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Old 4th January 2011, 01:48 PM   #1
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Default Morgan Jones' RIAA Pre-Amp

Hi,

I wonder if anyone can assist me. I'm starting to put together the RIAA pre-amp covered in MJ's Valve Amplifier book (3rd edition) (Figure 7.27 on page 543). I have started right at the beginning by running the calculations for my particular phono cartridge and that is where I've hit a problem. The basic issue is that when calculating the frequency response of the cartridge and valve input combination I end up with a -3db point at approx 17.7kHz. This is way lower than what I want (I want it to be well above 20kHz) and I don't know if I'm doing something wrong. For example, the equivalent circuit that I'm using might be incorrect or the formulae might be wrong etc. I've attached a PDF that shows the cartridge parameters, equivalent circuit, formulae and plotted frequency response. I would really appreciate it if someone on this forum who has some spare time to look over what I'm doing and offer any advice.

Many thanks.
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Old 4th January 2011, 02:26 PM   #2
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Take a look at this link for how the "motor" is modeled:
B2 Spice and RIAA Equalization
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Old 4th January 2011, 04:04 PM   #3
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Quote:
Originally Posted by skajam66 View Post
Hi,

I wonder if anyone can assist me. I'm starting to put together the RIAA pre-amp covered in MJ's Valve Amplifier book (3rd edition) (Figure 7.27 on page 543). I have started right at the beginning by running the calculations for my particular phono cartridge and that is where I've hit a problem. The basic issue is that when calculating the frequency response of the cartridge and valve input combination I end up with a -3db point at approx 17.7kHz. This is way lower than what I want (I want it to be well above 20kHz) and I don't know if I'm doing something wrong. For example, the equivalent circuit that I'm using might be incorrect or the formulae might be wrong etc. I've attached a PDF that shows the cartridge parameters, equivalent circuit, formulae and plotted frequency response. I would really appreciate it if someone on this forum who has some spare time to look over what I'm doing and offer any advice.

Many thanks.
No, you're not doing anything wrong, and the formulae you are using appear to be correct.

The response you are seeing is simply a consequence of running a source with fairly high inductance into a lowish load resistance. The high load capacitance doesn't help either.

If you re-run your analysis using 60k and 100pF as the load elements, you will see that (up to 20kHz) a much flatter response is obtained. In practice it is not hard to achieve such values. The unanswered question is - would you want to?

The peaked response and vigorous roll-off seen with 47k/250pF loading may well be intended to complement a non-flat response from the pickup. However, it seems fair to assume that a higher-quality MM cartridges will generally have a reasonably flat internal response, and will therefore probably benefit from loading with a higher resistance and lower capacitance than the 'traditional' values. Certainly it can't hurt - and it is a simple procedure to add parallel resistors and capacitors if you subsequently want to change the pickup loading.
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