Trying to find a resistor value for JFET RIAA Phono Preamplifier schematic

[Frankentronics]

Hello,

I've searched the web before posting here, but can't find the answer.

I would like to build a JFET RIAA Phono Preamplifier described here:

http://diyaudioprojects.com/Solid/JFET-Phono-Preamplifier-Kit/

Here is the schematic.



Everything is clear to me, except the value for the R2.

Does anyone have any idea what that value might be?

The other 2 components that are not specified are the C6 and C7 caps. I've been able to determine that they must be 220uF, 50V. Does that sound right?

Thanks...

[Paul Moir]

I'm only just acquainted with this end of electronics, but R2 should depend on the phono cartridge you're using.  It provides the recommended load impedance.
220uF for the power supply bypass caps should be plenty.  Half that would be plenty too.  50V seems a little unnecessary but I don't know what the power supply is.

[MasterT]

I 'd set R2 = R10.

[Frankentronics]

Thank you, both.

I guess I can just put a 100K trim pot and adjust the value, depending on the cartridge (I'm planning on using a Moving Magnet cartridge). Not sure how exactly I would set it, though. Perhaps I would listen for distortion, as I'm setting it?

Regarding C6 and C7, I just zoomed in on the image of the parts and I figured out those were 220uF, 50V caps. The recommended power supply is 18VDC. I'll start with 2x9V batteries and if it performs well I'll build a power supply later.

Thanks again...

[Frankentronics]

Well, here's an update...

After some more Googling I found this schematic...



...which is different from the schematic I posted initially. The initial schematic appears in this assembly manual...



..some other resistors also have different values in the new schematic and it looks like the new schematic is rev. 3, while the one I first posted is rev. 2.

But I guess I got my answer.

Thanks...

[Lightages]

The load resistor should be what the cartridge manufacturer suggests. Each cartridge will perform better when the resonances are damped out. The nominal value for phono preamps is 47K, and a capacitance is usually specified too. Too small a load (high resistance and low capacitance) and the cartridge response will rise at the high end and you can get ringing. Too high a load and the high end response can drop.

The best thing to  do is to build the preamp and use the suggested load resistance and capacitance, and then adjust until you get a flat response and/or it sounds right.

[floobydust]

R2 is the load resistor for the phono cartridge. Your phono cartridge datasheet will specify, for MM it's almost always around 50k ohms; R2=47k is fine there and common.

Sometimes there is a small capacitor across R2, also part of the cartridge's load (example below).
That (load) capacitance is usually obtained from the tonearm and RCA cable wiring. Thorens TD165 total measures 270pF. But if the pre-amp is close, inside the turntable, you'll have less (cable) capacitance and need to then add a small capacitor across R2, to meet the cartridge's load.

Example Shure V15 Type IV datasheet:

"Optimum Load: 47,000 ohms resistance in parallel with 200 to 300 picofarads capacitance per channel. Load resistance can be up to 70,000 ohms with almost no audible change in frequency response. Total capacitance includes both the tone arm wiring and amplifier input circuit. (Most amplifiers, tone arms and cables meet this requirement)."

[tautech]

Early audio inputs were matched for crystal pickups and output levels were much higher than magnetic pickups.
The RIAA spec and preamps became available to offer some standardization in the industry.
Early amps offered both crystal and magnetic phono inputs.

[Frankentronics]

Thank you all for your information. This is all helpful.

What I'm actually doing is a project with my kid. We are going through some turntables all of various makes and from various eras that people tossed out on the street (to mention some, we found Technics, Bang & Olufsen, Panasonic, Sony and some more). I want build a setup to test them all and just have some fun with my kid. I know I should be changing cartridges, but this is just a project with my kid and we'll also be using a bunch of old records that were also found on the streets.

In due time, once we settle on the turntable we like, we will get into the finer tweaking and optimizing.

If I understand correctly what you are all saying, it might be the best option at this early stage of this experimental project to put a variable resistor there and adjust for each turntable. Then, once we settle on the turntable (and mostly likely change the cartridge) we would replace the VR with a fixed value.

Does that sound like the best approach, for this project?

[Lightages]

No, it is not the best approach. The problem is that the pot will be very noisy. The best approach is to use a 47K resistor and perhaps a 470 pf capacitor in parallel. If this is just for playing around and not for critical listening then there is no real reason to go further.

[Benta]

The RIAA-specified load for MM cartridges is 47 kohm. Period.
No additional capacitance is needed, the coax cable between record player and amplifier will provide sufficient capacitance.

With moving-coil pickups it's a completely different story...

[Kleinstein]

47 K load resistance seems to be standard - so ne real need to adjust this.

With load capacitance one might want to add some, if the cable capacitance is low. e.g. if the preamp is directly at the source. 470 pF is too much, more like 100-200 pF added.

[CatalinaWOW]

Since OP wants minimum parts count, does anyone understand the purpose of C3?  This is an audio preamp and just about any garden variety capacitor should work fine over the frequency range.  Doesn't seem necessary to have C2 and C3 of different capacitor technologies to cover the entire frequency range. 

[Frankentronics]

OK, now I understand. I'll just put a 47k resistor (and perhaps a 470 pf capacitor in parallel) as explained to me.

I much appreciate all your quick and informative responses.

Thanks...

[Benta]

OK, now I understand. I'll just put a 47k resistor (and perhaps a 470 pf capacitor in parallel) as explained to me.

I much appreciate all your quick and informative responses.

Thanks...

Make space for the cap if you like, but don't mount it to start with. See how the circuit behaves first.

[MasterT]

Since OP wants minimum parts count, does anyone understand the purpose of C3?  This is an audio preamp and just about any garden variety capacitor should work fine over the frequency range.  Doesn't seem necessary to have C2 and C3 of different capacitor technologies to cover the entire frequency range.

I think, they put two caps C2&C3 in order to get better approximation with RIAA time constants. 3% correction of C2 seems useless  if not to build a circuit with 1% tolerance components.
 Googling over wiki for RIAA standard, I discovered an error in R5 value, it has to be 22 k. Definitely not a 28k, and tacking output impedance of the first stage amplifier, about  20.5 k 
 There is a calculator
http://www.mh-audio.nl/calculateRIAA.asp

and outputs: R1=22000 R2=3198.958 C2=34.09nF C1=99.4nF

Since 34 nF is not available, it explains a necessity for C3.

[CatalinaWOW]

The explanation for C3 makes sense from a math standpoint.  Since very few circuits in the time of LPs were built with 1% components I suspect the correction provided for C2 is a triumph of the calculator over the practical.  Remember this whole equalization scheme was invented when slide rules were king for calculation and thus 2 and possibly 3 digit accuracy was all that was normally achieved.

Perhaps someone will build a version of this with all Rs and Cs trimmed to their optimum 0.1% values and report on the perfection of its sound. 

[Frankentronics]

...I discovered an error in R5 value, it has to be 22 k. Definitely not a 28k...

Good catch...!

I think this is one of the things they addressed in rev.3 of this schematic. They changed that R5 to a value of 22.1k. I believe you were probably still looking at the rev.2 schematic (which is the first one I posed) when you caught that error. In one of my subsequent posts I posted rev.3, which I discovered at a later point.

Make space for the cap if you like, but don't mount it to start with. See how the circuit behaves first.

If you don't mind me asking, what kind of behavior from the circuit would I encounter in order to mount the cap?

Thanks...

[Lightages]

As far as needing a cap, it depends on the cartridge. Someone has already pointed out that the capacitance needed by be already supplied by the cable from the cartridge to preamp. If you can measure the capacitance of the cable or have the specs and can calculate then you can determine if you need to add any or ignore it.  You said that this is just a fun project so you can ignore it, or if you want to play you can adjust it and see what happens.

Many phono preamps had dip switches to change the resistor and capacitance on the input. You can do the same by putting an IC scoket t the input and plug in different resistors and capacitors and play around to your liking. You should use metal film resistors and polypropylene caps as the best for low noise. Stop obsessing and have fun and experiment.

[Frankentronics]

As far as needing a cap, it depends on the cartridge. Someone has already pointed out that the capacitance needed by be already supplied by the cable from the cartridge to preamp. If you can measure the capacitance of the cable or have the specs and can calculate then you can determine if you need to add any or ignore it.  You said that this is just a fun project so you can ignore it, or if you want to play you can adjust it and see what happens.

Many phono preamps had dip switches to change the resistor and capacitance on the input. You can do the same by putting an IC socket at the input and plug in different resistors and capacitors and play around to your liking. You should use metal film resistors and polypropylene caps as the best for low noise. Stop obsessing and have fun and experiment.

This is all useful info. Thanks. And at the right time, as I'm compiling the components through DigiKey.

I'm ordering 1% resistors. 1/4 W should be OK, right?

[MasterT]

...I discovered an error in R5 value, it has to be 22 k. Definitely not a 28k...

Good catch...!

I think this is one of the things they addressed in rev.3 of this schematic. They changed that R5 to a value of 22.1k. I believe you were probably still looking at the rev.2 schematic (which is the first one I posed) when you caught that error. In one of my subsequent posts I posted rev.3, which I discovered at a later point.

What I was trying to say, that even R5 = 22k value is not correct. Mathematically R5 + Zout = 22k, though to estimate R5 correctly you need to know an output impedance of the first amplifier. Roughly it's about 1.5k, you may try to simulate its in CAD or find an online some-kind calculator. Though R5 = 22 - 1.5 = 20.5k .  Point is you don't need a C3, and 1% tolerance in audio circuitry,  in most circumstances 10% is o'k. I also would change 221k - R10 (Rev. 3) to 1-2 MOHm, since it's loading RC filter.

[floobydust]

I think R10/C4 set a LF corner 16Hz, probably for rumble or warped LP's.
R10 needs to be low for low noise, and needs to be high to minimize loading on the passive EQ.

[TimFox]

Magnetic cartridges are velocity-responding, and the normal RIAA preamplifier equalization as used in the OP's circuit is designed accordingly.  Piezoelectric cartridges (crystal or ceramic) are displacement-responding, and need a different equalization circuit.
Magnetic cartridges are either moving-magnet or moving-coil.  The moving-magnet cartridges are more common and put out much higher voltage than the moving-coil units.  47 k is the usual terminating resistor for moving-magnet units, and the Shure cartridges work better into their specified capacitance in parallel with that resistor (better frequency response).  Moving-coil units usually use a lower load resistance, either through a step-up transformer or into a special low-voltage-noise preamplifier (often with paralleled bipolar transistors).

[Frankentronics]

Thank you all, once again, for taking the time to share your expertise.

I also discovered that those JFETs 2SK170 must be obsolete and I only found them on Amazon. Hopefully they are not fakes.

Thanks...

[Brumby]

I'm ordering 1% resistors. 1/4 W should be OK, right?

Perfectly.

In fact, the 1/4 W rating is going to be far more useful in handling (component size) than any power dissipation requirement in that circuit.