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Old 19th November 2003, 06:16 AM   #1
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Default Passive pre amp vs powered pre amp

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Hello,
After reading this thread i would like to know if useing a stepped attenuator instead of a pre amp would give you better sound quality?

I only use cd and only need a volume control

Cheers
Matt
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Old 19th November 2003, 08:08 AM   #2
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I use a passive attenuator for controlling the output level of my system but am thinking that there would be significant advantages to adding a simple active pre-amp/ buffer.

An active pre-amp would drive the cable with a near constant impedance regardless of level setting and the volume control would operate in a more ideal way, also the source would see a more constant impedance at it's output.

I think an active pre-amp would remove many of the inconsistencies is sound depend on volume setting. A simple transistor design (not op-amp) is all that's required so signal degradation due to the extra gain stage would be negligible compare to potential benefits.

A stepped attenuator followed by a simple active buffer would be a good solution.
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Old 19th November 2003, 09:36 AM   #3
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I've been using a simple passive volume control at the power amp input for years now, and don't feel the need for anything else. A preamp is just more circuitry to worry about, and I can't see what it actually contributes to the system.

I wouldn't use a potentiometer output to drive a long cable, though. The (comparatively) high output impedance may give HF rolloff problems with the cable capacitance, and make it more susceptible to some kinds of RF interference. Putting it at the amp input removes (or at least controls) these problems.

Cheers
IH
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Old 19th November 2003, 09:56 AM   #4
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The reason an active buffer is good is because it eliminates many of the inconsistencies of a passive attenuator alone. Look at the output impedance of a 10k pot for instance: towards the ends of its travel its impedance tends to 0 Ohm whilst in the middle its 5k Ohm! That's a significant ratio.

Also when you put this in parallel with the input impedance of your power-amp you will find that the characteristic of the pot is far from being the logarithmic ideal.

An effective buffer can be as simple as an emitter-follower with a constant current source load, two transistors is not much extra circuitry to worry about.
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Old 19th November 2003, 10:44 AM   #5
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I think you have it spot on there RichardC.

Passive 'pre-amps' are one thing that really can be improved upon greatly, simply, and cheaply.
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Old 19th November 2003, 10:57 AM   #6
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Oops, I should have said that the output impedance of a 10k pot at its central position is 2.5k NOT 5k. (5k||5k)
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Old 19th November 2003, 07:21 PM   #7
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Passive pots are the most transparent way of controling the volume, so long as a few rules are followed, the 100 to 1 rule i i`ve been told to call it.

1: The input impedance of the passive pot should be 100 times HIGHER than the output impedance of the CD player.

2: The output impedance of the passive pot (at it`s worst possition) should be 100 times LOWER than the imput impedance of the poweramp, if not the input impedance (resistor) of the power amp should be changed. If the input of the poweramp becomes greater than 200k to achieve this you should use low capacitance interconects (100pf or less) per foot, and not more than 1 meter long.

If you follow these steps you will have the the best volume control, some people have said that passives are a bit soft in the bass or rolled off in the highs or lacking dynamics, this is because they did not do the impedance matching that`s explained above.


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Old 19th November 2003, 07:46 PM   #8
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You are pretty spot on, but I must dispute the source loading at 100 times. This would give a typical source load of about 2k. You try loading any output with 2k and you won't get very good results.

Also, increasing the resistance to ease the load results in more noise and problems with cable capacitance.

Passive is the most transparant, but in practice it's almost always impossible to implement properly and you will get better results with a buffer.
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Old 19th November 2003, 07:48 PM   #9
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Ok, so a typical CDP output impedance could be 100 Ohm, multiply by 100 = 10k: this is our pot value. At its worst its output impedance is 2k5 multiply by 100 to get a suitable input impedance for our amp =250K.

Now, 250k seems rather high for the input impedance of a power amp, you might be able to change resistors in the power amp but the maximum input impedance is not solely dictated by resistors but also by the active devices in the input stage itself and so it may not be possible to increase input impedance without a major redesign of the amplifier.

worse still is that the output z of many CDPs is >200 Ohm meaning you would require a power amp input z of 0.5M! unlikely and undesirable.

Not only this but the interconnect is now becoming a dominant factor.

I don't see how this is better than a simple active buffer that would work consistently well with any source, any load and any interconnect.
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Old 19th November 2003, 08:03 PM   #10
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"Passive pots are the most transparent way of controling the volume"

Why is this? Passive attenuators are affected by noise and capacitive and inductive loading just like everthing else.
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