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Old 30th September 2012, 08:02 AM   #1
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Default Passive pre for my AudioSector

I am waiting for the rest of my parts to come in the mail, chassis and som other sruff. In the meanwhile, I want to finish my pre for my LM3875. I have a line selector switch and pcb from Mr.Broskie and a 50k Alps Blue. DO I just put it all together? I have seen som peaople using input resistors and output caps on their passives, why? Do I need to? Whats the benefit? Any advice?

"Simplicity is the key to brilliance"
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Old 30th September 2012, 09:51 AM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
A passive pre-amp is just a pot. It does not need anything else.

But, you must consider all the connections to the Source and to the Receiver as part of the Input and Output circuits.

The Source feeds the pot via some cables and a selector switch. That is a complete Audio Circuit. One end is the Source the other end is the Receiver. Design it as such. Similarly the pot feeds the Receiver via cables. That too is a Source and Receiver combination and must be designed.

It is best to keep the output cables very short. Really very short, else the filtering effect of cable capacitance severely alters the frequency response and the phase of the treble signals.

It is not simple.

I always recommend a buffer on the outputs of every Source that cannot properly drive the cables to the next Receiver in the string of equipment.
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 30th September 2012 at 09:55 AM.
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Old 30th September 2012, 11:58 AM   #3
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Thanks AndrewT.

I have a buffer I plan to use with my source, once I get it working.
I also plan to make the interconnects as short as possible, using 20/22ga solid core copper or silver.

Why then, do some people put caps and resistors on their volume control boxes, when the box is only a pot and a selector?

So, can I make one buffer with two inputs to buffer both dac and riaa, or do I need a standalone buffer for both?
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