Tube Preamp Paired with Tube Phono Stage?

Lewm
Dear Actusreus, If what you say is true about the JC3 load resistor for MC, I share your chagrin, but one might always open the chassis, find said 100R load resistor, remove it, and replace it with a resistor of the desired value. I do agree that the task is tedious, if one wants to play with a variety of cartridges and/or experiment with different load values.

I'm usually reluctant to mess with a product that is designed in a specific way. I always think there was a reason why a designer made his choices the way he did. I'd think that someone like John Curl knew exactly what he was doing when he fixed the impedance at 100 ohms, even though it is rather curious given other phono preamps in this price range.

Atmasphere

This is almost as frustrating as John Curl's JC-3 phono preamp offering only one impedance setting of 100 Ohms for MC. That one I'll never understand.

The reason is if the preamp is resistant to RF at its input, the load on the LOMC is not critical. Its only there to reduce RF caused by the inductance of the cartridge interacting with the capacitance of the interconnect cable, which can form a tuned RF circuit. The resistor destroys the 'Q' of the circuit. IOW if the preamp works right, the value of the resistor is not important since it only works at RF frequencies.

Atmasphere, I have to admit I don't fully understand your explanation as I don't have a technical background and my understanding of technical intricacies involved in designing playback equipment is rather limited. This particular aspect of the JC-3's design baffled me as the great majority of phono preamps in this price range (and certainly above) offer adjustable impedance settings for MC cartridges. It was also my understanding that it is so because impedance always mattered with MC carts, just like capacitance with MM carts; a mismatch can result in a less than optimal sound that the cartridge is capable of (usually expressed as too harsh on top or too mushy at the bottom). Admittedly, I played with load impedance settings in my friend's setup once with his remotely controlled Aesthetix phono preamp so any effect should have been immediately audible, but the differences were very subtle, if any. However, we both preferred his Dyna XV-1s at about the same setting, and we reached that conclusion independently. Fluke? Perhaps.

Jmcgrogan2
Actusreus, I would think that 60 db of gain from a phono stage would be enough for any cartridge, unless possibly if you are considering running through a passive controller instead of an active preamp. Most will run the Chinook through an active line stage preamp (as the OP is here) which will add more than enough gain for any cartridge.

If you use the KAB calculator, 60 dB of gain is optimal for MC cartridges with an output of 0.4 mV or higher. If that's true, the Chinook does not have enough gain for most Dynavectors, just to mention one popular manufacturer. The KAB calculator does not seem to take the gain of the line stage into consideration. From my experience, I certainly had phono preamps that simply did not have enough gain to produce a sufficient volume level with the cartridge I used (in this particular case a MM), even though the same amplification produced more than adequate volume levels for CDs. Doing the math, the KAB calculator seemed to be pretty spot on. My current phono stage has 55 dB of gain in the MC setting and I feel anything less would not be sufficient for my Lyra Delos's 0.6 mV despite the fact that my line stage has a variable gain. It appears that having the optimal gain at the phono stage level is more crucial than having enough gain downstream. At least in my less than expert opinion...

Good point by Ralph (Atmasphere) about resistive loading. Marek (Actusreus), although this reference is somewhat technical, it may help to clarify what Ralph is saying.

Concerning gain, it should be kept in mind that all 60 db gains are not created equal. In a given system and with a given low output cartridge, some 60 db phono stages will be adequately quiet and some will not. What makes the difference is signal-to-noise ratio. A general purpose gain calculator will not be able to take that into account.

Along the lines of some of the earlier comments, as long as the gains and sensitivities of the downstream components are not particularly low, chances are that with most low output cartridges 60 db of phono stage gain will be adequate to avoid running out of range on the volume control, or having to turn the volume control up to levels that will cause preamp-generated noise to become objectionable. But the key question is how much noise the phono stage itself generates. It is easy to design a phono stage that can provide high gain, and even sell at a low price point, if noise performance is compromised.

Assessing s/n performance from specs can be difficult, because different manufacturers tend to specify that parameter based on different reference levels and with different frequency “weightings.” And in a lot of cases they don't even indicate what those reference levels or weightings are.

Fortunately the s/n specs for the Chinook are presented in a meaningful manner. For the 60 db gain setting the spec is:

Noise Floor at 60dB gain setting with shorted input: -75 dBu, A-weighted

Based on some quick calculations that spec strikes me as encouraging, with respect to use with cartridges having rated outputs significantly less than 0.5 mv. But of course the only way to know for sure is empirically.

Best regards,
-- Al

Al, Thank you for linking to this article. I believe I actually have seen it before as it looks familiar. Interestingly, the author posits that a loading value of 100 ohms might indeed be suitable for MC cartridges in general. If so, are audiophiles unnecessarily obsessed about resistive loading, and manufacturers simply cater to that obsession? As always, I am sure the answer is more complicated than that.

Great point about the noise issue that's tied to increased gain. My line preamp has five gain settings and definitely noise increases as the gain is increased, so I try to find a setting that will provide an optimal balance of (low) noise and the weight, presence, and immediacy of the sound. It is still unclear to me why, but increasing the volume at a certain gain level will not render the same results as upping the gain at a given volume level. IOW, the sound is much better at a higher gain setting and lower volume, than lower gain and higher volume. The sound just lacks the heft and presence that it has at a higher gain. Perhaps that is why I am so focused on gain when it comes to phono stages...

05-16-12: Actusreus
... are audiophiles unnecessarily obsessed about resistive loading, and manufacturers simply cater to that obsession? As always, I am sure the answer is more complicated than that.

Hi Marek,

Not necessarily. As Ralph indicated, it can be presumed (or at least suspected) that JC's phono stage design can handle frequency response peaks in the ultrasonic and RF region with relative grace (i.e., with minimal interaction with audible frequencies). But it is probably safe to assume that many other designs will not be able to, which would make the choice of resistive loading more critical in those cases.

And I'm personally not sure if the effect Ralph described is the only means by which differences in resistive loading may affect sonic results when LOMC's are being used. But it is certainly a major one.

It is still unclear to me why, but increasing the volume at a certain gain level will not render the same results as upping the gain at a given volume level. IOW, the sound is much better at a higher gain setting and lower volume, than lower gain and higher volume.

There are a great many variables, many of them unpredictable, that can influence that tradeoff. Some of them are the signal-to-noise characteristics of the phono stage and the preamp; the distortion performance of the two components at various signal levels; side-effects of the preamp's volume control mechanism, that may worsen as the amount of attenuation is increased; ground loop issues that may exist between the two components, whose significance may be lessened if the signal level is greater at the interface between them, etc., etc.

Best regards,
-- Al

Actusreus, how much gain does your preamp have? As Layyi mentioned, his preamp has 13 db of gain, mine has 22 db of gain. Either when combined with a 60db phono stage would be more than enough for most any cartridge I can imagine.
I just sold a phono stage with 66 db of gain while using a cartridge with a 0.24 mV output. Combined with my 22 db of gain from the line preamp, I never turned the volume knob much past 10 o'clock. Together that combo provided 88 db of gain, I'm sure I could have gotten by on 82 db of gain very easily. While doing your KAB number crunching, don't forget to add in your line stage's gain, unless of course your line stage has no gain.