Do you have to use grid stoppers with D3A's? High gm tubes usually oscillate without. If yes, nV/rtHz vaporizes soon. Also what is the highest gain including RIAA 20dB loss you could get with a two stage D3A? Because one thing is Neq but without 60dB gain we can't play Low MC, and the transformer ratio will bring up En over spec again no matter the MC territory noise goodness of the tube. Anode load scheme? If highly resistive, loss again. Some pentode CCS?
Bingo. The grid stoppers are indeed critical.
With a mu of 73, max gain from two stages is 55dB at 1kHz, which is more than I need. Pentodes make lousy current sources (but excellent sinks); MOSFETs are better that way. Someone ought to design one with cascoded DN2540...😀
Had good anode side noise with Mu follower non the less. Did any like that?
P.S. 55dB straight without trafo does normal MC swell. Circa 0.5mV. Good for a big group of fine carts. But yours is 0.2mV? Which one is it?
P.S. 55dB straight without trafo does normal MC swell. Circa 0.5mV. Good for a big group of fine carts. But yours is 0.2mV? Which one is it?
0.2mV. With the line amp, I had to actually drop gain a bit. But that's a story for a different day with a surprise ending.
Fortunately, since it's a 10,000 hour tube, a lifetime supply is relatively small, especially compared to something like the 5842.
Martin,
If you want to build something at modest expense that performs competently, you will be (IMO) very hard pressed to do better than the tweaked RCA design. While, it's not SOTA, it is quite competent. Those of us who worked on the setup were determined to correct as many weaknesses as is possible in the RCA original, without "getting crazy". The MOSFET follower at the O/P provides substantial drive capability. Grid leak biasing the 2nd gain block improves bass extension. High Miller capacitance at the I/P is a fact, but many MM carts. work well. Properly selected 12AX7s yield satisfactory noise performance.
FWIW, folks who have built the design are smiling, as they play their records.
If you want to build something at modest expense that performs competently, you will be (IMO) very hard pressed to do better than the tweaked RCA design. While, it's not SOTA, it is quite competent. Those of us who worked on the setup were determined to correct as many weaknesses as is possible in the RCA original, without "getting crazy". The MOSFET follower at the O/P provides substantial drive capability. Grid leak biasing the 2nd gain block improves bass extension. High Miller capacitance at the I/P is a fact, but many MM carts. work well. Properly selected 12AX7s yield satisfactory noise performance.
FWIW, folks who have built the design are smiling, as they play their records.
Conspiracy! Those two guys have advanced the state of the vacuum art in the dark, while everybody was sleeping.😀
A really nice rework Eli. Nothing's as much precious to DIY as an easily repeatable inexpensive project with proven tone. Those kind of projects are giving value and widen the appeal of the hobby.
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