FET Audio | Hi-End Audio & Electronics DIY Electronics & Repair

This is a Borbely Erno design SE all jfet line amp in one of his article.  I have been using this as a pre-amp since 2006 with Pass power amplifiers like Aleph 60,  F4 & F5.  The gain of this pre-amp is quite high and thus it can be used to drive F4 to sufficient loudness.

The distortion level is also very low at around 0.003% at 1V and 0.026% at 4V output at balance mode.  I am only using +28V dc due to transformer only at 25V x 2 AC.

This pre-amp sound very smooth due to no global negative feedback!  But you see the distortion will increase quite fast as the output level increase!

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Last update: 20th Sept, 2010: This is the pre-amp that I am still using at this moment as I like the sound of NO Feedback!!

Borbely Erno All JFET SE line amp Details Updated Sept 21, 2010

Wiring diagram

Was completed 2006/9/16, update FFT measurement on Oct 19, 2009.

FFT jfet SE Bal 1V

FFT jfet SE Bal 4V

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This preamp is same as the All Jfet Buffer Line Amp except it is only for Single Ended input and output (RCA input/output sockets).  An old Alps volume ( 100K type A) is used at the input instead of step attenuator.

From the picture, you can see there is one single 25V x 2 30VA transformer and two Borbely low noise regulators and two buffer modules are used.  It is very simple but it sound also wonderful when driving a F5.

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The FFT measurement show that the hum noise is very small and also distortion is very good at 0.0035% at 1Vrms output level.  At 10Vrms output, the distortion rises to about 0.009%.  Hum and noise is at about 1.2mVrms at maximum volume.

FFT output at 1V, 12k load lin

FFT output at 1V, 12k load log

FFT output at 10V 12k lin

FFT output at 10V 12k log

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This is a clone kit from China.  It is a class AB amp and the clone is only for the power amp part.  The measured result shows a very good design amplifier!

1. Gain: 18x
2. Supply Voltage +/-50V; with 36V x 2 AC transformer
3. Distortion at 1W 8.2ohm: 0.0044% 30kBW
4. Current drain at idle: 160/153mA.
5. Current drain at 1W output: 260/253mA
6. Op-amp: 4.5mA
7. 470 ohm resistor: 1.75mA
8. 33 ohm resistor: 24mA
9. Output offset: 0.8mV
10. Current diode is 2mA 100V, metal can

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1W 8 ohm fft with hp ps

50W 8 ohm fft with 500va

MBL distortion vs input level

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This is a photo type for a Ipod amplifier.  It use 6DJ8 with supply at about +44V as input buffer and then drive a LM1876 IC power amplifier.

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Circuit

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This is another Borbely Erno design op-amp using all jfets.  This is the most complicated one with high gain and thus a servo is required to maintain the output at near 0Vdc.  I have tried this op-amp but I have not make it up to the best sound.  As of 2009, I think the all jfet buffer line amp sound the best.

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All Jfet op-amp Circuit

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