PRR said:
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gain, part of the passive EQ, gain, other parts of the passive EQ, gain. ...have you seen a design like that
Been done.
A design which is not what _I_ would do, yet I respect VERY much, is the
Hagerman Bugle.
Not bad... they spread the 60 dB of total gain across 3x 20dB modern high performance op amp gain stages (I approve).
I don't see a real pole so there are zeros above 20 kHz... not a big deal since RIAA stops caring above 20kHz, and cutting lathes don't boost all the way up to light.
I am not warm and fuzzy about the price. I never sold a complete preamp for that much (maybe I'm just jealous? and cheap?)
As a bonus (for this or ANY phono preamp), under "Tools" he has an "Inverse RIAA" which you can use to check the flatness of your preamp EQ. Most commercial preamps are quite unflat. I was astonished at the differences _I_ could hear with 1dB errors. Nothing I could point to, but different feeling. Testing RIAA curve is insanely tedious without an Inverse RIAA.
I bought an inverse RIAA network from somebody associated with TAA back in the 70s. Very handy on the test bench... I could feed regular program or sine waves into a phono preamp prototype for measurement and listening tests, without extra errors from turntable/cartridge/tone arm, etc. (IIRC that inverse RIAA had a HF zero too, but probably up around 200 kHz, so not a problem in practice.)
Measure RIAA with something better than a sound card or H-P 200AB. Some of the Dynas had unsuspected peaking near 0.5Hz, which is "record warp" at 33RPM. Most "simple" topologies "flatten out" above the audio band, which relative to a velocity pickup amounts to a response rising to infinity. The defense is that classic 600mH cartridges with R-C loading fall-off badly past 20KHz. But then there are Grados and MC which don't.
Back in the '70s I was selling companding tape NR kits. Too much LF content from turntable rumble, wow, whatever, could modulate the envelope of the compressed signal going to tape, but cassette tape and even open reel recorders were notorious for scraping off LF content. When the NR playback expander tried to reconstruct the recording with missing LF content , there would be a phantom modulation caused by the missing signal. Most noticeable during quiet parts.
I eventually fixed that with a sliding pole dynamic HPF built into my NR compressor that rolled off LF only when things were relatively quiet (compressor was at max gain), then restored normal full LF response when music was happening (and compressor was running at lower gain).
I also embraced a additional -3dB @ 30Hz pole added on top of RIAA EQ curve, proposed by the IEC but never accepted by the RIAA, even back in the 80s vinyl was old news and RIAA wasn't interested in tweaking it. It was enough work for them to extend the curve to 20Hz-20kHz
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Jung's Audio book has a very well-considered phono preamp. At the time, it looked extravagant. Today it would be just a few bucks.
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The Self "2Q" link is interesting. I have some more examples from the US side. However with just two transistors it is not possible to provide enough gain to integrate a conventional hi-fi and cover RIAA with "large" NFB over and beyond the audio range. Yes you can get by with penny transistors for a $10 preamp or dollar transistors in a $50 preamp, and millions of these were sold and used with ignorant satisfaction.
Adding a 3rd transistor seems obvious, but often the improvement is very slight.
RIAA typically needs 60dB boost at 50Hz, but only 20 dB by 20kHz so not really all that hard to maintain loop gain margin despite falling open loop gain caused by dominant pole compensation, but yes more than two transistors to be comfortable.. An issue with some early dedicated bipolar phono preamp ICs (LM381 etc) is that they weren't unity gain stable so hard to maintain a true 75uSec pole. (perhaps a personal problem of mine, worrying about what it's doing up at 200kHz).
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> pick up an NAD preamp
I'd listen to Gold.
I would NOT just buy a popular-price phono preamp by a "good brand". A *good* phono preamp is not just matching a gain-curve 50-15KHz.
I have liked many things from ART (especially when market-driven mis-features are avoided). So I got ART's low-price "disco" preamp. Plain simple design; I've lost the details. But when hooked up and brought to "realistic level" (I had 2 grand pianos and 50 piano LPs in the rooms so comparison was obvious), the woofer cone shook madly and then the power-amp shut-down. The 'scope showed huge random sub-sonic noise on the output. A little study showed this was caused by the chip's 1/f bias current noise and the small input cap. It probably worked fine with PA speakers and amps, but our high-class "domestic" system did not like it.
As I've shared already how you handle 20 Hz and below matters for several applications.
JR