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Old 7th July 2008, 03:06 PM   #1
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Default 1.st pream, basic questions, caps in series etc

Hi all,

after a successfull low cost VSPS build and a CD player MOD, my next project will be a high quality preamp, a high quality VSPS + The westhostaudio Preamp MK2 (but with single opamps, not dual ones) in one case, part sourcing is almost done, so please no discussion on the designs of these two circuits, but i have some very simple general questions, maybe you can help me.


1. The VSPS has a 4,7 uF output Cap, the preamp MK2 has a 1uF input cap ... well regarding to my basic electronics knwoledge two caps in series will result in a total cap smaller then the smallest cap. That would be below 1uf, so my question is : can i safely drop the output cap of the VSPS and R7 of the VSPS? R7 mainly serves to drain off charge from the output side of C3 ... and increase the Input cap of the westhostaudio MK2 Preamp to 4uF? Here are some nice 4uF OIP caps available and i would like two use them for this project.

2. Decoupling the OpAmps of VSPS / Preamp 0.01uf or 0.1uf, i think 0.1uf is better or am i wrong

3. I want a LED two show which of the three inputs is active, so i need a selector switch with 3 positions and at least 3 poles, 2 for the inputs, one to switch the LEDs right?

I know thats really basic stuff but i dont want to make anything wrong so i better ask first.

Thanks for your help
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Old 8th July 2008, 11:15 AM   #2
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1. In that case you are not simply connecting two capacitors in series. C3 and R7 in the VSPS output form a high-pass filter (~1,0Hz). C1L and R1L form another one at the preamp input (~1,6 Hz). There are several threads in this forum, where the reason for having those filters, and for their different tuning frequencies are explained. Leaving either of them out may work fine, because every capacitor less in the signal path usually improves the sound, but may give you problems, if you want to match the components with others that were also built without filter.

2. Depends on the frequency of the noise you want to filter out. The higher the frequency, the smaller the capacitor. http://www.ti.com/sc/docs/apps/msp/j...000/aug_09.pdf page 5 explains this. Your alternatives are to measure the noise and calculate the right capacitor or to use any capacitor from 100 pF to 100 nF and trust that it will be okay or to try different values and see, if you find any sonic differences that might indicate the best filter effect.

3. Right. The easiest solution among many.
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Old 8th July 2008, 12:03 PM   #3
AndrewT is offline AndrewT  Scotland
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Default Re: 1.st pream, basic questions, caps in series etc

Quote:
Originally posted by Razorblade
.......1. The VSPS has a 4,7 uF output Cap, the preamp MK2 has a 1uF input cap ... well regarding to my basic electronics knwoledge two caps in series will result in a total cap smaller then the smallest cap. That would be below 1uf, so my question is : can i safely drop the output cap of the VSPS and R7 of the VSPS? R7 mainly serves to drain off charge from the output side of C3 ... and increase the Input cap of the westhostaudio MK2 Preamp to 4uF?
you only need to block DC offset once between two pieces of connected equipment.
Keep the better cap in series, whether due to sound quality or bandwidth.
Short out the other superfluous one by installing a bypass RCA input/output terminal, just in case you change equipment at some time in the future and need the DC blocker.
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regards Andrew T.
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Old 8th July 2008, 12:45 PM   #4
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Thanks for the input, especially that PCB Design PDF is very interesting and a must read. I will redesign my pcbs according to this, I have not etched them yet and changes can still be made.

Since all OIP caps are these large blocks mounted next to the PCB not on the PCB i will make the connection points bridgeable.

Thanks for your help, that was exactly what i was looking for.
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