Hi Uberwaltz,
On page 13 (pdf page 8) of the
manual for the VTPH-2A, near the upper right-hand corner, the following statement appears:
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.
If as I suspect the input impedance of the 2170’s unbalanced analog inputs is in the area of 5K to 10K, or even if it is somewhat higher, it will not be a good match for the majority of tube-based phono stages. There are some exceptions, of course, including the Chinook for which the website description recommends a minimum load of 2.5K, and specifies a nominal output impedance of 91 ohms (which is much lower than is typical for a tube-based phono stage).
More generally, you’ve probably seen a rule of thumb guideline stated that to assure impedance compatibility of a line-level interface the input impedance of the destination component should be 10x or more greater than the output impedance of the component providing the signal. What is often not stated, unfortunately, is that the 10x ratio should be applied at the frequency for which the output impedance of the component providing the signal is highest. Most output impedance specs are based on a mid-range frequency such as 1 kHz. It is very common for the line-level outputs of tube-based components to have output impedances at deep bass frequencies that are much higher than that specified value, often 2K or 3K or even 4K ohms. That rise at low frequencies results from the output coupling capacitor that is used in the majority of tube-based source components and preamps (and also in some solid state preamps). The impedance of a capacitor increases as frequency decreases.
So in the case of a line-level output supplied by a tube-based component the highest output impedance within the audible frequency range often occurs at 20 Hz. If Stereophile has reviewed the component the measurements section of the review will usually indicate that output impedance. If that impedance is not known, and is not indicated in published measurements, then to be safe a considerably higher ratio than 10x should be applied to the specified nominal output impedance, IMO something like 50x or 75x.
Also, to clarify a common misconception I should add that failing to meet that guideline does not always mean that there will be an impedance compatibility problem. It depends on how much **variation** there is in the output impedance over the frequency range. But meeting that guideline (at all audible frequencies) assures that there won’t be an impedance compatibility problem. In this case, though, as I said earlier the majority of tube-based phono stages will almost certainly have problems dealing with a load impedance of 10K or less, and in many cases dealing with even higher load impedances such as 20K or 30K.
Finally, it might be a good idea to contact Lyngdorf and ask them what the unspecified input impedance of the 2170’s unbalanced analog inputs is. Chances are it is very low, since as I mentioned the balanced input impedance is very low, but if that is not the case you would have a much greater number of suitable phono stages to choose among.
Best regards,
-- Al
