It has taken a while but let’s pick up where we left it. Today, part three of the phono trilogy. We are reading MONOphono by Diego Nardi, from sound practices issue 16. In it, Diego bounces the ball back on 8 out of the 9 points raised last time by Allen Wright. He then adds some comment about cascode input stages and proceeds to introduce the MONOphono design. This is done mostly by means of practical advice for playing pre-RIAA curve records, but there is also some notes on the circuit.
teaser quote: ‘To me, the highest transconductance tube that is really useful for audio signal is the 6DJ8/6922, but due to insufficient amplification factor and microphony, it is at its best as a power amp input stage, and I discarded it for phono input.’
my take
In Diego’s eight return points there is some good stuff. Point #2 correctly highlights that the final-generation hi-gm tubes aren’t perfect. Overall, the stories of the people working with them always remind me of working with transistors (need to be selected for µ, low voltages/headroom, fanning curves = iffy linearity). Furthermore point #2 touches upon the RIAA network loading of the preceding gain stage. With no loading (relatively high-Z EQ networks), there is really no current/power flowing into the EQ network, and no power is available to control the grid of the next tube.
Point #8 shows the irony of using an unbypassed kathode resistor with an hi-gm/lo-rp tube: it reduces gm and raises rp. This can turn an exotic tube into a mundane one (e.g. into an equivalent of an ecc81/12at7), but with all the exotic hi-gm side-effects like microphonics and propensity to oscillate, plus the ones mentioned before.
It is a nice exercise to compare the schematics of the MONOphono and the Φ42; ignore the complication of the EQ switching of the former by ‘mentally hardwiring’ the RCA settings, and spot the differences. Also interesting is a comparison of the MONOphono to Joseph Esmilla’s phono stage, a further development of the RCA phono stage.
A lot more educational and fulfilling would be to build both the MONOphono and Φ42, using the common power supply as detailed in the article. If it covers all your needs you can now really hardwire the RCA/RIAA settings on the MONOphono. Relate differences in schematics to what you hear. Next step would be to start your own mods and experiments on one of the two, keeping the other one—I suggest the one you like most—intact as control; a stable reference to make sure you are not slowly, but surely, deluding yourself. The control will also provide music while your experimental stage is in tatters, or took a turn for the worse.
You can build either a mono (cool!) or stereo version of both the MONOphono and Φ42. Apart from the obvious (one or two channels) the differences are only the input plug configuration and input resistor (47k or 22k) as shown on the two schematics.
bonus tracks
Diego Nardi has a homepage! The reason that deserves an exclamation mark is that only a couple of years ago, word on the street was that Diego was an absolute internet refusenik. Plenty of interesting stuff on his site, like phono stages with transistor-cascode input stages (!) and a transistor-assisted kathode follower that pops up everywhere.
And with that we have reached the end… of the phono preamps topic. From next week we will survey speakers, refined choices.
Now go and read the article, see you next week.
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