Have to agree with Herman, the topic seems to come up once a month.
When discussing passives you need to break them out into a few different camps. There are those that use resistors to attenuate the volume, those that use transformers, and those that use autoformers (a transformer with a single winding). Within the resistor camp there is a sub group that used opto-couplers in the design, thus eliminating any effect the switch may have on the sound.
While the benefit of a pure signal at a fraction of the cost of an active preamp is an ideal goal of a passive preamp. The reality is some cost more, much more. Audio Consulting is a good example. Not only that, but passives, while providing a high level of transparency, can induce their own coloration and sonic signature. One example would be Audio Consulting's copper and Silver Rock transformers. The silver wound sounding more detailed and transparent IMO. Another example were S&B transformers of which there were the MkI - MkIII series. I preferred the copper MkI versions of these transformers as there was just a touch of warmth added to the sound. Resistor colorations are also possible - Vishay, Mills, Caddock, Tantalum, etc. Then there are the switches.
So all in all, while we hear how transparent and pure the signal can be with passives, there are some caveats to that. For me, the big issue is reduced noise. I also feel the level of transparency cannot be beat versus actives. If you have enough gain from your source and the rest of your system is passive friendly then a passive preamp may make sense. However, if you're someone who prefers coloration and what some will say is more "weight" to the sound (maybe its just another form of pleasing distortion), then I doubt a passive will be your cup of tea. Better to "tune" your system to you liking in other ways.
In my system the passives I have/had are:
S&B MkI TVC (custom built by K&K Audio)
Lightspeed Attenuator (boutique unit built by George Stantschleff)
Slagleman Autoformer Volume Control (prototype built by John Chapman)
The S&B was used with the solid state TRL D-225. The other two have been used with VAC Auricle Musicblocs, VAC Vintage Williamsom 35/35, and now a Music Reference RM-10 MkII. All of these passives worked fine with the respective amps. |
Active preamps are for those who can't figure out what it takes to integrate a passive into their system,... While this statement does apply to some, I do think there are those out there that just prefer the sound/coloration of tube preamps or other active preamps for that matter. No harm, no foul. Right now I'm playing with a Berning Mico ZOTL as a preamp and do enjoy the sounds of the various 6SN7 tubes I'm rolling with it. I also enjoyed my time with a Jeff Rowland Capri. Overall, I still prefer both my passives, but I can see why some might just prefer an active circuit. Some passives include an active buffer stage to solve potential mismatch issues. The SMc VRE-1 and Pass B1 come to mind. Both are fine preamps and an alternative for those whose systems might not be passive friendly. |
I used to think the TVC/AVC designs were better. The Lightspeed Attenuator changed that thinking. Again, it probably comes down to system matching, and of course the actual design and parts used, but my system can easily work with both types. |
Paul,
After listening to the RM-10 MkII for the last few days I'm beginning to believe Roger is the genius he thinks he is ;) I haven't even tube swapped yet and I may not. I may try my hand at building one of his PITB's, but after I build my balanced passive though. I have the parts now, just need to put it together. I live near Jack Elliano of Electra Print and I'm using his transformers and schematic for the project. He's been kind enough to allow me access to his workshop to build it as well. Great guy, very knowledgeable and some excellent SET amp designs, with which he uses passive preamps.
If all goes well the Lightspeed may be available soon. Of course if you ever just want to listen to it let me know. |
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Dave Slagle is selling them too now: http://intactaudio.com/atten.htmlI spoke to Jeffrey about redoing my Slagle box. He is very detailed and does great wood work. I still may pursue this with him but for now need to finish my balance passive project. I've been hearing a lot about the Sonic Euphoria. They have a balanced version I'd like to try. However, I'd be concerned about support as I don't think they are manufactured anymore. |
Well you would think with minimal parts and no active circuitry (non-buffered designs) a passive would be low maintenance, but I've run into a few scenarios where owners of both Sonic Euphoria and Promitheus expressed issues related to noise/hum. I imagine you can occasionally get a bad transformer, or in the case of my Lightspeed a bad opto-coupler. So they are not maintenance free.
Also, I'm already gray so that probably increases my risk:) |
Tvad, thanks for the invite. I'll most likely be heading out for the Brooks Berdan event May 22. Monrovia is not that far from Burbank from what I recall. I'll contact you as the date nears. |
I am not aware of one single amplifier manufacturer that designs a matching passive preamp to compliment their amplifiers. Three manufacturers were mentioned in this thread: Roger Modjeski - Music Reference, Jack Elliano - Electra Print, and Jeffrey Jackson - Experience Music. Also, a number of manufacturers have implemented passive volume controls in their integrated amp designs. A couple that come to mind are Vinnie Rossi - Red Wine Audio and Paul Weitzel - Tube Research Labs. Ralph Karsten at Atma-Sphere offers a passive volume control upgrade for his OTL designs too. |
The manual for the RM-10 has a nice discourse on cable voodoo, not to mention ground loop hum.
I'm going to finish building my passive today. Sanded down, drilled all holes, and painted the chassis yesterday. Today its plugging in parts and wiring. Hopefully by tonight I'll have a new passive to try out. |
Paul,
Transformer based using Electra Print nickel core copper wound transformers. I was going to do balanced, but single ended for now until its fully tested (just easier to start). If all goes well I may build some more and sell a few here and there. |
The VRE-1 is not a true passive, it is a hybrid design with a JFET buffer stage that eliminates impedance matching issues. Great concept along the lines of the Pass B1 (JFET buffer) and the Bent Tap-X (user selectable buffer outputs). |
Paul, I also heard the Pass B1 at RMAF that year we met up. I would have tried one sooner, but its not balanced and I already have too many single ended passives lying around.
I'd love to hear the VRE-1. Next time I go to one of the LA/Orange County Audio Society events I'll see if I can visit Steve before or after and listen to it. |
Rrog, there are quite a few people who chimed in here whose experience and opinions I trust. As with Pubul57, I can say for a fact the man has tried a number of different components and if he prefers active preamps so be it. I myself have tried a Cary SLP-98, Joule Electra LA-100MkIII, TRL Pre-1.5, Jeff Rowland Capri, all of which I have owned, and have auditioned a number of others including a very fine Herron preamp. I prefer the passives and that was whether I was using my 88db Spendor 1/2e speakers, 92db Audiokinesis Jazz Modules, or 95db Tonian Labs DL-1 speakers.
I think we've probably beat this subject into the ground. So I'll thank Phd for starting this thread, and people like Herman, Pubul57, Tvad, Almarg, and the others who offered constructive advice and opinions for contributing. |
Herman,
What formula are you using for the calculations.
From your original example, I would have never thought an amp with !0k in put impedance to be a good match for a passive. More like 100k ohm in my mind. |
The Clio 9 is finished and in the system breaking in. Early returns are favorable. This design requires a bit more attention to impedance matching than other TVC designs, but the benefits are very easily heard. |
Many passives have a Zin of 10K or less and this is further reduce by the Zin of the amp. At full volume they are in parallel so a 10K passive and a 10K amp would result in a 5K load for the pre. That is problematic for many CD players and phono stages no matter how much voltage they can generate with no load applied. Reviewing my response I see that my point was not clearly stated. What I meant was that in your example it would be obvious to me based on my own personal criteria that an amp with 10k input impedance would not be a good match for a passive. I'm not saying that you a stating otherwise. I understand you were just using the numbers to illustrate an example, which is of interest to me and why I was wondering about the formula you used to come up with the calculation. |
...if the impedances and cable capacitance are right, so that a transformer doesn't really solve a problem, wouldn't a resistor based passive, like my Goldpoint, perform better than a TVC, especially in terms of bandwidth? Theoretically it should and that is what I found in my comparison of the Slagle autoformers and Lightspeed attenuator. However, something about the sound of a transformer or autoformer type passive is still very appealing. |
At full volume (worst case) the combination of amp Zin and passive should be at least 10X source Zout but let's go with 100X. Pubul's 50 ohm source should see at least 5K so a 10K passive looks to be a great match. A 10K passive in parallel with the 100K amp is 9K. My source is 50 ohms Zout and the amp is 100k ohms Zin. The passive I just built uses transformers to step up the signal from the source. These transformers are wound for sources that are ideally 100 ohms Zout or less (but could go as high as 300 ohms). Also, the sources must be opamp driven, as many CD/DACs are, not transformer driven. The passive also uses 10k Alps pots. So basically, the 9k number listed above would apply giving me about a 200:1 ratio. Also, and I think that I have read this above, just want to make sure I understand, with a 10k pot the output impedance rises as the knob is rotated clockwise until it reaches 10k. Did I get this right? I too wish like AA pictures can be posted here. If you would like to email me offline with one that would be fine. |
Looking at this from Roger Modjeski's pot-in-the-box concept, which he recommends for the RM-10 MkII, and I have seen him use with the RM-200, the specifications are as follows:
Input Impedance 50K Ohms
Output Impedance 0 - 25K Ohms
Frequency Response DC - 100K Hz
Do I assume correctly that the Noble pots are the 25k version? What does this do in my case where Zout of source is 50 ohms and Zin of amp is 100k ohms. |
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Al is correct, not a TVC but a slightly different animal. Considering I have spent a lot of time with Jack Elliano the last few weekends (we are Las Vegans) I can say the guy knows his stuff and his SET amps sound great (with which he uses a PVA with 1:8 ratio). However, I think in this case Al and I are in agreement. I've fiddled around with this and a 1:1 or 1:2 ratio would be best. My main problem right now is I can't get the volume control past 9 o'clock, which supports the case for a lower ratio IMO.
Question: Jack tells me the transformers are wound for 100 ohm impedance, meaning the source must have an output impedance of 100 ohms or less. Any of this make sense? |
Almarg: The higher the output impedance of the source component, the greater the losses that would result from that transformer impedance. While I only quoted part of your response Al, Jack did mention DC resistance and sound degradation, particularly in the bass the higher the output impedance of the source. The PVA is really designed for opamp driven sources and when I asked if I could use my 600 ohm Zout Otari tape deck with the PVA I was told that I would get some bass distortion and potentially other losses due to the mismatch. With my Kenwood KT-8300 tuner and digital set-up the sound is fine. |
It has been a long time since I read Stereophile, but Sam's Corner in the February issue has a write up of the old Halcyon PVA and the Lightspeed Attenuator that I own. The article prompted me to put the Lightspeed back in my system for a whirl. I'm not regretting it. This opto coupler designed passive is truly remarkable. As usual careful system matching applies. I believe minimum amp Zin is 50k ohms and high sensitivity is best. Here is some additional information for anyone interested: Lightspeed AttenuatorSingle input/single output only. I have a version with dual attenuators. One of the designers comments is that this type of design removes the sonic effect of the switch from the equation. |
I think you will like it. Definitely makes you think about what it takes these days to get high quality sound. As an example, the combined cost of my Lightspeed and RM-10 MkII is $1450. That's a lot of sound per dollar in my opinion. |
I remember a conversation I had with Kevin Carter when he was building me a TVC, my first passive. I had made some comment to the effect that passives have the reputation of leaning out the sound. It took him less than a second or two (meaning he didn't even have to think about it) to reply that its active preamps that add something to the sound.
I'm leaning towards the opinion these days that a lot is in the engineering and mixing and that active preamps are more like tone controls. I was hanging out with some audio engineers this past weekend and one guy liked having his speakers against the wall to improve the bass response. I commented that to me that would hinder depth of sound stage and that is why I prefer having my speakers out in the room. His comment was that much of the depth in the sound stage comes from how the recording was done, not where the speakers are placed. I'm not sure I buy that one yet, but considering the number of fine speakers out there that are corner/wall placed, maybe there is something to it.
I agree with the argument that its the lack of distortion that creates the notion that something is missing in the sound. A tube amp should provide all the distortion one needs. |
Mine cost a little extra because I had George wire it with 2 volume controls. He referred me to Allied Electronics for a Stancor regulated (very important) power supply. Mine is also wired with the center pin negative on the PS since that is the way the Stancor is wired. I've always been curious as to the battery PS, but this time I think I'll leave well enough alone. http://www.alliedelec.com/search/productdetail.aspx?SKU=9289895 |
Given the wide array of monitoring positions and speaker locations used by mixing engineers while twiddling the pan pots, it would seem logical that reproducing any perspective of depth and width would vary based upon these unknowable factors. I'm starting to understand some of this through my friends here, as well as the effect of miking. I'm going to change my speakers out in the near future so I can listen to my Tonian Labs TL-D1's for a bit. At that time I will try the near wall, or even corner placement. |
