@Analogluvr, while there are of course ways of designing a tube-based line-level output stage to provide low impedance without using coupling capacitors (for example transformer coupling, or Atma-Sphere’s patented differential output circuit which uses a servo-based method of nulling out DC offsets), I’m not sure how or if a simple conventionally designed single-ended cathode follower circuit might be designed to be direct coupled. That might be a good question for Roger or Ralph in Roger’s "ask an amplifier designer" thread.
By definition a cathode follower has a resistor (or resistors) connected between the cathode and (at least usually) the ground of the circuit. The bias current flowing through the tube and consequently through that resistor will cause a significant DC voltage to be present at the cathode, which must be prevented from being sent into the power amp or other connected device. A capacitor is of course a simple and likely the least expensive way of doing that.
The DC voltage that is developed at the cathode, btw, will be positive and typically serves the purpose of biasing the tube. In effect it makes the grid negative with respect to the cathode, and thereby biases the tube at a desirable operating point. There are other means of developing bias, of course, by directly applying a negative voltage to the grid, but that kind of approach is less commonly employed as it adds complexity, has tradeoffs of its own, and a cathode resistor would still be required anyway if the circuit is to function as a cathode follower.
Best regards,
-- Al
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Hi Analogluvr, I believe you are referring to the following statement by the OP in the last of his posts dated 10-7-2017: My buddy told wanting tubes try and find a tube amp that can both get the desired output impedance and preferably one without a cathode follower. Those tend to cause different impedance swings verse a pre uses tubes in paralell [sic] to lower impedance. You are quite correct that cathode followers are commonly used in the output stages of tube-based preamps and other tube-based components which provide line-level outputs, because cathode followers can provide lower output impedances and better drive capabilities than other circuit configurations employing the same or similar tube types. Usually the impedance swings he refers to are mainly the result of the coupling capacitor that is used in the majority of such designs between the output of the cathode follower circuit and the output of the device. The impedance presented by a capacitor rises as frequency decreases, so as you probably are aware the output impedance of a tube-based component, or even a solid state component if it employs a capacitor in series with its output (and some do), can be far higher at deep bass frequencies than its specified output impedance. Output impedance specs usually being based on mid-range frequencies such as 1 kHz. That variation in impedance can cause deep bass rolloff and unwanted phase shifts if the input impedance of the connected component is not high enough. A capacitor having a relatively large value (i.e., a large number of uf, meaning microfarads) will result in that effect being much less significant than with a capacitor having a smaller value. However choosing a capacitor having a relatively large value can often bring tradeoffs into play involving the size, cost, and sonic quality of the capacitor. The bottom line is that if the power amp has a high enough input impedance to be a suitable match for the output impedance of the preamp at 20 Hz all of this will be a non-issue, and there will be no reason to avoid preamps which use cathode follower output stages. And if the input impedance of the power amp is ten or more times greater than the 20 Hz output impedance of the preamp the match will be suitable. If that ratio is less than 10x the match may or may not be suitable, depending on how the output impedance of the preamp varies over the frequency range. Best regards, -- Al |
I guess the bottom line re impedance compatibility with the BAT is that a reasonable case could be made on either side of the issue.
Earlier in the thread I referred to the possibility of audibly significant phase shifts occurring as a result of the impedance interactions that would arise using the BAT with your amplifier, in addition to a slight amount of deep bass rolloff. If I recall correctly Ralph of Atma-Sphere has said in past threads here that to be sure that audibly significant phase shifts will not occur frequency response should extend beyond the audible range by a factor of 10, meaning 2 Hz to 200 kHz. While his designs meet that requirement, at least at the low frequency end (and at both ends in the case of his preamps), obviously many well regarded designs do not. All or nearly all SET amps for starters. (I note, btw, that your X350.5 is specified down to 1.5 Hz).
And in this case, given that the preamp’s output impedance rises from 414 ohms at mid-frequencies to 4600 ohms at 20 Hz, it will not come close to supporting bandwidth down to 2 Hz in conjunction with a 30K load.
So it seems to me that while use of the BAT in this application represents a compromise in terms of impedance compatibility, the degree of that compromise is sufficiently marginal that its acceptability is something reasonable (and knowledgeable) people can differ about.
Regards,
-- Al
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Fsmithjack, re your question just above, that would be no problem.
BTW, I see that the K-1xe has a relatively low specified input impedance of 10K per phase, which means a balanced input impedance of 20K. And I suspect that it probably also means that the unbalanced input impedance is 10K, although that is not stated explicitly.
Your Bricasti DAC will have no trouble with either input impedance, but if you envision the possibility of ever adding a tube-based source component to your system chances are good it would have the same kind of impedance compatibility issue with the preamp that we’ve been discussing about the preamp-to-amp interface.
Regards,
-- Al
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Fsmithjack, I’m not in a position to recommend a choice between the K-1xe and the ET3SE, and the recommendations you’ve received in favor of the Ayre come from members whose opinions and directly relevant experience certainly command great respect. To add, though, to my earlier statement that: The CJ CT-5 is ok in terms of impedance compatibility, based on JA’s measurements. The 17LS is as well, so I suppose the 17LS2 and the ET3SE are likely to be also. ... I subsequently found the following statement in the manual for the ET3: MAIN OUT:Connect to the input of your amplifier (or crossover in a bi-amplified system). We recommend the use of an amplifier with an input impedance of 20k ohms or higher. Since this preamplifier inverts phase, it may be necessary to invert the speaker leads to maintain correct absolute phase. Also, given the reference in CJ’s description of the ET3SE to the use of Teflon capacitors it wouldn’t surprise me if similarly to the 17LS2 it uses two 2 uf output coupling capacitors in parallel, for each channel. That would probably result in an output impedance at 20 Hz of a little more than 2K, the exact value depending on the output impedance of the circuitry that lies ahead of the cap. That seems consistent with a 20K minimum load impedance recommendation, which appears to be satisfied by your amp, albeit just barely. Good luck, however you decide to proceed. Regards, -- Al |
Further update: I see that in the latter part of Sam Tellig’s writeup it is stated that the 17LS2 design does in fact use two of the 2uf caps in parallel, on each channel. Which is fine in terms of impedance compatibility with a 20K load.
Regards,
-- Al
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Hmm, after looking a little further I see in "Sam Tellig’s" comments about the 17LS2 that one of the differences between it and the 17LS involved the introduction into the design of ten 2uf Teflon capacitors. **If** one of those is used as the output coupling capacitor for each channel (rather than two of them in parallel on each channel, or something else altogether being used as the output coupling capacitor), that would translate to an output impedance upwards of 4K at 20 Hz, too high to be optimal with a 20K load. So I’m uncertain about the suitability of the 17LS2. Regards, -- Al |
The title of the ad for the Canary preamp refers to it as a CA-800, which is a one-chassis design (having a specified output impedance of 650 ohms), but the photo and the text indicate that it is a C800, which is a two-chassis design (having a specified output impedance of 1200 ohms). Also, I’m pretty certain that the word "maximum" which appears after the reference to a 1200 ohm output impedance applies to the subsequent spec on gain (meaning the gain that is provided when the volume control is at max), not to the output impedance spec. So I would be wary of the possibility that the output impedance could be considerably higher than 1200 ohms at deep bass frequencies.
The CJ CT-5 is ok in terms of impedance compatibility, based on JA’s measurements. The 17LS is as well, so I suppose the 17LS2 and the ET3SE are likely to be also. CJ preamps tend to have high gains, which in some systems can limit the usable range of the volume control to undesirably low settings, but in combination with the moderate gain of the Pass amp and the not particularly high sensitivity of the Magico S5 I suspect you’ll be ok in that respect.
So given also your desire for a warmish sonic character I would vote for one of the CJs.
Good luck! Regards,
-- Al
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... so I am committed to make the BAT work if you guys think it will. It will certainly work in a fairly reasonable manner, but as I said it would be introducing a known compromise, that IMO represents a significant risk that you would ultimately be disappointed. I’ll add that in addition to the relatively small but likely perceivable amount of roll-off of the deep bass that would result, undesirable phase shifts could very possibly occur to an audibly significant degree in the bottom octave or two, that would vary as a function of frequency. In any event, good luck however you decide to proceed. And thanks for the nice words. Regards, -- Al |
Fsmithjack, re the BAT VK-50SE (the reference in JA’s measurement writeup to VK-500SE appears to be an error), given my previous comment you would have a balanced output impedance of 4.6K at 20 Hz driving a balanced input impedance of 30K (or perhaps 29K), which is a ratio of a bit more than 6. You would also have a very wide variation in output impedance over the frequency range (from 414 ohms to 4.6K). Not ideal, and personally I wouldn’t risk introducing a compromise like that.
The Atma-Sphere preamp that was suggested would certainly be fine in terms of impedance compatibility (Ralph’s designs do not have capacitively coupled outputs), but I don’t know if it would provide the warmish sonic character you appear to be looking for.
Regards,
-- Al
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Jmcgrogan2 10-8-2017
I believe that you have those two numbers reversed. The way I read it on the Pass Labs website:
Input Impedance (SE & BAL Kohms) 30/20.
The way that reads to me is that the Input impedance is 30Kohms for SE, and 20Kohms for BAL. John, I’ve noticed that apparent anomaly in the specs of the Pass .5 amps in the past, and I’m pretty certain it is a mistake in the specs, and what was intended was 20K single-ended/30K balanced. All of the Pass .5 amps are spec’d the way you stated, but John Atkinson measured the input impedance of the XA30.5 as 20K single-ended/29K balanced. Also, the fact that the .5 amps require a jumper between pins 1 and 3 of the XLR connector when the RCA input is used would seem to indicate that the same input stage is being used for both inputs, with one of its two "legs" being grounded when the RCA input is used, and the balanced input impedance (representing the sum of the impedances between each leg and ground) should therefore be higher than the unbalanced input impedance. Best regards, -- Al |
To add to my previous post: What can often be more significant than the ratio of the two impedances, as defined at the worst case frequency (which is usually 20 Hz for preamps having capacitively coupled outputs, which means most tube preamps as well as at least a few solid state preamps), is how much **variation** there is in the preamp’s output impedance over the frequency range. And in that respect what is by far the worst case situation among the two preamps that are being considered occurs at the balanced output of the Calypso, where "the balanced output impedance was 112 ohms at 1kHz and above, this increasing to 3900 ohms at 20Hz due to the finite physical size of the output coupling capacitors."
Differences in how preamp output impedance **varies** over the frequency range are IMO probably the main reason that some people report good results when the 10x guideline is compromised, and others do not. The less variation the better, if the 10x guideline is not met at 20 Hz or any other audible frequency.
Regards,
-- Al
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If the design of the Calypso hasn’t changed significantly since Stereophile measured it in 2005, especially with respect to the value of its output coupling capacitor, neither it nor the Hovland is an ideal match for the Pass X350.5. Not so much because of distortion concerns, but primarily because of the deep bass rolloff and other sonic issues that are likely to result from the output impedance rise of both preamps to high values at 20 Hz. As Tony (Tls49) correctly emphasized. It is of course always possible that a non-ideal match might be subjectively preferable in a given situation, depending on listener preference, and especially if the non-ideality works in the direction of compensating for an issue elsewhere in the system or the room. But why take a chance by introducing a known and predictable issue into the system? It’s usually best to address the root cause of any issues that may be present, rather than trying to compensate for them. Regards, -- Al |
