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Old 20th December 2007, 08:22 PM   #1
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Question Strange preamplifier output stage

This is the an output stage of my Technics SU-C1010 six channel preamp. I want to change the opamp for a LM4562, but I have no idea how this circuit works.

Click the image to open in full size.

Would it be wise to give it an own symmectrical power supply? Whatfor are the 330P caps across the inputs? Whatfor is the voltage from Q511 to inputs? Or why does it have no symm. power?

Your help is strongly appreciated
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Old 20th December 2007, 08:44 PM   #2
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The FET is used to bias the opamp inputs at half supply. As there is only a single supply, this is necessary to get max pos and neg output swing from the opamp.
If you want to change this to a double supply system you should also take the other parts of the amp in consideration. My guess would be that it isn't worth it, but check it out.

Jan Diudden
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Old 20th December 2007, 08:47 PM   #3
KSTR is offline KSTR  Germany
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it's not so strange as it seems...

Q511 provides a virtual GND, about half the supply voltage (look at the voltages stated, 4.6V at the source).

The amps run in standard inverting config, with high gain (-3300/47=-70 which is a little too high IMHO, there probably is another source impedance in series not shown here).

Those caps do the high-frequncy stuff, the 330p rolls off the gain to zero for high frequencies and the 100p shunt HF to the virtual GND node.

The outputs must be capacitively coupled, not shown here, because they swing around the virtual GND voltage, not zero.

One would need a little more info about the enviroment of the circuit to make proposals for replacement.

- Klaus (arrgghh, lost the race to Jan, I see)
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Old 20th December 2007, 09:02 PM   #4
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Wow, that went FAST! Thank you a lot. Indeed there are some more source impedances in series, how do you know?

So here´s the whole thing, schematics begin at page 17:
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Old 20th December 2007, 09:41 PM   #5
KSTR is offline KSTR  Germany
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Hhm, give THAT "short" ( ) signal chain, I also would say it's maybe not worth the effort. Are there any problems with the current stage that you can nail down to exactly that stage, so a change at this point in that big circuit would improve things?

- Klaus
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Old 21st December 2007, 01:54 AM   #6
Leolabs is offline Leolabs  Malaysia
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Single supply inverting gain stage,nothing strange.
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