What do you think about these RIAA parts value ?

[legarem]

Hi

I recently got a phono stage I plan to upgrade. I checked the RIAA values of the parts and they seems to be far away from ideal values.
When doing RIAA calculation, to determine R!, should I take the value of R1 (270K) in parallel with R3 (347K) ?

I looked at many RIAA circuits and it seems most of them don't respect the ideal values for correct RIAA.

Any opinion how to correct the parts value of this phono stage ?

Thanks

 

[rayma]

When doing RIAA calculation, to determine R!, should I take the value of R1 (270K) in parallel with R3 (347K) ?

Yes, that's right, use the Thevenin equivalent value. This circuit does lose 5dB of gain.
It would be better to have R3 before R1 to avoid that attenuation.
Or instead, make R1 smaller and R3 larger, so the parallel combination
is the same but the loss is smaller. Gate resistors may be useful, too.

http://www.mh-audio.nl/calculateRIAA.asp

Use this inverse RIAA network for testing.

http://www.hagtech.com/pdf/riaa.pdf

 

[DF96]

I looked at many RIAA circuits and it seems most of them don't respect the ideal values for correct RIAA.

Two reasons for that:
1. some people don't know how to calculate values, and can't be bothered to find out - there are plenty of resources on the web for this;
2. some people don't wish to calculate values, but instead 'tune by ear' thus ensuring that they get the RIAA curve wrong.

 

[legarem]

Finally, i'll go with these values. It seems it is close enough to the real RIAA curve. With the past schematic showed, I had enough gain.

 

[jackinnj]

From my Excel spreadsheet:

 

[legarem]

Hi Jackinjj

And what to do with R3 (1M) ?

In my calculations I paralleled R1 (270000) with R3 (1M) so R1 become 2125984

Am I finally right or wrong when I take R3 in my calculation ?

Thanks

 

[rayma]

I paralleled R1 (270000) with R3 (1M) so R1 become 2125984. Am I finally right or wrong when I take R3 in my calculation ?

Yes, that's necessary to get a correct value for the RIAA components.
Also, the output impedance of the first stage should first be added to R1,
before paralleling it with R3.
The input capacitance of the second stage should be added to the value of C2 as well.

 

[legarem]

What's the output impedance of the first stage of the circuit showed in my first post ? or how can I find it ?

Thanks

 

[SY]

About 2k6. The resistor shunts the much higher drain impedance.

 

[legarem]

Would it be correct to inject signal to this first stage and load it with resistors so when I get half the output signal as compared to the input, the resistor needed to get half signal at the output is the output impedance ?

 

[SY]

You already know the source impedance. If you're going to set up a measurement, use the effort to measure the RIAA curve and trim the parts to conform.

 

[jackinnj]

Looks like your values are pretty good.
I adjusted the spreadsheet to take into account Zin and Zout

 

[legarem]

Hi Jackinnj

Considering output impedance of the first stage is 2.6k and C in of the next stage is 52 pf,

Here are may calculations results.

 

[jackinnj]

Hi Jackinnj

Considering output impedance of the first stage is 2.6k and C in of the next stage is 52 pf,

Here are may calculations results.

52pF isn't even a rounding error! The values look excellent, however.

Anyone who wants the spreadsheet just let me know via PM