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Old 23rd April 2013, 10:53 PM   #31
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The wipers are tied to pin 3 of the pots as shown in R.G.'s article. I think there are some online calculators that let you change the center frequency of the filters, then you just add more filter sections as needed, as they are all wired in parallel.

Jaz
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Old 24th April 2013, 02:00 AM   #32
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ah, i thought that looked slightly familiar...that's such a great article he did. and great job updating it for the tube stages, Jaz. As i recall, Keen has the formula on that article for calculating the target frequencies. i will check it out.

By the way, in my limited experience, i've never seen a diode hooked up to the cathode before, what's the story on that?
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Old 25th April 2013, 06:12 AM   #33
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Seems you can get a range of 4.5:1 per simulated inductor. So, one inductor can go from 100hz to 450hz, the next one from 450hz to 2025hz, and a 3rd from 2025hz to 9112hz. And he does have the math in that article for selecting the right cap values.
Funny, if you look at my very first post, it mentions merging tubes with this schematic of RG's. And here i had come to think it was a daft idea....Thanks Jaz.
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Old 28th April 2013, 11:36 PM   #34
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I sim'd that on CircuitMaker 2000 and am getting similar results as you did.

I changed tube to a 12AU7 and put a pot on the output, so gain is in line now.

One thing I'm seeing is that when you increase the frequency, the Q changes (it's not as sharp, and is gutting a lot more sidebands) and the overall boost amount goes up too. But down at the low end of what this 'inductor' will do, it's nice and sharp.

Are you seeing this, too?

Attached are some screenshots: All i did was change the frequency pot from extreme low in one to extreme high in the other.
Attached Images
File Type: jpg eq-lowend.jpg (34.4 KB, 180 views)
File Type: jpg eq-highend.jpg (40.2 KB, 175 views)
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Old 28th April 2013, 11:50 PM   #35
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Hi there,

Yes, R.G.'s article the grand-daddy of the EQ articles online (there are many in textbooks though), anyway if you refer to Wikipedia, you can see that the Q is a function of f, so changing the center frequency will effect the Q.

As for LED or diode bias, it's nothing new, all you are doing is using the forward drop (typically 0.7 to 3V depending on the type used) to bias the stage, the advantage is that the stage can be fully bypassed without a capacitor, which is usually a good thing - also one less part to deal with...

Jaz
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Old 29th April 2013, 12:34 AM   #36
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I guess I am used to software EQ's where you get this even Q across the whole range!

I'm not dismayed though, i call the discrepancies 'color' :-)
Makes you get to know your gear better!

By the way, adding a couple more 'inductors' is working out really well.

Here's a screenshot....

I just wish i could get tighter Q in general. It's kinda gouging out sidebands. I've tried changing value of Q pot and a variety of other tweaks. Anyway, I'll keep plugging away...
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File Type: jpg 3-bands.jpg (40.7 KB, 175 views)
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Old 5th May 2013, 01:51 AM   #37
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Not sure if anyone can help, but I am trying to get a "flatter" response out of that low-frequency gyrator. I have everything over 256hz "kind of" flat (there are 6 bands now), but this low end has been a bugger. I've tried adjusting all kinds of values, but can't get it quite right.

All settings are at half-way (freq and gain), Q at minimum.

Screenshot attached...
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File Type: gif response-5-4-13.gif (5.4 KB, 144 views)
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Old 5th May 2013, 03:33 AM   #38
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i should have said...i don't mind the roll-off under 64hz really (i could go flat, too), but i'm trying to smooth out from 64hz to 256hz. so i did try to tweak the next gyrator up (which is around 256hz), but not happnin for me.
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Old 6th May 2013, 10:20 PM   #39
PE2WDO is offline PE2WDO  Netherlands
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I'm also experimenting with the parametric tone controls for my tube based bass guitar preamp.
This is what I initially came up with. Simulation results looks fine in Microcap,
but I will need to do some further research on (IRF510?) output stage, choice of opamps etc.

Low end control: 30-120Hz (variable Q)
Low/Mid control: 150-600Hz (variable Q)
High control: 1.5kHz fixed.

Any opinions?

BRs

Willem
Netherlands
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File Type: pdf BassPreampSchematic.pdf (35.4 KB, 86 views)
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Old 15th May 2013, 02:00 AM   #40
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Hi PE2WDO,

Please keep me posted on how this goes. I am learning myself so i could hardly make worthy suggestions.

I'm curious - do you not need to buffer the output because it's going to go before the output stage in an amp? Or is that what "x20" is for (the last opamp before the output)?

In other news, I figured out the issue i was having. In my endless tweaking, i never put the Q pots back to 10k. Once i did, it 'sort of' worked out.

Attached is a screenshot. I added a LP filter on a switch, so that's what the 2nd line is on the image.

I guess the last issue i'd like to work out before ordering the parts is this: The first (lowest) band needs to be cranked to 80% boost to keep it 'flat-ish'. So, any suggestions from ye geniuses slumming it on my post are welcome as always!
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File Type: gif response-flatish.gif (8.7 KB, 112 views)
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