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Old 14th December 2003, 10:07 PM   #1
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Question question on rolloff below 100Hz

Hi,

I made a kit soundcard preamp, initially to allow me to do basic cro measurments with my pc. I'm now thinking I'll also use it with speaker workshop, as a safegaurd between the amp and my soundcard.

I measured it's freq response using rmaa 5.2 and it starts to rolloff at about 150Hz and is down about 1db at 30Hz.

http://home.exetel.com.au/wintermute...mp_compare.htm is a comarison between the soundcard in loopback and with the preamp in circuit.

I don't think I can post the circuit diagram, due to copyright but Its using a TL074 and the only cap in the signal path is a 10nf coupling cap on the input, I suspect this is the culprit, but I'm only guessing, so thought I would ask if I should increase the value of this cap, and if so what to?

Regards,

Tony.
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Last edited by wintermute; 15th July 2011 at 11:56 PM. Reason: updated url to new host.
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Old 15th December 2003, 03:57 PM   #2
sreten is offline sreten  United Kingdom
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47nF to 100nF sounds reasonable to me.

sreten.
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Old 15th December 2003, 09:57 PM   #3
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Thanks Sreten I'll give it a try

Tony.
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Old 16th December 2003, 02:49 AM   #4
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Thumbs up Thanks Sreten

I put in a 68nf wima cap (replacing the no brand 10nf) and not only is the freq response now almost identical to what I get doing a loopback test on the soundcard, but the distortion went right down to almost nothing.

I'll put the new graphs up just as soon as my ISP's web server comes back online

It might get slightly better if I put the 100nF caps I bought at the same time, but they are no brand, and I really can't complain about the freq response improvement the 68nF ones made!!! So I think I'll leave it as is. The only thing that bugs me now is the huge spike at 50Hz with the SC preamp. Not sure where that is coming from since it's running off a 12V battery. probably radiated, the kit came with a cheap plastic box, may be necessary to stick it in a metal box.

OK ISP is back on line new url is http://home.exetel.com.au/wintermute...mpare_MKII.htm

Tony.
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Last edited by wintermute; 15th July 2011 at 11:58 PM. Reason: update url to new host.
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Old 16th December 2003, 03:23 AM   #5
sreten is offline sreten  United Kingdom
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Distortion across capacitors is related to the AC drop across
them (higher=higher) and their voltage rating (higher=lower).

but you seem to be sorted /sreten.
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