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Old 24th August 2005, 03:14 PM   #1
crissty is offline crissty  Greenland
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Default All About Amplifiers

I have been searching and making some small Amp Project since past one month or so, especially sparing some time to make these interesting little things, & spice up our lives.


ALL I want is to be as this thread for knowing the basics of the amplifiers, what is what. .
1. Different Basic Stages & Types(i.e. VAS and ........)
2. Where & which type of amp shines for which frequency band.
3. Pros & cons for the particualar type of amp described.

I did not get proper info on surfing the net and wasting lots of my time & money

All this is to save people time, for exactly what they wanna make.

example - I wanted to make a 150W robust amp for driving my mids, and i needed that amp to shine in that particular band (using crossover) & made ZETA (still having problem), then ended up making a chip amp from LM3886 Bridge to get 120W of power in 8 Ohms.
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Old 24th August 2005, 04:28 PM   #2
tlf9999 is offline tlf9999  United States
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1) a typical linear amp has three stages: input, voltage amplification and output stage (current amplification). There are some variations of that but it probalby covers 90% of class AB SS amps out there, including those IC amps.

there are also digital amps but they are mostly pwm amps. Historically, they are used for bass / subwoofer applications but now they are getting better, to the point of being used for full spectrum applications as well.

2) not sure which "types" you were referring to but a decent SS amp will have flat frequency over (and beyond) the entire audioble band. so as far as your frequency response is concerned, anything will do (for the most part).

3) you will have to identify for us what "types" of amps you have in mind before we can answer that question for you.
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Old 25th August 2005, 09:18 AM   #3
crissty is offline crissty  Greenland
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I meant to say different types like Quasi Complimanary and bla bla bla ......

And different circuits for input stages, Current Mirrors & VAS, with the Advantages & Disadvantages, for better understanding.

There are so many types of designs for input stage OR VAS.
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Old 26th August 2005, 02:25 AM   #4
CBS240 is offline CBS240  United States
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Quasi complementary circuits were popular in the 'olden days' before high quality complementary trasistors were available. It is still a useful circuit design though. More symetrical AB circuits are popular today and there are some high quality audio devices out there now.

Sometimes current mirrors are used in the differential amplifier stage, and maybe to boot strap the VAS transistor(i e current source). Instead of using a resistor on the collector of VAS transistor, when a current source is used, the transistor has to only drive the next stage instead of the resistor + next stage so the current change may be more linear.

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Old 26th August 2005, 05:45 AM   #5
sam9 is offline sam9  United States
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Quote:
I meant to say different types like Quasi Complimanary and bla bla bla
For a first effort use an emitter-follower output stage. They are more stable and a quasi-complementary or a CFB arrangement. Both of these later ones are more likely to have oscillation problems. It is a real bummer to be working out things in the input and VAS stages while at the same time the output section is trying to commit suicide.

Get experience with the small signal stages and a stable if not very interesting output stage. Then, for a follow on project experiment with the more interesting and troublesome output topologies.

(I wish I had followed this advice -- I wasted between 9 and 12 months trying to get a CFB with parrallel devices to behave. They looked good in the book. Either I was incredible persistent or incredibly pig-headed - youir call.)
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Old 26th August 2005, 06:43 AM   #6
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Bah!

It's challenges that keep the juices flowing - and the rewards are there!

Cheers,
greg
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Old 26th August 2005, 03:35 PM   #7
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Quote:
Originally posted by sam9

[...]
(I wish I had followed this advice -- I wasted between 9 and 12 months trying to get a CFB with parrallel devices to behave. They looked good in the book. Either I was incredible persistent or incredibly pig-headed - youir call.)
I had a hard time with one channel of the amp I built (Slone design 3) that uses CFB output. The problem ? One of the collector resistors present in the CFB pair was heating up a lot. It was caused by a bad case-to-heatsink interface problem (one of the transistors wasn't properly screwed to the heatsink so it didn't follow thermically the other one).I applied some more thermal grease, screwed the transistor tighter and the problem was gone. I found that amazing. How can a topology be that sensitive to those issues. I think it is due to the fact of the tight local feedback loop constituted by the output transistor and its correspondent driver.

The CFB output has it merits when considering distortion levels, sound quality, Vbe multiplier thermal track,etc . But surely it is a beast you have to tame to get it working.


Best regards,

Joćo Pedro
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Old 26th August 2005, 04:38 PM   #8
sam9 is offline sam9  United States
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Quote:
One of the collector resistors present in the CFB pair was heating up a lot. It was caused by a bad case-to-heatsink interface problem (one of the transistors wasn't properly screwed to the heatsink so it didn't follow thermically the other one).I applied some more thermal grease, screwed the transistor tighter and the problem was gone. I found that amazing. How can a topology be that sensitive to those issues.
In CFB, it's because the output devices and the drivers form a very tight feedback loop. In some configurations there is not even a resistor in between. One of the benefits is that the feedback frim the output device to the driver causes the driver to contol the thermal response out the output device and prevent runaway. If the junction to the heatsink is not effective the ability of the driver to compensate is overwhemed. The hot resistor was probably not oscillation but the output device transconductance going sky-high due to temperature. An EF topology would probably do the same thing only faster.

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Bah!

It's challenges that keep the juices flowing - and the rewards are there!

Cheers,
greg
One step at a time. It's a good idea to learn to swim before takinging scuba lesons. And it's a good idea to get your scuba certification before trying cave diving.
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Old 27th August 2005, 11:57 AM   #9
jaycee is offline jaycee  United Kingdom
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I don't get why the triple darlington stage is not used more, as in the Leach. When I tested this it was a whole lot more stable than CFP and had little crossover distortion as seen in an EF stage. Whereas with a CFP stage adding more than one output transistor is a pain, with the triple it is as easy as it is for an EF.

Worth the extra 2 predriver transistors I think
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Old 28th August 2005, 05:29 AM   #10
CBS240 is offline CBS240  United States
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I have a crude class G circuit I am playing with using a triple darlington that I plan to use for frequencies 5-7Khz and below. Since this is where most of the power is, the 5-7Khz and above will be a smaller different topology.(not made yet...keeping with true DIY spirit ) Compensation and perhaps the 'G' operation combined with cheap parts (for now...) seem to limit the bandwidth at higher gains. Besides, this way will be more efficient.
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