My Borbely All-Fet Preamp with Subbu DAC

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SB it seems a shame to AC couple the Borbely preamp through an electrolytic cap. The Borbely system I run is totally DC coupled all fet/jfet from dac or phono input to power amp output. The power amp uses Borbely’s dc protect / mute delay board for DC protection - the protection board design is in the Borbely DC 102 article. If you want to protect the preamp output from DC output and avoid any capacitor coupling you can use the DC 102 protect board and make it more sensitive by adjusting the reference voltage setting resistors to maybe 10 to 50mv. It also gives you a mute delay for turn on.

Don’t increase the value of any of the power suppy Electrolytic capacitors on the DC 102 DC protect board like any diyaudio guy would normally do. The cap values are delibereratly small so it turns off (mutes) right away when the preamp is turned off or the AC power fails. The input to the protect board can be a 10uf /63 volt polyester instead of the back to back 22uf/40 volt electrolytics.

Updates to the preamp design are to use OPA 140 or AD 4625 op amps for the servo. They have 1/5 or less than the noise of the AD 820. Replace the 33K2 current source resistor for the VAS cascode with a E102 jfet current source diode. Replace the VAS cascode’s LM385 voltage references with 2 series connected green LEDs ( not blue ones like Erno first used, they are too noisey).

AD 4625 op amps exist?
 
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