Looking inside one of my 42-2101A, R03/04 appears to be Blue/Gray/Orange/Silver (68kohm +/- 10%, 1/4watt) and measures 67.8 kohm with my pocket Radio Shack voltmeter.
Its similar to the Monacor. Class A Discrete. Warm and sweet. Can be run off a battery. Because it's cheap some are suspicious
My apologies. You are correct. R03/04 is definitely DC blocked and just sets the input impedance. TR1/3 is biased by R13/14, which is interesting because that makes it Voltage Feedback Biased. My understanding is that this was a common design in the low hFE Germanium transistor era, but wasn't really necessary once Silicon transistors became readily available. Because of the Voltage Feedback biasing, the bias changes slightly with signal peaks, creating a very VERY subtle compression. I think that's the most significant difference between the 42-2101a and most other MM phono pre. And a tiny bit of subtle compression/limiting would explain why it seems to add a touch of warmth and clarity.
Excellent find! I'll update that schematic, as with the correction, we can now fit an RIAA curve. For the 2N5088 substitute transistor, and with 430mH of cartridge inductance (averaged some AT MM) the 56k R03/R04 seems to give a better low-end accuracy: We can also analyze that barely-legible C13 would be .0027uF (2.7nf):
Just bought another of your rebuilt ones last night on fleabay. Looking forward to seeing what it can do with Jupiter Copper caps
I think the earlier units had R03/04 = 68k, C03/04 = 500pF, and all transistors were 2SC1327, just like the schematic (assuming that that is 68k and not 6.8k). I just traced out my unit and it has 56k, 470pF, and 2SC1570. Everything else matches the schematic. I don't know how to date these, but I think I have a later unit. Makes sense. 56k seems to be an improvement. And the other two values were likely just substitutions due to 470 becoming the more standard value and 2SC1327 probably became obsolete while still these were still in production.
Upload pic at imgur dot com. When the picture displays, right-click inside the pic and choose "Copy Image" (not "Copy Image Address"). In your message here, with the cursor where you want the pic, right-click and "Paste". Done.
I was wondering about the bias for the first stage. Since there is a bypass cap from the emitter of TR2/4 to ground, would there be any audio present on the R13/R14 feedback and/or bias resistor?
The left side of R13 has the phono input after coupling cap filtering (brown), the same levels as phono input after this low-pass filtering. DC 627mV. The right side of R13 (green), a 30Hz bump 10dB above input levels in low bass, but quickly descends into microvolts at bypassable higher frequencies. 21 microvolts at this node with a 1mV 1kHz phono input. DC 719mV. Upload to Imgur. Right-click on the actual photo after it is uploaded, and select "Copy Image Location". Press the picture button in this forum's rich text editor, and paste the photo URL.
I uploaded to an album I have on Facebook caller forums which is shared to the public. Then open the image uploaded full screen and copy the hyperlink. Enter that for the image link and you're golden. Images don't expire on Facebook , unlike other free hosting services.
Another phono preamp that had the 2SC1570 transistor in it -- the Realistic SA-102 mini-amplifier, which you can see next to our little hero in this 1982 catalog: If you want to build a lo-fi phono pre out of old transistors, its schematic seems a better design base if you want to hack something out of parts. The design is similar, but with different component values and a bit different output config. It can run off 12 volts (it is filtered by 16v caps). More importantly, this gets much closer to an accurate RIAA curve (with a push in the right direction): - Green is the 42-4202a phono pre, a significant deviation in the treble, and a hump before its premature sub-filtering, not fixed with different transistors; - Purple SA102 is the preamp circuit from the Realistic integrated amplifier, after I gave it a <47k load with less input filtering, 36dB gain to match the other RatShack, and tweaked the values for the extra digit in 1% resistors; - Pink "RIAA" is actually an op-amp-based design with 15dB less noise, known to be +/-0.1dB when built out of the components simulated. Realistic SA-102 phono stage (L channel), small mods: All polarized caps are 50V except C09 10V, but might not need to be that high.
That's pretty neat. The circuits are almost identical. It would be pretty easy to modify the 42-2101a for this...well...not really. You'd essentially need to replace every single component, so you might as well just build it from scratch at that point. I would assume removing the input capacitor before the impedance resistor made the biggest difference. And the value changes to C107, C109, R115, and R117 brought it closer to the RIAA curve. So maybe you don't need to change every component on the 42-2101a, just those. That would make modifying it a little more realistic (no pun intended). Anyhow...the model of the SA-102 certainly seems to perform better, and it has the voltage feedback biasing that I think gives the 42-2101a its "mojo", but you really would have to DIY just the phono pre to try it out for real, since you can't use it without using the amplifier. I wish I had p-spice. I just requested the trial copy. Do you mind plugging these values into the 42-2101a model and seeing what happens? C15 - jumper R03 - 47k C11 - 2.2n C13 - 8.2n R17 - 33.2k R19 - 332k tack on 100k output impedance resistor at the end
I PM'd him, those values aren't an improvement. I'd want to verify any mods by analysis on a real unit before I would make any recommendations, and I already have good phono preamps being unused.
Really needs a recap for decent performance.... If you're going to splurge in any top caps use them in the 4.7uf stage. Makes the biggest difference. Super keen to see how Jupiter Copper Foil caps work in that position. The Mundorf Silver Gold are pretty spectacular as is
I still have 3 of the Shure V15-RS, I picked up 4 of them when Radio Shack was fire saleing them out the door for $14.99. I had a friend who purchased one bac in the late 70's or was it the early 80's, he paid $79 for his, and that was a fire sale price. He also pissed me off, I would buy new albums and stop by his place, all the lp's tracked like a dream. Finally after playing them on my table, I noticed the distorsion on the same record. Went back and that was when I was introduced to the Shure V15-rs.
I built a 42-2101a from scratch on a proper PCB. A lot of people on a Facebook group were talking this thing up, so I tried building one on some strip board to see what it sounded like. I found this forum looking for the schematic. Anyhow...I couldn't get the strip board build to work, so I just bought an original. I liked the way it sounded so much, that I figured I'd make a PCB for it and build it from scratch with better components. I was going to build an A/B switch box to compare this with the original, but the difference was so apparent, there really was no need. This sounds much more clear, but still has all the warmth of the original. I added a common mode choke and fixed inductor for DC line filtering between the rectifier and regulator. Aside from that, there was no deviation from the original schematic. I'm going to build another to SA-102 specs. The circuits are similar enough that it should be easy to use the same PCB.
