Seldom has there been any discussions on passive preamps in the forums and although my experience with them has been limited I have found them so far to be very enjoyable and refreshingly different. They seem to fall into their own category, somewhere between solid state and tube. Finding a preamp that is satisfing has been difficult. Some active solid state preamps can be very good but they seem to inject grain to some degree in the upper registers and some tube preamps are not too far behind. So far I think they should at least be matched up with an amp that has sufficient gain which is often overlooked. Which passives are you using and with what amp? Why do you like them?
From a measurement standpoint resistors should beat transformers, but an awful lot of people claim they like the sound of the transformers better. My opinion is either one can sound very, very good if properly implemented and this is one of those issues where it comes down to personal preferences.
I tried a Placette in a system before I was biamping and it sucked the life out of it. I talked them into building me one that had an input impedance of 100K instead the stock 9K and it sounded wonderful. I compared it to some Slagle transformer units and to tell the truth couldn't say say which I really preferred, leaning a bit to the transformers but they were both very good. Now that I biamp I don't use a preamp as I control the volume via digital processing in Pure Vinyl software.
I know, not much help but at this point I think you need to try both and see which you prefer.
It did help. I ordered a 50 kohm resistor passive from Goldpoint. I do think that 9kohm (I did have a Placette too)wa a bit problematic, but I was not A/Bing with a TVC at the time. But I did have the Placette Active (buffer, no gain)and it was signifncantly better in the bass - but that might simply have been an issue with the 9kohm Zin. Well, I'll have it to listen to soon enough. Thanks.
I am using a DIY ALPS 10k passive to feed an Audio Research VT50 and a Powered Sub. The IC's to both amps are low capacitance. This has resulted in a very spacious lifelike and tonally neutral sound with all the dynamics I need.
To summarize some of the posts here and in addition to other aspects I have learned I didn't know before that there are two kinds of passives, resistor and transformer based. Argubly folks swear by both designs and the use of passives require more attention to the proper selection of associated gear and cabling. But once set up properly the results can produce a dynamic but superb sound.
I fell into this not knowing alot about passives but in my particular case the guess work was somewhat taken out by pairing a McCormack TLC-1 to a McCormack amp which has a high input impedance and was obviously designed to accomodate passives. Coupled with highly efficient speakers to the tune of 102db produced a very dynamic, smooth, and spacious sound. I can reach sound levels that challenges actives. It is my opinion that I have not heard a solid state active preamp that compares. The only other type of preamp that I would ever consider as an equal would be a tube preamp but I am leaning more towards the passive because I can leave it powered up all the time and is ready when I am plus it sounds damn good. I will eventually have Smc upgrade my TLC-1 and DNA Deluxe amp or possibly try other passives like the VRE-1.
It is all about system building and matching between components, to the point where it come sometimes be a little misleading to even say what a particular tube sounds like absent any information of the circuit it is used in, I know the Roger Modjeksi's EL34 sound is different the CJs EL34 sound, so your left with the question, what does an EL34 sound like, and answer of course is, "it depends".
So I've been listening to the Goldpoint for a few hours, and without comparing at the this point, I would say that for $2,300 to get a Goldpoint ($312) and a Music Reference RM10MKII ($1950) you could be pretty darn happy running a pair of Merlin VSMs. For $312, the Goldpoint, in the right system is "stupidgood". As good as the Joule or Atmasphere preamps (or the CAT I used to own?)? I don't know yet, but you would be surprised how darn good a $312 preamp can sound.
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.
Chatta, that is beyond question. I've been going back forth on this a long time, I've concluded that great active linestages are better than passives, but that passives are better than many actives. But there is no doubt, that if you are on a budget, you will get exceptionally good performance from a passive for far less money than getting something comparable from an active at any where near the price point. A nice feature of the CD player is that it has a low and high output level, I can tell you with 4v output stage, there is no loss of dynamics with the passive setup (not reall a problem with 2v either).
I suppose that if your on a budget looking for the biggest bang for the buck than a passive could be the solution. On the other side of the coin there are some audiophiles that have enormous amount of money to spend towards this hobby and still go passive. Even a TLC-1 would require a substantial investment to get Smc's upgrades.
That is very true, for some no active beats a passive in their system. So to answer the original question, many use them becasue they are a very viable opton to SOTA sound, though some still prefer actives for their own reasons, and it not a right or wrong difference, just two approaches and choices.
Ngjockey, not everyone who owns a passive is passive. I live an exciting life. After slopping the hogs I return to my listening room where I quickly fall asleep enjoying my passive preamp. My corn field don't care what kind of preamp I buy!
I have a P-L-P (pathetic little preamp) which is a 25k TDK motorized pot. At the risk of being called tin ears, or worse, my Adcom GFP-750 could switch between active and passive and the differences were virtually inaudible. Maybe becasue I listen at 60-70dB. Active a tiny bit more dynamic, passive a tiny bit clearer/ less grainy.
The issue of dynamics is sometimes commented on when discussing passives, but I'm not even sure what that means. Does it mean that passives sound compressed, homogenizing the differences between soft and loud volumes in a performance? I'm not sure why that would be. I can understand frequency abberations due to impedance matches, but why would a passive ever be any less dynamic?
Pubul57, I thought this thread was closed a long time ago and here you are still carrying one.
Maybe dynamics can suffer because of what I said in my very first statement here. The source is not designed to drive anything. The source signal is designed to flow freely with it's signal boosted by the preamp.
I think of it as an amplifier driving a difficult speaker load. The sound of the amplifier is compromised opposed to the same amplifier driving a very easy load. Now it sounds better in every way.
Maybe, but my CD player puts out 4volts in high setting, I would think that would drive anything without any help from a preamp. Impedance matching I get, "drive" seems to be a strawman as far as dynamics.
I can understand frequency abberations due to impedance matches, but why would a passive ever be any less dynamic?
One explanation which I suspect underlies those perceptions in many cases would be upper treble rolloff caused by the high output impedance of a passive preamp interacting with cable capacitance that is excessive in relation to that output impedance.
Upper treble rolloff would result in dull and sluggish transients, which very conceivably may create the subjective perception of reduced dynamics.
Pubul, the output voltage isn't the issue, just about any CD player has enough voltage to drive any amplifier to full volume. The problem occurs when the CD player output impedance (Zout) is too high compared to the load it sees which is a combination of the input impedance (Zin) that it sees from the passive and the amp.
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.
Even with the volume turned all of the way up the CD player is loaded with 33K ohms and the Zout of your player is only 50 ohms, a ratio of about 700:1. Conventional wisdom says at least 10:1 with higher better and no benefit beyond 100:1. I think 10:1 is a bit low but you are in a very good place. I would be tempted to try a Placette or a transformer control with those ratios.
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.
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.
Herman, if the impedances and cable capacitance are right, so that a tranformer 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? Anthony?
I once did have the Placette RVC and then tried the Placette Active, and what I notice was more articulate bass, I now wonder if the issue was the the RVC simply has too low an input impedance to be optimal in my setup - but it sure was transparent.
Clio, I wish they would allow pictures so I could post a drawing, but you are looking at a series-parallel circuit with the Zin of the amp in parallel with part of the passive. When the volume is all of the way up the whole passive is in parallel with the amp Zin so you use the math for parallel resistors. In my example they were both 10K so it is 1/2 of the 10K. With the volume close to zero it rises to about 10K. and falls as the volume goes up.
Pubul, yes, on paper the resistor is better but many ears say differently, If you look at the results with test equipment in the audio band it is pretty much a wash anyway. What measures better does not always sound better.
Only to the extent that they offer pots with different ratings. Goldpoint has 10, 20, 25, 50, and 100kohm attenuators, and they'll recommend the right one for your system source and amp.
Pubul57: Does the 50kohm of my attenuator address that issue, or does it cause problems on the other end into my 100kohm amp load?
Herman's comments accurately addressed the impedance seen by the cdp looking into the resistive volume control, which as he explained is dependent on the rvc, it's setting, and the amplifier input impedance.
A separate issue, which I think is what the quoted question is addressing, is the relation between amplifier input impedance and the output impedance seen looking back from the amplifier into the output of the rvc. That will be dependent on the rvc, it's setting, and the source component's output impedance (if it is high enough to be significant).
With the volume control turned all the way down, the impedance looking into the output of the rvc (I'll call it Zo) will be essentially 0. With the volume control all the way up, it will be the parallel combination of the volume control's end-to-end resistance (in this case 50K) and the output impedance of the source component. If the source component has a low output impedance, such as 50 ohms, then the impedance of the combination will be essentially the same as the source component's output impedance.
Zo will be at a maximum when the volume control is set to the mid-point of its resistance range (which is NOT the 12 o'clock position; it will be just a few steps down from the maximum volume position). At that setting Zo in this case, where the rvc has an end-to-end resistance of 50K, equals the parallel combination of 25K with (25K + 50 ohms), or about 12.5K.
The ratio of your amp's input impedance (100K) to the 12.5K worst case output impedance that is driving it is 8:1. That is slightly short of the so-called 10x rule that you are no doubt familiar with, but considering how close it comes to satisfying that rule, and that the ratio will be better at any other volume control setting, that all seems comfortable.
The other criterion that Zo should satisfy is that it should be considerably lower than the capacitive reactance (which is an impedance, measured in ohms) of the cable that connects the rvc to the amp, at the worst case frequency within the audio band, which is 20kHz.
That number can be calculated by taking the capacitance per foot of the cable, multiplying by the number of feet, and plugging into the formula Xc (capacitive reactance) = 1/(2 x pi x f x C), where f is 20,000 and C is capacitance in farads. The resulting Xc will be in units of ohms.
For example, a 5 foot cable having a low capacitance of 20 pf (picofarads) per foot results in Xc = 1/(2 x 3.14 x 20000 x 5 x 20exp-12) = 79.6K, which would be a satisfactory result in relation to Zo = 12.5K.
so 2 meters of Cardad Golden Reference 12pf/ft should be no problem for the passive approach, with with a 2 meter IC?
Yes, that is excellent, due to the very low cable capacitance per unit length. Xc calculates to 101K at 20 kHz for the 2 meter length, which I calculate will result in a roll-off of 0.066 db at 20kHz, in combination with the worst case source impedance of 12.5K. In other words, completely negligible.
Can't argue with your math but I think there is a simpler way to look at it, something I should have brought up before. You need to consider 2 factors.
To make a match first look at ratio of source Zout to amp Zin. Convention says 1:10 minimum, I say closer to 1:100 is much better. In Pubul's case with 50 Zout and 100k Zin he is way above minimum at 1:2000.
Then pick a passive as low as you can get away with. 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.
For the reasons your math points out going with a passive much greater than required can cause problems just like going too low with too low being worse. That's why you see Placette building 9K passives. In Pubul's case many would argue a 50K pot is too high.
Yes, that strikes me as a good way to look at it, Herman.
I notice that at this page of the Goldpoint site the following statement is made, which I believe is incorrect and misleading:
When choosing the stepped attenuator value for an in-line level control or "passive preamp" (such as the Goldpoint Level Control Boxes), the attenuator value is chosen to match the input impedance of whatever it will be controlling. example: If the amplified monitor speakers or power amplifier you will connect the output of your your passive preamp to has an input impedance of 20K, then order a 20K stepped attenuator for that application.
Although they then qualify that with this statement:
You can usually use a level control value which is LESS than the rated input impedance of the gear it will be controlling.... 25K is usually a good choice for both vacuum tube and solid-state equipment
...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.
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?
No, Herman's relevant statement was:
In my example they were both 10K so it is 1/2 of the 10K. With the volume close to zero it rises to about 10K. and falls as the volume goes up.
In your case, with a 10K pot and a 100K amp Zin, at minimum volume control setting the input impedance (not output impedance) looking into the input side of the pot would be 10K. With the control turned all the way up, the impedance would be 10K in parallel with 100K, which is 9.1K.
The source component would see a load impedance equal to that value (dependent on the volume control setting) factored by square of the ratio of transformer primary turns to secondary turns (assuming the transformer is "ahead" of the volume control).
As I indicated a couple of posts ago, the OUTPUT impedance of the passive, which must be kept low in relation to the amp input impedance, will be at its worst case maximum when the volume control is set to the mid-point of its resistance range (assuming the source component's output impedance is small). Apart from the presence of the transformer, with a 10K pot and a 50 ohm source impedance, that output impedance would be 2.5K. The step-up transformer will raise that value a little, by stepping up the (very small) contribution of the 50 ohms. But for any reasonable turns ratio the resulting output impedance is likely to still be well under 5K, and therefore fine in relation to the amp's 100K Zin.
BTW, calculating the resistance of a parallel combination of two resistances is easy. It's just the product (multiplication) of the two numbers, divided by their sum. If more than two resistances are in parallel, it's a little more difficult, the result being equal to the reciprocal of the sum of the reciprocals of the individual resistance numbers. (Reciprocal = the number divided into 1).
Almarg: In your case, with a 10K pot and a 100K amp Zin, at minimum volume control setting the input impedance (not output impedance) looking into the input side of the pot would be 10K. With the control turned all the way up, the impedance would be 10K in parallel with 100K, which is 9.1K.
The source component would see a load impedance equal to that value (dependent on the volume control setting) factored by square of the ratio of transformer primary turns to secondary turns (assuming the transformer is "ahead" of the volume control).
To make sure it's clear, I should add that if you have a transformer ahead of the volume control which steps UP the incoming voltage from the source component, the source component will see a load impedance that is REDUCED from the 9.1K or 10K number, in proportion to the square of the turns ratio.
For example, if the transformer provides a 4x (12db) voltage step-up, the source component would see a load impedance of only 568 ohms to 625 ohms, depending on the volume control setting. Obviously that gets into worrisome territory.
Is it safe to assume that the Goldpoint is shunt-type?
No, they are series types. From the Goldpoint website:
Goldpoint Level Controls used to offer Ladder and Shunt type stepped attenuators too. The fact is that with the very low noise, 0.1%, thin film resistors we are now using, we believe that there is no real advantage to Ladders and Shunts - and so have ceased offering them.
And a schematic of their Mini-V series attenuator:
I know it is a good thread when I understand half of it. On the issue of impedance matches and best pot ratings, I see the theoretical, and perhaps measureable difference, but are they audible (to most people)? I suspect that in my setup, whether the pot is 10, 25, or 50 I would ne hear the difference, though my test equipment might.
I took a look at it. I see that several different transformer ratios are offered. 1:1 and 1:2 will certainly be no problem for low impedance sources. The other two ratios, 1:8 and 1:13, will, as I speculated earlier, result in an overall input impedance that is extremely low. 1:8 will divide the 10K attenuator impedance down to 156 ohms. 1:13 will divide it down to 59 ohms. Addition of a 100K amplifier load may reduce those numbers slightly further, depending on the volume control setting.
Whether or not that heavy a load will result in good sound is dependent, of course, on how well the source component can handle having to supply relatively high currents, and on how flat its output impedance vs. frequency curve is. But in general I don't think that those two ratios can be counted on to perform well.
Also, I don't understand the statement about output impedance remaining constant as a function of attenuation setting. If the attenuation is set for minimum volume, the output impedance looking back from the amplifier into the pva will be zero or very close to it. If the attenuator is set fully clockwise (max volume), the output impedance will be 10K in parallel with the source component's output impedance times the square of the transformer's step-up ratio. As I indicated in one of my earlier posts, if the attenuator is set to the middle of its resistance range, the output impedance will be around 2.5K (the worst case), assuming the source component's output impedance and the transformer's step-up ratio are small.
BTW, moving the transformer to the output side of the pot, as you may realize, would most likely not be helpful, because it would probably raise the pvc's output impedance to levels that would be too high relative to the amp's input impedance. Also, it would increase the range of amplitudes over which the transformer would have to operate, which MIGHT compromise its performance to some small degree (I'm not knowledgeable enough in that area to be able to say).
The bottom line, imo: Assuming (as I do) that parts quality is good, it looks like an excellent product at 1:1 or 1:2 transformer ratios, but performance at 1:8 or 1:13 will be highly dependent on the characteristics of the source component, and good performance at those ratios cannot be counted on.
big caveat, I've never tried this but those who's ears I trust, like Dave Slagle and Jeffrey Jackson tell me performance is compromised when you go with transformer ratios in TVCs that exceed 1:1. In other words trying to get voltage gain from a transformer volume control deteriorates the sound. No proof from my end but these guys have yet to steer me wrong.
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