[jmsun]

I'm looking for a good preamp to work with my Odyssey amp. Someone suggested Purist 500. I've never had a passive preamp before, so I don't know what to expect. Does anyone have experience with this preamp? I'd appreciate if you can share your thoughts.

[JoshK]

Why don't you ask Klaus about his passive preamp?  I think it is relatively inexpensive too.  One thing to remember is the input impedance of the Odyssey amp is 10K, so when mating this amp with a passive preamp it would be good to know if all your sources are suited for the task.

[jmsun]

Thanks Josh. I will ask Klaus about that. But what I'm struggling with is what is the benefits of using a passive preamp. If a cheap Purist can do the job so well, why people spend $$$$ on active preamps? Sorry for the newbie question.

[Marbles]

The passive that I use is not very cheep by most standards.  

The reason many use active is because they have a source that may have less than 2V output, and or amps that have a tough input impedance.

Also since most all actives (some exceptions of course) color the sound in one way or another, these colorations may be pleasing to the purchaser and or synergistic with the system.

Who knows, it could also be that they like how a pre-amp looks too.  It could be any reason why one would like a pre-amp over another, much less an active over a passive.

[JoshK]

Boy, I'm afraid that is a monstrous question!  I am not going to even attempt to give all the answers, but one obvious one is that they are not suited for all situations, such as high output impedance of the source and low input impedance of the amplifier.   Long interconnects are also a time when passives may not work best.   If you systems does not have enough gain, i.e. your speakers are relatively inefficient and your amp does not provide sufficient gain then a passive will not be appropriate.  Many more reasons but often the biggest reason is deep seeded traditions, marketing hype and resistance to change.  *ducks*

[mcrespo71]

*throws another punch at Josh and misses!*

I've got no reason to disagree here, but you did give me an opportunity to throw a punch and I had to take it.

Michael

[dado5]

Nutshell response-

Passive advantages:

- less circuitry=lower failure rate.

- no added distortion=possibly cleaner sound.

- no unneeded voltage gain=possibly more useful range on the volume control.


Passive disadvantages:

- high/variable output impedence=possible high frequency rolloff/possible lack of current delivery=possible dull sound

- no needed voltage gain=possible lack of desired system volume (probably only an issue with single ended or super simple amps).


Active advantages:

- needed voltage gain=useful volume control range.

- low output impedence=more current drive/no HF rolloff=possible dynamic sound.

- possible increase in signal to noise ratio due to higher voltage signal input at power amp.

- possible additive sonic character.


Active disadvantages:

- more circuitry=higher failure rate.

- too much voltage gain (very comon)=little useful volume control range

- added distortion=possible muddier sound

- possible distractive sonic character


Beyond this you just gotta try things for yourself.  You can build a passive for peanuts (less than $10.00) with Radio Shack parts if you are so inclined and this might answer some of the above concerns.


Good luck,
Rob

[PhilNYC]

great summary, Rob.  I'll say that I ran my cdp (with volume control) direct to my amp for a while and thought it did some great things.  Then I put a (active) preamp back into my system and thought it did some great things.  At the end of the day, I strongly preferred using a preamp...possibly it has something to do with the kind of music I like to listen to that works better with the added gain and other characteristics.  But asides from the technical considerations others have talked about, it is definitely about preference and tastes...

[JoshK]

In all fairness, Phil, I think the volume control out of the CDP versus your preamp have more to than just active vs passive.  Quite often CDP volume controls are not implemented very well.  I cannot speak to the Resolution player but I am guessing it isn't quite on par with the shallco switchs and premium resistors in the attenuators of your BC.

But I still think you would probably come to the same conclusion, it just might not be as evident or dramatic.

[tkp]

If you are going to check out passive preamp, then please do not use the RS pot.  The RS pot is low cost but it will hurt the signal and have very poor trackability between the left and right channel.  At the very least, use some 12 position switch to build a 12 steps series stepped attennuator.  This will give you a taste of a properly implemented resistive passive preamp sound like.  There are many things to consider when building and using passive preamp.

[jmsun]

Thanks for the tip. But I am not skill enough to build my own pre. I'll need to decide on either passive or active before I select a brand. Because I am relatively new to two channel stuff, I'll rely heavily on my reading here and elsewhere. Thanks for the input.

[PhilNYC]

In all fairness, Phil, I think the volume control out of the CDP versus your preamp have more to than just active vs passive.  Quite often CDP volume controls are not implemented very well.  I cannot speak to the Resolution player but I am guessing it isn't quite on par with the shallco switchs and premium resistors in the attenuators of your BC.

But I still think you would probably come to the same conclusion, it just might not be as evident or dramatic.

Josh, that's a valid point.  But also note that on the Resolution Audio player, you can't bypass the volume control without using a special DIN-RCA cable (which I didn't get until recently), so my comparison of running direct vs. running through the preamp BOTH went through the Resolution Audio's volume control...and therefore were both subject to the effects of that volume control.  

Like I said, I think that beyond some specific technical considerations, it's all a matter of taste...

[dado5]

I would respectfully disagre with tkp here.  The basic RS 100k stereo pots I have used tracked well and sounded decent.  Noble plastic pots will sound smoother and more open, but the difference is pretty subtle.  Switched attenuators are not my bag. I made 4 of them and did not care for the results.  They seem to rob the music of some body. Could be the metal film resistors or the multiple solder joints but I don't care for them in general. Now I have not heard high end units like the DACT or the Swiss switch L & H pads, but these are pricey and begin to approach the  cost of consumer passives.

The other option is transformer based attenuation, which is said to eliminate the output impedence issue.  The weakness here may be  varying frequency response, the design bugbear of all transformers.

Personally I prefer active pre amps.  If you look for these, I would stick with low gain units.

Thanks,
Rob[/quote]

[Marbles]

The weakness here may be varying frequency response, the design bugbear of all transformers...

I think you have painted ALL transformers with the same wide brush.

You might want to look at the specs of the Stevens & Billington Transformers made for Bent Audio before making such claims.

At least in the 5 hz to 100KHz they are ruler flat.

[zybar]

Which is why I am willing to wait months to get my Bent NOH.  

BTW, I get better dynamics with the Placette than I am getting with my active Electrocompaniet EC 4.7 preamp.

George

[dado5]

Marbles,

I was not intending to rule out transformers as a valid choice.  Just raising one possible downside to their use in this application following the pro/con approach of my first response. Level performance at all frequencies (not just the extemes) is one of the big hurdles in transformer design and it gets tougher with smaller transformers. It can be achieved of course, but small xfmrs with top notch full spectrum performance will be expensive.

All that aside, I have not heard any xfmr based volume control so I cannot comment directly on their sound. Just bringing up another option for jmsun to consider.

Thanks,
Rob

[tkp]

I would respectfully disagre with tkp here.  The basic RS 100k stereo pots I have used tracked well and sounded decent.  Noble plastic pots will sound smoother and more open, but the difference is pretty subtle.  Switched attenuators are not my bag. I made 4 of them and did not care for the results.  They seem to rob the music of some body. Could be the metal film resistors or the multiple solder joints but I don't care for them in general. Now I have not heard high end units like the DACT or the Swiss swit ...

I wish I was as lucky as you were.  I purchased roughly 20 pots from RS to check out the trackability between the left and right channels accross the whole resistance band and most of them were off between 5% to 30% depending on the selected position.  However, I did find one that was off about 5% on most position.  What I am trying to say here is that the likely hood of some one getting a good RS pot with trackability of 10% between the left and right channels is very low.

As for stepped attennuator, the quality of the switch is very important.  The low quality type with brass contact and low pressure wiper are pathetic.  You will get bad sound if you build the stepped attennuator out of them.  The symptom here is that the sound will be some what flat.  All contact takes away some thing even a very good one.  The Elma Switch use in the DACT is decent but not great and yes they are expensive if you don't know where to buy.  You can get a good dual deck 12 positions Grayhill switch for about $ 20 and will get decent performance out of them.  If any one wants to try out the DACT unit, make sure you get the CT1 version not the CT2.

All resistor alters the sound in some way.  I have played with SMD, generic metal film, Dale and old Holco resistors.  SMD has a metallic sound on the high.   Generic metal film is a bit better but still has a bit of metallic sound.  Dale is more balanced but some what flat.  Old Holco is abit warmer but is my favorite.

As for topology, the laddered stepped attennuator is the best for resistive type.  If you want to know how good a properly build resistive passive preamp sound, just get your hand on the now discontinue Welborne Lab dual mono passive preamp.  Keep in mind, some Welborn lab passive preamp built with Old Holco, some with new Holco (inferior) and some with Dale.

All the above make one assumption that you know what resistive value is appropriate for your gear.  Most often, the generic RS pot beats out a good stepped attennuator is because the resistive value for the stepped attennuator is mismatched with the system.  The pot from RS will work with virtually all system because it is 100Kohms.  This does not mean 100Kohms pot is good for your system.  There is no free meal.

Keep in mind, you need to know your system very well before you can buy or build a passive preamp, whether it is resistive or transformer, that give your system optimal performance.

FYI, the best passive preamp I have built and heard is the Bent with the SEIDEN switch (now this is a real switch).

[dvb]

I found this link to a discussion by Frank Van Alstyne of the active/passive issue interesting.

That said, I am still evaluating Frank's T7SL with phono against the FT-LW-1 Little Wonder Passive, through which I had been playing a George Wright WPP100C phono stage.  Bolder Mensa DAC through both.   The very low capacitance Bodgan Spirit Reference ICs are used.

I am trying to reserve my evaluation 'til later -- a rough comment might be that the active is more brilliant and dramatic,  but perhaps more fatiguing than the passivel  The passive is very transparent -- the WPP100C through the T7 comes out sounding more like the T7 phono!

Then again, this whole exercise is teaching the importance of comparing pieces over weeks, rather than days or hours.  





http://www.audiocircle.com/index.php?topic=3067&highlight=passive

[dado5]

Thanks much for the amplifying info tkd.

I did build my stepped units using cheap switches ( a 4, 6 and 2 12 position units) so that may well have been the problem. Also used Mouser sourced generic metal films.


What is the difference between the new and old Halco resistors?

What is the difference between the two types of DACT units?

Thanks,
Rob

[tkp]

Thanks much for the amplifying info tkd.

I did build my stepped units using cheap switches ( a 4, 6 and 2 12 position units) so that may well have been the problem. Also used Mouser sourced generic metal films.


What is the difference between the new and old Halco resistors?

What is the difference between the two types of DACT units?

Thanks,
Rob

Rob,

The name is TKP not TKD .

The old HOLCO resistor was constructed out of non-inductive material and the new one is not.  I don't know for sure how this effect the sound (magnetic field created by electrical field due to signal going through inductive material?).  The old HOLCO sounds more musical and less metallic.

DACT has two version of switches/attennuator.  One is CT1 and the other one is CT2.  CT2 is a cost reduced version of the CT1.  

It is hard to explain but if you look at the pictures of the CT1 versus CT2, you will see that the wiper is making contact with gold-plated rivets on the CT1 where as in the CT2 the wiper is making contact with PCB copper pad.  The end result here is that the CT1 has higher reliability and the wiper tention could be set much higher without the fear of completely wear down the gold plate rivet over time.  It is not the case with the CT2.  Sound wise, higher tention wiper minimized micro-vibration hence less background noise which is translate into more life and transparency in the music.