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Old 22nd January 2002, 07:39 PM   #1
rickpt is offline rickpt  Portugal
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Question current feedback pre-amp

hello

It's my first post here at DIYaudio.com. it's a great forum!!!!

I'm using this schematic as a line preamp
(note that the inverting input is wrong in the schematic)

But i have one problem: when i put the volume pot to zero(47kohm alps blue series) the dc output offset is about 0.2v
but whem i put in normal listening position the offset is about 20mv

This means that i cannot put the volume pot to zero. i had luck because my amplifier has dc protection(elektor project using using: ssm 2210,2220,bd911 and bd912. i dont know the name of the peoject in english).

What can i do to correct this?

thanks
(sorry about my bad english)
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Old 22nd January 2002, 07:56 PM   #2
rickpt is offline rickpt  Portugal
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Default schematic

here is the schematic.
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File Type: zip current feedback.zip (3.6 KB, 272 views)
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Old 22nd January 2002, 09:09 PM   #3
mlloyd1 is online now mlloyd1  United States
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Rick:

Have you check to see if it is oscillating? Also, how much matching did you do for the input devices (HFE and VBE at operating point)?

As I look at this again, it looks like something is missing in your circuit. How do you set the gain? Are you missing a resistor from the emitter junctions of the second set of devices back to the output?

Michael
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Old 22nd January 2002, 10:13 PM   #4
rickpt is offline rickpt  Portugal
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Wink feedback resistors

thanks for the reply:

in that schematic the feedback resistors are missing. i use one 680ohm resistor from the output to the inverting input and 100ohm resistor from the inverting input to ground.i also use a 22pf capacitor in paralel with the 680ohm resistor for bandwith limiting, and i made 100ohm positive rail resistor ajustable so that i can adjust de output offset.

i used this values because i add them at home.is anithing wrong whit them?

i also use a cascode single ended folower for output stage in the feedback loop.

i cant see if it's oscillating because i dont have access to a lab and i dont have an osciloscope.
i only matched the transistors for hfe whit a multimeter.

(apart from this problem the sound is fantastic!!)

ricardo
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Old 23rd January 2002, 02:03 AM   #5
jam is offline jam  United States
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Ricardo,

You can control dc offset by capacitor coupling the input, before the volume control. The other method is using a dc servo.
The last method I can recomend is to use a design that has j-fets in the input.

Jam

P.S. see attachment for an example of a fet input amplifier.
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Old 23rd January 2002, 02:07 AM   #6
jam is offline jam  United States
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The voltages can be scaled to the required values for a line stage.
The output mosfets can be replaced with a bipolar pair.

Jam
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Old 23rd January 2002, 04:43 PM   #7
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Default Trimpot

Wouldn't a trimpot bettwen the J-FETs, say 500 ohms, do the job better than two 220 ohm types for zeroing offset at the input?


Carlos
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Old 23rd January 2002, 09:01 PM   #8
jam is offline jam  United States
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Carlos,

That would work for the pre-amp. with bipolar outputs.
If it was built as a power amplifier or preamp with mosfet outputs, using both trimpots you could adjust for both bias and dc offset.

Jam
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