First impression: Herron VTPH-2A phono preamp

almarg

I can see where your coming from Al, the 400ohm is fine, as you say if the output coupling cap is too small there would be some rolling off the bass too early with a 10kohm passive.
 
I just did some calculations and if that coupling cap is 1uf or more all is fine. Do you know how big it is maybe you have one? I have sent an email and asked Keith I will post it up when he reply's. 

And if it is less than 1uf there's no trouble or much cost making it bigger, and at the same time a better quality one.

And the gain can be either: From Herron
Gain MC Mode
(2 X 12AX7, 3 X 12AT7) 64 dB
(4 X 12AX7, 1 X 12AT7) 69 dB

Cheers George 

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 

that the manual recommends 50K or more as being optimal

Yes as I said before that could very well be because of the output cap size, as you know 400ohm is fine to drive a 10kohm load. 1:20 ratio is no problem even 1:10 also
We’ll find out that cap size if Keith emails me the info.

Cheers George

Keith just emailed me, here’s what he said.

"Hi George,

Thanks for the note. The output capacitors of the VTPH-2 and VTPH-2A are approximately 7 uF (polypropylene) and the low frequency response is RIAA flat down to 10 Hz (within 0.1dB) into a 10k Ohm output load."

As you can see no problems with bass rolloff as the cap is 7uf and he’s done the measurements flat to 10hz, into the same load as a 10kohm passive represents!
My calculations with 7uf into 10kohm load are -3db at 2hz, perfect!

All looks fine to me for it to be used with a passive preamp, as the gain is very high for this phono stage, so you may as well utilise it all, instead of knocking it back because of the gain in an active preamp, this way you’ll get a definite lower background noise level.

Also Keith said the Distortion: Less than 0.03% at 2.45 volts RMS output at 1kHz at 20k Ohm, this also "to me" bodes well for passive preamp use. If you’ve got’em smoke em! to see how it sounds. We’ll see after this weekend what rob67 thinks of it with the Lightspeed as it mimics a 10kohm passive volume control.

I’ve got other customers of mine with tube phono’s that aren’t spec’d as good as this for passive use, they say it’s the best they’ve heard their vinyl sound, once they got rid of the active preamp.


Cheers George

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