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Old 14th December 2013, 09:31 PM   #1
BigE is offline BigE  Canada
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Default Simply sub Z for R in electrical equations?

HI!

Does anyone know if the standard forumlas here apply if all resistances are simply replaced with impedances?

http://www.ti.com/ww/en/bobpease/assets/AN-31.pdf

I'm trying to get a grip on how the Peter Daniel phono section works, and am trying to put a spreadsheet together that would calculate the gain at various frequencies.

The calculation for the differential amp portion is a bear when using complex numbers. I keep messing it up.... I hope I am not barking up the wrong tree!

Thanks for your help!
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Old 14th December 2013, 11:31 PM   #2
Mr Evil is offline Mr Evil  United Kingdom
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If you don't care about phase, then you can use the magnitude of the impedance instead of resistance in those gain equations. It would be much easier to just use a simulator though...
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Old 15th December 2013, 12:27 AM   #3
BigE is offline BigE  Canada
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Thanks! It would be more accurate though to use the actual complex number..... It's a bit of a chore, as I have not done this in some time, but it is coming along.

I have checked all the parallel impedances against online calculators, and they work out....

Only the final differential remains. That and checking against RIAA.

I'm thinking that once the gains are finished, to just to use 20*log(Vout/Vin) and check against the RIAA standards.

The Peter Daniel phono section is a simple I to V convertor feeding a difference amplifier, with the same voltages applied to both legs of the op amp. However, both legs have complex impedances.
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Old 15th December 2013, 08:31 PM   #4
BigE is offline BigE  Canada
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I can't seem to get this close to RIAA. Sure there is more gain in the bass region and less in the treble, but it's not a big enough spread.

The calculations are based on these two formulas:

1) Stage 1: Vout = -Zf * Iin. Where Iin = Vcart/Rcart, and Zf is R1||C1 from the phono board schematic. (R1=300 Ohm, C1 = 0,047 uF)

2) Vout = (Z1+Z2)/(Z3+Z4) * (Z4/Z1) V1 - (Z2/Z1)*V2

From: http://www.ti.com/ww/en/bobpease/assets/AN-31.pdf

Since V1=V2=Vin and Z1=Z3 (=300 ohms), formula 2) reduces to:

3) Vout = (Z4-Z2)/(Z1+Z4)*Vin

Where Z1 = R1 = 300 Ohms, Z2 = R4 || (R5 + C2) (R4 = 330K, R5=33k, C2=0.01 uF) and Z4 = R6||C3 (R6 = 330K, C3 = 10uF).

All impedances have been verified on this online calculator:

Impedance


The final equation is a combination of 1) and 3) by subbing Vin = -Zf*Iin = -Zf*Vcart/Rcart into equation 3.

I'm getting no love at all from these equations....

Any obvious errors? I mean this *is* the right approach isn't it? Thanks for reading! Any help would be very appreciated -- I am about to give up!

Last edited by BigE; 15th December 2013 at 08:34 PM.
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Old 15th December 2013, 10:22 PM   #5
BigE is offline BigE  Canada
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FOUND IT! Error in parameter entry for a capacitor. Put in 0.1 instead of 0.01. It's very good now.
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