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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

PC Speakers' Preamplifier
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Old 7th April 2019, 10:31 AM   #1
StevenStanleyBayes is offline StevenStanleyBayes  Canada
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Default PC Speakers' Preamplifier

I have made a simple, yet, very necessary, PC Speakers' Preamplifier :

PC Speakers Preamplifier - Google Drive
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Old 25th November 2019, 03:04 AM   #2
StevenStanleyBayes is offline StevenStanleyBayes  Canada
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Simple or not, there is a nasty mistake with the schematics :

The input capacitor, C1, with a value of 1nF must NOT be there and must be removed.

This is because the previous stage, the PC, would have a high value electrolytic capacitor in series to the output in order to remove the DC component. Because the PC analogue audio output is to be used by headphones with a typical impedance of 32 Ohms, this capacitor is extremely large. This capacitor requires a DC path to ground to charge. C1 prevents this and the output capacitor never charges ( or in a very long while ), because C1 is a standard ceramic capacitor which has very low leakage.

Although C1 blocks the DC, the fact the PC output capacitor cannot charge may introduce noise.

Thus, much better is to remove C1 and, then, the big PC output capacitor would charge through R1, R2 and the OA's output and or the schematics.

To rely on the OA's output and the schematics may not be a good idea. Thus, a better approach may be to use a non inverting amplifier with a 10K resistor from input to ground.

The inverting amplifier can still be used after the 10K input resistor to ground, but, R1 must have a large value of 100K and, R2, even much larger.
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Old 25th November 2019, 02:51 PM   #3
Mark Tillotson is online now Mark Tillotson
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My guess is you have shares in a capacitor factory! Don't think you need quite that many decouplers for an audio circuit. For a CPU chip perhaps, but a few opamps, no. Still they don't hurt.
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Old 25th November 2019, 04:32 PM   #4
Douglas Blake is offline Douglas Blake  Canada
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Quote:
Originally Posted by StevenStanleyBayes View Post
I have made a simple, yet, very necessary, PC Speakers' Preamplifier :

PC Speakers Preamplifier - Google Drive
Why is it "very necessary"?

Most PC sound cards are fully able to drive 30 ohm headsets to about 3 volts (p/p) which would easily clip any amplifier designed for Consumer Line Level (CLL) operation.

It seems to me that, while it sounds louder, all you would be doing is increasing the distortion in your PC speakers.
And especially so with a voltage gain of 25.
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Old 26th November 2019, 06:16 AM   #5
PRR is online now PRR  United States
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Quote:
Originally Posted by StevenStanleyBayes View Post
...fact the PC output capacitor cannot charge may introduce noise. ...
How??
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Old 27th November 2019, 09:37 AM   #6
MarsBravo is offline MarsBravo  Netherlands
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Quote:
Originally Posted by StevenStanleyBayes View Post
...C1, with a value of 1nF...must be removed...
The circuit works. C1 reads 1 uF (not 1 nF) and is sufficient.
Better keep it there but add a bleeder in front of it (between the inputconnector and R1), say 100k (input imp is 9.1 kohm) to gnd.
If the outputcap is 1 mF, it has to be charged to Vr by the output stage of the opa2134; check current limits! Also, same cap-value to expect on powerrail.

Why do you want to amplify the pc output 25 times? On a 9V rail (with floating ref on 4.3V), inputsignal is ~ (9/3)/25 = 120mVac MAX!
Consider the audiocircuit in pc's are not neccessary high end, so why this 'golden' lunch box???
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