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The top 15 bands of the Buchla 296 Programmable Spectral Processor bands consist of three bandpass multiple feedback stages in series with staggered tuning.
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<div class="">I typed in the various component values and wrote a MATLAB script to plot the frequency responses of the bands, using the schematics here:</div>
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<div class=""><a href="http://fluxmonkey.com/historicBuchla/buchlaSchematics/296spectralProcessor-scheme_oct1978.pdf" class="">http://fluxmonkey.com/historicBuchla/buchlaSchematics/296spectralProcessor-scheme_oct1978.pdf</a></div>
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<div class="">I included a gain factor to 150/4.7 which I think should arise from the op amp circuits on the left side of the second page.</div>
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<div class="">For the resistor to ground with the trim pot, I reverse engineer the total resistance value needed to given the exact peak frequency. I double checked to make sure the resulting resistances are achievable with the given fixed resistor and trim
pot.</div>
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<div class="">Anyway, as you see in the attached graph, I’m getting something weird —there seems to be a downward slope, when all the plots I’ve seen people made of outputs of actual 296 show a flat response. In particular, the lowest bandpass band is waaaaay
hotter than the rest. So, I must be overlooking something. If anyone could point me in the right direction I’d greatly appreciate it.</div>
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<div class="">I’ve pasted my MATLAB script below. You can run it in Octave, which is a free open source MATLAB compatible sort of thing.</div>
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<div>Aaron Lanterman, Prof. of ECE, Georgia Tech<br class="">
My blog on Education and Innovation: <a href="https://edupocalypsenow.wordpress.com" class="">https://edupocalypsenow.wordpress.com</a></div>
<div>My blog on Electronics and Programming: <a href="https://lantertronics.blogspot.com" class="">https://lantertronics.blogspot.com</a></div>
<div>My YouTube channel on Electronics and Programming: <a href="https://www.youtube.com/c/lantertronics" class="">https://www.youtube.com/c/lantertronics</a></div>
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<div>don_fc = [105 185 138; <br class="">
210 285 245;<br class="">
310 415 335;<br class="">
450 545 490;<br class="">
580 690 530;<br class="">
730 870 790;<br class="">
920 1080 1000;<br class="">
1160 1360 1250;<br class="">
1460 1730 1600;<br class="">
1850 2200 2000;<br class="">
2360 2880 2600;<br class="">
3100 4100 3550;<br class="">
4500 6100 5200;<br class="">
6700 9700 8000;<br class="">
10600 17000 13200];<br class="">
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% capacitors <br class="">
c19_20_21_22 = 1E-6*[0.068 0.033 0.015 0.01 0.0068 0.0056 0.0047 0.0039 ...<br class="">
0.0033 0.0022 0.0015 0.0015 0.001 0.001 0.001];<br class="">
c1_2_10_11 = 1E-6*[0.047 0.022 0.01 0.0068 0.0056 0.0056 0.0047 0.0033 ...<br class="">
0.0022 0.0022 0.0015 0.0015 0.001 0.001 0.001];<br class="">
c3_4_12_13 = 1E-6*[0.18 0.068 0.033 0.022 0.015 0.01 0.01 0.0056 ...<br class="">
0.0047 0.0047 0.0022 0.0033 0.0022 0.0022 0.0022];<br class="">
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% input resistors<br class="">
r69_74 = 1E3*[91 180 180 220 240 240 220 240 240 220 220 180 150 82 56];<br class="">
r1_37 = 1E3*[15 39 75 100 120 100 100 100 100 91 91 36 36 22 8.2];<br class="">
r6_41 = 1E3*[10 33 39 75 91 100 82 120 100 68 150 44 47 24 10];<br class="">
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% feedback resistors<br class="">
r72_77 = 1E6*[0.430 0.620 1 1.2 1.5 1.5 1.5 1.5 1.2 1.5 1.8 1 1 0.680 0.330];<br class="">
r5_40 = 1E6*[0.390 0.680 1.2 1.5 1.5 1.2 1.2 1.5 1.5 1.2 1.5 0.820 0.820 0.620 0.300];<br class="">
r10_44 = 1E3*[62 130 200 200 300 330 240 390 300 220 470 180 180 120 56];<br class="">
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% ground resistors (without trim pot)<br class="">
r70_75 = [680 470 680 470 680 470 330 330 470 680 680 680 680 470 330]; <br class="">
r2_38 = [470 330 820 680 680 330 330 330 680 470 330 330 220 100 100]; <br class="">
r7_42 = [220 150 330 470 470 680 470 820 1000 820 1000 680 680 330 100];<br class="">
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q = zeros(15,3);<br class="">
wc = zeros(15,3);<br class="">
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f = 50:20000;<br class="">
s = 2*pi*j*f;<br class="">
h1 = zeros(15,length(f));<br class="">
h2 = zeros(15,length(f));<br class="">
h3 = zeros(15,length(f));<br class="">
h = zeros(15,length(f));<br class="">
<br class="">
for k = 1:15,<br class="">
c1 = c19_20_21_22(k);<br class="">
c2 = c1_2_10_11(k);<br class="">
c3 = c3_4_12_13(k);<br class="">
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ri1 = r69_74(k); ri2 = r1_37(k); ri3 = r6_41(k);<br class="">
rf1 = r72_77(k); rf2 = r5_40(k); rf3 = r10_44(k);<br class="">
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rg1 = ri1 / (rf1*ri1*(2*pi*don_fc(k,1)*c1)^2 - 1);<br class="">
rg2 = ri2 / (rf2*ri2*(2*pi*don_fc(k,2)*c2)^2 - 1);<br class="">
rg3 = ri3 / (rf3*ri3*(2*pi*don_fc(k,3)*c3)^2 - 1);<br class="">
r70_75c(k) = rg1;<br class="">
r2_38c(k) = rg2;<br class="">
r7_42c(k) = rg3;<br class="">
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rp1 = ri1 * rg1 / (ri1 + rg1);<br class="">
rp2 = ri2 * rg2 / (ri2 + rg2);<br class="">
rp3 = ri3 * rg3 / (ri3 + rg3);<br class="">
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wc(k,1) = 1/(c1*sqrt(rp1*rf1)); fc(k,1) = wc(k,1)/(2*pi);<br class="">
wc(k,2) = 1/(c2*sqrt(rp2*rf2)); fc(k,2) = wc(k,2)/(2*pi);<br class="">
wc(k,3) = 1/(c3*sqrt(rp3*rf3)); fc(k,3) = wc(k,3)/(2*pi);<br class="">
q(k,1) = sqrt(rf1/rp1)/2;<br class="">
q(k,2) = sqrt(rf2/rp2)/2;<br class="">
q(k,3) = sqrt(rf3/rp3)/2;<br class="">
a(k,1) = rf1 / (2*ri1);<br class="">
a(k,2) = rf2 / (2*ri2);<br class="">
a(k,3) = rf3 / (2*ri3);<br class="">
h1(k,:) = a(k,1)*(wc(k,1)/q(k,1))*s ./ (s .^ 2 + (wc(k,1)/q(k,1)) * s + wc(k,1)^2);<br class="">
h2(k,:) = a(k,2)*(wc(k,2)/q(k,2))*s ./ (s .^ 2 + (wc(k,2)/q(k,2)) * s + wc(k,2)^2);<br class="">
h3(k,:) = a(k,3)*(wc(k,3)/q(k,3))*s ./ (s .^ 2 + (wc(k,3)/q(k,3)) * s + wc(k,3)^2);<br class="">
h(k,:) = (150/4.7) * h1(k,:) .* h2(k,:) .* h3(k,3);<br class="">
end<br class="">
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fc = wc / (2*pi);<br class="">
semilogx(f,20*log10(abs(h'))); axis tight</div>
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