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Old 15th February 2007, 06:50 AM   #1
6f6 is offline 6f6  United States
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Default Moving Magnet Phono Load

I just built a new phono preamp. This was Jeremy Epstein's design. The circuit can be seen at this website:
http://home.earthlink.net/~jeremyepstein/riaa.html
Overall the performance is good to me, but I have a problem with parasitic oscillation and hand capacitance. It becomes louder as I put my hand near the cartridge. Should I add say 100pf in paralell with the input resistor in the interest of reducing oscillation coming from the cartridge? (I have been hesitant to do this for fear of reducing high frequency response.)
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Old 15th February 2007, 10:26 AM   #2
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Quote:
reducing oscillation coming from the cartridge

Cartridges most definitely don't oscillate. You may indeed have an oscillation problem but it doesn't seem very likely. Increasing the grid stoppers should deal with that.

Another possibility is grounding. Do you use some kind of a star-grounding scheme? Are i/p and PS grounds separate? Choosing the point of the input ground can have a strong effect on hum.

What is the frequency of "oscillation"?
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Old 15th February 2007, 01:02 PM   #3
AMV8 is offline AMV8  United Kingdom
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Hi

I suspect the arm / headshell ground wiring.

Don
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Old 15th February 2007, 09:53 PM   #4
6f6 is offline 6f6  United States
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Here's what I did:
Foreseeeing potential hum problems, I did increase the value of the input grid stopper to 1000 ohms. Yesterday evening, I did also add 150pf in paralell with the input resistor-and got no noticeable decrease in high-frequency response, but did reduce the effect of hand capacitance when putting the needle on the record. I do have a little hum and some oscillation. My guess is that the tube itself is acting as a resonant tank of some sort. I hear about a 18khz-21khz squeal in earphone and some hum. is it possible that there is some heterodyning going on at supersonic frequency result in sum and difference frequencies which create this hum. As far as ground is concerned, the Dual 1225 that I am using is grounded to its chassis and connected to the metal chassis on which the amplifiying circuits are built. The PSU is not on the same chassis. My best guess is to try and isolate further the amplifiying circuitry by decoupling the PSU with some resistors and caps. Any other suggestions would be welcomed.
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Old 16th February 2007, 09:46 AM   #5
AMV8 is offline AMV8  United Kingdom
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Hi
From your comments I still suspect the earth conections. They can be tricky with turntables and arms and cartridges.
I would check the turntable and power supply earth conections - I assume there is a connection to mains earthfrom the turntable.

If you then have a connection to the earth on the pre amp via the cartridge wires you may have a loop. The easiest way to avoid this normally is to insert a low value resistor - say 5/10 ohms 3w in one of those earth connection circuits. I recommend trying the resistor in the cartridge earth connection. Or try temporarily removing that erth connection and see what happens.

Yiu could also have a double connection to earth via the cartridge wires if you have a connection to earth via each channel. ( That is if the both channel earths are connected at both the cartridge end and also the pre amp end ). I hope that is clear.

Don
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Old 16th February 2007, 11:10 AM   #6
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Quote:
Originally posted by 6f6
I hear about a 18khz-21khz squeal

I can hear this only in my wet dreams. You may indeed have oscillation.

To tell the truth i don't have much experience with glowing regulators but trust they don't tolerate any capacitance at all. So, further PS decoupling on the board can only be done if you use additional RC filtering after the regulators.

If i had to troubleshoot this i'd replace the regulators with an RC filter and check (preferably with a scope) if any hum/oscillation is present after the input stage. Normally PS induced oscillation is at a very low frequency so stage interaction should not be the cause.

Let me ask again: are you using star grounding?
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Old 16th February 2007, 09:25 PM   #7
6f6 is offline 6f6  United States
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I am not using a star ground system . The common negative for the amplifying circuit IS the steel chassis. Here I connected the turntable's ground wire. When the turntable's ground wire is removed I hear an awful "whiz" noise. The turntable is fine. I connected it to my B&K Pro-5 preamp and it gave me no problems. As far as the PSU is concerned, I did not use the original circuit. Instead, forseeing potential hum problems affiliated with connecting the center tap to B- (of amp circuit), I made a capacitor-input filter and then used a couple of r-c filters with resistors on both the postive and negative leg of the PSU. This was then fed to an l-c filter. Add then to the vr tubes. I apparently was overzealous with the rc filters as the vr tubes are not igniting with 270v between them when the unit is on (start-up voltage for a vr tube is 30v above rated voltage). The vr tubes are basically not part of the circuit unless I choose the decrease the value of the shunt resistor.
What leads me to believe that the oscillation is tube-generated is the fact that putting my finger on the 6sl7 tube results in "whiz". As far as the hum is concerned, any miniscule residual hum could easily be amplified with such a high-gain device as the 6sl7.
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