Realistic 42-2101/42-2101-a Phono Preamps

Discussion in 'Audio Hardware' started by Robin L, Aug 12, 2015.

  1. Classic Car Guy

    Classic Car Guy Analog Recording to the Next Level

    Location:
    Northwest, USA
    How long did it took you guys to overhaul the preamp? I bought the kit too last January but I haven't install it yet....:rant:
     
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  2. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    I've thought about that, but in trying to isolate the preamp I'd also have to make sure I'm maintaining the same cabling capacitance and maybe output impedance?

    I think it would be difficult for a neophyte like myself, maybe easier for someone like you.

    I guess I'm interested in the overall response anyhow. So if the preamp ends up compensating for the cartridge a little, that's okay.
     
  3. patient_ot

    patient_ot Senior Member

    Location:
    USA
    I have an older Signet cart with similar electrical specs. It will have a treble lift at ~200pf + 47K no matter what traditional phono preamp you use. Only way to get rid of that lift is to play around with non-traditional loading.
     
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  4. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    It went pretty fast. I decided to remove the front label of the preamp to get access to the screws so I could remove the RCA jacks (they're attached to little PCBs). This allowed me access to the board (which is mounted upside down in the enclosure) w/o having to desolder the eight wires between the main PCB and the RCA jacks.

    To remove the old components, I wrapped a piece of string around the leads, and heated the pads from the other side while I gently tugged the string to lift the lead. Once I had the component removed, I used a small drill bit in a pin vise to clean the solder from the hole so I could install the replacement component. Just select a drill bit that is just larger than the lead of the component you're installing.

    The key is to use a soldering iron with some balls and crank the heat, so you're only on the pad a second or two before the solder is liquid. The mistake most people make, is using too small an iron and throwing down heat a long time.

    I'd say the process took me an hour or so, start to finish.

    One of these days, if I'm truly motivated, I'll CNC a new front label replacement that has through-holes for the screws, so I can easily disassemble/reassemble the thing. The screws that hold the RCA jacks in place, would actually hold the faux front panel on.
     
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  5. Davey

    Davey NP: Broadcast ~ Haha Sound (on repeat, as usual)

    Location:
    SF Bay Area, USA
    The cable capacitance and output impedance of the signal source (provided it is relatively low) will have negligible effect on the inverse filter response, and the inverse filter has a relatively low output impedance to feed the phono preamp, so input capacitance won't have an appreciable effect either.

    Anyway, just a thought, but looks like you've got it under control :)
     
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  6. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Gotcha. I should toss my Super OM30 on here and see how that looks. I suspect a similar lift, I think the issue is maybe 1.5db of lift from the preamp, and some lift from the cartridge. Maybe.

    I'll try "loading it down" to 36k or so and see how the Signet measures.

    I'm more curious than anything, I don't know how the bass bump is gonna sound if the top is too flat.
     
  7. Classic Car Guy

    Classic Car Guy Analog Recording to the Next Level

    Location:
    Northwest, USA
    Okay I got it. That definitely makes a lot of sense and I'll do the same procedure you made. I have a 40 watt iron and I can make a schedule on Monday morning. I'm a bit excited for I cant believe I have a perfectly running 40 year old recording station. Well I worked on most of them but I haven't done the preamp yet and lastly, I'll have to recap the speakers before it start giving me trouble. This preamp definitely made a big different in my system.
    Thanks,,,
     
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  8. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Well nobody has answered my question about replacing the 10v transformer with a wall wart. Can I likely get away with using a 9v AC wall wart?
     
  9. patient_ot

    patient_ot Senior Member

    Location:
    USA
    Can you try the Signet with 39k? IME it worked well, at least for me.
     
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  10. Classic Car Guy

    Classic Car Guy Analog Recording to the Next Level

    Location:
    Northwest, USA
    I suppose the reason why you want to go wall wart, the ac cable is in the way for some reason?
     
  11. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Sorry, I missed the Q's at the bottom.

    I plot from a file. I have an inexpensive USB interface attached to the fixed outputs on my ZPre3 preamp. I connect this to my Android phone and use an app called USB Recorder Pro to capture a WAV file, which I then grab on my computer and open in REW. REW is free/shareware (there is a paid pro version I believe). The USB Recorder Pro app was like $5. The USB device I use was $20, made by Xitel. It checked-out flat when using digitally sourced pink and white noise. Limited to 48k but I normally just leave it at 44k, for these exercises it is fine.
     
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  12. Davey

    Davey NP: Broadcast ~ Haha Sound (on repeat, as usual)

    Location:
    SF Bay Area, USA
    I don't know how close the AC voltage is now, it uses a voltage doubler circuit to get a high enough DC voltage to regulate down to 28VDC. You could just supply a higher voltage AC and get rid of the doubler circuit, feeding a more conventional full-wave rectifier bridge, or use an external 28VDC supply and get rid of all the internal power supply, would just need a decent power supply capacitor inside for decoupling.

    But assuming you are looking for a simple solution, yes you could replace the internal transformer with a AC output walwart (though not sure if 9VAC would work), or maybe just remove and repackage it for remote use with the low voltage AC cable you want to use.
     
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  13. Classic Car Guy

    Classic Car Guy Analog Recording to the Next Level

    Location:
    Northwest, USA
    Id be interested in getting your results because I'm using a vintage Audio Technica signet and I thought it sounds great. I was a bit skeptical since this is 25+ years old and hoping its not oxidized.
     
  14. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Yeah for my application it is a PITA. I have a wall-mounted rack that I keep my turntable on. And I can route cables through it, but it would be handy to be able to leave the cable routed and just pull the preamp either to make changes or just switch to something else temporarily.

    Perhaps moving the transformer to a wall-wart could reduce noise, but I have no noise issues with the thing. Theoretically it could. I guess it could make it worse, too.

    Or well, don't sweat the cord on yours, my application is unique.
     
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  15. Classic Car Guy

    Classic Car Guy Analog Recording to the Next Level

    Location:
    Northwest, USA
    you can make a simple circuit for the wiring going inside your preamp to eliminate any noise by putting a ceramic capacitor between the 2 wires. I gotta look at the circuit first and tell you the value that will work on the amperage load of the preamp.
    Now you got me so curious. I might do the same too since I have a power source on my recording station. As far as I remember I have one on the back of my DBX 400 router.
    [​IMG]
     
    Last edited: Oct 29, 2021
  16. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Simple would be preferred, especially as the power supply now seems fairly quiet and my expertise limited.

    I can measure at the transformer, I'll just check it connected to the circuit, and disconnected.

    The thing is, those wall wart type transformers always seem to exceed their rating anyhow, at least when there is no load.
     
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  17. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Somewhere around here I have loading plugs already made up, just can't remember their value. Could be whatever was used was to get 39k with this thing, I may have copied something you indicated previously. Once I find them and get it tested, I'll let you know the results, just in case you're curious.
     
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  18. patient_ot

    patient_ot Senior Member

    Location:
    USA
    I'm definitely kind of curious. My last test when I was using that cart was on an 11g gimbaled arm, nothing too exotic like you have. But across 3-4 different phono preamps, the results were basically the same, even though the phono preamps had different specs and designs.
     
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  19. Davey

    Davey NP: Broadcast ~ Haha Sound (on repeat, as usual)

    Location:
    SF Bay Area, USA
    Yes, the voltage rating is for the full rated current output, so the smaller the transformer, the bigger the difference will be between no load and full load voltage. Hard to say what the transformer inside is producing, the current draw is probably pretty small.
     
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  20. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Wow that is good to know, thanks. I'll post back to this thread a ping you when I get any #'s.
     
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  21. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Thank you, as always you've been extremely helpful.
     
  22. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Yeah I'm curious how the Super OM 30 will perform on this unit as well.

    Maybe a day or two before I can check, but I'll post back to this thread.
     
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  23. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Just in case you're curious, 11.1v while connected to the phono amp board. I'm guessing 9v would be insufficient.

    I'll look around for a 10v.
     
  24. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    I found the wye cables and loading plugs I had used previously with this thing. The resistors are 82k which when combined with the 50k on the preamp yields approx. 31k.

    And here is the resulting capture of the Hi-Fi News track.

    I will listen like this a while and then circle back and try the @patient_ot suggestion of 39k and give that a listen.

    My prediction is that I won't like this much (I honestly can't recall), that the bass lift w/o any treble lift at all is going to sound less like a smile and more like a smirk. :)

    [​IMG]
     
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  25. harby

    harby Forum Resident

    Location:
    Portland, OR, USA
    The response looks pretty reasonable - too much even. Your plot has the opposite of the expected and verified 0.75dB hump 3k+.

    1. I would just bypass the transistor regulator and put external 24v-30v linear regulated DC right into the C17 power cap. Keep AC out of the box.
    Here's about the cheapest "linear regulated" wall wart: DFU240050F1121: Jameco Reliapro : AC to DC Power Supply Wall Adapter Transformer Single Output 24 Volt 0.5 Amp 12 Watt : Power Supplies & Wall Adapters

    2. This preamp doesn't take to having its internal "loading" monkeyed with. With the DC-blocking input capacitor, the load resistor also forms the "subsonic filter".

    [​IMG]
    (yes, I had to re-make the deleted phono preamp simulation, no longer interested in such toys, and give you verification of 24V supply performance. The beta of the 1740 transistor is as close as I could ascertain, which also affects low frequency hump.)

    2a. Those calculator's resonance is a "free resonance", not the realized resonance with the load damping.

    3. (I have such a preamp I've been plugging away at. The subsonic is non-defeatable due to integration, and very sharp. Optimized for only VM540/740 (or VM95 with jumper). Remote input module at the turntable to allow short cables. Price point/features TBD.)

    25-25kHz:
    [​IMG]
     
    Last edited: Oct 29, 2021
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