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Old 15th September 2011, 12:52 PM   #1
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Default Identifying the stages of an amplifier (understanding)

The attached is a DX Trust amplifier schematic with MY OWN categorization, which maybe wrong, but i'm learning to understand the circuit. HOpe someone can point out my mistakes.
I have categorize in term of colour. Red = Input stage(transconductance), blue = VAS stage (transimpedance/predriver), green = output stage, brown = feedback line ?

But i don't know the function of Q1, Q2 & Q3 repectively, as normally i saw is pair input style, which used in dual supply. (or they just confusing me ?)
It seems that the input section have most of the component (half left), suppose because to supports its linearity ?(improve THD)

The output stage seems pretty straight forward and simple, so no much to explain/understand, but i'm not clear with the other section of the amplifier, esspecially input and feedback.

Another question is that where is the local decoupler ?
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Old 15th September 2011, 01:14 PM   #2
DF96 is offline DF96  England
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Good! You are asking the right questions. Your breakdown of the circuit is about right. The blue VAS stage around Q2 is basically a common-emitter voltage amplifier, but with local feedback via C6.

Most modern amps start with a long-tail pair (LTP), but older circuits often had just one transistor.

What do you mean by "the local decoupler". There may be many or few. A decoupler is something which stops unwanted transmission of signals between stages, normally by acting as a low pass filter. For example, R3+C4 or C18+C19 (which uses the tiny resistance and inductance of the supply wire from the power supply).

Other on here are more expert than me on old transistor amps, so I will leave it to them to continue. Happy learning!
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Old 15th September 2011, 01:15 PM   #3
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Q1 is the input stage, which subtracts the NFB signal (at the emitter) from the input signal (at the base).
Q2 is the VAS, with simple Miller compensation applied via C6.
Q3 is used as the bias spreader. The transistor (and R15/R16) , together with the resistors in the base circuit, produce a defined voltage difference between the bases of Q4 and Q6. This voltage difference causes a bias current to flow through the output transistors.
You also need to understand the 'bootstrap' circuit around C2 and R9.
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Old 15th September 2011, 01:37 PM   #4
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Quote:
Originally Posted by DF96 View Post
Good! You are asking the right questions. Your breakdown of the circuit is about right. The blue VAS stage around Q2 is basically a common-emitter voltage amplifier, but with local feedback via C6.

Most modern amps start with a long-tail pair (LTP), but older circuits often had just one transistor.

What do you mean by "the local decoupler". There may be many or few. A decoupler is something which stops unwanted transmission of signals between stages, normally by acting as a low pass filter. For example, R3+C4 or C18+C19 (which uses the tiny resistance and inductance of the supply wire from the power supply).

Other on here are more expert than me on old transistor amps, so I will leave it to them to continue. Happy learning!
wahaha, this amp was based on old amplifier design i guess. I'm happy that the breakdown was not all wrong
I should say is "Local HT Decoupler", so i guess is the C18+C19 ?
ah... today learn alot of new words

Quote:
Originally Posted by Ouroboros View Post
Q1 is the input stage, which subtracts the NFB signal (at the emitter) from the input signal (at the base).
Q2 is the VAS, with simple Miller compensation applied via C6.
Q3 is used as the bias spreader. The transistor (and R15/R16) , together with the resistors in the base circuit, produce a defined voltage difference between the bases of Q4 and Q6. This voltage difference causes a bias current to flow through the output transistors.
You also need to understand the 'bootstrap' circuit around C2 and R9.
yay, input stage, but i heard others said that there are 2 NFB, is it both return to the emitter ?
Wow, bias spreader, haven't heard of it (this world is so big ! ) So if i wanted to adjust the gain, i should adjust the value of R11 or R10 ? (i dunno anything )
Bootstrapping.... heard alot of time, but haven't learn about it, will look into it ! Today and Tmw is going to be a study day !
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Old 15th September 2011, 02:04 PM   #5
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....... i read something about bootstrap, it stated it is a PFB(positive feedback), and i thought positive ones was not good for the circuit ? (which easy to oscillate and other problem)
And what is the purpose of raising the input impedance ? (sorry that i'm weak)
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Old 15th September 2011, 03:01 PM   #6
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The bootstrap circuit is indeed a type of +ve feedback mechanism, but has a gain of less than 1, so it does not affect the audio performance. What it does is ac couple the output voltage to the top of the the 'bias spreader' (the dc bias circuit for the output stage). This ac-coupled signal makes the VAS appear to be driving a higher impedance load than if it was driving R9 and R10 on their own. You could get the same effect by replacing R9 with a constant-current source, but this would limit the maximum output pk-pk voltage from the amplifier. The bootstrap circuit is a neat way of providing most of the benefits of a constant current source while also allowing a higher output signal level before clipping. The technique only works if C2 has a fairly constant voltage across it, so it doesn't work at very low frequencies.
I'm sure there will be a lot of information on the web if you serched for "VAS stage with bootstrapped load" or something like that.
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Old 15th September 2011, 03:09 PM   #7
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Quote:
Originally Posted by Ouroboros View Post
[snip]You also need to understand the 'bootstrap' circuit around C2 and R9.
I don't think it's a bootstrap, although it looks like it.
C2 is connected on ground at one side.

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Old 15th September 2011, 03:28 PM   #8
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No Jan, it's not a ground symbol. It is labelled with a letter 'A'. There is a matching letter 'A' at the output. (+ve end of C3).

The circuit appears to have two NFB paths, one from this 'A' point to the emitter of the input transistor, and one taken from the dc-blocked output via a 2.7k resistor. This second path has a lot of attenuation, and I'm not really sure what is is doing.
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Old 15th September 2011, 06:27 PM   #9
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I suspect that this circuit is wrong. The link back from 'A' looks like it is intended to bootstrap the VAS, yet it also attaches to the input stage emitter.

The diagram is dated 2009, yet the circuit looks like it comes from the 1980s. My guess is someone has attempted to reverse-engineer the diagram from a PCB and made a few mistakes.
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Old 15th September 2011, 11:28 PM   #10
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So this amplifier is full of mystery ?
The both "A" are intended to connected together, instead of grounding (i'm confused at the first time i build this amp too !)
You mentioned that the second NFB is (through R18?) kinda unknown, then should i just leave it there ? (was finding ways to adjust gain)
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