help with my ramsa eq

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buschfsu

Well-known member
Joined
Dec 31, 2004
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760
Location
jacksonville FL
ive got my ramsa WR-s4416 sounding good by rolling chips and bypassing caps.  while i have it pulled apart id like to give some though to the eq.  the circuit is built on  the NJM2068 running at +-15V. i would describe it as usable but would love to sweeten things up.  the low shelf stops at 70 and the mid parametric starts at 200.  this leaves the 100hz mudzone untouched.  what caps should i be looking at to lower the mid c27?

thanks
 

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ok i think the high and low shelves are baxandall circuits so i know how to change the threshold freqs on those.  the sweepable mid i think is a transversal allpass filter but dont have the equation for the threshold freq. anyone have it?
'
 
Those are the most important caps but also use decent (NPO) ceramics or film for c24 and c26 (specifically make sure voltage coefficient is low). Nonlinearity in these small caps could cause IMD down in the passband.

JR

PS: Minor point but the Wein-bridge  EQ is called sweep or perhaps quasi-parametric, not parametric since Q is not adjustable too.
 
Ok I got it!!!  Sorry for all the math but this should help anyone stumbling into this.

Left side of the circuit is the high and low pass based on the Baxendall circuit.  Frequency cutoffs are
1/2Pi*RC
so.. for the high side its 1/2Pi*5600*8.2E-10 = 35K Hz  (wow this doesn't look right?? should be 12.5K)
low side is 1/2Pi*55000*4.7E-8=  62Hz  (close 70Hz)

Right side is a sweepable mid based on the Wien Bridge
 
1/2Pi*sqrt(r46*c27*r47*c28)  when the pot is at zero ohm. (high sweep)

So its 1/2Pi* sqrt(3300*1.2E-8*3300*4.7E-9) = 6.4k Hz (spot on)

1/2Pi*sqrt(r46+vr4*c27*r47+vr4*c28) when the pot is at 55K ohm. (low sweep)

So… 1/2Pi*sqrt(103300*1.2E-8*103300*4.7E-9) = 205Hz (spot on)

If I want to lower the 205Hz I can double C27 and C28 but it also reduces the high sweep to 3.25k

1/2Pi* sqrt(3300*2.2E-8*3300*1E-8) = 3.25k Hz
1/2Pi*sqrt(103300*2.2E-8*103300*1 E-8) = 104 Hz

Or I can double only C27 and get 150 hz and 4.74, sound like a nice compromise????

1/2Pi* sqrt(3300*2.2E-8*3300*4.7E-9) = 4.74k Hz
1/2Pi*sqrt(103300*2.2E-8*103300*4.7E-9) = 151 Hz

Either way they are all getting replaced with NPO or mylar.  thanks guys.

sources
http://www.headwize.com/projects/equal_prj.htm  fig 1b
http://freecircuitdiagram.com/2008/09/15/wien-bridge-oscillator-the-circuit-schematic-diagram-and-formula/

 
i replaced the opamp with a signetics ne5532 and wow. huge improvement.  the caps alone weren't enough. now its musical without the brittle in the high and smear in the low.  i ordered some ti 5532's for the other channles.  lets hope they sound the same as i only have one signetics.

thanks
 

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