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Simplistic NJFET RIAA
Simplistic NJFET RIAA
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Old 29th November 2020, 08:05 PM   #18631
Salas is online now Salas  Greece
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Simplistic NJFET RIAA
Hi, one of those starting from post #1812 in May 2009?
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Old 29th November 2020, 08:15 PM   #18632
jazz is offline jazz  Netherlands
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Hi, yes that one.
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Old 29th November 2020, 08:20 PM   #18633
Salas is online now Salas  Greece
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Alright, to make it sound heavier up R3. When not sure about the exact filter components values due an about meter, R3 is the best bet because it affects the whole filter's base impedance. Start with 50-51K and evaluate if that helps. What is the PSU? LM317?
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Old 29th November 2020, 08:34 PM   #18634
jazz is offline jazz  Netherlands
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Hi,

Thanks, that's the sort of answer i was looking for. PSU is a 36V Switch mode wallwart, single clcrc filter to 32V, one lm7824 on top of a bypassed led and 47uF cap. Last a rc stage 10R/1000uF. per channel.

I know, heresy but funds are tight and i had to use what i had. The 10R should isolate the 1000uF enough i think and that's an elna silmic II. the b1 uses a single lm317, bypassed devider.

Joris
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Old 29th November 2020, 08:43 PM   #18635
Salas is online now Salas  Greece
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Those circuits had been voiced with the first gen simpler shunts on slower high biased TO-247 Mosfets that contributed oomph. So they may need some re-tuning with different PSU. Especially if SMPS based.
Only make sure on the scope there isn't any switching frequency residue finding its way to the phono's rail or ground because PSRR is almost null in such circuitry. That would decidedly thin it out. That's a noise pollution first subjective effect. Line filtering done carefully but switching noise can couple like electromagnetic signal to an antenna too.
Also if there is path or wiring distance between the second stage's output point and the buffer, stick a 100Ω inline isolation resistor at the output point. Q2's drain node in those schematics.

Let us know how tuning goes on and stay safe, best of health.
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Old 29th November 2020, 08:58 PM   #18636
jazz is offline jazz  Netherlands
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Hi,

Yes, i know the psu is not nearly as good as ccs shunt. I did scope various points but no clear signals up to 10mHz. The B1 has a 1k gate resistor after the volume pot. Should that not suffice for the output series resistor? There's about 15 cm of wire per channel.

Edit: I do use a coupling cap, 4,7uF, at q2's drain, 1m resistor to ground.

Anyway i will try and see. Probably later this week. For now it's spinning records and quite nicely. I like it! this was build for the second setup in the living room. It is already better than expected.

Joris
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Last edited by jazz; 29th November 2020 at 09:06 PM.
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Old 29th November 2020, 09:22 PM   #18637
Salas is online now Salas  Greece
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That 1K resistor is a gate stopper for B1. It should be best located very close to its gate pin. Without any it oscillates due to wide bandwidth and incoming wiring/layout inductance. But it can also use lower in value when no long interconnects are involved.

Since the distance is 15cm only and you don't see any weird riding waveform on the signal path all should be well as it is.
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Old 30th November 2020, 12:30 PM   #18638
jazz is offline jazz  Netherlands
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Hi,

Yes, i know what the gate stopper is for. Just did not see the use of an extra resistor in that line. Stray capacitance is low with the short wires.

Hooked up the scope and a function generator this morning. There is a very low 2mV frequency at about 10Hz but that is probably residual as it is measurable at any point. No other signals to be seen. Even with 1V/1MHz input for the line stage right next tot the phono inputs, no signal is measurable in the phono stage. max resolution about 0.2mV.

So i guess it comes down tot riaa components being slightly off. Thanks for the design and advice!

Regards,
Joris
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Old 30th November 2020, 02:24 PM   #18639
Salas is online now Salas  Greece
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Nice that your build is clean so to know where to aim for. In other words for tuning it. That second stage output resistor suggestion was just in case there was a damping problem directly at the second stage's output. Because it was originally made with its own single JFET buffer tightly put together. Not tested with an external buffer back then.

I am only very cautious with SMPS and phonos because I have even witnessed 20Hz-20kHz FFT clean as a whistle on the bench with such PSU and an op-amp 60dB phono (high PSRR chips) but when on the actual system it turned very audibly hissy. Replaced with linear PSU with nothing else touched, total silence. Those hundreds of kHz switching frequencies find crazy ways to couple back sometimes.
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Old 6th December 2020, 02:41 PM   #18640
jazz is offline jazz  Netherlands
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Hi,

Upped R3 with 1k5 to 48k2. Now Ella has got a body and the Bass/drum has lost that hollow aspect. it's still light, nimble and has a good detailed, dry quality on top. Before going further i want to try it in the main setup.

Thanks again,
Joris
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