Leben CS300x feedback loop cap - diyAudio
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Old 15th June 2014, 05:59 AM   #1
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Default Leben CS300x feedback loop cap

Hello Everyone,

Please look at this schematic, mainly at the FB loop electrolytic cap.

It is specified as a "copper leaf styrol" cap but I was not able to find a cap of this nature at these high values on the web, usually they are very small values and not directional.

Is this a mistake?

What is the reason for this cap?

Thank you,

Glass painter
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Old 15th June 2014, 08:58 AM   #2
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It is an electrolytic. I suspect the polystyrene caps you mean are the 0u47 and 0u22 ones.
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Old 15th June 2014, 09:30 AM   #3
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You can also find the small caps as "styroflex"

PS Is this a stable and non compromising the feedback way of applying bass boost?
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Old 15th June 2014, 03:56 PM   #4
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I assume you mean that 100 F electrolytic ?
It is just for DC-blocking purpose. Usually this capacitor is not required, but now it is there due to swithcable bass boost circuit.
It can not be an ordinary electrolytic, because there is very small DC voltage (~2 V) accross it but essentially higher AC, so the polarity varies during output voltage cycle.
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Old 15th June 2014, 09:44 PM   #5
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You are right, a bipolar is a lot better in that position. I really thought that mr. Leben would know better.

But my question is about feedback tone controls. When using bass boost in this example, we have less feedvack in low frequencies. it does not seem like a good idea to me.
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Old 16th June 2014, 04:59 AM   #6
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Quote:
Originally Posted by costis_n View Post
...When using bass boost in this example, we have less feedvack in low frequencies. it does not seem like a good idea to me.
This happens allways at low and high frequencies, even when GNFB loop has flat response. This phenomenon is emphasized with pentode connected output stages because of their high output impedance.

For example with classic Mullard 5-10 the open loop gain at 20 Hz and at 20 kHz is declined about 10 dB.
The GNFB at 1 kHz is 26 dB but at 20 Hz/ 20 kHz it is only some 16 dB.

Mullard 5-10. Ten Watt Amplifier

Click the image to open in full size.
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Old 16th June 2014, 05:56 AM   #7
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Default Leben cs300x FB cap

Hello everyone,

Thank you for all the information.

I was under the assumption that it can not be an ordinary cap as well.

What type of cap will you choose for this kind of application?

Thanks again.

Glass_painter
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Old 16th June 2014, 06:28 AM   #8
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Quote:
Originally Posted by artosalo View Post
This happens allways at low and high frequencies, even when GNFB loop has flat response. This phenomenon is emphasized with pentode connected output stages because of their high output impedance.

For example with classic Mullard 5-10 the open loop gain at 20 Hz and at 20 kHz is declined about 10 dB.
The GNFB at 1 kHz is 26 dB but at 20 Hz/ 20 kHz it is only some 16 dB.

Mullard 5-10. Ten Watt Amplifier

Click the image to open in full size.

I know this. What I am saying is that it does not look a good idea to have even less. Distortion and output impefance will rise .
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Old 16th June 2014, 03:12 PM   #9
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No, it is not the best solution, but if the amplifier were UL-type (or triode) and has low distortion by nature, then I would accept that.
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Old 16th June 2014, 03:16 PM   #10
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Quote:
Originally Posted by Glass_painter View Post

What type of cap will you choose for this kind of application?
I would try to connect two ordinary 220 F electrolytics in series but at reversed polarity and with 2 x 47 k resistors in parallel with each cacacitor.
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