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Old 9th October 2012, 07:57 AM   #11
maouna is offline maouna  Greece
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I forgot to say that i will use OPA2134 for stereo version and a gain of about 9dB.
The inpul filter (R1,C1,R2,C2) is to prevent RF from entering but in some designs on the net i have seen that they dont use one at all and omit R2,C2. necessary ?
As far as the compensation capacitor C3 and the bandwith of the opamp what do you suggest?
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Old 9th October 2012, 08:11 AM   #12
Mooly is offline Mooly  United Kingdom
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Unless you have a problem with RF (live near a transmitter etc) then I would omit the input filter. 22pf looks OK but what I would advise is that you determine the best value by observing squarewave response on a scope. This is an old thread but shows what you should be looking at. In practice layout and stray capacitcance means the optimum value often has to be determined in situ,

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Old 9th October 2012, 10:35 AM   #13
maouna is offline maouna  Greece
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The problem is that i dont have a scope.Just simulation results from LT Spice which is just an approximation to reality but not reality.
Here's a picture of my pcb with some parts missing.What capacitor should i choose for the input? I have these orange MKT and the smaller yellow ones.I read that these big sized capacitors can act as 'microphones' sometimes.Space is provided for both types of these capacitors.
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Old 9th October 2012, 11:50 AM   #14
Mooly is offline Mooly  United Kingdom
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I wouldn't worry over microphonic pickup for line level signals. The larger caps could be more prone to pick up stray fields (hum) in some circumstances.
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Old 9th October 2012, 12:00 PM   #15
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Hi
Here are some links to previous projects using OPA2134 as a pre :
SOAP MKIII (Simple OpAmp Pre) With onboard PSU
SOAP EVOLUTION - Simple OpAmp Pre

Phil
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Old 9th October 2012, 12:49 PM   #16
maouna is offline maouna  Greece
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and what about the sound quality of the yellow one and the orange? Which one you prefer ? the yellow is 2.2uF and the other is 5.6uF.
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Old 9th October 2012, 01:02 PM   #17
Mooly is offline Mooly  United Kingdom
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With 150K input impedance either is fine as regards low frequency cut-off. Sound quality wise I'm probably not the right person to ask as I'm a non-believer

I would choose on technical merit probably going for the physically smaller cap as it has a smaller footprint and area to minimise noise pickup.
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Old 9th October 2012, 01:05 PM   #18
maouna is offline maouna  Greece
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thank you .i will come back with the preamp assembled.
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Old 9th October 2012, 01:12 PM   #19
Mooly is offline Mooly  United Kingdom
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Your welcome.

It's a classic text book application so you won't have any real problems. The OPA2134 is one of the best opamps available at the moment for applications like this.
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Old 10th October 2012, 06:00 PM   #20
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Mooly View Post
Unless you have a problem with RF (live near a transmitter etc) then I would omit the input filter. .............
I am of the opposite opinion.
I prefer to see an RF filter at the input of every receiver.
I generally set the source equipment input filters to ~ twice the frequency of the Power Amp RF filter, i.e. for my PA I use ~680ns and for Sources I use ~330ns. That extra octave takes very little off the required audio signal, but does give extra and useful attenuation of RF.

Modern homes are full of RF transmitters.
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