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Old 23rd January 2012, 05:46 AM   #1
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Default Passive pre, adding buffers

My current setup is DVD player – selector switch – passive master vol – passive line level xo – amps
I’m thinking of adding a couple buffer op amps to compensate for any losses in all that passive wiring,
My question is - where in the chain is the best place to put them?
I’m thinking after the pllxo, so I have some line driving capability & have my dvd player & vol control close to my beer, while the amps are close to the speakers…
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Old 23rd January 2012, 06:36 AM   #2
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Without seeing the actual circuit, probably after the VC.
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Old 23rd January 2012, 06:53 AM   #3
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Yes , after the potentiometer . But...which value is this pot ?
And...what impedance it sees ?
The rule that in line stages the output impedance should be at least 1/10 than the next stage become true when a pot is in the circuit :the series resistance and the parallel ( to ground ) resistance , they form a lowpass filter when 'seeing' the capacitance associated to the circuit following ( also Hp , but it is less audible ) , so this brings to BW restrictions , depending at which point the cursor is .
So it's better to use pots that are less than 50 KΩ for this reason . The buffer makes the series resistance ( plus capacitance...hidden ) negligible .
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Old 23rd January 2012, 08:59 AM   #4
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re:'which value is this pot ?' - good question, I'll have to get out the screwdriver & have a look, & work out the impedance levels at each stage...

after the pot makes sense, but the pllxo likes to see a constant impedance too

edit: the Vol pot is 50K, the 1st order xover LP filter is 15K6 in line, .047uF to ground,
HP filter is .139uF in line & 5K6 to ground, as long as the HF amp has an input impedance ~> 10x that I should be ok???
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Last edited by PeteMcK; 23rd January 2012 at 09:06 AM.
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Old 23rd January 2012, 09:23 AM   #5
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Pete
I'm not very versed into electronics (neither in speakers )
but I know the impedance of an amplifier is given by the resistor at its input , that's why you contemplate it into the equation for obtaining R and C values in the filters . I see that ,looking at some posts about Nelson Pass buffer , most people put buffers before and after the RC nets . That consists in only two ( well matched ) active components ,fet or bjt , that allow to have a high impedance at their input and capable of driving loads > 1KΩ ( better 2 KΩ ) . About potentiometer value ... it's got also to do with the Johnson noise . I don't know what a fully feedback configuration with an IC
could do with the 'sound' . At this point ,it would be better to put the IC into the path of the filter ( Sallen & Key ,LR ,etc ),and see/listen to the active crossover .
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Old 23rd January 2012, 09:49 AM   #6
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I'm not phobic about opamps, so I'd just use an OPA2134, one stage after the PLLXO, and one after the pot.
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Old 23rd January 2012, 10:24 AM   #7
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the specs for the DVD play say 10K load impedance... so anyone have opinions on the optimum value of volume pot?

Pinkmouse, I'm probably going to use OP27s (scavenged few recently), your solution is probably optimal, but with 6 op amps, it's almost at the point where I might as well go fully active.... hmmm, perhaps I could make the volume pot part of the feedback network of the first op-amp if I could maintain relatively high input impedance... I'm going to have to read up on op amp circuitry....
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Old 23rd January 2012, 12:14 PM   #8
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A buffer becomes necessary if the source equipment cannot properly drive the load/ receiver equipment.

That load seen by the source is a combination of the Rin of the receiver plus the RF attenuation fitted to the receiver input plus the cable connecting source to receiver.

One could have, at the high end, an effective 100k//100pF as the total load seen by the source, ranging down to 1k//2nF (or worse) for the most demanding of loads.

A passive pot does not perform well into low resistance loads. Similarly the Passive pot does not perform well into high capacitance loads.

A poorly designed piece of equipment may similarly not be capable of driving some commonly seen loads.

This inability of the source to drive the load adequately is met by fitting the buffer at the output of the source. The buffer cannot work at the input to the receiver, since the source will still see the cable as part of the load.
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Old 23rd January 2012, 05:09 PM   #9
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DVDP - 10k pot - LM4562 - parallel passive filters - amp inputs.

If you have no analogue sources,

DVDP - DSP xover - 4 gang VC - poweramp inputs.
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Old 23rd January 2012, 08:55 PM   #10
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re:'DVDP - 10k pot - LM4562 - parallel passive filters - amp inputs' - that looks like the simplest & most effective option (& easiest for me to implement )
Thanks for the input guys
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