difference between an active and a passive preamp?


hi,
I have a nad c272 amp and am looking for a good preamp to go with it, but I am on a very tight budget. I see lots of preamps that are acive and some passive - I have no idea of the difference? I have quad 22L speakers and listen to cd only. Any help understanding these differences would be great. I just want simple 2 channel preamp, with as tube like sound as possible. Please help, and many thanks,
jason
audioflyer67
The passive has no active circuitry and therefore no gain.
And, as a result of having no active gain, passives are generally more transparent, and provide more bang for the buck if gain is not an issue. If you have only one source, check out Luminous audio axiom
Passive has no gain stage.
Active has a gain stage (at least one)
In all other ways they are about the same physically.
electrically though, they are different.
The passive preamp depends entirely on the source component to 'drive' the amp.
So using a passive preamp means you MUST have a robust source device. (or the result will be thin, weak sound)
Generally most CD players can drive a passive preamp 'good enough' That is it will sound no worse taht an active preamp.
For a passive preamp to excel.. the source must have a great ability to drive the amp.

With an active preamp none of that matters. any source can be used and a good result will occur.

Some sources actually state i nthe specs do not use with a passive preamp. (nice to have that info. My Audio Research PH-2 specifically states do not use with passive pre._
So.. IF you can find out the ability of your source to drive the amp, a passive will be good..
If it cannot do it well, then you either have to get a source that can drive a passive, or get an active preamp.
When you get up there in money spent on a preamp.. the actives get pretty good.
In the lower ranges is where the passive shines in comparison to an Active pre. IMO,
King Rex preamp under $400. As transparent as a passive with tube like mids and deep bass of the solid state pre it is. I believe it has 15 db of gain. A real fine sounding unit the sounds well above its price.
There are a lot of caveats when using a passive preamp. As Elizabeth said, you have to make sure your source component has a very robust analogue output section with low output impedance. In addition, your interconnect lengths from source to preamp to amp (short as possible) and capacitance of the interconnects (lower the better) and the input impedance of the amplifier (higher the better) are critical as well. There are passive preamps made of pots, stepped resistors, LDRs, transformers, and autoformers. I prefer the latter three. If all of these are taken into consideration and executed correctly, a passive preamp is better sounding than any active preamp I have ever heard. But I have not heard the mega expensive $15k+ preamps like darTZeel, Atma-Sphere MP-1 with upgrades, etc.
Very nice clear responses...the way they build upon one another...nice to see someone get a direct answer(s) to their questions. Educational too....One for the archives!
The input impedance of your amp is 20k ohms. IMO this is not a great match for a passive preamp. There could be exceptions, but I am not aware of any.

On the other hand you could use a buffered preamp. This uses active circuitry with no gain. The active circuitry greatly minimizes the risk of impedance mismatches. There are tube buffers and solid state buffers. A few solid state options that come to mind are the Pass B-1, Burson Audio, and Horn Shoppe Truth. The latter of which I own and can state that it is extremely fast, detailed, and transparent. It gets out of the way which is what I believe a preamp should do. However, I would not classify it, as well as other solid state buffers, as having a tube like sound.

Also note, not all buffers have a volume control. If you desire something "tubey" you should probably stick to an active tube preamp or a tube buffer with volume control.
Should I be depressed that I am tempted to recycle posts I wrote 7 years ago, and that I remember them? 'Cause I am....

In any event, from the way-back machine in '04:

Just to add a couple more simplistic thoughts to the already excellent discussion from an absolute layman’s perspective, "volume control" generally can be achieved by two basic means, either attenuating (that is reducing) the signal that comes into the control or by amplifying it (which I have understood as adding gain). An active preamp is called that because the volume control can do both of these things--it can both reduce the signal from the input level towards zero and it can (because it has an amplification stage) amplify the signal above the input level. A "passive" preamp, on the other hand, is passive because it can only reduce the input level -- it will not have an amplification stage at all.

For most sources, the output level of the signal (which is the input at the preamp) is relatively high. Which is to say that at zero gain at the preamp (no attenuation or amplification of the signal) the volume level will be relatively loud. (Which is how passive preamps/volume controls can work -- based on the assumption that the volume associated with zero gain will be louder than you ever want or need, therefore attenuation or reduction of the signal level can provide all of the volume control range you would ever want).

Compared to just about every other source out there, however, the output signal that comes off of a vibrating pin scratching across a rotating platter of vinyl is tiny. So tiny that, without additional amplification, the signal would be too weak for a volume control designed to provide a useful range for things such as CD players to do much anything useful with at all. Thus, in order to jack the tiny output from a typical phono cartridge up into the range the your average preamp volume control was designed to operate in, you need some additional amplification. This is why a phono preamp is a separate or additional piece of equipment (or stage in your existing preamp) from your plain vanilla preamp -- it independently amplifies the signal coming from the cartridge up into a range useful for your average volume control. The gain level of the cartridge dictates how much more work the phono stage will have to do to get the signal up into this useful range. Did my layman’s understanding screw that all up horribly?
Not at all, Mezmo. I'm not technical and it made perfect sense. Audioflyer: I was recently sort of in your position. Just a few weeks ago I was posting about my passive pre. As someone who went in blind, I can tell you what I encountered. Once I had it hooked up, the level of transparency people keep talking about was immediately apparent. My cd player was completely wrong, however, because of the impedance issues already mentioned in the thread. There is a loss of that in-your-face sense of detail and some roll-off. Still, I prefer it immensely to my previous solution. I ended up replacing my cd player almost immediately for better impedance. Alternatively, I probably could have gotten a buffer.

I'm not experienced with tubes or even the "tube sound." Some friends of mine more experienced with this have said that I've achieved a bit of a tube sound with the passive--I believe other passive pre users on this site have often reported the same thing. It's been interesting how loud it can get without being uncomfortable. Before, with my active pre, which wasn't a stellar piece of audio equipment anyway (Lexicon DC-1), the high end was just too sharp and fatiguing, especially at high volume. Now, more volume just yields more rich sound.

As has been said, an input sensitivity of 20 k0hms is a bit low. What's the output impedance of your cd player? What's your budget? Knowing nothing else, if you're interested in the passive solution, I'd do what Clio09 said.
Note: I misunderstood your last sentence, Mezmo. No wonder your layman's understanding suited me so well. ;)
To be on the safe side when using a passive preamp, I would want my source to have an output impedance less than 500 ohm, source output voltage at least 2V (this also depends on the input sensitivity of your amp), my amplifier input impedance greater than 50k ohm, and total interconnect length from source to amp not longer than 6 feet. In my system, I have a autoformer passive preamp, USB DAC with output impedance of 50 ohm and output voltage of 2.5 volts, amplifier input impedance of 100k ohm (with input sensitivity of 1.25 or 2.8V depending on the amp), and total interconnect length of about 5 feet. This way I can have all the transparency, naturalness, and openness of a passive without significantly losing dynamics of an active preamp.
Dracule1 said:
source output voltage at least 2V (this also depends on the input sensitivity of your amp)

Can you expand on this?
For example, if amp X has an input sensitivity of 1.25 V, 1.25 V from the source (eg,CD player) is need to achieve full rated power from your amp X. So if amp X is rated at 100 watts, it needs 1.25 V to produce 100 watts. If amp Y has input sensitivity of 2.8 V, then your standard CD player output of 2 V is not enough to get full power out of your amp. In this case you need active preamp, to provide that extra output to get full power out of your amp. Most passive transformer or autoformer preamp can provide an extra 6 dB of gain (stepped resistor, pot, or LDR based passives can't), so you can mitigate some of this. I'm not too certain on about how the length of the interconnects affects this. But I think longer interconnects will decrease the amount of effective volts the amp sees from the source. So I try to keep my interconnects as short as possible which also helps to prevent significantly losing high frequency response. If anyone more technical can find any error in my explaination, please correct me. I'm not that technical.
@audioflyer67
+1 for the Luminous preamp. It's about $150 with the better caps and they'll use your systems measurements to get the right resistor. You can't beat it for the money.

@dracule1
What am I missing? My amp has a sensitivity of 775mV (VTL compact 100), and I've been running 20 foot interconnects between the amps and various passive pre's with great results. Not sure of the numbers for my dac (Levinson360s). For cables I've used Apogee, Discovery, and Crimson Musiclink. My current, favorite, and last passive is a Bent NOH based on the music first design and transformers. I've also used the Luminous and the Prometheus passives.

Go figure, it sounds great!
Larry to tower...get ready for the flames...here comes a statement which will be trounced on...

Personally, I've NEVER liked passives...ever.
They sound 'washed out' to me. It's almost as if the input signal to the pre is somehow absorbed more than passed and that the result is an anemic output--dynamically.
I suppose that someone on the other side of the ledger would simply claim the opposite--more pure no 'changes made by poor circuitry in the gain stage.'
I've heard several along the years and just don't get what I like.
For example--growing up in audio, my favorite preamps were,

Conrad Johnson's, (I know they use lower case) Premier 3
Gryphon's LS1 (Model may be wrong) $9000. in the late '80's.
They were both different, but both incredible.
The cj was a beautiful sounding, (though colored) preamp, with space within space along with beautiful textures.
The Grypon was a work of art, both asthetically and musically (electrically) employing two outboard power supplies. It DROVE, DROVE, DROVE, the amplifier like no other up until that point...by a wide margin.
One day, slow day in retail, a guy came in from New Jersey...he'd been to every store in North America, by his own admission...anyway, he came in to hear what we were doing. For giggles, I said, I'll bet you've never heard the Adcom GFA 535 (60wpc). It was a $299. amp back then as I remember.
I hooked it up to the THIEL CS5..(which dropped to 1.5 OHMS in the bass, very tough load to drive).
I played it with the Adcom, probably GTP500 Preamp/Tuner combo piece, that it was normally paired with.
He yawned...of course so did I.
Then, hooked the Grypon to it and he was open mouth stunned. WOW!
He said, I've never heard a preamp that controls like that.
'Me neither'.
Gryphon sent 13 into the United States that year...I sold 11 of them in Louisville, KY, the Mecca of High End Audio.
lol.
Buy a regular preamp not a passive...all things being equal, you'll be happier.

Good listening,
Larry
I never met a passive preamp I could live with...dynamics are severely squashed
Ts0711, you have a very sensitive amp so it won't take much to get your amp to full power. However, I would think you would lose some dynamics and high frequency response with interconnects that long. Have you tried to get your passive preamp and source close to your amp so you only have 3 feet from preamp to amp and about same from your source to preamp? I would think you would hear a noticeable difference. Your present setup may sound good to you now, but using shorter cables may significantly improve the sound.
04-12-11: Dracule1
I'm not too certain on about how the length of the interconnects affects this. But I think longer interconnects will decrease the amount of effective volts the amp sees from the source. So I try to keep my interconnects as short as possible which also helps to prevent significantly losing high frequency response. If anyone more technical can find any error in my explaination, please correct me. I'm not that technical.

04-12-11: Ts0711
@dracule1
What am I missing? My amp has a sensitivity of 775mV (VTL compact 100), and I've been running 20 foot interconnects between the amps and various passive pre's with great results.
With resistive and LDR-based passive preamps, the need for short interconnects derives from their high output impedance, and applies to the cable between preamp and power amp. I can't envision a reason why the length of the cable between source and preamp would be any more critical than with an active preamp.

The reason cable length on the output side matters is that its capacitance forms a low-pass filter in conjunction with the output impedance of the preamp. Cable capacitance, besides being dependent on the cable type, is proportional to cable length. If it is too high, the bandwidth of the low-pass filter will be low enough to fall within the audible spectrum, resulting in some amount of roll-off of the upper treble.

The degree to which that is significant will also depend on where the volume control is set, since the volume control setting will have a major effect on the preamp's output impedance. For a passive preamp consisting of a potentiometer or a resistive stepped attenuator, the worst case output impedance will occur at a setting corresponding to 6db of attenuation, which will typically be very loud if the source is a cdp or dac.

As was noted, it is also important for the output impedance of the source to be low (in relation to preamp input impedance), and for the input impedance of the amp to be high (in relation to preamp output impedance). Otherwise frequency response irregularities will result, to the extent that the impedances that are involved are not constant as a function of frequency.

TVC's are a completely different story, that I'm not particularly familiar with. But since a transformer provides an impedance transformation corresponding to the square of the turns ratio (with attenuation or gain being directly proportional to the turns ratio), I would expect that as long as the volume control position that is used corresponds to a significant amount of attenuation (i.e., it is not near the top of its range), cable length and component impedance matching would be much less critical than for a resistive passive preamp.

Regards,
-- Al
Al, thanks. My understanding of TVC or AVC vs resistor/LDR volume control is that as you apply more attenuation, TVC will decrease its output impedance whereas resistor volume control will increase its impedance. That is one argument for TVC because your suppose to get better sound as you attenuate more with TVC, while you get worsening sound as you attenuate more with a resistor based volume control. At least that's the argument I heard, but I don't know if in practice this is true.
04-12-11: Almarg
TVC's are a completely different story, that I'm not particularly familiar with. But since a transformer provides an impedance transformation corresponding to the square of the turns ratio (with attenuation or gain being directly proportional to the turns ratio), I would expect that as long as the volume control position that is used corresponds to a significant amount of attenuation (i.e., it is not near the top of its range), cable length and component impedance matching would be much less critical than for a resistive passive preamp.
I should have added to this statement that it assumes the transformer itself does not introduce significant resistance or impedance into the signal path. I have no particular knowledge of how true that assumption may be for typical TVC's.

Regards,
-- Al
04-12-11: Dracule1
My understanding of TVC or AVC vs resistor/LDR volume control is that as you apply more attenuation, TVC will decrease its output impedance ...
True, which is why I referred to TVC sensitivity to cable length being less "as long as the volume control position that is used corresponds to a significant amount of attenuation."
As you apply more attenuation ... resistor volume control will increase its impedance.
That's not quite true. Assuming the resistive volume control is configured as a conventional voltage divider, it is true that the lowest output impedance will occur at the maximum volume position (where it will to a very close approximation be equal to the output impedance of the source). However, the worst case (maximum) output impedance will occur when the volume control is set to provide approximately 6db of attenuation, which in terms of subjectively perceived volume is not greatly different than the maximum volume position. As the volume setting is reduced from that point, output impedance will then gradually decline.

Best regards,
-- Al
I have to admit that the common detractors' rant of diminished dynamics rings true for me. For what I put into my system, however, I consider it an acceptable trade-off (for now). Plus, my system isn't perfectly optimized for a passive preamp, like the op's. This brings me back to the, more or less, main point of the thread as expressed in the original post, which is to determine when one should use a passive preamp versus an active preamp given a certain set of non-negotiable pre-existing system components.

This thread seemed to point out an Option C, however: the buffered passive. It seems that if enough gain is provided by the source output and amp(s), then a passive or buffered passive should be sufficient to accurately reproduce the source material. The only issue is impedance and possibly voltage(?). (Based on my reading here, this is my impression). If this is so, then shouldn't optimal dynamics (e.g., "attack") be achievable with a buffered passive, if not a discreet passive? For the proponents of actives, I suppose a more direct question is, what specific attribute of a proper active preamp achieves that vivid "attack" that a passive does not? Is it more than an impedance issue? If so, then what?

In short, it's clear to me that not all systems are passive-friendly, but a buffer rectifies the situation. That is, a buffer seems to be *the* Band-Aid for passive-hostile systems, based on any thread I've found on this subject. Or is it? When is a buffer plus a passive still not enough, and how common might this situation be? If this question deserves another thread, please say so and I will post anew. But this seems like a natural progression from the current conversation following from the op's expressed concerns.
I wouldn't necessarily consider a buffered device a band aid for passive-hostile systems. If you look at the designs of a Pass B-1, Burson-160, and Horn Shoppe Truth as examples, these buffered devices are anything but band-aids. They are also technically active devices as the buffers require a power supply, but in general they do not add gain to the system. The main purpose of the buffered output is to provide a consistent and low output impedance, generally under 100 ohms, but this could vary depending on the buffer design. Those using opamps or other solid state devices will tend to have very low output impedance, while tube buffers will in most cases have greater than 500 ohms, perhaps even over 1000 ohms.

IMO optimal dynamics can be achieved with both passive and active buffered preamps, my Lightspeed attenuator, Truth, and Silicon Arts Design preamps being examples in my system. However, system matching with a passive is a bit trickier than an active buffer (which if the output impedance is very low and due to the active circuitry may also provide the benefit of using longer interconnects). To really determine if your system is passive friendly and more importantly if you like the resulting sound, go to Arthur Salvatore's web site and read up on the Bolero test. This should give you a good idea if your source has enough output voltage to drive your amp. If so you might be ready to experiment a bit with resistive, magnetic, or LDR passives, as well as active buffers (preferably those with built in attenuation).

I could go on about how component specs can influence the sound, as can cable length and capacitance, but there is nothing like hearing it for yourself (although I think your system would benefit more from an active buffer). Lots of options out there at very reasonable prices to try both passive and/or active buffers. I know Ed Schilling offers a money back option on The Truth, and at $800 or so it is well worth a test drive. There are others that may offer trial periods as well. Take advantage of those opportunities to listen for yourself.
Although I have not heard the buffered passives Clio09 mentioned, the one's I have heard did not sound as transparent as the pure passive preamps. But this was a long time ago, and I don't even remember the brand of the buffered passives I auditioned. On my Bent Audio Tap X, I was thinking about putting in a Burson active buffer on the output of the Tap X, but this is akin to putting a transformer on a OTL amplifier. Seems ass backwards, but I've been known to do ass backwards stuff in the past.
My need for an active buffer with attenuator is due to a need to run long interconnects to an amp that sits in between my speakers. I have another set-up where I just use the Lightspeed with short interconnects to the amp and long speaker cables. Quite frankly, both units are very transparent. There are slight sonic differences, but nothing to split hairs over.

As for the TAP-X, IIRC the 6 input version John Chapman made has a board where the two sets of outputs can be operated with an active buffer by the flip of a switch. Quite a nice feature.
Yea, I know about active buffer option on the Tap X. John told me the active buffer degrades the sound of the Tap X and stopped including them on his later runs of the Tap X. I think the buffer was an opamp.
If you look at the designs of a Pass B-1, Burson-160, and Horn Shoppe Truth as examples, these buffered devices are anything but band-aids.

I think we're using the term differently. I didn't intend a pejorative. I meant it the other way: something that easily mends. In fact, based on your response, I must have made myself very difficult to understand in that whole post, so I'll rephrase. I'm not trying to find out how a passive is with my system. I have an older passive and I'm hooked on the direction it's taking me. It's already pretty clear that I would benefit from a buffer. In fact, Almarg was quite patient and helpful in another thread where I was struggling with this. Now, I'm just wondering what the active preamp crowd would say remains unprovided in a merely passive or buffered preamp that a traditional active preamp provides. On other words, I'm not just learning how configure my system; I'm attempting to learn the conversation.
A very good question, After having owned some of the finest tube line stages,
and almost every type of passive, and given that I have low impedances
sources with strong analog output, short, low capacitance IC from "pre
to amp, and a sensitive (1v) amp with 100 kohm input impedance, I'm not
sure what an active, any active can do to make the sound truer to the source -
warts and all. What an active will certainly do as add a coloration (many
different flavors) that, while undoubtedly a distortion of the source signal can
be, for many people, very pleasant to listen to, and it does not matter if they
believe it is a move from "true to the source" - they like the way
their actives sound and that's all that really matters. Me, I prefer to take the
preamp (as best I can) out of the tone equation and deal with the Source,
amp, and ICs for the sound I want. The best passive I've tried? The Lightspeed
Attenuator. But I have not stopped looking......
Wow, guys,this has been really helpful and insightful. I'm learning! Actually, I should have read this before I made my purchase!!! I got the canary passive 200, and what a disaster! the sound was awful. A huge mistake! I chalk it up to a learning experience. Now, I am thinking of either a nad 165bee preamp to go with the nad 270 amp or maybe a tube pre that would be COMPATIBLE SYNERGYSTICALLY! I don't want to make another mistake. Because I don't really have an engineering background, and for that matter, don't understand the "electrical concepts" to ensure comptability, I'm going to first clear it with you guys, if that's ok. I love this hobby!
Can you guys recommend a good tube amp that would compliment nicely my nad c270 amp? Or, should I just sell the amp and go with a nice tube integrated? I prefer the former option for now. My budget is 500 to 1,000 max.
Thanks very much.
You tried the Canary with the NAD270? What is the sensitivity of the Quad
22s?
They don't need much power, do they? As for passive, I think you would need
something like the Goldpoint with a 10kohm pot and short ICs to drive the
NAD properly. I may be wrong about this, but I think tube amps are generally
better suited to passives, but if the Canary had a 50kohm pot, it would not
work too well with an NAD amp. I wonder if a used Cayin integrated will be
enough power (40 watts of so tubes) to drive the Quads. IMHO, if you want
tube sound you are not going to do with an SS amp, certainly not at your price,
and really even at higher price points.
Another active tube buffer that is getting some attention is from Dodd Audio.
I have found that my system when using some passives is more succeptible to noise (ie. RFI, grounding issues, etc).
Since this thread has in part become a "passive and buffered pre roundup," I thought I'd throw the Superphon SP100 into the mix. It may be older, but it seems to have some kind of following. Plus, there're currently two available in the Classifieds. Seems to be a passive volume control with a buffer stage.

Audioflyer67, have you looked at the Decware site? I haven't heard their stuff with my own ears, just noticed their reputation. If you're looking to gradually get into tubes and you're starting at just the preamp at present, take a look at this one: http://www.decware.com/newsite/CSP2.html

Until recently, the forum there had a bit of banter. I think Steve Deckart (owner) is happy to talk you with you about your specific needs. It's a fun site to explore and has plenty of interesting articles by Steve. They also offer a tube buffer, tube gain stage, and input selector. Put those together, and you have a versatile 6-input tube preamp.
I've got my eyes on the Decware preamp with adjustable voltage. Talked to Steve about it and it is a well designed piece of equipment. Can be an OTL headphone amp as well.

Stan Warren designed some great pieces. The SP100 is one of definitely one of them.
I can't envision a reason why the length of the cable between source and preamp would be any more critical than with an active preamp.

With passive controls, the cable length is quite critical as is the construction of the cable. The interaction is between the source impedance, the installed variable of the passive control, the characteristics of the cable that goes from the passive to the amplifier, and the input impedance of the amp. Its a bit complicated, but with passives what you will universally hear is that as you turn down the control, the bass and impact in the system will be diminished. It does not matter the quality of the control itself- that part is really not a variable, although the *value* of the control is.

The problem there is that a lot of sources may not be so happy with a 10K passive control hung of its output; even then you will still hear effect.

TVCs overcome some of these disadvantages. They can present a much lower impedance to the interconnect cable at their output- this reduces the effect of the cable and can reduce the variables in that rather complex interaction I pointed to above. The price of TVCs is that they must be designed to be loaded properly at each tap on the transformer, else they can 'ring' or distort. In addition, if improperly loaded, the inter-winding capacitances of the device will come into play. This is more or less like saying a random capacitance is being used to bypass the TVC. This will be a path for high frequencies.

So great care has to be exercised in the design of a TVC. Even so, If you want the system to play bass, the TVC must have bandwidth to 2 Hz if the system is the play to 20Hz without loss. If the unit cuts off at 10Hz, effects that will be heard as losses will extend to 100Hz. Making transformers that go that low without problems in the highs is a really challenge!

Active preamps have advantages of cable control as well, although if the designer of the active line stage does not recognize that that advantage may not be realized. Now f things are done right the effect is that the cable will have no sonic contribution at all, and the length of the cable will be irrelevant. I think you can see from this last statement that many line sections fall well short of this ability! Quite often in the use of a line stage the hidden cost of the line stage is the cost of the cable that makes it sound right. In such cases its easy to see how a passive or TVC would be preferred.

But if an active line stage is properly executed, the length of the cable and its cost will have no bearing on the sound of the system at all. The advantage here is you can avoid the colorations of a passive, and potentially get greater bandwidth and lower distortion (read: smoother and more detailed) than is possible with a TVC.

Now in the case of DACs and CDPs, quite often the output of such units is prodigious, far more than is needed to overload even the least sensitive amplifier made. This requires that the output of most DACs and CDPs must be reduced (attenuated) by some means. A digital control is not a good way to do that- such controls work by essentially subtracting bits from the signal. By the time you get below about 85% of the range of the control, a significant loss of resolution is audible.

A simple solution would be for digital equipment manufacturers to go back to the 1V peak standard that has been in place for 40 years. It pretty obvious now that LP will not be going away as the digital community had predicted (I remember seeing on TV an RCA executive saying that they were going to discontinue LP production in 1987... lol!). At any rate its not to the advantage of anyone using an all-digital setup to have the output voltage be so high that all amps made are overloaded by it.

Active buffers are a good choice in such a case. They avoid interactions with the cable by keeping the volume control intimate with the rest of the circuit and have the possibility of cable control just like an active preamp (which in reality is what they really are- active, but with no gain). We've built a few such devices by getting rid of the gain stage in our preamps and they have worked quite well, but in such a case you really do need the extra output that DACs and CDPs provide. The problem there is that if you want to use a phono section as well, it will either have to have a very high output or it will not be usable...

So actives remain my first choice but you knew that by now :)
04-12-11: Almarg
I can't envision a reason why the length of the cable between source and preamp would be any more critical than with an active preamp.

05-18-11: Atmasphere
With passive controls, the cable length is quite critical as is the construction of the cable. The interaction is between the source impedance, the installed variable of the passive control, the characteristics of the cable that goes from the passive to the amplifier, and the input impedance of the amp. Its a bit complicated, but with passives what you will universally hear is that as you turn down the control, the bass and impact in the system will be diminished. It does not matter the quality of the control itself- that part is really not a variable, although the *value* of the control is.
Ralph, thanks for your comprehensive and characteristically informative response. Could you explain a little further, though, what parameters of the cable connecting the source component to a passive preamp would interact with the things you mentioned, beyond interaction with the output impedance of the source component that would also occur with an active preamp?

Best regards,
-- Al
Could you explain a little further, though, what parameters of the cable connecting the source component to a passive preamp would interact with the things you mentioned, beyond interaction with the output impedance of the source component that would also occur with an active preamp?
For a transformer-based volume control, it's possible that certain combinations of high source impedance and a high-capacitance input cable, that this could affect the transformer's self-resonance point, and maybe the in-band HF transient response. In the case of a potentiometer-type resistive circuit, the only thing I can think of is the fact that the source component will also see the output cable capacitance at the highest volume settings, like above -6dB.

But this is really grasping at straws . . .
Hi Al, The cable between the source component and the passive control plays a reduced role on account of the output impedance of the source. When the series resistance of the passive control is inserted into the circuit, the character flaws of the cable are essentially magnified. So most of the issue is downstream from the passive control.

The rather obvious solution is to install the volume control at the input of the amp, but this could be really inconvenient if you have monoblocks. A remote could solve that problem though...
I am bumping an old thread here!

I am leaning towards going with a passive preamp. Although, I am not into numbers too much because as long as you like the sound, who cares but in this case, I think it is important to check some numbers.

Is passive preamp suitable for my setup?

DAC: 
  • Output Level: 3.0V RMS Max. (Analog).

  • Outputs: Stereo (RCA) 300 Ohms.
AMP:

Input Sensitivity (RMS) - @3Ω, 500W,1KHz 1.36V

Input Impedance - 33K ohms



@celo , maybe.

The real number to look at is the output impedance of the DAC at 20hz. If that is the same number then I would say your chanced are good with a passive control, if you keep the cables short.
I think that the output impedance is only half of the equation here. The other half is how the waveforms are generated from the source. Can you tell us the make/model of your DAC? Also, what kind of amplifier? I general don’t like to recommend passive preamps unless the source device has a very good Class A output stage or tube output stage. Generally, sources that use op-amps only as output stages may still have somewhat of a digital sound. It really depends what’s on the output stage. Sometimes there are excellent op amps and circuits (i.e. AD797 or something similar). The OPA2134, which is very popular, is okay but has more of a laid back sound and doesn’t have the resolution or separation-of-instruments as other circuits available.
My DAC is Musatex Bitstream. Amp is Class D. Thanks. 
Interesting.  If you have the "Data II" update, it looks like you have a Class A circuit.  The Musatex probably uses FET devices (the MELIOR BS-1 used FET).  A passive preamp may work well here.  If you're not sure, you can check out the Schiit Freya.  With that, you have your choice of passive, JFET or tube gain stages.  It's probably in the budget that you're looking for.  15 day return policy if it doesn't work out.
check out the Schiit Freya. With that, you have your choice of passive, JFET or tube gain stages.
+1  auxinput

Cheers George
Thanks! No, mine is a stock unit.

I am leaning towards the Tortuga Audio but have not made my mind yet.