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Old 9th January 2013, 05:55 AM   #11
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Eh, decided to just go ahead and do the sim. I used 180K in the feedback loop where 100K/220K is spec'ed, since that's what I have in the one in my bass (Travis Bean fretless). 1uF output cap, going to 10 doesn't do much at all and I wanted to use nonpolar coupling caps. 330K on the voltage divider resistors since I'm using LT1351 which is more current hungry.

So here are some curves that LTspice threw out. Scale for bass and treble controls is 0-10, I use 01 and 99 in file names to reflect min and max rotation because that's how LTspice specs them. Freq sweep is 20Hz-20KHz.

T=0, B=0

Click the image to open in full size.

T=10, B=10

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T=4, B=5

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T=4, B=3

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T=4, B=7

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T=10, B=0

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T=0, B=10

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Hope this helps?
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Old 9th January 2013, 07:57 AM   #12
JayGunn is offline JayGunn  United States
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No, using the Uncle Fluffy board would be the easy way and I usually do things the hard way. Plus I want to do a lot of customizing and experimenting, which is why I was so eager to find out how the EQ sections really work.

Here's the work in progress:

Click the image to open in full size.

The huge resistor is the 220K, which I ordered from Digikey and somehow it sneaked through my filter for 1/4 watt parts--it's a 3-watt. I'll replace it when I go shopping for more parts locally next week. I'm in India until March and there are somethings you can't get here, like exotic op amps. The white ready mate connectors are there so I can modularize the parts that won't change--pickup and blend control (3 pins), +4.5, 0, -4/5v supply with battery (3 pins), treble and bass pots (the 6-pin connector) and output (3 pins, only need 2). That way I don't need a full set of pots for each preamp I solder up.

Sometimes the hard way is more fun. I take JM Fahey's point about starting off plain before making changes, but I am being faithful to the parts values of the original, except for the rail splitter, which is a no-brainer, and using a dual op amp because of the input buffer.

And to make things really challenging I am winding my own low impedance split coil pickups...
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Old 9th January 2013, 08:14 AM   #13
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Originally Posted by JayGunn View Post
Sometimes the hard way is more fun.
Yes, I can totally appreciate that. For me, learning to do Spice models is a lot harder than just huffing a lot of solder smoke and physically swapping out components. I've done plenty of that though, and still will.

You might be able to get one of the big opamp manufacturers to send you some engineering samples, even in India.
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Old 9th January 2013, 03:45 PM   #14
JayGunn is offline JayGunn  United States
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That sim is fantastic! I was planning to try to do those measurements with a laptop freeware signal generator feeding the circuit, then a desktop (nearly) freeware real time analyzer to plot the results. It looks to me like I really need to learn how to do simulation.

I'm sure MM worked hard on the component values and crossover points. I like the even transition from bass to treble, like in the T=4, B=3 and the T=4, B=7 curves. You don't get a notchy or scooped result except in the T=10, B=10, which is what you'd be trying for with those settings.

And looking at the T=0, B=0 curve at the top of your post, I see the evidence for the claim that although the bass pot is supposed to be boost only, it actually has a cut (very small, like -2dB) when set to zero.

And the T=10, B=0 is interesting because this is a shelving EQ and would rise indefinitely, except that the high end is limited by the 120pF cap, and maybe also by the slowness of the op amp. I'm thinking I'd like to try a bigger cap, maybe 150pF to bring this peak a little lower. On the other hand unless you have a tweeter, you're not going to reproduce that 10-12K stuff anyway. And unless you slap and pop a lot I don't think you'd miss it.

Thanks again--this is really helpful.
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Old 9th January 2013, 07:36 PM   #15
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Quote:
Originally Posted by JayGunn View Post
That sim is fantastic! I was planning to try to do those measurements with a laptop freeware signal generator feeding the circuit, then a desktop (nearly) freeware real time analyzer to plot the results. It looks to me like I really need to learn how to do simulation.

I'm sure MM worked hard on the component values and crossover points. I like the even transition from bass to treble, like in the T=4, B=3 and the T=4, B=7 curves. You don't get a notchy or scooped result except in the T=10, B=10, which is what you'd be trying for with those settings.

And looking at the T=0, B=0 curve at the top of your post, I see the evidence for the claim that although the bass pot is supposed to be boost only, it actually has a cut (very small, like -2dB) when set to zero.

And the T=10, B=0 is interesting because this is a shelving EQ and would rise indefinitely, except that the high end is limited by the 120pF cap, and maybe also by the slowness of the op amp. I'm thinking I'd like to try a bigger cap, maybe 150pF to bring this peak a little lower. On the other hand unless you have a tweeter, you're not going to reproduce that 10-12K stuff anyway. And unless you slap and pop a lot I don't think you'd miss it.

Thanks again--this is really helpful.
Jay, keep in mind that this sim more or less includes the effect of your buffer section too, as I just used a low impedance sig generator to feed the model. I haven't really fully sussed modeling a pickup as the signal source yet, just one of many things I hope to get to eventually. PM me if you want the LTspice files, this is a pretty simple circuit and could be a good way to get your feet wet. LTspice itself is freeware and there is wide support online.

I do use RTA software (True RTA, Holm Impulse, Room EQ Wizard) quite a lot to test DIY builds, and RMAA as well. On Talkbass there's a DIY tube bass preamp thread where I compared modeled and measured frequency response curves, the correlation was excellent in that particular case.

Last edited by Passinwind; 9th January 2013 at 07:39 PM.
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Old 9th January 2013, 08:05 PM   #16
sreten is offline sreten  United Kingdom
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Hi,

(Bass) guitar pickups have resistance and inductance, values are not hard
to find. Split coil P type bass pickups can have a series / parallel switch
as can the less common humbucker types, e.g. Gibson's / Musicman's.

(Jazz type pickups can be split to allow the same arrangements.)

The S/P switch is rare on actives, but works fine on passives. Series
is deeper, with more output, parallel brighter with less output, they
interact with the usual passive volume and tone controls.

rgds, sreten.

Higher the output, usually means the higher impedance and inductance.
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Old 9th January 2013, 08:37 PM   #17
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Quote:
Originally Posted by sreten View Post
Hi,

(Bass) guitar pickups have resistance and inductance, values are not hard
to find. Split coil P type bass pickups can have a series / parallel switch
as can the less common humbucker types, e.g. Gibson's / Musicman's.

(Jazz type pickups can be split to allow the same arrangements.)

The S/P switch is rare on actives, but works fine on passives. Series
is deeper, with more output, parallel brighter with less output, they
interact with the usual passive volume and tone controls.

rgds, sreten.

Higher the output, usually means the higher impedance and inductance.
Hi Sreten,

There's rather a bit more to it, actually. Nice discussion here.
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Old 9th January 2013, 09:13 PM   #18
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Passinwind View Post

There's rather a bit more to it, actually.
Hi, No there isn't if you actually know your stuff, rgds, sreten.
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Last edited by sreten; 9th January 2013 at 09:16 PM.
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Old 9th January 2013, 09:35 PM   #19
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Quote:
Originally Posted by sreten View Post
Hi, No there isn't if you actually know your stuff, rgds, sreten.
No worries, I don't bruise easily...
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Old 10th January 2013, 01:47 AM   #20
JMFahey is offline JMFahey  Argentina
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Quote:
I'm sure MM worked hard on the component values and crossover points. I like the even transition from bass to treble, like in the T=4, B=3 and the T=4, B=7 curves. You don't get a notchy or scooped result except in the T=10, B=10, which is what you'd be trying for with those settings.
As you see, the "original" in any commercial product which became a Classic is usually very good.
H*ck! There must be a reason behind that!
It can't be just good luck!!
Which it never is, of course, but *lots* of hard work.
That's why I always suggest everybody to start with the original one, whatever it is, learn to use it and what it does, and only after that, you should start tweaking.

Last edited by JMFahey; 10th January 2013 at 01:49 AM.
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