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

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

  1. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Thank you for your help.

    Did you catch that I mentioned that this unit has undergone a parts upgrade that I ordered on eBay? So not stock, and I'm not sure if some of the component values were altered. Perhaps that could explain the 3k hump that is missing? If you have any interest, I can likely find the instructions that accompanied the parts kit and see if I can find substitutions.

    On the power supply issue, I think I may just cut the power cord to maybe 6" or so and then use a short single-receptacle extension cord. Checks all the boxes, cheap (my favorite box).
     
  2. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    I missed that, it sounds very interesting to me, please somehow keep me in the "loop."

    I listened for a while with the 30k loading, which is essentially flat, and it sounds very good. It also sounds good (more like fun?) without being loaded down, that smile puts a smile on your face.

    I was surprised to see that I didn't effect any substantial changes in the bass with the parallel resistors, I thought you had mentioned that loading it down would effect the bass/subsonic?

    Again I appreciate everyone's help.
     
  3. harby

    harby Forum Resident

    Location:
    Portland, OR, USA
    If you just want a short cord and not a noise improvement, then yea, pull it back into the enclosure and chop off and re-solder.

    There's one "upgrade" selling on eBay. Cleverly turned away the cap values in the picture. However, think I see 11k resistors, which would be crazy random happenstance, since I published this response-flattening mod including that value two years ago (with even more resistor changes though) (original schematic first):


    [​IMG]
    [​IMG]
    (4.9 is a typo, makes no difference vs 4.7)

    Today, the mod re-analyzed with my current inverse RIAA.
    With a VM540 sim, and only 70p for cabling and 100p internal, the high frequency peak:
    [​IMG]
    Why a +/- 0.1dB Sutherland is only nice on paper...
     
  4. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    @harby, I checked the instructions that accompanied the 42-2109 kit, they say "First, replace the capacitors. The capacitors in this kit are the exact same capacitance (uf) ratings as the originals, and the voltages are the same or higher. They are much higher-quality, but are smaller because of advances in electronics design."

    In the meantime, I finally got my $12 (I think, eBay purchase) TC-750 quieted. I found a thread at Superbestaudiofriends.org (I know, I know) that provide some guidance on improvements. Unfortunately, it appears some of the posts are by people that really don't know much more than I. So in the end, I ended-up reversing some of the changes. The unit now powers from the same linear power supply that runs the turntable motor.

    An interesting thing to me, is that I did pull the approx. 220pf load capacitors in the TC-750 and so my capacitance is whatever is contributed by the circuit, and my cables (cables approx. 155pf), and I still have a substantial peak. I guess either the capacitance from the circuit is substantial or that is just the way it is. I can lower resistance and tame that peak, I just wanted to see what I was starting with.

    I am pleased to have less bass hump and be able to see how changes to my arm effect the low-frequency performance of the arm.

    Also somewhat interesting, is that the TC-750's output is approx. 2db higher than the 42-2109. I've adjusted the curves so they overlay, but the TC-750 is certainly louder.

    Perhaps we can start a TC-750 thread, and you can help with RIAA improvements if you ever have the time, I know you are busy and maybe the idea doesn't even seem that interesting to you.

    I do have one of the 24vdc regulated adapters on the way, I plan to try that on this TC-750. What sort of advantage may I see in running these sorts of discrete circuits with 24v instead of 12?

    [​IMG]
     
  5. harby

    harby Forum Resident

    Location:
    Portland, OR, USA
    Connecting 24V to the TC-750 will bring back a "bass hump". It will also blow up the 18V power filter capacitors.

    [​IMG]
    Above response shows just 155pF total load - on an AT 460mH/800 ohm(you don't say what model).


    More voltage gives more headroom, here the output of +20dB over 5mV RMS input at 1kHz. 12v (brown) clips.
    [​IMG]
     
    Ingenieur and Phil Thien like this.
  6. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    Again, thank you for all your help.

    I've removed the 18v power filtering capacitors, in anticipation of using a 24v supply.

    A 1.25db hump may actually flatten the bass a little, it seems like it is slightly rolled off now. I'm hoping for flatter, not enhanced as in the Realistic.

    I guess I'm going to find out.
     
  7. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    I've been powering the TC-750 from the same Pyramid 12vdc linear regulated power supply that drives my turntable's motor. I just snipped the cable off the low-end linear (but unregulated) AC adapter that accompanied the TC-750, and wired that to the binding posts on the Pyramid.

    Yesterday I received the 24vdc adapter (the Jameco one mentioned) and figured I'd check my HiFi News pink noise track with both the Jameco 24vdc and the Pyramid 12vdc.

    Not much to see, actually, the curves overlay fairly nicely. No real bass hump coming from the 24vdc as was expected based on comments from @harby. I do note that having read quite a few discussions on these, the component values don't necessarily match schematics, and the unit I'm using is the TC-750LC, which has a variable output (more on that below). So @harby is at a disadvantage in making projections.

    Anyhow, here is the 24vdc vs. the 12vdc:

    [​IMG]

    I had read in a thread some time ago, don't remember where, that the variable output on these actually alters the response curve along with the output level. So I tried testing for this by recording the same pink noise track at both the 75% output level as well as 100% output levels. REW (the software I'm using) allows me to shift curves up/down so I can overlay curves at different amplitudes. So I did that here, and found the 75% results in a more even high-frequency output, almost as if I've modified the loading lower. I'll play with this some more in the future, as I can give-up gain at the phono stage, I have plenty of gain further down the chain.

    So here is that curve...

    [​IMG]
     
    Ingenieur and JohnO like this.
  8. Slack

    Slack Forum Resident

    It would be nice to see this thread return to topic.
     
  9. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    In what way exactly?

    No talk of the TC-750? It is the kissing cousin of the 42-210x, and there have been lots of other preamps/receivers discussed?

    No attempt at objective data/measurements/comparisons?
     
    patient_ot likes this.
  10. Phil Thien

    Phil Thien Forum Resident

    Location:
    Milwaukee, WI
    So @harby, I was able to go back and do some better measurements of the 24v vs. 12v on the TC-750 and was able to see the slight boost in the LF as you indicated.

    Do you have any ideas on flattening the slight rise in HF starting around 15k on the TC-750? I don't know if a similar RIAA EQ change can be made, as you made for the 42-2101a.

    Sorta academic because I can't hear it anyhow, but if it can be flatter, I'd give it a shot.
     
    Ingenieur likes this.

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