Yes. In such a scenario the switch is selecting a different pair of resistors for each volume control position.
A shunt attenuator typically only switches one resistor.
A shunt attenuator typically only switches one resistor.
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Thanks Al, I can think of a few more nits, for example analog volume controls are not linear, so the halfway point is anything but half of the control! I was assuming a traditional potentiometer, which is a series-shunt configuration. Our output impedance of the source is an unknown however. I did point out that this problem has more to do with higher resistance value controls than those of lower resistance (10K being my example). I am sure that variables such as this as well as the input impedances and cable capacitances are really pointing to the need for a database for best results. |
Ralph, thanks for your characteristically informative, knowledgeable, and detailed input. It all makes perfect sense to me. One small nit, though: A typical cable might have 15pf per foot. With a 3 foot cable and a 100K volume control set halfway across the scale, this results in a rolloff (-3db point) at 70KHz, meaning that phase shift artifact is going all the way down to 7KHz.I think you are assuming that the halfway setting would result in a 50K output impedance. But provided that the output impedance of the source component is small in comparison, and assuming that the two ends of the pot are in shunt rather than in series with the signal source, it seems to me that the impedance seen looking back into the output of the pot would be close to 25K at that setting, and the 3 db point would be around 140 kHz. 10-28-14: CzariveyNote that my comment to which you were responding specifically referred to tube preamps. I think it is safe to say that a substantial majority of tube preamps utilize coupling capacitors at their outputs (Ralph's designs being notable exceptions). And as can be seen in the measurements John Atkinson usually provides in conjunction with reviews in Stereophile, it is not uncommon for the output impedance of tube preamps having capacitively coupled outputs to rise from a few hundred ohms at mid and high frequencies to 3K or 4K or even more at 20 Hz. 10-28-14: Czarivey***In properly configured system*** I think that a better comparison would be between the Bimmer and a Porsche Boxster or Cayman. I have never taken sides in the never-ending active vs. passive controversies because it seems clear to me, based on what I perceive to be an overwhelming preponderance of the anecdotal evidence, that either approach can work well in a well chosen and properly configured system. Regards, -- Al |
A common problem with passive controls (IOW variable output impedance) is a coloration at settings that are less than full output. This can be reduced in some situations by using a control that is itself a lower value, like 10K. You can't count on all sources having an output impedance of less than 100 ohms. Many phono preamps and vacuum tube CD players have output impedances that are higher than that. This can often limit the utility of lower value passive controls. The trick is really in the setup, if you get it right you can get excellent results. IOW its not a guarantee that the passive will work in all situations- you have to be careful. In the case of controls with higher resistance values, the source impedance that results when the control is set to lower values is really a function of the interconnect cable preceding the control, the source preceding the control and of course the control itself. This is of course true of a volume control installed inside a preamplifier, the difference is that there is literally no interconnect cable. This is important as the interconnect cables have some small capacitance per foot (which becomes a larger factor when higher impedances are involved); this is a non-issue in a dedicated preamp where such capacitances can be 1/100th or less as seen in an interconnect. The high frequency rolloff thus produced is likely not in the audio passband unless the amplifier has a very high input impedance. The control's series resistance can mess with the Miller Effect of the input gain stage, independently of the rolloffs generated by the cables involved. Generally speaking, this will be more likely somewhere in the middle to upper 3rd of the control range where higher series resistance of the control combined with a high resistance to ground begins to interact with the input capacitance of the input gain stage of the amplifier. A high frequency rolloff has phase shift effects manifesting as low as 1/10th the cutoff frequency according to the engineering rule of thumb. The cutoff frequency (-3db point) is defined as: f= 1/ RxCx2Pi where R is resistance in Ohms, C is capacitance in Farads. To get a more meaningful calculation, it is useful to express C in microfarads, thus the resulting quotient 1,000,000 instead, the resulting f frequency is thus in Hz. A typical cable might have 15pf per foot. With a 3 foot cable and a 100K volume control set halfway across the scale, this results in a rolloff (-3db point) at 70KHz, meaning that phase shift artifact is going all the way down to 7KHz. At the points of mechanical contact (audio connectors) there is usually some sort of primitive diode issue to overcome due to slightly dissimilar metals used in the connectors. The higher the impedance driving such contacts the more audible their effects become. This will be expressed as an intermodulation, and is not present when the connections are hardwired. This is why the choice of cable is so important when dealing with a passive control! Now some of you may have noticed that there is a more complex situation then just the simple example I gave above! In addition to that particular rolloff, we have the interaction of the output coupling cap with the volume control, interconnect cables and amplifier input all in parallel. From the output coupling cap point of view, this is likely to be a negligible value, but from the amplifier input its another story. The input capacitance of an amplifier is in parallel with the input, which is to say it acts as a rolloff factor. An input capacitance of 25pf is not uncommon and can be considerably higher in solid state amplifiers. This value is operating independently of the Miller Effect of the input stage (which also contributes to rolloff). The same formula applies. If you did the math, you will see that the rolloff at the output of the passive control is lower (if set at the same place in the prior example), in this case about 45KHz. Now artifacts are occurring down to 4.5KHz, on top of those going to 7KHz. This makes things tricky. Obviously keeping the cable capacitance down is important- if the cables were 6 feet instead of 3 feet, the cutoff frequency would be cut in half, putting the phase shift artifacts an octave lower! If the control is a higher value, the cutoff frequency is the example above is reduced and can actually be **inside** the audio passband! You can easily see that higher value controls are going to introduce a coloration no matter how good the parts are in the control, as the coloration is a phase shift introduced by the math and is incontrovertible. Practically speaking, this limits all passive controls to no more than 100Kohms unless fidelity as defined in the traditional sense is not a goal. Now it is a fact that phase shift in a simple signal like a sine wave is inaudible. This is not true if the phase shift covers a band of frequencies- the wider the band, the easier it is to hear. The human ear/brain system uses phase to establish where a sound is coming from, i.e. in contributes to soundstage perception. In addition, phase shift is interpreted by the ear as tonality- a simple 6db/octave rolloff at 50Khz will be heard as a darkness. Some years back I had this demonstrated to me in spades, where I was trying to find out (for one of my dealers) why an old MFA preamp was so bright in the phono section. It turned out that there was a step in the EQ curve at 50KHz, resulting in a 6db per octave error in a frequency band that is often thought to be inaudible. But phase shift being what it is, the ear was easily able to detect the problem even though our hearing does not go that high! |
I'll add that what can often be an issue with **active** preamps or sources is variation of output impedance as a function of **frequency.** Especially if the design utilizes a coupling capacitor at its output, as most tube preamps do, which commonly results in a large rise in output impedance in the bottom octave. Almarg, what year/century is it now?? I thought that today we mostly use direct coupled outputs unless it's a tube preamp(many already use direct-coupling as well). It's pefectly legit for me to say that Most of good preamps are direct coupled and than compare:
Did you ever check input impedance of Pass poweramps? Do you think Pass poweramps are good or not? Are you sure that 10kOhm passive volume control will be able to mute?? Unity gain solid-state preamps such as McCormack, Wyred will kick S to all passives ez and swapping A/B is the best way to hear and know. It's as easy and similar as to swapping Chevy Aveo with BMW Z4 roadster. Numerically 30...300Ohm output impedance which is substantially less variable vs. passive, but for some reason haven't been presented. |
+1 on the importance of system synergy. In addition to impedance related issues, it is also important to pay attention to system gain as well. You don't want to have too much or too little gain overall. Too little and obviously you aren't hitting the volumes you would like. Too much and you need to do too much attenuation. Not enough fine tuning in your volume control, and attenuator performance is usually at its worst at the low end of the spectrum. IME, active and passive both have their strengths and weaknesses. Passives can be excellent for detail retrieval, but actives (again, IME) tend to have more dynamics and oomph. To get a preamp that had the detail I was hearing with passives and the dynamics of active linestages, I had to move to a much higher priced preamp. More tradeoffs need to be made the lower the price (though this isn't without exceptions). But to come full circle: Yes, synergy. And your musical tastes and listening priorities also come into play here too. |
I'll second George's point that the variation of passive preamp output impedance as a function of volume control setting doesn't matter, as long as the worst case (highest) output impedance at any volume setting is suitable for the application. I'll add that what can often be an issue with **active** preamps or sources is variation of output impedance as a function of **frequency.** Especially if the design utilizes a coupling capacitor at its output, as most tube preamps do, which commonly results in a large rise in output impedance in the bottom octave. Again, however, as long as the worst case (highest) output impedance at any audibly significant frequency is suitable for the application, that won't matter. One minor clarification to the example George provided: The 2.5K figure for the impedance seen at the output of a 10K pot set to its mid-point will be increased slightly as a consequence of the output impedance of the component driving the pot. But if that component has a low output impedance (as it should, if it is to be used with a 10K passive), that addition will be essentially negligible. 10-27-14: Response34Well said! Regards, -- Al |
"Variable output impedance is "heavy disadvantage" of any passive preamp even if you have substantially high input impedance of your amplifier." Wrong Czarivey, ^you stated^ and the reason is. A 10kohm passive can have a varying output impedance, at it's highest (worst) it's 2.5kohm at mid position. Any amp/s that have input impedance 47kohm and especially higher will have absolutely no ill effect on the sound with 47kohm it's close to 1:20 impedance ratio. And if 100kohm input impedance the ratio is then 1:40 ratio. And if the volume is lower or higher than mid point, the ratio/s are even higher, still with out any effect. The only thing that needs to be watched is interconnect cable capacitance that can cause a HF filter. The HF filter caused by the interconnect capacitance and this high 2.5kohm output impedance. (BTW: same goes for many tube preamps as well as they can be that high or even higher) If we look at a bad quality interconnect cable that has 200pf per foot of capacitance, that equates to say 600pf for a meter. This combined with the 2.5kohm of the passive pot will give you a -3db at 106khz! Well beyond our hearing. All good quality interconnect that I have measured are below 100pf per foot, and this equates to -3db at 212khz! Up in bat hearing territory. There's just some of the math on this without any voodoo. I ask once again show the math without the voodoo to counter this if you can? Cheers George |
No one type of preamp is going to be ideal for every system. There are many variables to consider such as the type of source being used, gain of the amplification and output voltage or gain of the preamplifier. We all have our views on passive vs active and this subject will be debated till the end of time but one thing we will never get away from is system synergy and choosing components that work together properly for "our" personal system requirements. I, for one, believe providing such specs is important for the consumer to make better buying decisions. We always show all the specs on our Musica Bella Audio preamplifiers and have always offered to customize to the personal requirements of any given customer's system. |
Czarivey you make a lot of sense. I agree with you, an active preamp with just the right gain to work. Most high gain preamps don't allow you to use the volume control at a higher level. An active preamp keeps the imp. constant is what you want. Yes passive preamps sound ideal on paper, but to me I have never liked one in my system and I have tried a few. |
Like I asked, show the electronic math? And then post a link to a "quality" interconnect that has high capacitance more than 200pf per foot. And I'll show you a rubbish interconnect? Because it will create HF filter with tube preamps as well as passive pre's. And the HF audio band filter that it creates is detectable with just listening let alone using math or bench tests. Cheers George |
"Variable output impedance is heavy disadvantage of any passive preamp even if you have substantially high input impedance of your amplifier." Please give your reasons why (using laws of electronics) 1: If using a low (<100ohms) source impedance (which most are). 2: Into a 10kohm passive pot with 1mt of low capacitance interconnect (which most good ones are). 3: Into a power amp with 47kohm or higher input impedance (which most are) "Nelson Pass would also agree with me."Your assuming a lot with this bold statement, quoting words that Nelson Pass has never made. In which he's been quoted to say the opposite 6 posts back. Cheers George |
P.S: An example of what I am describing would be these specs for the Parasound JC2: Input Sensitivity:Note that 20log(1 volt/200 mV) = 20log(1/0.2) = 14 dB, corresponding to the indicated gain. Regards, -- Al |
10-24-14: CzariveyI'm not sure this is correct. For preamps and line stages, when input sensitivity is specified it usually represents (with the volume control set at max) the input voltage required to drive the output to an indicated level, such as 1 volt, that is representative of a realistic operating condition. Not the input voltage required to drive the output to its specified maximum. The specified maximum is the maximum output voltage the preamp or line stage is capable of providing without clipping or gross distortion, and would usually and hopefully be a far higher voltage than would ever occur under normal operating conditions. You may be thinking of power amps, where the specified input sensitivity usually represents the input voltage required to drive the amp to its rated maximum output power. Also, I believe that maximum output voltage specs for preamps and line stages are usually expressed in rms terms, not peak, so the 0.707 factor would usually not be applicable. Regards, -- Al |
Vintage Accuphase E202 integrated have volume control that compensates line vs. phono. I've used in the past Classe CP30(80's) that also didn't have any problems and volume had been compensated from both outputs. On the other end, you should look for cartridge with sufficient output so that your line volume level will match to phono. Separate phono boxes with adjustable gain is certainly best way! I'm not audiophile so I simply loop my phono preamp (Musical Fidelity X-LPS) that somewhat needs a-bit of higher boost vs. line signal through the Nakamichi tape deck that has its own recording level gain stage by simply engaging casset to pause-record mode. This way I get 'step up' stage to compensate with line stage. The electronics of the Nakamichi's recording level are simply superb! |
Anywhere from 0db(unity gain) typical to McCormack preamps. Low-gain preamps such as Wyred4Sound will do about 6db, Medium-gain such as Classe somewhere in 12dB there are high gain preamps such as AudioResearch, Bryston will blast 20dB of gain Passive preamp will vary the impedance while active preamp will keep it same. Variable output impedance is heavy disadvantage of any passive preamp even if you have substantially high input impedance of your amplifier. If there's enough gain from source to poweramp, I'd choose either low-gain or unity-gain active preamp to achieve perfection and Nelson Pass would also agree with me. It will give you larger dynamic headroom and more pleasure at substantially larger volume range to dial REGARDLESSS of quality of passive preamp. Manufacturers instead of gain would often provide you input sensitivity and max output voltage. 2 ways can be worked around this problem: 1. Contacting manufacturer about gain info 2. Use following formula to calculate gain: a) Max_Voltage * 0.707 = RMS Voltage b) 20Log(RMS Voltage/Input Voltage) Personally, I would prefer purchasing preamps that have DETAILED specifications provided. Ones that aren't provided, would trigger my suspicion that they're not tested properly before going to the market. |
These days with sources giving out 2v or even more with low output impedance, and amps only needing 1.5v or less for full output. There is no need to do any preamplifying. All you need is a passive volume control with a unity gain buffer after it if the amp has unusually low input impedance <33kohm, or your driving 10mt of interconnect. Or a 10kohm passive pre if the amp has >33kohms or more input inpedance, with up to 1.5mt of good interconnect. And if you don't believe me. A Quote from the master Nelson Pass "Nelson Pass, We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more. Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up. Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control. What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection. And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp." Cheers George |
