buschfsu

help with my ramsa eq
« on: July 16, 2009, 02:23:34 PM »
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
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick


buschfsu

Re: help with my ramsa eq
« Reply #1 on: July 16, 2009, 11:55:04 PM »
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?
'
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick

abbey road d enfer

Re: help with my ramsa eq
« Reply #2 on: July 17, 2009, 08:17:14 AM »
The mid EQ is just a Wien-bridge type. You may double the value of C27/C28, and halve the value of R46/47.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

buschfsu

Re: help with my ramsa eq
« Reply #3 on: July 17, 2009, 09:07:05 AM »
thanks,  and those changes would lower the bottom end of the sweep? to what? 100Hz?
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick

JohnRoberts

Re: help with my ramsa eq
« Reply #4 on: July 17, 2009, 10:52:45 AM »
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.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

buschfsu

Re: help with my ramsa eq
« Reply #5 on: July 17, 2009, 12:39:22 PM »

Yes noted.  Also did you mean c25 and c26? 
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick

buschfsu

Re: help with my ramsa eq
« Reply #6 on: July 17, 2009, 04:36:08 PM »
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/

« Last Edit: July 17, 2009, 05:33:11 PM by buschfsu »
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick

abbey road d enfer

Re: help with my ramsa eq
« Reply #7 on: July 18, 2009, 04:23:30 AM »
If I want to lower the 205Hz I can double C27 and C28 but it also reduces the high sweep to 3.25k
That's why I suggested also halving R46 & 47, so you retain the high point at 6k.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

buschfsu

Re: help with my ramsa eq
« Reply #8 on: July 18, 2009, 05:07:52 PM »
ok thanks!!

the caps are all http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+PZ+3+ABD0008+ECQV1103JM+7+WW  no surprise since ramsa is  panasonic.  im thinking replacing these may not be necessary since they are film?  c25 and c26 are not film so those are getting replaced with npo ceramics.


also any recommendations for replacing the 2068 opamp?
« Last Edit: July 19, 2009, 01:52:44 AM by buschfsu »
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick

buschfsu

Re: help with my ramsa eq
« Reply #9 on: September 02, 2009, 10:04:21 AM »
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
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick


 

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