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In Reply to: RE: A million questions. Before 'n Before posted by 1973shovel on September 17, 2015 at 16:25:49
The schematic as Eli posted below is the one sometimes included with these, inside their boxes, along with very few specs and info, on one page. However, this schematic is not right for most mm carts. The original RIAA EQ feedback circuit lacks high frequency deemphasis and yields too much mid bass emphasis. The original phono EQ relies on a low input resistor to cut some highs, which was popular in the 1950s for the Variable Reluctance carts. Thus, the original chrome cutie, like original GE UPX-003 and Fisher PR-4,5 and6, are actually feedback and passive EQ equipped. The cartridge that the Japanese designers seemed to have designed these units around was not imported as often as the chrome cuties. There are a few USA and Euro made carts that will "work" with these preamps; but not as favorably as their potential abilities.
The classiest, classic '60s vintage preamps, like the Marantz 7 and McIntosh C-20, C-22 and C-11, as well as many others, favored feedback EQ solely. My preference seems to agree. Standard RIAA EQ requires certain "turnover" bass emphasis and "rolloff" treble deemphasis. If you were to study and/or list and chart the many active feedback EQ schemes of yesyteryears' 12AX7 preamps, you will quickly notice significant resemblances to one another. In fact, in spite of the differing voltages and topologies(ie: whether using cathode followers before the feedback, cascode or cascade, etc.)the EQ parts' values have many more similarities than uniqueness among any vintage 12AX7 feedback equipped phono preamps.
Rather than jump into a technical discussion here, let me answer your other few questions. Having repaired, restored, upgraded, modified and built from scratch for many lunar eclipses, I have been rebuilding the chrome cuties for over a decade. I have tried many EQ arrangements. We can use test equipment, including signal generators, test records and inverse phono EQ circuits along with a quality o'scope to provide absolute precision of our chosen circuitry. We can also use tried and tested, proven circuits. Ultimately, if you DIY and listen to well known discs on a well known playback system, plus really know the sound of instruments, your ears and brain will probably become the final answer.
I can tell you that using a full wave bridge rectifier arrangement might sound better to you; maybe not. I get excellent results with a half wave or full wave rectifier within this small chassis. I bypass the selenium rect, but it might work fine for another few decades. In my unit above, I used the selenium leads, folded and wrapped to support other leads. There really is only small space to work with. The AC power in leads are disconnected from their tie posts, then spliced and insulated cleanly, resting under the power supply caps. That freed up space for the power supply cap connections and positioning.
Please take some time to check the archives in AA, as I have discussed the variances in equalization values and my thoughts on AC filaments more than a few times. If you like, you can email me thru the asylum or the asylum trader. In my systems, hum level is low enough with the AC filaments' center tapped winding grounded. You might get lower hum with DC filaments or biasing them with a low B+ value. Keep in mind the truly limited space in these chassis.
For the DIY crowd that is watching the progress of the chrome cutie projects, let me first recommend dual mono power supplies. Two power transformers for each channel, even with high resistance B+ windings, will sound best. The dual cap banks for each channel sounds great, but, separate trannies providing high voltage and filamnets for each channel will and do sound better. Finally, I guess this is a good time to summarize some EQ choices I have made. Different cap types, mica vs. mylar vs. polystyrene vs. polypropylene vs. ? will all sound different and the defining cap values will differ among the types. I prefer the feedback to be arranged with the high frequency rolloff EQ pole first(thus lowering the noise from the later stage; then the bass boost EQ pole connected to the earlier stage. So, the classic typical RIAA EQ pole value, providing 75 usecond treble deemphasis, is 750 pf across 100K Ohms. 820 pf across 91,500 Ohms might be your preference. 680 pf across 110K might be mine, with certain cap types. More modern 12AX7 preamp phono designs might use 450 pf across 165K or 390 pf across 192,300 Ohms. These values provide the necessary 75 microsecond RIAA EQ for treble rolloff or deemphasis. The bass pole EQ differs even more among classics and modern designs. The classic vintage bass EQ pole (connected in series to the treble EQ pole of 750 pf across 100K) is nominally 2750 pf across 2.2 Megohms. Variations I like have been 2200 pf-2500 across 2.2-3.9 Megohms, depending on the EQ cap types as well as the slight bass boost some prefer. 3300 pf across >2.2 Megohms seems to have been a choice among two peers with bass heavy speakers. Modern designs show many deviations and listed precision EQ tolerance frequency response.
The chosen EQ and coupling cap values you end up with are your choice; literally. It is your musical playback enjoyment factor at stake here...So...ENJOY !
Follow Ups:
As Crazy Dave said, thank you for an excellent post. I appreciate you taking the time and effort to share so much information with us.
I will save the information you've posted, and check the archives for your other posts on the subject.
Thanks again for sharing your work.
"You won't come back from Fletcher-Munson curve"-Jan and Dean
Thank You for the kind words.
I truly enjoy "sharing the knowledge" so we can all enjoy "sharing the wealth (of enjoyable sound)."
I believe these sensibly modified chrome cutie phono preamps could be the best budget phono stage available; especially if you can DIY. I often watch them disappear quickly on the bay, offered reasonably as a "buy it now" in clean cosmetic condition. With some resto skills and spare parts to upgrade the circuit, these sound competitive and totally enjoyable...ENJOY !
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