A question about the value of the R27. With a resistor value of 750 ohms, I get the values 3.97 mA on one circuit board and 3.74 mA on the other. There is therefore a difference of approximately 0.23 mA between those channels. Are these values good enough or should I match them more closely?
Hello,
Give us a peek at the headroom.
Turn up the input voltage until the harmonics are up out of the noise floor.
If that RTX6001 analyzer has an oscilloscope function turn up the input voltage until we see a little clipping.
Thanks DT
I haven't decided yet. I'll wait to see how it sounds and decide then how much I want to invest. In any case, I'll almost certainly keep the PSU out of sight; and only have the RIAA on display. I've been using a Salas Simplistic NJFET RIAA that I built around 2016. That's in a Hammond walnut enclosure. If the Pearl 3 is a keeper, I'd consider redoing the Salas enclosure to house the Pearl 3. I'm waiting on a few connectors then the Pearl will be ready to text.
Of course headroom depends on the jumper and switches settings. I measured with jumper OFF (high gain) and all switches OFF. For these settings clipping starts at just over 3 mV rms (8.6 mV peak-peak) input. Pearl 3 output remains clean (THD < 0.004 percent) until 9.1 Vrms out. Clipping occurs (THD ~ 0.2 percent) at 9.4 Vrms i.e. when the output swing of +/- 13.3V approaches the +/-15V output stage supply rails.
Bottom line, no surprises. Nothing to see here.
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If you purchased a Pearl 3 kit and need the parts for the power supply please see the link below and/or send me a PM. I ended up buying 65 transformers from a pro audio manufacturer who went another direction on a product. It happens to be the exact transformer used in the Pearl 3 design. As of now I have over 50 kits available. I may end up putting these on my Etsy store, but you can save a few bucks sending me a message directly.
Pearl 3 Power Supply Kit Group Buy
https://www.diyaudio.com/community/...o-crcrc-power-supply-parts-kit.406461/page-12
Pearl 3 Power Supply Kit Group Buy
https://www.diyaudio.com/community/...o-crcrc-power-supply-parts-kit.406461/page-12
Hello,
I have built and measured multiple iterations of the Pearl 3 Vinyl Pre-Amplifier.
The Capacitance multiplier, cascode, JFETS and feedback loops that include discrete transistors are all fun to dissect and wrap your head around.
The Pearl-3 has two levels of gain; high and very high. Reducing the gain by adjusting the resistor values in the feedback loop has the high cost of increasing distortion.
If you use the jumper to select the P-3 low gain of 50 db's and use a mc cartridge this setting will allow overhead for vinyl clicks and pops.
It was pointed out that the level of gain depends on your system.
The common system gain is summed across a Vinyl Pre-Amplifier, a pre-amplifier and a power-amplifier. The final output is controlled by at least one Volume Pot some where in the chain.
A issue that I see is that most of the gain is in the pre-pre-amplifier not leaving any headroom for clicks and pops on the vinyl. The pre-amp even if it has only a few dB's of gain is still too much and needs a lot of attenuation to avoid overdriving the power-amp.
My vinyl collection is mostly well used over the years and or purchased used. My TT is a DJ workhorse and the cartridge is a mid range mm.
Typical method. When I tried that method on BA2018, worked great. When I tried it here, the first tiny pad I touched vaporized! No problem just resoldered the area, tested continuity just fine.
For this piece, I just stuck the back to a thin piece of tape. Arranged it so the two legs in middle of pads. Iron solder on work and tiny pad, solder flows, done. Remove tap, do third leg and touch up. These parts aren't that tiny as far as and parts are concerned and soldering them was easy.
Respectfully, I don't think that for a same low level input signal (same cartridge) this is correct.
Measurement taken off single ended output.
Jumper on (low gain)
Switch 1 ON (50 ohm input impedance)
fc 1kHz
Vin 0.328 mVrms
Vout 0.11 Vrms
Gain 50.5 dB
THD < 0.0002 percent mostly second harmonic
Looking at the RTA, and for a same Vin, I see no difference in distortion compared to post #3166 showing the high gain setting. In either case, it is so low as to be very hard to measure reliably.
Hello,
Same as my results.
The Pioneer kit P-3 has a gain:
of 70ish with the J2 jumper removed.
of 50ish with the J2 jumper in place.
Try placing a 100R resistor at the C16 location in parallel with R22. The gain is reduced by 4 dB to about 46 dB while the distortion takes a big hit.
Why?
When the feedback loop resistance is reduced the U1:A current increases past a point where the OP-Amp will operate with low distortion. I used a OPA 1656 that on the data sheet is good for 100ma.
The short story is that there is a limit to gain adjustment by switching to lower resistance values in the OP-Amp feedback loop.
If you want the long story take a look in D Self's discussion of active Op-Amp volume control in "Small Signal Audio Design" FOURTH EDITION.
Thanks DT
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