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.
preamp_module_3.jpg
preamp_9.jpg
preamp_only_bass&amp;treble_bw_CHA.jpg
preamp_only_bw_CHA&amp;B.jpg
preamp_only_%THD_CHA.jpg
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.
preamp_layout_R1.jpg
preamp_module_2.jpg
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
preamp_sim_1.jpg