@mulveling , thanks for providing the comprehensive response to the question posed by @jw944ts. In doing so you saved me a lot of time, and I of course agree with what you wrote. JW’s calculation reflects a commonly held misconception that a "voltage db" and a "power db" both correspond to 10x the logarithm of the ratio between two voltages or two power levels, respectively. But they don’t, at least in terms of generally accepted usage, if not etymology as well. 10x is the proper multiplier to use when computing the difference between two power levels in db, but 20x is the multiplier that should be used in computing the difference between two voltage levels in db. A db is a db. It is not either a voltage db or a power db. The numerical result will be the same regardless of whether the number of db is calculated from voltages or from power levels, assuming impedances are the same in the two cases. That is a consequence of the fact that if impedances are the same power is proportional to voltage squared, as mulveling indicated. And therefore squaring a 2x increase in the voltage provided into a given impedance corresponds to supplying 4x the power, not 2x. (In the interests of simplicity I’m putting aside effects that occur when the load impedance is not purely resistive). And assuming the speakers and the rest of the system are operated within limits that allow them to perform in an essentially linear manner, a gain of 66 db corresponds to a voltage multiplication of about 1995x, that commonly being rounded off to 2000x. (20 x log(1995) = 66 db, where "log" is the base-10 logarithm). And if everything else is equal that results in approximately 4,000,000x (2000 squared) as much power being delivered to the speakers, and correspondingly to an approximate SPL increase of the same 66 db. (10 x log(1995 x 1995) = 66 db). Regards, -- Al |
+1 brf
Another bit of information that would be good to know is what kind of music Rob67 used in this evaluation. Volume levels tend to be set based on the average volume of the music, rather than on the volume of brief dynamic peaks. My understanding is that the majority of pop and rock recordings are dynamically compressed such that peaks require less than 10x as much power as the average level of the recording (corresponding to a 10 db peak-to-average ratio). While many well engineered classical symphonic recordings may require 1000x as much power for brief dynamic peaks as for the average level (corresponding to a 30 db peak-to-average ratio), which will usually result in the listener setting the volume control much higher than for a highly compressed recording.
In any event, what is undeniable is that 64 db of gain will boost 0.25 mV to 0.396 Volts. Although as I mentioned earlier the dynamic peaks of some recordings may result in a cartridge output significantly greater than under the standard test conditions upon which the cartridge's output rating is based.
Best regards,
-- Al
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Regarding the 8 db spec difference in S/N ratio between the VTPH-2 and VTPH-2A that George pointed out, I can say that one thing which has particularly amazed me about my VTPH-2 is how quiet it is. In fact when I listen with my Stax electrostatic headphones and no music is playing I hear absolutely nothing even with the volume control on the Stax amplifier at max (which is **way** higher than I would ever set it while listening to music). (With speakers I do hear some slight noise at high volume settings if I get close to the speakers, but that is clearly being introduced elsewhere in the system).
I see that the 80 db spec for the VTPH-2 is accompanied by the words "noise level will be tube dependent." Perhaps the 88 db spec for the 2A simply reflects a change Keith made in his choice of tube manufacturer at some point in the evolution of the design, and/or a change in the criteria he applies in the tests I assume he performs to weed out noisy tubes.
Best regards,
-- Al
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I hope you don’t mean it’s going to be more dynamic if it’s plenty loud enough, but can’t clip the amplifier in theoretical terms, as some shonks here have tried to make out What I primarily mean is that the user will have paid for amplifier power that cannot be utilized. And it would have been better to direct the $ that went toward watts that can’t be utilized toward something else in the system. Perhaps toward a lower powered but higher quality amplifier, if the lower power capability is in fact adequate. Best regards, -- Al |
I actually had to run a splitter off the back of the tube buffer because it only has one output.
So, one goes to my subwoofer amp and the other goes to the main amp.
It may be wrong technically (I don`t know) but it seems to work. Whether or not that would have a sonic downside would depend on the output impedance of the buffer, the input impedance of the sub, and the input impedance of the main amp. And if either or both pairs of cables that are involved are especially long it may also depend on the the length and the capacitance per unit length of the particular cables. These things can all be calculated if the relevant parameters are known. Best regards, -- Al |
Thank you, Professor.
George, there are no hiccups in my calculations. Note that I referred to not being able to drive the power amp to full power, in some and perhaps many circumstances. I did not say that users in those circumstances would necessarily be unable to achieve a volume level that is satisfactory to them.
A quick search I did for specs on the ATI 3002 seemed to turn up specs on the ATI 3000 instead. But presuming the specs for the two model numbers are the same the 1.8 volt sensitivity spec you cited is correct, and the gain of the amp from its unbalanced input is 34 db, which is probably about 8 db higher than average for a power amp.
The gain provided by the LOMC input of SCM’s VTPH-1MC+ is 66 db. That corresponds to a voltage multiplication of 1995x. That would raise a 0.24 mv cartridge output to 1995 x 0.24 = 479 mv, far less than the 1.8 volts that is necessary to drive the amp to its maximum power capability. To be precise, about 11.5 db less than what is necessary, which means that his amp that is rated at 300 watts into 8 ohms would only be putting out about 21 watts into 8 ohms when the cartridge is providing its rated output and the passive preamp’s volume control is set at max!
But let’s assume that high volume dynamic peaks of some recordings may cause the cartridge output to be in the area of 3x the rating, corresponding to about 10 db more than the cartridge’s output under the standard test conditions. Those peaks would result in an output from the phono stage of 479 mv x 3 = 1.44 volts, which would still be significantly less than what is required to drive the amp to full power.
And, again, the power amp would only see voltages as high as those numbers when the volume control of the passive preamp is at max!!
IMO, in most circumstances if the front end of a system cannot drive a power amp to full power the system has not been configured in an optimal manner.
Best regards,
-- Al
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Yes I know Kim and Al very well, they are 200mts down the road from me in Brookvale. Awesome! It's a small world, as the saying goes. I've spoken with Al (Alan Langford) at length on the phone on one occasion, and communicated via email on several other occasions. He's always been very helpful, and a pleasure to deal with. Professorsvsu, apologies for the digression. Best regards, -- Al
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Correction: In the second paragraph of my previous post when I said "For my preamp I used a DEQX HDP-5" the word "used" should be "use."
Regards, -- Al
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For normal listening levels in your setup where is your vc positioned on your active pre and do you know it’s gain? Does it have a log pot? And what is the gain of your amp and efficiency of your speakers? Hi George, My speakers are Daedalus Ulysses, which are rated at a high 97.5 db/1 watt/1 meter, and have a 6 ohm nominal impedance. I don’t know what the gain is of my VAC Renaissance 70/70 MkIII amplifier, but it is certainly not low, especially for the zero feedback setting of its feedback select switch, which is the setting I use. For my preamp I used a DEQX HDP-5, which as you may be aware is produced by some of your fellow countrymen Down Under, and provides numerous DSP-based functions including improving speaker time coherence and room correction. To make that possible analog inputs are converted to digital, but the unit is renowned for its transparency and I have high confidence that it does not manifest any audible bit-stripping at any of the volume control settings I use for either CD or LP playback. I say that based on my own tests with both the speakers and Stax electrostatic headphones, as well as on numerous testimonials to the transparency of DEQX units by experienced audiophiles that are stated in the long running thread here entitled " Is DEQX A Game Changer." It provides 50K input impedance, and at its factory default settings I use I believe its gain is small, just about 1.5 db from unbalanced analog input to unbalanced analog output. The volume control characteristic is described as follows: Each button press increments or decrements in 1dB steps from 0dB to above -24dB, then in 2dB steps above -36dB, then in 3dB steps above - 48dB, then in 6dB steps down to -120dB. Much of my listening is to classical recordings having wide dynamic range, and for those I commonly have the volume control set in the area of approximately -20 to -26 db or so, when listening to LPs. Those who listen to dynamically compressed pop and rock recordings would presumably use lower settings in most cases. Again, I am using the 64 db version of the VTPH-2 and a 0.5 mv cartridge. Best regards, -- Al |
He’s given you massive 64db or 69db of gain, use it, some poor saps only get 35db of gain with their phono stages and they have to listen to the preamps output gain stage’s noise.
Hi George, I’m not sure you’re realizing that the 64 or 69 db of gain is just for use with low output cartridges, primarily low output moving coil cartridges. Moving magnet or high output moving coil or high output moving iron cartridges are connected to a different input of the phono stage, which provides either 43 or 48 db of gain depending on the version of the phono stage. 64 db of gain is not "massive," it is an amount that is appropriate for the majority of low output moving coil cartridges. While 35 or 43 or 48 db are reasonable figures for use with most high output cartridges. Using 64 db of gain with a moving magnet cartridge would most likely result in an overload condition. Best regards, -- Al |
As for the 2.5v output figure into 20kohm at 0.03% distortion this is far less than what a cartridge has as a distortion, they are closer to 1% so to me that doesn’t mean much. Hi George, To be sure it’s clear, the point I was making had absolutely nothing to do with the distortion percentage. I was making the point that the 2.45 volt figure has no relation to how much voltage the phono stage puts out in normal use. It just relates to how much voltage it is **capable** of putting out, without exceeding the distortion spec. And the 64 db version of the phono stage, which is what most people choose and which is what Keith recommends as being the best version to use with LOMC cartridges that do not have unusually low outputs, will produce an output far less than 2.45 volts under most circumstances. In fact well under 1 volt under most circumstances. Which in turn will be too little to drive many and probably most power amps to full power, even with the volume control at max, unless the preamp provides additional gain. As I said I have customers with tube phono stages that have lower gain around 48db and 1kohm output impedance they don’t have any issue with not enough volume. I make it a point of asking where the volume control is for normal/loud listening, and they say around 2pm which gives them plenty of range up or down. Surely that is with moving magnet or other high output cartridges, not with LOMCs! Speaking more generally, given the credibility Keith has earned over the years as a result of his universally praised design work and sincere dedication to customer support, if he says that 20K is a bare minimum and 40K or more is preferable, that’s good enough for me. Best regards, -- Al |
Wow! So bass rolloff would not be an issue that would result from driving low impedances such as 10K, with Keith’s design. But I would still take to heart his comments recommending against anything less than 20K. And his suggestions that for other reasons 40K or more is preferable with the 64 db configuration (400 ohms nominal output impedance), and 50K or more is preferable with the 69 db configuration (500 ohms nominal output impedance).
Regarding the 2.45 volt figure, while he has conservatively chosen that number for use in specifying distortion, keep in mind that a gain of 64 db will raise the output of a cartridge rated at 0.5 mv under the standard test conditions (that being the rating of the AT-ART9 which I and the OP and some others here use in conjunction with what I presume in all or nearly all cases is the 64 db version of the VTPH2 or 2A) to only 0.792 volts under the standard test conditions. Not nearly enough to drive many power amps to full power, even with the volume control of a passive preamp at max. Although very high volume transients on some recordings can exceed the standard test conditions severalfold, as I understand it.
In any event, thanks for obtaining the good info about the cap.
Best regards,
-- Al |
Hi George, No, I don't know what the value of the coupling cap is, as I've never had occasion to open my unit. One reason being that I've never felt tempted to do any tube-rolling with it, and in the process perhaps messing up the voicing Keith has achieved. But in addition to what I mentioned earlier, that the manual recommends 50K or more as being optimal as well as Keith's statement that he would not recommend less than 20K, a few months ago he explicitly recommended to another member
that the VTPH-2A not be used in conjunction with a 10K load, stating as follows (quoted by member Uberwaltz in a post dated 4-2-2018 in this thread):
Hi Kevin,
Thanks for the note. I like to see, as a general rule in high end
audio, an impedance ratio of at least 1 to 100 (output impedance
to input impedance) when using tubes or solid state as loading the
circuits down generally reduces the quality of the sound. I have
seen similar impedance ratio recommendations from Audio Research
and other high end audio companies.
Solid state circuits generally have much lower output impedances
than tube circuits. 10k Ohm input impedances are a standard
typically used in professional audio, but this does not work as
well for tube based high end audio equipment.
The two 12AX7, three 12AT7 version of our VTPH-2A has an output
impedance of 400 Ohms which performs best into a 40k Ohm or higher
line stage input. Some people are using them with line stage
inputs that are a little less than that with reasonable results,
especially if their cartridge has a lower output voltage. Your
integrated with a 10k or 5k single ended input would likely
restrict dynamics substantially, even with a very low output phono
cartridge. I can't recommend that combination. Audio is supposed
to be fun.
Another thing to watch for is that some line stages (particularly
digital ones) have a very low input voltage limitation which can
cause overload (clipping) with some analog sources. That can sound
nasty.
For the reasons above I design our line stages with a 100k Ohm
input impedance and a wide input voltage margin in order to get
the best sound quality from tube based sources such as phono
stages, tape machines, tuners, and DACs with tube output stages.
I wish everyone would do that.
I hope this short explanation helps.
Best Regards,
Keith
Best regards, -- Al |
Professorsvsu, congratulations and continue to enjoy! I know you will, being one of several other members here using the VTPH2 or 2A/ART-9 combo. Regarding passive preamps, the Herron phono stage is not suitable for driving a low impedance passive preamp. One reason being that its nominally 400 ohm output impedance undoubtedly rises to much higher values at deep bass frequencies, due to the use of a coupling capacitor at its output. (Also, btw, the configuration providing a 400 ohm output impedance provides 64 db of gain, not 69 db as was stated above. And I assume you have the 64 db version, which is what I and most VTPH owners have purchased). From the manuals for the VTPH-2 and 2A: We recommend that the VTPH-2A be used with a line stage having an input impedance of 50,000 ohms or higher for optimum performance. And a direct quote of a response Keith Herron provided just yesterday to a member who asked him if the VTPH-2A could be used with a 20K load impedance (see this thread): "We have VTPH-2A customers that are using preamplifiers with 20k Ohms RCA single ended input impedances that are pleased with the sound.
I don’t recommend going any lower than that. The 2 X 12AX7, 3 X 12AT7 version of the VTPH-2A has an output impedance of 400 Ohms so it will drive the Ayre preamplifier much better." Also, I suspect that the degree to which those driving 20K with the VTPH-2A are "pleased with the sound" depends on how much deep bass extension their speakers provide, since one of the major consequences of this kind of impedance issue is deep bass rolloff. Best regards, -- Al |
