Hi Al,
so, let’s look at some simulations of the RIAA stage input.
Let’s assume that a minimum capacitance has been achieved of 100pF, which as it includes the cartridge winding capacitances, the internal wiring of the pickup arm, the interconnect to the preamp, the wiring to the preamp input and the preamp input capacitance. is probably optimistically low and beyond absurd for anything that involves a SUT or a tube amp. Let’s also assume that mechanical resonances do not exist and the only thing that matters is the electrical response.
Let’s also use our ideal MC cartridge with 5mH and 5 ohms series resistance and lets start with a 47k load.
The peak is at 663kHz and the magnitude is 28dB, the 10MHz rolloff (relative to 1kHz) is 48dB, the boost at 20kHz relative to 1kHz is 70mdB.
For RFI- conducted RFI is generally c. a few kHz to 30MHz. Conducted RFI can be converted to radiated RFI in the power cords, power supplies etc. Radiated RFI is generally considered to be 30 MHz and above, except where conversion occurs. What domain are we concerned about?
I’ll choose the loss at 10MHz as a metric.
OK, lets increase the capacitance to 1000pF, which is a realistic cap based on the values originally presented as available, and see what happens.
The resonant frequency decreases to 223kHz, the magnitude drops to 26dB, but the 10MHz roll off is now 65dB! The gain at 20kHz is 68mdB.
Which of these would be more benign to RFI while keeping an acceptable audio response? I would argue the 1000pF case.
Now lets change the load R to 1k.
In the 1000pF case the peak is c. 3.5dB, and, of course, the loss at 10MHz remains the same at 65dB. The 20kHz boost has decreased to 53mdB.
In the 100pF case the peak has gone and the gain is now 4mdB and the -3dB bandwidth is about 400kHz. The 20MHz loss is 48dB.
Again, the 1000pF case is better from an RFI perspective, provided you don’t care about a 49mdB increase in RIAA error at 20kHz.
Now change the R to 100 ohms. For the 100pF case the gain at 20kHz is -1.3dB and the 20MHz loss is 51dB.
For the 1000pF case the 20MHz loss is now 66dB, but the 20kHz loss is slightly less than the 100pF case, so it appears that a bigger cap might be better, so lets try that.
Increase the cap to 10000pF. The 20kHz loss is now 0.8dB, and the 20MHz loss is 77dB!
Now increase it to 28600pF- the error at 20kHz is now 0mdB, the attenuation at 10MHz is -77dB, and the -3dB point is c. 40kHz which is generally at about the limit that the cartridge manufacturers specify.
So which of these scenarios gives the flattest 20-20kHz ELECTRICAL response AND the highest rejection at 10MHz?
And who knows, a real cartridge might actually have a slightly more ideal audio frequency response with a small loss at 10kHz and a larger one at 20kHz, depending on where the mechanical resonance lies.
By the way, I am the owner of two Miyajima Madake cartridges, one that’s approaching end of life and can’t be retipped and a second one with 12 hours of play, and I’ve been going through this exercise with them both with a SUT/tube amp combo and the AD797 based preamp and it’s proving very hard to reach a conclusion as to what is best...
Wyn
so, let’s look at some simulations of the RIAA stage input.
Let’s assume that a minimum capacitance has been achieved of 100pF, which as it includes the cartridge winding capacitances, the internal wiring of the pickup arm, the interconnect to the preamp, the wiring to the preamp input and the preamp input capacitance. is probably optimistically low and beyond absurd for anything that involves a SUT or a tube amp. Let’s also assume that mechanical resonances do not exist and the only thing that matters is the electrical response.
Let’s also use our ideal MC cartridge with 5mH and 5 ohms series resistance and lets start with a 47k load.
The peak is at 663kHz and the magnitude is 28dB, the 10MHz rolloff (relative to 1kHz) is 48dB, the boost at 20kHz relative to 1kHz is 70mdB.
For RFI- conducted RFI is generally c. a few kHz to 30MHz. Conducted RFI can be converted to radiated RFI in the power cords, power supplies etc. Radiated RFI is generally considered to be 30 MHz and above, except where conversion occurs. What domain are we concerned about?
I’ll choose the loss at 10MHz as a metric.
OK, lets increase the capacitance to 1000pF, which is a realistic cap based on the values originally presented as available, and see what happens.
The resonant frequency decreases to 223kHz, the magnitude drops to 26dB, but the 10MHz roll off is now 65dB! The gain at 20kHz is 68mdB.
Which of these would be more benign to RFI while keeping an acceptable audio response? I would argue the 1000pF case.
Now lets change the load R to 1k.
In the 1000pF case the peak is c. 3.5dB, and, of course, the loss at 10MHz remains the same at 65dB. The 20kHz boost has decreased to 53mdB.
In the 100pF case the peak has gone and the gain is now 4mdB and the -3dB bandwidth is about 400kHz. The 20MHz loss is 48dB.
Again, the 1000pF case is better from an RFI perspective, provided you don’t care about a 49mdB increase in RIAA error at 20kHz.
Now change the R to 100 ohms. For the 100pF case the gain at 20kHz is -1.3dB and the 20MHz loss is 51dB.
For the 1000pF case the 20MHz loss is now 66dB, but the 20kHz loss is slightly less than the 100pF case, so it appears that a bigger cap might be better, so lets try that.
Increase the cap to 10000pF. The 20kHz loss is now 0.8dB, and the 20MHz loss is 77dB!
Now increase it to 28600pF- the error at 20kHz is now 0mdB, the attenuation at 10MHz is -77dB, and the -3dB point is c. 40kHz which is generally at about the limit that the cartridge manufacturers specify.
So which of these scenarios gives the flattest 20-20kHz ELECTRICAL response AND the highest rejection at 10MHz?
And who knows, a real cartridge might actually have a slightly more ideal audio frequency response with a small loss at 10kHz and a larger one at 20kHz, depending on where the mechanical resonance lies.
By the way, I am the owner of two Miyajima Madake cartridges, one that’s approaching end of life and can’t be retipped and a second one with 12 hours of play, and I’ve been going through this exercise with them both with a SUT/tube amp combo and the AD797 based preamp and it’s proving very hard to reach a conclusion as to what is best...
Wyn