Preamplifier board

[nre]

The two-channel, preamplifier board is complete.  This is a break-out board from the Scott LK72A preamplifier.  Plots show CHA and B bandwidth and effect of tone controls on bandwidth.  Preamplifier output is directly input into FFT analyzer input is cross correlated MLS.  The board requires a 235-250VDC input for the 12AX7 anodes and a -45VDC for the heaters.  Center-tap and chassis ground tie back to the main amplifier.  The preamp can be configured to operate as a standalone as well.

FFT analyzer input impedance 128kOhms, output impedance 660Ohms.  Input is cross correlated MLS, no load on output, tone controls flat.

Last plot show %THD v. voltage output at five test frequencies:
dark blue 50Hz
light blue 100Hz
violet 500Hz
green 1000Hz
straw 5000Hz

Note typical use in service will be <2VRMS.








     

[nre]

PC board Gerber shows top and bottom traces.  Photo shows board about same size as a Fluke.  The on-board tone controls can be relocated to an enclosure front panel.  I've designed a simple power supply module for the board to allow for it to be used as a standalone preamplifier.

A low-cost tube preamp of exceedingly high performance is the goal.



 

[nre]

LTSPICE simulation of the treble potentiometer.  Vacuum tube model of 12AX7:

*            Plate
*            | Grid
*            | | Cathode
*            | | |
.SUBCKT 12AX7 A G K
BGG  GG  0 V=V(G,K)+0.59836683
BM1  M1  0 V=(0.0017172334*(URAMP(V(A,K))+1e-10))**-0.2685074
BM2  M2  0 V=(0.84817287*(URAMP(V(GG)+URAMP(V(A,K))/88.413802)+1e-10))**1.7685074
BP    P    0 V=0.001130216*(URAMP(V(GG)+URAMP(V(A,K))/104.24031)+1e-10)**1.5
BIK  IK  0 V=U(V(GG))*V(P)+(1-U(V(GG)))*0.00071211506*V(M1)*V(M2)
BIG  IG  0 V=0.000565108*URAMP(V(G,K))**1.5*(URAMP(V(G,K))/(URAMP(V(A,K))+URAMP(V(G,K)))*1.2+0.4)
BIAK  A    K I=URAMP(V(IK,IG)-URAMP(V(IK,IG)-(0.00058141055*URAMP(V(A,K))**1.5)))+1e-10*V(A,K)
BIGK  G    K I=V(IG)
* CAPS
CGA  G    A 1.7p
CGK  G    K 1.6p
CAK  A    K 0.5p
.ENDS