DIY phono preamp - cheap and simple! - Page 11

rayfutrell · 11th January 2005, 07:37 PM

This Shure cartridge has an inductance of 720 mH. 450 pF of shunt capacity is way too high. You will have a big electrical resonant peak around 10kHz. The maximum shunt capacity for the Shure V15 (about 500 mH inductance) is around 120pF so the maximum shunt capacity for this cartridge should be around 100pF.

Regards,
Ray

DJNUBZ · 11th January 2005, 07:40 PM

why does the site tell me

"Recommended Load
- 47 kilohms in parallel with 450pf"

then? I don't understand why they would tell me the wrong thing.

rayfutrell · 11th January 2005, 08:31 PM

I don't know why Shure tells people that. If you model the cartridge as a low pass filter with that termination you will find the frequency response has a peak on the high end and the cutoff frequency is well below 20 kHz.

For example, Shure recommended a 250 pF termination for the V-15. It should be about 120 pF for a maximally flat frequency response and for a maximally flat phase response the value should be about 60 pF. That is the total capacity in the cartridge, tonearm wireing, interconnects and preamp. That low a capacity is difficult to obtain.

Properly terminating a high inductance cartridge is difficult because they require very low capacitance or you will have a resonant peak in the frequency response and the bandwidth will be less than 20 kHz.

Regards,
Ray

matjans · 11th January 2005, 09:16 PM

q: how do you calculate the optimum capacity/impedance for termination of different cartridges?

rayfutrell · 12th January 2005, 05:04 PM

You use the formula Q=R*SQRT(C/L) where SQRT means to take the square root. This is the formula for the resonant Q of a parallel resonant circuit. R is the terminating resistance, typically 47K, L is the cartridge inductance and C is the terminating capacitance. The Q should be between 0.5 and 0.7. A Q of 0.5 gives a maximally flat (no peak) phase response and a Q of 0.7 gives a maximally flat frequency response. At these low values of Q the bandwidth is approximatly the resonant frequency of thegiven by F=1/(2*Pi*SQRT(L*C)).

These formulas are approximations, the mathematically exact formulas for the Q and bandwidth are more complicated.

Regards,
Ray

maxw · 13th January 2005, 02:36 AM

Thanks rjm for the design, I have finished mine

Unfortunetly I wont be able to use mine for a while yet as I have to unpack my TT from storage but it will be used soon.

maxw · 13th January 2005, 02:39 AM

Here are my eagle PCB files just in case anyone in the future wants to use my design.

ebijma · 13th January 2005, 07:38 AM

Very nice pcb design maxw.

Hope it sounds as good as mine.
Takes some time to run in.
Look forward to your listning tests.

ran_ph · 19th January 2005, 06:48 AM

1. Has anybody tried using silvered mica capacitors for the RIAA feedback loop? These are available in 0.5% tolerance at Farnell.

2. Is the 4.7 uF Blackgate non-polar output cap an arbitrarily chosen value? Only 10uF is available in our place and I'm wondering if this will suffice.

ebijma · 19th January 2005, 03:07 PM

Quote:

Originally posted by ran_ph
1. Has anybody tried using silvered mica capacitors for the RIAA feedback loop? These are available in 0.5% tolerance at Farnell.

2. Is the 4.7 uF Blackgate non-polar output cap an arbitrarily chosen value? Only 10uF is available in our place and I'm wondering if this will suffice.

You can get them at the following link

http://www.acoustic-dimension.com

Hope this helps.

11th January 2005, 07:40 PM	#102
DJNUBZ diyAudio Member Join Date: May 2003 Location: Chicago	why does the site tell me "Recommended Load - 47 kilohms in parallel with 450pf" then? I don't understand why they would tell me the wrong thing. __________________ http://www.caraudioconnoisseur.com/

11th January 2005, 09:16 PM	#104
matjans diyAudio Member Join Date: Aug 2003 Location: Utrecht, NL	q: how do you calculate the optimum capacity/impedance for termination of different cartridges? __________________ Si fractum non sit, noli id reficere!

13th January 2005, 07:38 AM	#108
ebijma diyAudio Member Join Date: Mar 2004 Location: Huizen	Very nice Very nice pcb design maxw. Hope it sounds as good as mine. Takes some time to run in. Look forward to your listning tests. __________________ Music is what i want, music is what i need

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11th January 2005, 07:37 PM	#101
rayfutrell diyAudio Member Join Date: Sep 2004 Location: virginia	This Shure cartridge has an inductance of 720 mH. 450 pF of shunt capacity is way too high. You will have a big electrical resonant peak around 10kHz. The maximum shunt capacity for the Shure V15 (about 500 mH inductance) is around 120pF so the maximum shunt capacity for this cartridge should be around 100pF. Regards, Ray

11th January 2005, 08:31 PM	#103
rayfutrell diyAudio Member Join Date: Sep 2004 Location: virginia	I don't know why Shure tells people that. If you model the cartridge as a low pass filter with that termination you will find the frequency response has a peak on the high end and the cutoff frequency is well below 20 kHz. For example, Shure recommended a 250 pF termination for the V-15. It should be about 120 pF for a maximally flat frequency response and for a maximally flat phase response the value should be about 60 pF. That is the total capacity in the cartridge, tonearm wireing, interconnects and preamp. That low a capacity is difficult to obtain. Properly terminating a high inductance cartridge is difficult because they require very low capacitance or you will have a resonant peak in the frequency response and the bandwidth will be less than 20 kHz. Regards, Ray

12th January 2005, 05:04 PM	#105
rayfutrell diyAudio Member Join Date: Sep 2004 Location: virginia	You use the formula Q=RSQRT(C/L) where SQRT means to take the square root. This is the formula for the resonant Q of a parallel resonant circuit. R is the terminating resistance, typically 47K, L is the cartridge inductance and C is the terminating capacitance. The Q should be between 0.5 and 0.7. A Q of 0.5 gives a maximally flat (no peak) phase response and a Q of 0.7 gives a maximally flat frequency response. At these low values of Q the bandwidth is approximatly the resonant frequency of thegiven by F=1/(2PiSQRT(LC)). These formulas are approximations, the mathematically exact formulas for the Q and bandwidth are more complicated. Regards, Ray

13th January 2005, 02:36 AM	#106
maxw diyAudio Member Join Date: Jun 2004 Location: London	Thanks rjm for the design, I have finished mine Unfortunetly I wont be able to use mine for a while yet as I have to unpack my TT from storage but it will be used soon.

19th January 2005, 06:48 AM	#109
ran_ph diyAudio Member Join Date: Jun 2004 Location: makati	1. Has anybody tried using silvered mica capacitors for the RIAA feedback loop? These are available in 0.5% tolerance at Farnell. 2. Is the 4.7 uF Blackgate non-polar output cap an arbitrarily chosen value? Only 10uF is available in our place and I'm wondering if this will suffice.

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