Simplistic NJFET RIAA

Using highest-gain MC preamp as example

The 2nd stage uses 6.2Kohm, with delta_Idrain of +- 0.1milliAmp to produce +- 0.62 volts peak. [schem in post#1814]

The deltaPower in that 6.2Kohm is 3.15mA * 6.2 = 20volts (my guess) times 0.1mA = 2 milliWatts. What will be the deltaTemperature?

At music frequencies, because of the thermal mass (2mm radius for RN70) of the core is 4Hertz F3dB, the changes in heat cannot propagate outside the resistor to the leads and the P2P wiring or to the PCB. All we can do is heat the periphery of the core, at music frequencies. See my prior post on reasoning for the 4Hz.

Assuming the resistor core is similar to silicon, with 140 watts/ meter*degC Rthermal, or 1.4watts/cm*degC, or 0.14watts/mm*degC, or 0.28watts/2mm*degC = 3.6 degC/watt for the resistor path from periphery to center.

Our 2milliWatts deltaPower causes 3.6degC/watt * 0.002 watt = 0.0072 degC change in temperature.

At 100ppm/degC, that's 0.7PPM change in resistance........at LOW frequencies. This change in resistance causes a change in GAIN, at low frequencies, which causes a change in gain for ALL of the music.....which comes across as Amplitude Modulation, by the low frequencies, of the high frequencies.

Small diameter resistors are the worst for this IMHO. And high-PPM TC resistors are the worst.

The 0.7PPM value, produced by legitimate computation with minimal handwaving (hah), is -123dBc Amplitude Modulation.

Smaller resistors, smaller in diameter than the 4mm diam of RN70, will be square-law faster. A 2mm diameter resistor will be 4Hz * (4mm/2mm)^2
or 4Hz * 2^2 = 4Hz * 4 = 16Hz F3dB.

I'd expect those tiny 1mm *2mm SMT resistors, with very thin bulk layers, to easily show this distortion to an FFT.

Again, I'm trying to quantify these distortion effects, which Walt Jung warned about and suggested LARGE RESISTORS be used for Feedback resistors.

tank
 
Anatech,
something seems to be mixed up between our posts.

You appear to be concerned about excessive dissipation in the NFB resistor/s.
That is exactly the same point I was making, when I suggested that the dissipation be limited to <10% of Pmax and showed some example values, i.e. 27K 600mW upper NFB resistor with a 41Vpk maximum signal from the output. It would be asked to dissipate a peak instantaneous 62mW, equivalent to 31mW average which is only 5% of the 600mW Pmax

Where are we different?
 
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www.hifisonix.com
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hello "Simplistic NJFET RIAA" forum

I've been reading, from post #1 forward, to learn about necessary properties of resistors and capacitors. Now at page 162 (post 1,620)
and still reading, I want to offer the attached attempt, intended to summarize what properties of resistors and capacitors are important for RIAA success.

Please be very critical of my ideas, because its a learning experience for me.
thanks for sharing your DIY efforts in the past
tankcircuitnoise

I would take a look at Douglas Selfs 'Small Signal Analog Design' - he discusses distortion in passives and shows plots.

The distortion on thin film SMD resistors bigger than 0602 lies at or below the floor of the AP SYS272 - i.e. better than -114 dbV. 0602 show a light increase in distortion due to heating effects and related to their small thermal mass.

Capacitors - quite a lot of data here from various sources so you need to choose carefully. Richard Marsh and Walt Jung wrote an interesting article on this subject about 30 years ago. For modern devices, NPO/COG show performance figures as good as anything - AP talk about this in one of their app notes and use them in low distortion circuits. Added to that, they are pennies from mouser in 1% tolerance. Parallel 5 or 10 and you have a high precision component.
 
Hi,

my measurements:

Raw PSU loaded with RIAA - 42.2v - 44.4v after warmup

for 3.6v TP1-TP2 - 32.15v after warmup

now, another voltage measurement that intrigues me - the output riaa VAC is 3.23vac in one channel and 4.83vac in the other. is this ok? or this value is not important? i have the teabag kits

BR
 
before i post some pictures, i want to ask your help to solve a hum problem that i think comes from the TT and not from the RIAA. I think! I have a lot of hum with the Riaa and with the built in pre-phono in my Marantz 6011 (new 2016 model). if i connect the earth cable to the marantz from the back of my TT, nothing happens. The hum persists. the samething happens with the Riaa. So, i'm guessing that the problem is in my TT ( an old one, but the one that i have right now) I'll post some pictures of the TT knowing that this is not the thread for it, but i'm not 100% convinced that the problem is in the TT. I am searching for ideas to service the TT.

pictures:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.


TT

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.



The white thick wires are for the AC home supply

The thin wires (taped) from the headshell were soldered on a common ground in the rca output. The other two thin wires are one from each channel. I don't know wich is left or right.

The earth cable from wall is the green/yellow that connects to the chassis and the blue is for the outside connection to the RIAA or other pre-phono, i think.

So, it is a hopeless case, or can it make sound without hum? even if it is horrible sound?!

Thank you

Congratulations Salas for the RIAA.

BR
 
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diyAudio Chief Moderator
Joined 2002
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Its that nobody can really make out hum issues from pictures but if you will draw the TT internal wiring schematic it will be easier. But maybe you found a solution already. Your FSP build looks good. Let us know what happened and how it all sounded or ask again.
 
Hi salas.

The hum is almost gone. It was a wiring issue from the tonearm and return signal ground. I now have some noise that i describe as a treble noise. I have it on my marantz and the RIAA. I think its related with the TT. It is old. Very. What do you think?

The sound from the RIAA makes me smile with hapiness. Thank you for your efforts.

BR
 
diyAudio Chief Moderator
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You are welcome.

This Riaa has very low own hiss. It uses the best low noise Toshiba input stage FET. Especially so when in MM mode that yours looks like its set. Because the gain is not so high. Since it appears with the Marantz as well I would firstly not suspect a build mistake in the electronics as you also say. Is it like hiss or like a high pitched buzz or whistle sound? Could it be RF interference? Are the TT output wires shielded? Clean cartridge contacts and any other contacts in its signal route. Also inspect for rotten old wires to replace or bad old wire joints in that route, redo with fresh solder.
 
I think its a high pitched buzz like you said.
When you mean TT output wires, its the right and left RCA interconnects to the riaa or the wires from the head/Stylus inside the TT?

If its RF interference, does a AC inlet with filter for the TT helps?

The cartridge connectors are clean but the Stylus pins look dirty.

Thks

BR