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Topic: Passive preamp theory? (Read 11138 times) previous topic - next topic
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Passive preamp theory?

Hey guys.

(As you might've guessed,) Im working on a 'reference audio system'. My attention has now turned to pre-amplifiers. And, while the market is full of shiny, expensive boxes that have pretty lights (or not), I have the strong impression that perhaps the pre-amp complexity is unneeded.

Enter the passive preamp.

From a simpler is better point of view, would you not say that a simple 'resistor' (potmeter or stepped attenuator), perhaps paired with an input selector relays and remote, would alter the sound less than any active preamp ever could?

Am I missing some fundamental theory here? Why would anyone want an active preamp?
- I am well aware that SOME output stages of audio components (like CD players) are not able to drive the input stages of the end amplifiers, but I think this situation is quite rare, especially with well-designed hardware on both ends
- Also SOMETIMES (with less than optimal components or sources) you would want to color your audio signal a bit.. but well.. call me a purist..

I'd love to hear some solid theory here!
Check out: http://www.creekaudio.com/noflash_main_pro...p?prolook=obh22 for example. This thing even shuts off its remote control circuitry when it's not needed...

Passive preamp theory?

Reply #1 –
Quote from: puntloos on
From a simpler is better point of view, would you not say that a simple 'resistor' (potmeter or stepped attenuator), perhaps paired with an input selector relays and remote, would alter the sound less than any active preamp ever could?


The "gut feeling" that this thought is based on is basically that more components/complexity automatically translate into more distortion. This is not quite correct.

Both active and passive pre-amps need to be properly designed. The "straight wire with gain" is a silly goal - the right goal is no distortion.

Quote
... This thing even shuts off its remote control circuitry when it's not needed...


In cheap components, you certainly see problems (think of onboard audio chips in PCs, which often buzz when you move the mouse and such) that lead to the thought that you should shut off everything that's not needed. But again, a properly designed circuit doesn't exhibit such problems.

Passive preamp theory?

Reply #2 –
Although, if complexity is unnecessary, as possibly generated by this guy's theory, surely this leads to cheaper, more reliable hardware?
hi

Passive preamp theory?

Reply #3 –
I think the problems you run into when you just use a variable resistance between the source and the power amp can be at least as complex as the circuit you're avoiding.

As a rule of thumb, you want to have every component in the system (except the speakers) to have a high input impedance and a low output impedance. The source and power amp will both be designed expecting the preamp to have these properties. A potmeter won't have a low output impedance...

The story is probably more complicated than this - but I'm not an electronics engineer, so I had never thought about this problem much before

Passive preamp theory?

Reply #4 –
The really big user-interface issue with passive preamps is that their gain is dependent on what you're plugging it into. If you have a 10k pot wired to an opamp with a 10M input resistance, the gain curve will be considerably different from a BJT front end with much less input resistance. This issue gets better with smaller pot resistances, but then you run up against the output impedance of your source, and vagaries related to that. You can quite easily increase distortion by driving down the pot resistance low enough. You'll also increase power consumption.

Similarly, because the impedance as seen by the amp changes quite a bit, depending on the amplifier in question you could see changes in frequency response or distortion.

I'm planning on building a phono stage right now with a volume control, and I'm planning on putting a buffer after the pot for all those reasons.

Passive preamp theory?

Reply #5 –
Don't forget the non-linearity of response based on frequency.

Passive preamp theory?

Reply #6 –
Quote from: pepoluan on
Don't forget the non-linearity of response based on frequency.
This is a problem. You are essentially creating an RC filter with the passive volume control and the input capacitors on the power amplifier. This shouldn't cause a problem for some power amps, but for others it can cause a change in frequency response. Buffering the signal before feeding it to a power amp is generally a good idea.

Passive preamp theory?

Reply #7 –
Quote from: cabbagerat on
This is a problem. You are essentially creating an RC filter with the passive volume control and the input capacitors on the power amplifier. This shouldn't cause a problem for some power amps, but for others it can cause a change in frequency response. Buffering the signal before feeding it to a power amp is generally a good idea.


What would such a buffer circuit look like? Won't that introduce the kind of sound coloring we're trying to avoid?

Passive preamp theory?

Reply #8 –
Quote from: puntloos on
What would such a buffer circuit look like? Won't that introduce the kind of sound coloring we're trying to avoid?
It'll colour less than using just a pot, in any case.
A powerful OP amp wired in unity gain will do the trick.. It's a very simple circuit

This might be what you're looking for : http://headwize.com/projects/showfile.php?...pawliw1_prj.htm

Passive preamp theory?

Reply #9 –
Quote
' date='Aug 3 2006, 02:59' post='417573']
Quote from: puntloos on
What would such a buffer circuit look like? Won't that introduce the kind of sound coloring we're trying to avoid?
It'll colour less than using just a pot, in any case.
A powerful OP amp wired in unity gain will do the trick.. It's a very simple circuit
Shade[ST] is right. A good opamp based buffer (the audiophile anti-opamp attitude is ridiculous) will introduce an amazingly small amount of distortion to the signal. As long as you choose the right opamp and feed it decent power (a good opamp will reject power supply noise very well, though) you will add very little noise and distortion to your circuit.

Getting rid of the effects of the pot and increasing the impedence the source sees and decreasing the impedence the power amp sees might even improve quality a fair amount.


The design that Shade links to is solid - but I don't know much about this IC. Here is an alternative with pre buffering on the volume control and optional balance and tone controls based on the respectable OPA2134.

Passive preamp theory?

Reply #10 –
Quote from: cabbagerat on
Shade[ST] is right. A good opamp based buffer (the audiophile anti-opamp attitude is ridiculous) will introduce an amazingly small amount of distortion to the signal. As long as you choose the right opamp and feed it decent power (a good opamp will reject power supply noise very well, though) you will add very little noise and distortion to your circuit.

You can also find this : http://headwize.com/projects/showfile.php?...=szeke1_prj.htm mosfet-powered amplifier.  It should be just as sonically neutral

Regarding amps and buffers, though, guys... How can I know how much output current a said opamp has?  I'm looking to build an amp with opa227s buffered with opa228s.  Would that give me enough current to drive pretty much any headphone?

Thanks,
Tristan.

Passive preamp theory?

Reply #11 –
Quote
' date='Aug 3 2006, 09:31' post='417633']
Regarding amps and buffers, though, guys... How can I know how much output current a said opamp has?  I'm looking to build an amp with opa227s buffered with opa228s.  Would that give me enough current to drive pretty much any headphone?
Texas Instrument's data sheets are excellent, they are usually the best place to find info about their products.

First - you can't use OPA228 as a buffer, because it is not unity gain stable (due the the fact that it is only lightly compensated) - you need a closed loop gain of greater than 5 for this opamp to be stable. OPA227 is unity gain stable and would be a good choice for a buffer, but you would need to parallel at least two of them to get enough drive for most headphones. See page 17 of the Ti datasheet for more info.

Passive preamp theory?

Reply #12 –
Quote
You can also find this : http://headwize.com/projects/showfile.php?...=szeke1_prj.htm mosfet-powered amplifier.  It should be just as sonically neutral

It's not. I measured about -50 dB THD at 2Vrms output due to lack of PSRR and loopgain. Even though it introduces a 'warm' sound audiophiles will like, it's anything but neutral.

Passive preamp theory?

Reply #13 –
I have to admit something.

While I'm a microelectronics major (I dropped the actual electronics stuff for computer tech though) I have to admit something: Im a beast of luxury and aesthetics.

More specifically:

- I can't build a case that doesn't look like I made it out of an old shoebox. That kinda won't look cute enough standing next to my prettypretty stereo set.
- Im a minimalist in principle, but I 'require' a remote, and input selection.. and preferrably mute.

Can anyone suggest a 'course of action' here?

Passive preamp theory?

Reply #14 –
Quote from: ultranalog on
Quote

You can also find this : http://headwize.com/projects/showfile.php?...=szeke1_prj.htm mosfet-powered amplifier.  It should be just as sonically neutral

It's not. I measured about -50 dB THD at 2Vrms output due to lack of PSRR and loopgain. Even though it introduces a 'warm' sound audiophiles will like, it's anything but neutral.


Well I prefer not to drift into the area of 'warm sounds' 'airyness' etc. Every coloring of sound can be done at every point in the audio chain (the recording artist probably has a 'airyness' filter at his disposal). ADDING coloration is Bad?, especially since more often than not the coloring will do more harm than good.. it'll sound amazing for some records but 'break' others.

Ultranalog, would you agree that adding an opamp in the chain (purely as a buffer) will improve things without adding coloration(sic)?

Passive preamp theory?

Reply #15 –
Quote from: puntloos on
- I can't build a case that doesn't look like I made it out of an old shoebox. That kinda won't look cute enough standing next to my prettypretty stereo set.
- Im a minimalist in principle, but I 'require' a remote, and input selection.. and preferrably mute.

Can anyone suggest a 'course of action' here?


- You don't have to build a preamp yourself...
- Having input selection using a remote does not mean you are compromising audio quality.

I was going to suggest to just go for a good integrated amp, but from your other topic I gathered you just bought a Bel Canto power amp... so then the course of action would be getting a preamp that has the features you need.

There are plenty of decent preamps - especially when you've got as much to spend as you have  - that do not "color the sound". That is, their distortion levels will be orders of magnitude below any audible threshold (and below that of the speakers ).

Passive preamp theory?

Reply #16 –
Quote from: Patsoe on
- You don't have to build a preamp yourself...
- Having input selection using a remote does not mean you are compromising audio quality.


And I wasn't going to unless there was some k-rad kit that came with everything and the kitchen sink.
No, the remote shouldn't influence things. Most pre-made passive pre's turn themselves off when they are not using the remote.

Quote
I was going to suggest to just go for a good integrated amp, but from your other topic I gathered you just bought a Bel Canto power amp... so then the course of action would be getting a preamp that has the features you need.

There are plenty of decent preamps - especially when you've got as much to spend as you have  - that do not "color the sound". That is, their distortion levels will be orders of magnitude below any audible threshold (and below that of the speakers ).


Im about 10 minutes away buying this one (used): http://stereophile.com/solidpreamps/133/

Passive preamp theory?

Reply #18 –
Quote from: Patsoe on
Quote from: puntloos on

Im about 10 minutes away buying this one (used): http://stereophile.com/solidpreamps/133/


I love the amount of measurements Stereophile always provide!
Looking at those, I wouldn't say it's a particularly special preamp, or is it? I think I've seen better performance if you look at high frequency crosstalk, for example.


2 things to keep in mind:

1/ This amp received the Stereophile A-class recommendation (the highest they give out)
2/ The measurements are mostly for this amp in ACTIVE mode. The measurements of the passive mode are 'moot'. Words like 'Ruler flat frequency response' were used etc.

Passive preamp theory?

Reply #19 –
I have a feeling that with these class d amps imput impedence has no reason not to be very low. Which would imply that a passive attenuator should be a good match. But I am no audio engineer and could be wrong.

Passive preamp theory?

Reply #20 –
Quote from: CSMR on
I have a feeling that with these class d amps imput impedence has no reason not to be very low. Which would imply that a passive attenuator should be a good match. But I am no audio engineer and could be wrong.


The main rule of thumb is:

- At any (relevant) frequency, the output's impedance of the source should be at least 100x as low as the imput impedance of the input of the next stage.
- The passive pre isnt the problem here (it does add an extra loop, but it should be designed in such a way that this is will not become a factor), the main issue is the impedance of the output of the source (CD/DAC).

My DAC's output impedance is rated at 47Ohm, my power amp is at 100Kohm, so IN THEORY I should be good.

Passive preamp theory?

Reply #21 –
Quote from: puntloos on
1/ This amp received the Stereophile A-class recommendation (the highest they give out)


That's not bad  On the other hand, an amp not getting that recommendation may still be a great amp...
I think that Stereophile favors exclusive and expensive gear - they provide excellent measurements and other information so you can still judge for yourself, but since they hardly ever have "normal people's gear" on test, you can't really compare how much better all their cool stuff is.

Quote
2/ The measurements are mostly for this amp in ACTIVE mode. The measurements of the passive mode are 'moot'. Words like 'Ruler flat frequency response' were used etc.


Oops, I didn't catch the part about the active and passive modes when I skimmed through it

More on-topic: I still don't think there's a clear reason why a passive preamp should be intrinsically better.

Passive preamp theory?

Reply #22 –
Quote
More on-topic: I still don't think there's a clear reason why a passive preamp should be intrinsically better.


I think I can answer that partially.

First, soundstaging and definition: there can be no loss of detail if you don't feed your signal through components (like opamps, tubes) that can actually be 'bad for it'.

Secondly frequency response (dropoffs at the extreme highs and extreme lows). All that a passive pre does is 'shift the problem elsewhere'. There can be little problem with the passive pre itself, however the quality of the output of the CD player/dac will become twice as important.

And lastly, I don't know if you've ever worked with audio or video editing, but my personal experience is that for example while touching up digital audio.. I apply this filter.. hmm that sounds better, less noise.. I expand that range.. yeah that pops nicely.. I do this.. I do that... there, done, much better.

Then compare it to the original..

Then throw all my hard work into the trashbin.

Less is more, is my motto. Im sure its not always true, but it is at least a reasonably safe bet.

Passive preamp theory?

Reply #23 –
Quote from: puntloos on
Ultranalog, would you agree that adding an opamp in the chain (purely as a buffer) will improve things without adding coloration(sic)?

Yes. Low output impedance is one of the best attributes of a preamp (that, and being able to adjust the level).

Passive preamp theory?

Reply #24 –
A capacitor is passive, and it will most definitly change the sound (acting as a highpass/ lowpass filter, depending on how you wire it)

Opamps are good, quality components, generally.  Especially if you get the "microwave RADAR, milspec" kind.