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Old 26th September 2012, 06:35 PM   #1
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Default Phono cartridge loading in balanced circuit

I'm compiling a transistor balanced phono stage.
The cartridge is connected between the (+) input and the (-) input of the stage, which go to the bases of the 2 relevant transistors.
The cartridge loading resistors are connected between the bases of the transistors in the (+) input and the (-) input (they are R1 and R2) and the ground.
The cartridge needs to see a 100 Ohm load across it.
What is the necessary value of each loading resistor (R1 and R2)? I guess each resistor should be 200 Ohm, is it correct?
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Old 26th September 2012, 07:03 PM   #2
DF96 is offline DF96  England
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If I understand you correctly then the resistors should be 50 ohms each, as they are in series.
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Old 26th September 2012, 07:31 PM   #3
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Thanks.
The basic circuit is attached.
Are R1 and R2 of correct value?
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File Type: jpg Phono_1.jpg (41.3 KB, 148 views)
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Old 26th September 2012, 08:12 PM   #4
DF96 is offline DF96  England
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Yes, that will give the cartridge a 100 ohm load.
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Old 26th September 2012, 09:10 PM   #5
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Thanks.
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Old 27th September 2012, 12:55 AM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Joshua_G View Post
Thanks.
The basic circuit is attached.
Are R1 and R2 of correct value?
Hi Joshua,
The shown connection will degrade your cmrr substantially, you can use tightly matched resistors 0.1% tolerance might get you into the 50dB range, but I suspect it would be better to separate the dc bias path from the cartridge load. I'd start with something like 10K from each set of complementary input gates to ground and probably 102 ohms in shunt across the input. Not sure of the noise implications but if you are looking for good cmrr this should be (a lot) better. All of this of course presupposes closely matched fets and resistors in that input stage.
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Old 27th September 2012, 05:55 AM   #7
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Thanks, Kevin.
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Old 27th September 2012, 07:04 AM   #8
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As a compromise between CMRR and input noise, I changed the input stage as in the attached image.
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File Type: jpg Phono_2.jpg (38.1 KB, 112 views)
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Old 29th September 2012, 02:13 AM   #9
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Not sure exactly what you're doing but consider using an LM833 chip as a preamp. Read the LM833 data sheet for RIAA equalization circuit. Also consider using a single-voltage power supply but use a virtual ground to convert it to + and - voltage. To do this use the TLE2426 chip rail-to-rail virtual ground chip. This chip enables use of a single voltage supply (9V battery, for example) and get plus and minus voltage between voltage supply rails. It's precise to less than 1% difference between rails. This will eliminate any DC voltage output due to common-mode imbalance.
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