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Old 7th August 2010, 08:41 AM   #11
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That right, there should be a complex network of Rs and Cs in place of R2 in your circuit, which give the 3180 & 75us pols and the 318us zero.
R1 is likely to have an electrolytic in series to reduce dc gain to unity.

If R1 is too big, you would expect overall low gain and the hf boost you are getting
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Old 8th August 2010, 12:59 PM   #12
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I can see the usual values present. R110 is 91K, should be paralleled by the 33nF,
R111 code 85B=7.5K should be paralleled by the 10nF, R107 470R should be in series with 100uF (this is R1 in your diagram). This is the standard Nat Semi app note from long ago.
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Old 8th August 2010, 01:46 PM   #13
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Thanks!

I need to get better equipment (multimeter to measure capacitence values) to get those checked.

Value of R107 (if it's part of the R1 circuit) isn't very big so, if there's a 100μF condencator in series with it, deoes it mean that it's not root for the issue or, do I just have to make it less?

Are those caps C62/C63 and C69/C70 (picture #2) part of the RIAA circuit since, If I see it right, either R118/R111 or R117/R110 are connected with them?


Juha
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Old 9th August 2010, 02:31 PM   #14
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Correct, C62,63,69&70 are the RIAA filter caps.
I can see some 47uFs by the switch which could be the electrolytics in series with the 470Rs. I was guessing 100uF, but 47uF is big enough
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Old 10th March 2014, 12:44 PM   #15
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Return to the topic...

OK, I did a LTSpice schematic for the RIAA stage of DuaFire.

Click the image to open in full size.

Measured those R's and C's David mentioned and those were OK. As picture shows, in theory, removing the R112 makes curve look quite good but, in practice, it didin't lead to situation which simulation shows (so I quess there must be more components involved).

Wondering, where to look next?
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Old 10th March 2014, 02:10 PM   #16
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With R112 150R , your circuit still simulates to have way too much gain below 100Hz. Changing R110 from 91 k to 51k gets the curve a lot closer to the RIAA
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Old 10th March 2014, 06:27 PM   #17
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I ran a simulation of the circuit myself, but with an inverse RIAA input so the response cure ideally would be a straight line. The circuit as it is given is anything but a correct RIAA response. Not sure what the 15K resistor is doing in this circuit; nearly all single op amp phono stage circuits I have seen do not have a resistor in series with the RIAA network in the feedback arm. Secondly, the values for the RIAA network are a bit off. I recalculated the RIAA network values, got rid of the 15K resistor, and added a low-pass filter on the output to cancel the unwanted zero in the response due to op amp gain being unable to fall below unity in a series feedback circuit such as this. Lastly, the 47F capacitor has been changed to 470F to reduce the amount of distortion that would otherwise be present at low frequencies. Better to perform subsonic filtering with a separate stage designed for the purpose than to rely on a wide tolerance and potentially high-distorting electrolytic capacitor. The simulation results and .asc files are attached.
Attached Images
File Type: jpg riaaproblem.jpg (175.9 KB, 51 views)
File Type: jpg riaafixed.jpg (174.4 KB, 48 views)
Attached Files
File Type: asc problemRIAA.asc (2.1 KB, 1 views)
File Type: asc problemRIAAfixed.asc (2.2 KB, 3 views)
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Old 10th March 2014, 07:59 PM   #18
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Quote:
Originally Posted by loepke72 View Post
...
Not sure what the 15K resistor is doing in this circuit; nearly all single op amp phono stage circuits I have seen do not have a resistor in series with the RIAA network in the feedback arm. Secondly, the values for the RIAA network are a bit off.
...
I recalculated the RIAA network values, got rid of the 15K resistor, and added a low-pass filter on the output to cancel the unwanted zero in the response due to op amp gain being unable to fall below unity in a series feedback circuit such as this.
...
Thanks!

I noticed that the C1 and C2 are little off in my linked schematic but, as the edit button isn't active after a while here... .

Could it be possible that the 15K resistor is just totally off as mentioned earlier? About the same time when I started this thread I took contact to ESI to get some schematics for the RIAA stage but they just send me specs for the phono amplier.

By following David's suggestion, R112 changed to 150R gives better responce in practice as well but ... still not enough low frequencies and peaks start to clip a bit. When totally remove the R112 then the gain is just too high.

I'll check if your suggestion helps in this.
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Old 13th March 2014, 10:13 PM   #19
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Is there any limit for load capacitance value?
By simulation (if I have done everything right), original RIAA circuit could be left untouched by just dropping the input capacitance down to 33pF (original value is 150pF and both cardridges I use say it should be less than 200pF). Would this be wrong way to go? If the schematic/simulation is done wrong, could you assist ...

Click the image to open in full size.

Cartridge specs:
DC resistance 500ohm
Inductance 240mH
Impedance 1.6kohm 1kHz
Recommended load 47kohm
Recommended load capacitance <200pF
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Old 13th March 2014, 11:34 PM   #20
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Simulating cartridge loading does not work well as the inductance is nothing like a fixed value.
You are correct that changing C3 from 200pF to a 33 or 47pF should improve matters. With 200pF plus the cable capacitance you will get a high treble excess.
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