Thursday, May 05, 2016

B1 Buffer Preamp – Audiophile Pre amp - Schematic diagram – Part details – Circuit description - DIY Audio

Category: DIY Audio 

Contents of this article 

  • B1 Schematic diagram 
  • Circuit description 
  • Substitute parts detailes  

 B1 Buffer Preamp

The sound of B1 Buffer Preamp extremely neutral without being clinical - just about what you were looking for when you were thinking about a passive preamp. A teensy bit of second harmonic and no noise at all. There’s just enough gain. If you were using any other power amp, you’d get 8 to 18 dB more gain, and would be able to break your lease or the speaker, or both.
There are two channels shown with a common power supply. Supply parts in common are numbered from 1 to 99. Parts in the right channel are 100 to 199, and the left channel is 200 to 299. With the exception of R1, all the resistors are ¼ watt –  used RN55D types, but you can use whatever you like. C1 and C2 are big electrolytic types, with a nominal rating of 15000 uF at 25 Volts. C100, 200, 101 and 201 are high quality film capacitors. You can use these values or substitute in your favorites.
For C1 and C2 used Digikey P6890-ND. The value is not critical, and you can use as low as 1,000 uF at 25V. C3, C100 AND C200 are 1 uF metallized polypropylene film capacitors (Digikey BC2076-ND) C101 and C201 are 10 uF polyester film capacitors (used Axon 10 uF metalized polypropylene from Orca Design). Feel free to use any comparable types. D1 is a generic 1N914 type diode, and D2 is a generic LED for indicating power the board.
All of the transistors are N channel JFETs. The stock parts are 2SK170’s, LSK170’s or 2SK370’s, and you can use substitutes having Idss between than 5 and 10 milli-Amps and transconductance numbers from 5 to 30 milliSiemens. The potentiometers are linear taper at 25 Kohm, but again you can easily use higher or lower values as you like.
The buffer uses an external power supply from 18 to 24 Volts DC. You can power it with batteries, but most convenient is an external regulated supply running off the wall. The preamp typically draws fewer that 0.02 Amps, so current is not much of an issue. A regulated supply is better, but the circuit is pretty good at ignoring noise on the supply and minor fluctuations.
The design uses RCA input and output connectors, and a DPDT switch for selecting one of two inputs. You are, of course, free to use a switch with many more inputs.
The input switch selects one of the two input signals, routing it to the top (clockwise position) of potentiometers P100 or P200. A divided input signal appears on the wiper. This signal goes to the Gates of Q100 and Q200 through a resistor R102, R202 and capacitor C100, C200. As a practical matter, the input impedance of this preamp is determined by this volume control potentiometer. A 25 K Ohm pot gives 25 K Ohm input impedance. R102 and R202 are there to prevent parasitic oscillation with the very wide bandwidth JFETs. C100 and C200 are there because the Gate of the JFETs needs to be set at ½ the DC voltage of the power supply – a voltage delivered to the buffer inputs by R2, R3, and C2 through R103 or R203.
D1 performs the service of drawing down this DC voltage with the power supply when the power is turned off, otherwise C2 may hold a charge for a long enough time to give you a turn-on thump when powered back on. By the way, the time constant of R2, R3, and C2 are long enough that it takes a minute or two for the circuit to reach normal operating values, so don’t get excited if there’s no sound for a few seconds when you turn it on. R1 and C1 filter noise coming from the external supply. Q100 and Q200 are JFETs operated as follower transistors. The Source pins of these transistors follow the voltage at the Gates. The input impedance of the Gate is exceedingly high – many millions of ohms, and the output impedance at the Source pin is about 50 ohms. Q101 and Q201 are constant current sources formed by simply attaching the Gate pins of the JFETs to the Drain pins. They provide without loading them down or creating significant distortion.
The best performance generally comes from matching the Idss of Q100 and Q101, and also Q200 and Q201. The Idss is simply the current that flows through the JFET when the Gate and Source are grounded and +10 volts or so is applied to the Drain. Often when you buy JFETs you can get them in Idss grades.
The Source DC voltage of the JFETs Q100 and Q200 is about the same as the Gate DC voltage (1/2 the supply voltage), and the output from the Source needs to have the DC voltage removed by C101 or C201, leaving the AC output signal, which passes through another safety resistor R104 or R204. Resistors R100, R200, R105, R205 are there to prevent the various potential thumps from switching inputs and turn-on transients.
The circuit of Figure has quite good performance specifications. Figure  shows the distortion at 1KHz. Below a volt, the distortion comes in at about .0007%, and about the time your amp is clipping, it measures about .003%
Figure  shows the full schematic of the B1 buffered passive preamp.