Douglas self four-band baxandall eq

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jammin

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Joined 2012
#1
I have just spent most of today building a little circuit from the book: Small Signal Audio Design, by Douglas Self. The circuit is figure 10.18 page 273. A four-band eq, now the low, low-mid and high controls seem to work perfectly but the high mid control seems to do absolutely nothing! I have checked my circuit over again and again, I'm sure I haven't made a mistake.

Would someone please be kind enough to have a look at the circuit and see if there is a mistake on the diagram?

I've been looking for hours on the internet tonight trying to find away to calculate the filters involved to see if the values on the high-mid section made sense but I can't seem to find the calculation to do this.

Please help, I really want to finish this circuit..

Many thanks
 
#4
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Thanks for the responses this is the circuit in question, anyone know how to sim / check the design??
 
#5
This is my little attempt at the circuit, I know its messy but it should work, everything's connected as it should be, I reckon there is a mistake in the component values for the high-mid section. All the other sections work fine.

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#7
Haha yes fair enough, as I say I'm 99% sure that I've followed the design exactly, would just really appreciate if anyone can work out from the circuit diagram what the centre frequency for each band is and whether the mid-hi section is as it should be.. the photo was just to show how I've gone about trying the design.

Any help with this would be massively appreciated...
 
#8
a long time ago before PC's and Lspice when I wanted to find the proper values for tone controls I would use 3 frequencies 20hz, 1khz and 20khz. I would calculate the Z of the capacitors at those frequencies and replace the caps in the tone control circuit with resistors (Z values calculated) and do the calculation. It didn't take long to realize to get ball park figures I could presume at 20hz capacitor is High Z or open circuit and at 20khz low Z or ) ohms. This estimate would get me close enough to start soldering.

In your case calculate the Z of the caps of the centre point of the tone control frequency range and see what happens when the controls are at one end of their travel or the other. Draw a little graph and plot your results.

to simplify even more, when a 20k resistor is in parallel with a 1 k resistor, presume 1k is the answer. When a 20k resistor is in series with a 1k resistor presume 20k. Remember not long ago 10% resistors were the norm and people used 5% resistors when things were critical and 1% resistors were used in test equipment.
 
#10
jammin said:
Haha yes fair enough, as I say I'm 99% sure that I've followed the design exactly, would just really appreciate if anyone can work out from the circuit diagram what the centre frequency for each band is and whether the mid-hi section is as it should be.. the photo was just to show how I've gone about trying the design.

Any help with this would be massively appreciated...
Click to expand...

Since the sections in the book figure are all the same in concept, I really would look at the implementation.
Have the wiring checked by someone who doesn't know how you did it, that most often gets you to the problem.

Jan
 
#11
Does anyone know how to calculate the values of components for these kind of controls? Is there an equation that will give the centre frequency of each band??

Or on the other hand can anyone suggest another good eq circuit that I can try?
 
#13
Thanks I will have a look for it..

I would really like to get this circuit working..

Anyone else fancy chucking one together to see if the diagram does work? It's such a simple circuit. It would be great to know I've made a mistake or if the drawing is wrong..
 
#14
My guess is that C5 (2.2nF) is too small - it may just have been copied from the first stage and not spotted in proof-reading. Rough calculation tells me that the 'hi mid' control will do little above 17kHz, and little below 15kHz. This seems to be far too narrow a bandwidth for a first order control. Try increasing C5 - 4.7nF or 10nF could be about right.
 
#15
Thank you for the suggestion. I just tried that and it made no difference. I was hoping a value had been written down wrong, putting the non-working band out of hearing range..

I also had an email today from D Self, he says the circuit is correct and suggested I might have a bad pot, checked that and its not that either. What am I doing wrong!!!??

Another thing- when I disconnect the power there is still signal coming through the circuit, should this be the case???
 
#17
Right sorry everyone!!! I just checked it again for the millionth time and I found a little fault, a tiny solder bridge between two tracks that I hadn't seen / identified before. Everything is now working perfectly and a great circuit, I am a happy man!!. Maximum respect to Mr D Self!!

Ok thank you to every one for your help and advice it is truly appreciated.

I am now going to remove the diagram from the thread

Once again THANK YOU
 
#20
Thanks for the help man. It was great to hear from Douglas Self today as well, he knew his diagram was correct and it was.
If anyones interested I'm going to be using the 4-band eq to adjust the send signal going into delay / fx. When I've completed the whole project I will post up some pics etc...
 
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