Stumbled on Randall G3 power amp schematic, found EQ inside, simulated it

[Kaz Kylheku]

Hi all,

I was looking at some power amp schematics recently and stumbled on the Randall G3 series schematic.

I noticed that instead of current feedback, it has a fixed, four band equalizer (five, but with the fifth band indicated in the schematic with unpopulated parts).

The first and third bands of the EQ try to simulate the familiar speaker-impedance curve EQ we see when the output impedance of the amp is raised. The choice of resonant peak is a bit high at around 125 Hz; not where I would want it. The mids scoop at around 1.3 kHz.

The second and fourth bands create two closely-spaced narrow cuts at around 190 and 220 Hz. Like they are trying to cut some cabinet or room resonances, or just trying ot balance the resonant peak or something.

Well, there you go.

I will put the .asc file in a reply post.

[Kaz Kylheku]

.asc file:


Version 4
SHEET 1 1768 1296
WIRE 544 32 384 32
WIRE 592 32 544 32
WIRE 752 32 672 32
WIRE 800 32 752 32
WIRE 960 32 880 32
WIRE 1072 32 1024 32
WIRE 1216 32 1152 32
WIRE 1264 32 1216 32
WIRE 1392 32 1344 32
WIRE 1392 64 1392 32
WIRE 384 112 384 32
WIRE 944 128 800 128
WIRE 1216 128 1216 32
WIRE 1216 128 976 128
WIRE 1456 128 1376 128
WIRE 1632 128 1536 128
WIRE 544 144 544 32
WIRE 608 144 544 144
WIRE 752 144 752 32
WIRE 752 144 672 144
WIRE 944 144 944 128
WIRE 976 144 976 128
WIRE 1280 208 1008 208
WIRE 1376 208 1376 128
WIRE 1376 208 1344 208
WIRE 1408 208 1376 208
WIRE 1488 208 1472 208
WIRE 1536 208 1488 208
WIRE 1632 224 1632 128
WIRE 1632 224 1600 224
WIRE 384 240 384 192
WIRE 1488 240 1488 208
WIRE 1536 240 1520 240
WIRE 800 256 800 128
WIRE 848 256 800 256
WIRE 960 256 960 208
WIRE 960 256 928 256
WIRE 1520 288 1520 240
WIRE 1632 288 1632 224
WIRE 1632 288 1520 288
WIRE 1488 352 1488 320
WIRE 384 384 384 304
WIRE 800 384 800 256
WIRE 864 384 800 384
WIRE 1008 384 1008 208
WIRE 1008 384 944 384
WIRE 1072 384 1008 384
WIRE 1216 384 1216 128
WIRE 1216 384 1152 384
WIRE 544 416 544 144
WIRE 624 416 544 416
WIRE 752 432 752 144
WIRE 752 432 688 432
WIRE 1456 432 1376 432
WIRE 1632 432 1536 432
WIRE 624 448 384 448
WIRE 384 496 384 448
WIRE 1280 512 1008 512
WIRE 1376 512 1376 432
WIRE 1376 512 1344 512
WIRE 1408 512 1376 512
WIRE 1488 512 1472 512
WIRE 1536 512 1488 512
WIRE 1632 528 1632 432
WIRE 1632 528 1600 528
WIRE 1488 544 1488 512
WIRE 1536 544 1520 544
WIRE 1520 592 1520 544
WIRE 1632 592 1632 528
WIRE 1632 592 1520 592
WIRE 800 608 800 384
WIRE 864 608 800 608
WIRE 1008 608 1008 512
WIRE 1008 608 944 608
WIRE 1072 608 1008 608
WIRE 1216 608 1216 384
WIRE 1216 608 1152 608
WIRE 384 624 384 576
WIRE 1488 656 1488 624
WIRE 1456 736 1376 736
WIRE 1632 736 1536 736
WIRE 1280 816 992 816
WIRE 1376 816 1376 736
WIRE 1376 816 1344 816
WIRE 1408 816 1376 816
WIRE 1488 816 1472 816
WIRE 1536 816 1488 816
WIRE 1632 832 1632 736
WIRE 1632 832 1600 832
WIRE 1488 848 1488 816
WIRE 1536 848 1520 848
WIRE 1520 896 1520 848
WIRE 1632 896 1632 832
WIRE 1632 896 1520 896
WIRE 800 912 800 608
WIRE 864 912 800 912
WIRE 992 912 992 816
WIRE 992 912 944 912
WIRE 1072 912 992 912
WIRE 1216 912 1216 608
WIRE 1216 912 1152 912
WIRE 1488 960 1488 928
WIRE 1456 1040 1376 1040
WIRE 1632 1040 1536 1040
WIRE 1280 1120 992 1120
WIRE 1376 1120 1376 1040
WIRE 1376 1120 1344 1120
WIRE 1408 1120 1376 1120
WIRE 1488 1120 1472 1120
WIRE 1536 1120 1488 1120
WIRE 1632 1136 1632 1040
WIRE 1632 1136 1600 1136
WIRE 1488 1152 1488 1120
WIRE 1536 1152 1520 1152
WIRE 1520 1200 1520 1152
WIRE 1632 1200 1632 1136
WIRE 1632 1200 1520 1200
WIRE 800 1216 800 912
WIRE 864 1216 800 1216
WIRE 992 1216 992 1120
WIRE 992 1216 944 1216
WIRE 1072 1216 992 1216
WIRE 1216 1216 1216 912
WIRE 1216 1216 1152 1216
WIRE 1488 1264 1488 1232
FLAG 384 624 0
FLAG 384 384 0
FLAG 1392 64 0
FLAG 1488 352 0
FLAG 1488 656 0
FLAG 1488 960 0
FLAG 1488 1264 0
FLAG 960 256 OUT
SYMBOL voltage 384 480 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC 1
SYMATTR InstName V1
SYMATTR Value 1V
SYMBOL OpAmps\\opamp 656 368 R0
SYMATTR InstName U1
SYMBOL res 688 16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R0
SYMATTR Value 22k
SYMBOL res 400 208 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R1
SYMATTR Value 4.7k
SYMBOL cap 672 128 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 47p
SYMBOL cap 400 304 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C2
SYMATTR Value 0.33µ
SYMBOL res 896 16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 100
SYMBOL cap 1024 16 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value 22µ
SYMBOL res 1168 16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 2.7k
SYMBOL res 1360 16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 100k
SYMBOL OpAmps\\opamp 896 176 M90
SYMATTR InstName U2
SYMBOL res 960 368 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 9.1k
SYMBOL res 1168 368 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R6
SYMATTR Value 820
SYMBOL res 960 592 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 10k
SYMBOL res 1168 592 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R8
SYMATTR Value 100
SYMBOL res 960 896 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R9
SYMATTR Value 100
SYMBOL res 1168 896 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R10
SYMATTR Value 10k
SYMBOL res 960 1200 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R11
SYMATTR Value 10k
SYMBOL res 1168 1200 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R12
SYMATTR Value 100
SYMBOL res 944 240 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R13
SYMATTR Value 2.7k
SYMBOL OpAmps\\opamp 1568 288 M180
SYMATTR InstName U3
SYMBOL cap 1344 192 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value .47µ
SYMBOL cap 1472 192 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C5
SYMATTR Value 4.7n
SYMBOL res 1504 224 M0
SYMATTR InstName R14
SYMATTR Value 15k
SYMBOL res 1552 112 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R15
SYMATTR Value 470
SYMBOL OpAmps\\opamp 1568 592 M180
SYMATTR InstName U4
SYMBOL cap 1344 496 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C6
SYMATTR Value .12µ
SYMBOL cap 1472 496 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C7
SYMATTR Value 82n
SYMBOL res 1504 528 M0
SYMATTR InstName R16
SYMATTR Value 110k
SYMBOL res 1552 416 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R17
SYMATTR Value 470
SYMBOL OpAmps\\opamp 1568 896 M180
SYMATTR InstName U5
SYMBOL cap 1344 800 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C8
SYMATTR Value .47µ
SYMBOL cap 1472 800 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value .15µ
SYMBOL res 1504 832 M0
SYMATTR InstName R18
SYMATTR Value 51k
SYMBOL res 1552 720 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R19
SYMATTR Value 470
SYMBOL OpAmps\\opamp 1568 1200 M180
SYMATTR InstName U6
SYMBOL cap 1344 1104 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C10
SYMATTR Value .12µ
SYMBOL cap 1472 1104 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C11
SYMATTR Value 82n
SYMBOL res 1504 1136 M0
SYMATTR InstName R20
SYMATTR Value 150k
SYMBOL res 1552 1024 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R21
SYMATTR Value 470
TEXT 488 512 Left 2 !.lib opamp.sub
TEXT 488 544 Left 2 !.ac dec 100 10 20000