Tube Preamp Paired with Tube Phono Stage?

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

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

I took a look at the KAB Gain Calculator.

First, it clearly states in its introduction that:

The optimum gain is based on achieving 325mV rms output at 5 cm/s. For the current crop of CD recorders, 300mV is required for 0dB recording level with the recorder's level control set at max. Aiming for 325mV gives a little margin.

325 mv is not close to being enough to drive most power amps to full power, so clearly the answer provided by the calculator applies just to the phono stage, and not to the combination of phono stage and line stage.

All the gain calculator is doing is dividing the cartridge's rated output in mv into 325 mv, and converting that ratio into db based on the standard conversion formula 20log(V1/V2), where V1 and V2 are two voltages, and "log" means the base-10 logarithm.

That is very simplistic at best. Not only does it not take into account line stage gain, but it also does not take into account power amp gain, speaker sensitivity or efficiency, listening distance, and speaker type (volume falls off more rapidly with increasing distance for box-type speakers than for planar speakers, for example). All of those factors will influence the position at which the volume control will be set.

And perhaps most significantly, it does not take into account the signal-to-noise ratio of the phono stage, as I discussed earlier. One phono stage providing a given amount of gain may result in an acceptable level of background hiss, while another phono stage providing the same amount of gain may not, even though both phono stages will cause the volume control to be used at the same setting. That is especially a concern if the cartridge has a particularly low output.

In other words, the calculator should be considered as providing at best a rough rule of thumb guideline. Nothing more.

Best regards,
-- Al

05-20-12: Chervokas
The load impedance of a device should be at least 10X the source impedance of the device driving it to minimize insertion loss. Even at 10:1 you have a 1/10th loss of voltage IIRC.

That's close. It's actually 1/11th, assuming that both impedances are essentially resistive at the frequencies being considered.

Now if you plugged a mid output Benz with a 24 ohm output impedance and 0.8 mV output into the 100 ohm phono stage I think you'd have around 0.5 mV insertion loss (if I'm remembering the math correctly)

Assuming both impedances are essentially resistive at the frequencies being considered, you would lose:

0.8 x 24/(24 + 100) = 0.15 mv

Resulting in (0.8 - 0.15) = 0.65 mv appearing at the input of the phono stage. Which can be calculated a little more directly as:

0.8 x 100/(24 + 100) = 0.65 mv

All of which brings to mind a pet peeve of mine. When the specs for a cartridge indicate "impedance," what are they talking about? Impedance at zero Hz (i.e., DC resistance), impedance at 1 kHz? 20 kHz? Other? For many and perhaps most LOMC's it won't make much difference, because of their low inductance. But it would certainly be nice if DC resistance and inductance were separately and explicitly specified (which they sometimes are, but all too infrequently), so we don't have to make assumptions and guesses.

Regards,
-- Al

05-21-12: Chervokas
I just installed an AT33EV at home and noted that the specs supplied in the manual were pretty thorough on those scores -- 10 ohms impedance at 1 khz; 22 uH inductance at 1khz -- but I gather that's not always the case.

Yes, those specs are unusually well defined.

What I don't quite understand is what, if any, disadvantages there are to lightly loading the cartridge. Like what are the problems that arise when loading a low impedance MC with a high impedance? Why would you want to load it down?

The one reason I am aware of (although I'm not completely certain that there are no other reasons) is the one Ralph (Atmasphere) described. Heavier loading will reduce the magnitude of the ultrasonic resonant peak you referred to, which as you realize is caused by the interaction of the inductance of the cartridge and the capacitance of the cabling plus the input capacitance of the phono stage.

The greater the magnitude of that resonant peak, the greater the amplitude boost that will occur, relative to other frequencies, of frequency components that may enter the phono stage that are near the frequency of that resonant peak. Depending on the design of the phono stage, that may increase the likelihood that intermodulation or other effects caused by those ultrasonic frequencies will affect audible frequencies. Extraneous ultrasonic or RF energy may be present as a result of tics and pops, ultrasonic musical information that is on the record, RFI pickup, etc.

Some references that may be of interest:

http://www.hagtech.com/loading.html

And the post dated 8-14-10 by JCarr (Lyra cartridge designer) in this thread

Best regards,
-- Al