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Szekeres with CCS NFB?
Szekeres with CCS NFB?
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Old 3rd August 2009, 02:27 PM   #1
kacernator is offline kacernator  Czech Republic
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Default Szekeres with CCS NFB?

Hi,

I have built Szekeres headphone amplifier with LM317 CCS. How do I implement the negative feedback? Should I just connect resistor after the output coupling cap back to the source of the mosfet?

Thanks.
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Old 3rd August 2009, 02:37 PM   #2
h_a is offline h_a  Europe
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This is a ClassA biased source follower - that already enjoys 100% voltage feedback. Not sure, why you want more

Have fun, Hannes
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Old 3rd August 2009, 05:57 PM   #3
kacernator is offline kacernator  Czech Republic
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Ah, didnt know that. But it soounds there is not enough bass and trebles, it sound towards mids. So if I would like to try more gNFB should I just connect the resistor defacto across the output cap?
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Old 3rd August 2009, 06:12 PM   #4
Bigun is offline Bigun  Canada
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Szekeres with CCS NFB?
Quote:
Originally posted by kacernator
Ah, didnt know that. But it soounds there is not enough bass and trebles, it sound towards mids. So if I would like to try more gNFB should I just connect the resistor defacto across the output cap?

I don't think that's going to work to your satisfaction. The output cap is there to ensure that only the alternating output signal finds it's way to your headphones. You don't want to be putting a resistor across it.

You could play around with some other things but with a single transistor amplifier you shouldn't expect miracles.

One thing that will have an effect on the low frequencies is the input filter of C1 and associated resistors. A larger value of C1 can improve low frequency response.

If I understand correctly the feedback is via Source degeneration - in other words it's set by the constant current source. You could put a capacitor in there, in parallel with the current source to reduce the negative feedback at higher frequencies. This might boost the treble but can also make it unstable - so you'll want to experiment.

I never saw this circuit before so hope I didn't provide some bad advice...
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Old 3rd August 2009, 08:47 PM   #5
Lumba Ogir is offline Lumba Ogir  Sweden
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Hi,
would this work?
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Old 4th August 2009, 04:59 AM   #6
kacernator is offline kacernator  Czech Republic
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Hi,

I have solved my problem by increasing the value of output to 4700uF. (I am using 32 Ohm headphones).

Thanks for your ideas.
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Old 4th August 2009, 05:15 AM   #7
Lumba Ogir is offline Lumba Ogir  Sweden
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kacernator,
sorry for asking, but what problem did it solve?
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Old 4th August 2009, 05:32 AM   #8
kacernator is offline kacernator  Czech Republic
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It solved my problem with bass response. Also overall sound is more focused, not so smeared, maybe because of better phase shifts too.
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Old 4th August 2009, 06:30 AM   #9
Lumba Ogir is offline Lumba Ogir  Sweden
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kacernator,
simplicity is a great benefit, but this degree of simplicity is not beneficial.
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Old 4th August 2009, 06:58 AM   #10
kacernator is offline kacernator  Czech Republic
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I know it is not the best, but now it sounds very very good to me. I am driving it with aikido front end...really great sound. I am missing nothing in sound. Mayeb add an external EQ to compensate for not so good (cheap) headphones.

PS: I lowered the output cap to 2200uF, as 4700uF was overkill.
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