moved because of a wave form being carried on the current in the circuit or something like that. This was a big eye opener to me.

The amp has a fair sized heatsink attached to the PC board, which is where four of the transistors are mounted because they get WAY hot. In fact the article states that these output transistors run hotter than they are designed to handle and still they work.

I am wonder if people have tried other methods for helping the heatsink to do its job. For instance, there is a device known as a Pertier
https://www.sparkfun.com/products/10080

I have used them in the past to keep an over clocked CPU cool.

You have to add a finned heatsink and a good fan to the hot side of the peltier. The result is the cold side gets real cold. I was always worried about moisture becoming a problem but it never was (to my knowledge).

BR
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Old 19th January 2013, 06:38 PM   #6
DF96 is offline DF96  England
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A useful article for the complete beginner, but bear in mind that the author is (hopefully!) still learning himself so some of the details are wrong or exaggerated. To be honest, it reads like it was written by an opinionated teenager. Good start, but then move on to Doug Self or Bob Cordell (via Horowitz & Hill).

Quote:
Originally Posted by badraven
I have learned from this article why an SS amp has a + power rail, - rail and a ground terminal coming out of the transformer. AND with that explained, it goes on to explain that the motion of the speaker cone in and out is the result of transistors switching the circuit from the - rail to the + rail back and forth at a very high rate of speed. I always assumed the speaker moved because of a wave form being carried on the current in the circuit or something like that. This was a big eye opener to me.
You (or he) may be confusing Class D with Class AB/B.

Last edited by DF96; 19th January 2013 at 06:40 PM. Reason: extend
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Old 19th January 2013, 06:41 PM   #7
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Mooley, you are correct. It is a lot to read. But its not really a simply subject. In fact you will find within these pages just how inefficient amplifiers really are. AND how we are deceived by retailers.
I figure you have to read this thing at least three times. The first time to get the general idea, second to pick up on all the stuff you didn't catch the first time and third and subsequent times to put it all together in workable use. But hey, its worth it if you want to understand how these beasts work. Being able to repair a given thing means learning how it works and how to recognize what's really happening or not happening.

BR
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Old 19th January 2013, 06:49 PM   #8
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Smile DF96

DF96

I would imagine that I am the one with the confusion. I'm not posting this to argue about whether the author is perfect. I am posting this because its a good start for me and other novices. No matter who authors something, someone else will always find faults, mistakes etc. Its not about that, its about learning. And this is the first tool I have found specific to SS guitar amps. So how about just enjoying the fact that someone out there spent a good amount of time to give us some insight. There will be plenty of time for finding the flaws. Thanks!

BR

Quote:
Originally Posted by ;3333323
A useful article for the complete beginner, but bear in mind that the author is (hopefully!) still learning himself so some of the details are wrong or exaggerated. To be honest, it reads like it was written by an opinionated teenager. Good start, but then move on to Doug Self or Bob Cordell (via Horowitz & Hill).


You (or he) may be confusing Class D with Class AB/B.
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Old 19th January 2013, 11:34 PM   #9
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Cool DF96

Quote:
Originally Posted by DF96 View Post
A useful article for the complete beginner, but bear in mind that the author is (hopefully!) still learning himself so some of the details are wrong or exaggerated. To be honest, it reads like it was written by an opinionated teenager.
I have been reading the article and thinking about this "Opinionated Teenager" comment. I noticed when you posted that it was minutes after I posted the article. Unless you have seen the article previously you didn't have time to read anything more than a page or two. I think you skimmed over a few pages and based your assessment on the first page that could be construed as being biased towards solid state amps. If that be true that means you are biased toward tube amps and you took offense to the perceived statement. I'm not going to defend the author because that is for him to do. But I will say this: after having read the article and beginning back through, this is not about proving SS amps are better, worse or the same. It is simply an essay in amp electronics. It is quite fascinating for those willing to read and learn. I can not attest to it being 100% accurate because I am a novice concerning electronics. But I can say this: it is well written and most certainly NOT come off like someone with an arrogance issue. Its simply a training tool.

How about giving it a chance by reading it before you pass judgment. Then when you are finished we can all discuss the discrepancies.

BR
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Old 20th January 2013, 12:01 AM   #10
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Are you trying to fix a guitar amp?

Or learn about solid state amplifiers.

I'd not suggest trying to mix the two together at the precisely same time.
They are related, but *not* quite the same skill sets or information sets.

I took a look at the schematic of your amp, it's a complicated circuit.
Also it is a $200-$300 retail unit, making the cost of repair close to the cost of replacement. So I can see why you might want to try to repair it.

If you state the exact symptoms, in reasonable detail, like what works and what does not, a few things that might work based on what you say.

Also state what the amp was doing when it "stopped working".

First steps:

Check fuses
Check output from preamp. BE CAUTIOUS, if there is a blown output component, there may be enough DC present on the pre-out to blow what you plug it in to.

SO, take ur DVM and check the DC level of the SPEAKER OUTPUT, with all input levels turned all the way down. Also check the DC level of the PRE-OUT, same conditions.

Assuming all are under 100mv MAXIMUM, you may have only and open component and not a shorted component.

It may be possible to find that and repair. But it is still rather unlikely.

To properly troubleshoot this amp, one would need at minimum a signal source, a scope, DVM and preferably a "Variac". Soldering iron, of course.

To be 100% direct, this sort of repair is likely way past your current abilities, and unless you either get very lucky or have the ability to acquire and apply new knowledge at an amazing rate, it's going to be so for a long time to come.

Otoh, if this provides the impetus to learn about solid state amps and venture into building circuits and trying them, all the better. This is a great place to do that!
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