passive preamp

[R-Carpenter]

don't know anything about amps and preamp. As a general question, is there an advantage to a passive preamp at all? Some say, it's a resistive switch box, some say it's a shorter pas sway to an amp.
Opinions?

[Michael Chua]

A Passive Preamp does not amplify. No active devices. No transistors, opamp etc. Uses fixed resistors, like in a stepped attentuator, or a potentiometer to control volume.

The main advantage with a passive preamp is a purer signal since no transistors are involved. The disadvantage is it cannot amplify. Its not an issue if the source is a CD player with 2Vrms output but will be if its 0.3V. The other disadvantage is an apparent lack of dynamics.

[tvrgeek]

I was a fan of passive for years. I can't explain why, but by buffering with a modern op-amp ( MCoy circuit for example), yes, it seems more dynamic. This bothers me when I know the source has plenty of current drive. Extra stage, extra cables, power supply, connectors and all other things bad. Can't explain it. I sure as heck can't explain what all the hub-bub is over an optical attenuator. Has to be placebo effect for something new. I have not heard it, so this view is total conjecture.

[Michael Chua]

I believe the lack of dynamics is due to the low impedance that a passive preamp imposes on the power amp.

Let's take a 10K passive preamp and a power amp with an input impedance of 10K as an example.

When we turn the preamp volume control full clockwise, the wiper of the preamp potentiometer is at the top. The power amp input impedance is now halved to 5K (from two 10K in parallel).

In normal use, the preamp volume control is not at full clockwise as this will be too loud. Let's assume its at 12 o'clock. Let's also assume that with a log pot, the wiper is at 7K(in series) +3K(shunt). At this position, the power amplifier's input impedance is even lower at 2K3 ohms (3K parallel with 10K).

Low input impedance doesn't sound nice. High impedance does. That's one area tube preamps excel in.

[sakis]

my 2 cents in a few words

a simple active preamp even with a basic tone control based on on transistor and single supply will introduce except phase and impentance issues capacitors on the signal and distortion coming from the tone control

simple may be is, but introduces too many problems to be audiophile

most of these problems are gone with the use of an op amp and split supply ...BUT it seems that also this can be a problem compaired to a single CD driving an amplifier directly with next in quality a potentiometer , then an op amp , then an op amp with tone control , and then a one transitor preamp with or without tone control .....

having a long discussion about this with forum memebers and sleeping on it over nights the only solution i come up with ( since i have an application that tone control is a must ) is :

to design a tone control probably baxandhall based that is going to be totaly passive with cut and boost of only a few db ( probably 2.5 is the max ) and then beef up the gain of the amplifier just enough to achive full drive from a cd ..... ( probably not needed since the average amp wil be driven around 1v and average cd will produce a max of almost 2 volts )


will see this in the next few days and will come back with a schematic i belive that use of very good parts and > 100K pots will do the trick

lets see

[AndrewT]

Originally Posted by Michael Chua

I believe the lack of dynamics is due to the low impedance that a passive preamp imposes on the power amp.

Let's take a 10K passive preamp and a power amp with an input impedance of 10K as an example.

When we turn the preamp volume control full clockwise, the wiper of the preamp potentiometer is at the top. The power amp input impedance is now halved to 5K (from two 10K in parallel).

A power amp with a 10k input impedance requires a source impedance <=500r.
A 10k passive pot has an output impedance between 0r0 and 2k5. This is a bad match.
I would recommend Rin=50k and the RF attenuator capacitance must be <<680pF to avoid rolling off the top end.

[mp9]

Originally Posted by sakis

my 2 cents in a few words

a simple active preamp even with a basic tone control based on on transistor and single supply will introduce except phase and impentance issues capacitors on the signal and distortion coming from the tone control

simple may be is, but introduces too many problems to be audiophile

most of these problems are gone with the use of an op amp and split supply ...BUT it seems that also this can be a problem compaired to a single CD driving an amplifier directly with next in quality a potentiometer , then an op amp , then an op amp with tone control , and then a one transitor preamp with or without tone control .....

having a long discussion about this with forum memebers and sleeping on it over nights the only solution i come up with ( since i have an application that tone control is a must ) is :

to design a tone control probably baxandhall based that is going to be totaly passive with cut and boost of only a few db ( probably 2.5 is the max ) and then beef up the gain of the amplifier just enough to achive full drive from a cd ..... ( probably not needed since the average amp wil be driven around 1v and average cd will produce a max of almost 2 volts )


will see this in the next few days and will come back with a schematic i belive that use of very good parts and > 100K pots will do the trick

lets see

you lost me, i think you're saying you'll post a schematic for a high parts quality 100K or greater pot "passive" attenuator, or are you designing a "tone control"?
if it's a passive attenuator i would be interested in doing a ≥100k, not sure if i need higher for my Fi2A3 amplifier / 470kΩ input impedance. actually, Don Garber of Fi told me to go with a 100k pot passive. any benefits going >100k ?

[Michael Chua]

Originally posted by AndrewT
A power amp with a 10k input impedance requires a source impedance <=500r.

A power amp is most happy with zero source impedance. 500R is very high. 33R-100R are the values normally used.

A 10k passive pot has an output impedance between 0r0 and 2k5. This is a bad match.

I agree a 10K passive pot is a less than ideal match for a 10K input impedance amp. That is why I used it as an example.

I would recommend Rin=50k and the RF attenuator capacitance must be <<680pF to avoid rolling off the top end.

If I were to use a passive preamp, a 100K pot would be my choice.

[AndrewT]

Originally Posted by Michael Chua

I agree a 10K passive pot is a less than ideal match for a 10K input impedance amp. That is why I used it as an example.



If I were to use a passive preamp, a 100K pot would be my choice.

100kpot to 10k input impedance is even worse. The amp see Rs between 0r0 and 25k.

I am suggesting 10k pot and 50k or greater input impedance.

[nelsonvandal]

What's the input impedance of a typical pre-amp? If one builds a power amp with the same input impedance using eg JFETs in the input stage, would it still be beneficial to use some kind of buffering stage between potentiometer and poweramp? If yes, why?