My AKSA 'Paris', JFET-based head amp has no input coupling cap. I've never had any issues with my Benz LP.
I thought DC offset occurred at the output of a gain stage ... not at the input?
Regards,
Andy
The question is, where the input is by your jFET based head amp.
If you have a common source topology (gate = input), you will not have any trouble and calvin is right:
In case of a present common gate topology (source = input), read post #19 and #20 about
http://www.diyaudio.com/forums/tubes-valves/10635-audio-research-mcp-33-a-2.html
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Dear Mr. Carr,
concerning the "KLEOS" from your company I need help concerning DC coupling to a head amp - i. e. without a capacitor in the signal pad.
The thread, which I started in this case, is here:
Lyra Kleos & Jean Hiraga's Le Pré-Pré (from Magazine "L'AUDIOPHILE")
Thank you very much for your advices.
You care about the current that damages the coil (smokes the insulation, or mounting adhesives), fusing is the coup-de-grace! The damage
current could be quite a bit less than fusing current.
Basically microamps is fine, most amplifier configurations only need microamps of bias - the exception (common base/common gate) do need careful consideration of DC issues, and a balanced configuration or coupling capacitor considered.
Another way to look at the issue is guess at a maximum power dissipation and figure out the current limit from that. Say your coil is 100 ohms and you guess 3mW max dissipation. That puts Imax as 5.5mA.
And there's the magnetic effect to consider too - from the coil turns and geometry you can calculate the flux density the DC current would produce - if you know the permanent field its in you can figure out the amount of distortion of the magnetic field the DC produces. 1% magnetic distortion isn't going to a big deal, 50% would be.
Of course the mechanical displacement might dominate over magnetic field disruption anyway - you'd basically need all the physical parameters of the cartridge to analyze this.
However one issue that may eclipse all the others is power-on and power-down transients - if the preamp misbehaves even briefly during these phases it might put large currents (ie several mA) into the source even if it only draws nA in normal operation. Sequencing of dual supplies might matter, for instance.
current could be quite a bit less than fusing current.
Basically microamps is fine, most amplifier configurations only need microamps of bias - the exception (common base/common gate) do need careful consideration of DC issues, and a balanced configuration or coupling capacitor considered.
Another way to look at the issue is guess at a maximum power dissipation and figure out the current limit from that. Say your coil is 100 ohms and you guess 3mW max dissipation. That puts Imax as 5.5mA.
And there's the magnetic effect to consider too - from the coil turns and geometry you can calculate the flux density the DC current would produce - if you know the permanent field its in you can figure out the amount of distortion of the magnetic field the DC produces. 1% magnetic distortion isn't going to a big deal, 50% would be.
Of course the mechanical displacement might dominate over magnetic field disruption anyway - you'd basically need all the physical parameters of the cartridge to analyze this.
However one issue that may eclipse all the others is power-on and power-down transients - if the preamp misbehaves even briefly during these phases it might put large currents (ie several mA) into the source even if it only draws nA in normal operation. Sequencing of dual supplies might matter, for instance.
Surely, a "SUT box" is no more bulky than a "head amp box"?
But they severely limit your ability to provide optimal loading to the cartridge (given the "Zin divided by the square of the turns ratio" equation).
I would suggest 5 to 30.
Andy
Most SUTs attempt to provide a 100R to 47k match, which is about 21:1 turns ratio.
A lower ratio of about 5:1 would match 100R into 2k5. An input resistance of 2k5 is an impedance where bipolar semiconductors can give an excellent noise figure, while the much smaller number of secondary turns would make the transformer far easier and cheaper to make and get better bandwidth at both ends
A lower ratio of about 5:1 would match 100R into 2k5. An input resistance of 2k5 is an impedance where bipolar semiconductors can give an excellent noise figure, while the much smaller number of secondary turns would make the transformer far easier and cheaper to make and get better bandwidth at both ends
check out question on post #2 under
https://www.diyaudio.com/forums/ana...e-pr-pr-magazine-laudiophile.html#post6663060
https://www.diyaudio.com/forums/ana...e-pr-pr-magazine-laudiophile.html#post6663060