NTMB & OBP-3 low frequency pot values

[MysticMichael]

Folks,

For a variety of reasons, I wouldn't know where to begin to search for an existing thread on this topic. So I'm just gonna put it out there:

I've got a couple of Carvin LB70 basses - one fretted, the other fretless. The fretted instrument has an Aguilar OBP-3 onboard preamp, the fretless has a Bartolini NTMB.

I'd like to see about having each preamp rewired to add a 3-way switchable mid-point function for low frequencies, that would function just like the switchable mid-point function for midrange frequencies that each unit already offers (400 Hz & 800 Hz for the Aguilar; 250 Hz, 500 Hz & 800 Hz for the Bartolini). So those could be something like 40 Hz, 80 Hz & 120 Hz...and 30 Hz, 60 Hz & 90 Hz, respectively.

Here's the questions:

1) Have any of you fellow TB members ever had such a modification done?

2) Can anyone tell me with certainty the appropriate pot values to use for each of these low frequency ranges?

3) Are there alternate low frequency points I should select that would provide for maximum sonic impact and flexibility in the lows, while avoiding problems such as phase cancellation with the mids, etc?

Any relevant suggestions, corrections, and/or other feedback gladly welcomed...

MM

[ubnomnar]

Bump for an interesting tech question.

[SGD Lutherie]

Looking at the schematic for the NTMB... you might be able to do it by inserting a cap between each output lug of the bass pot and the preamp. Or two in parallel. On the mid control, this is inserted on the wire that goes to the wiper on the pot.

The bass control has two same value caps wired from each outside lug to the wiper. So you can try either putting two in parallel, i.e. from the wiper to the outside lugs, or in series.

The schematic I have says the caps are .1µF.

I don't know how accurate this is, but here it is. I suspect the OBP-3 might have a similar tone section, but who knows?

[Kwesi]

That sounds like a cool idea! I'm the farthest thing from a techie but I think something that would be really cool for the low freq's would be some sort of knob to change the Q of whatever point you're boosting at. Would that be possible?

[line6man]

The Aguilar preamps are epoxy sealed, so it's impossible to modify them.

[SGD Lutherie]

Originally Posted by line6man

The Aguilar preamps are epoxy sealed, so it's impossible to modify them.

So are the Barts. As I pointed out you can try and retune the bass control by adding caps, which is the same way both preamps alter their mid frequencies. Parallel or serial caps would alter the value of the internal caps.

[MysticMichael]

Thanks for the replies thus far, guys. Especially to David for providing that substantive potential solution.

Electronics at this level are a little over my head, so I've taken the liberty of sharing a link to this thread with my tech for this project - Jeremy of Rudy's Repair Shop here in NYC. I'll be conferring with him again about this later in the week, or early the following week.

In the meantime, please keep the informed suggestions coming. There must be an ideal way to approach this project - somewhere.

MM

[lowfreqgeek]

Well, as David said, you could put caps in parallel with C20/C21, effectively raising their capacitance (parallel caps add). However, since fc = 1/(2pi*R*C), raising C lowers fc, or the filter frequency. So it seems that by adding caps in parallel, you'd be lowering the shelving frequency. Putting caps in series with the legs of the pot will create a band-pass filter instead of shelving. It won't raise the LPF corner frequency, though.

I'll have to look more closely at the schematic and analyze it to get a better feel for what values you should use. My current statements are based on an intuitive approach, which isn't always the *right* approach.

BTW: 100nf = 100x10^(-9), or 0.1x10^(-6) or 0.1uF.

[MysticMichael]

Thanks, Geek. Please feel free to add whatever additional data you feel could be helpful. It's all good...

MM

[FunkMetalBass]

Originally Posted by lowfreqgeek

Well, as David said, you could put caps in parallel with C20/C21, effectively raising their capacitance (parallel caps add). However, since fc = 1/(2pi*R*C), raising C lowers fc, or the filter frequency. So it seems that by adding caps in parallel, you'd be lowing the shelving frequency. Putting caps in series with the legs of the pot will create a band-pass filter instead of shelving. It won't raise the LPF corner frequency, though.

I'll have to look more closely at the schematic and analyze it to get a better feel for what values you should use. My current statements are based on an intuitive approach, which isn't always the *right* approach.

BTW: 100nf = 100x10^(-9), or 0.1x10^(-6) or 0.1uF.

Instead of using caps to raise the bass frequency, you could probably create a workaround by throwing a resistor in parallel (should the circuit allow for it). It would alleviate the HPF tendencies of a series capacitor. But then you run into issues of controlling it with a simple 3-way toggle.

[lowfreqgeek]

Originally Posted by FunkMetalBass

Instead of using caps to raise the bass frequency, you could probably create a workaround by throwing a resistor in parallel (should the circuit allow for it). It would alleviate the HPF tendencies of a series capacitor. But then you run into issues of controlling it with a simple 3-way toggle.

Resistors in parallel with caps C21/C20 will also increase the denominator value of the fc equation and lower fc.

So the way I see it (and I could be wrong), the two .1uF caps are basically in series with each other across the 50k pot. That give an fc of 63Hz. EDIT: The 100nF cap is in parallel with the 50k pot, giving an fc = 31Hz at max boost/cut. At the mid-way point on the pot (25k), fc = 63Hz, but the gain = 1, but as the pot is turned up or down, the frequency moves toward 31Hz.Since you can't make those caps smaller, you have to make the pot smaller to raise fc. If you leave the 2 caps alone and put in a 25k pot, you get an fc of 63hz @ max boost/cut. Now you can add more caps in parallel to increase the capacitance and lower fc back toward the 30Hz target.

The down-side is that you also decrease the gain with a lower pot, which means lower boost/cut. Now if the most you ever do is ~8db, you're probably ok.

I'll look more closely when I have time. I *could* simulate it, but where's the fun in that?
Aw, screw it! I'm making time... far more interesting to me than whatever it is I *should* be working on. Besides, it's lunch time.

EDIT: Silly me... See changes above.

[FunkMetalBass]

Originally Posted by lowfreqgeek

Resistors in parallel with caps C21/C20 will also increase the denominator value of the fc equation and lower fc.

I was thinking of a resistor in parallel with the pot, to effectively lower the resistance and therefore increasing the frequency. I think the idea of lowering the pot value and compensating with resistors sounds better though, even with the gain loss.

EDIT: It should be mentioned that while I understand the basics of circuit theory, my experience in design is limited.

[lowfreqgeek]

Originally Posted by FunkMetalBass

I was thinking of a resistor in parallel with the pot, to effectively lower the resistance and therefore increasing the frequency. I think the idea of lowering the pot value and compensating with resistors sounds better though, even with the gain loss.

EDIT: It should be mentioned that while I understand the basics of circuit theory, my experience in design is limited.

You're on the right track. I honestly don't have a lot of experience building this stuff, so I'm coming at it from the theory side, as well.

It helps to break it down into various discrete states and analyze for each state. So start with an extreme, like the pot cranked all the way in one direction, and then go from there.

[SGD Lutherie]

You can try this parametric preamp:

NOLLelectronic ONBOARD EQ TCM 3P

The best feature of the TCM3P is a concentric adjustable frequency range for all controls.

[lowfreqgeek]

Originally Posted by SGD Lutherie

You can try this parametric preamp:

NOLLelectronic ONBOARD EQ TCM 3P

That's pretty cool. I've been toying with the idea of building my own version of the Raven Labs TrueBlue EQ, which is a 5-band semi-parametric EQ. But having 3 bands onboard would certainly be useful for at least 2 of my basses.

[FunkMetalBass]

Originally Posted by SGD Lutherie

You can try this parametric preamp:

NOLLelectronic ONBOARD EQ TCM 3P

Leave it to Noll to offer YET ANOTHER product that I want.

Originally Posted by lowfreqgeek

That's pretty cool. I've been toying with the idea of building my own version of the Raven Labs TrueBlue EQ, which is a 5-band semi-parametric EQ. But having 3 bands onboard would certainly be useful for at least 2 of my basses.

I'm glad I'm not alone in the "moar mid controls" territory. I've considered buying an East 2-band preamp and then adding two or three of his separate variable mid controls to really lock in the shape around 300, 600, and 900 Hz.

[SGD Lutherie]

Originally Posted by lowfreqgeek

That's pretty cool. I've been toying with the idea of building my own version of the Raven Labs TrueBlue EQ, which is a 5-band semi-parametric EQ. But having 3 bands onboard would certainly be useful for at least 2 of my basses.

Ooh, Raven Labs. Steve W. Rabe designed some nice stuff over the years.

Do you have the schematic for that?

I installed the 3 band Para Noll for someone, and it's a real nice sounding preamp.

[lowfreqgeek]

Originally Posted by SGD Lutherie

Ooh, Raven Labs. Steve W. Rabe designed some nice stuff over the years.

Do you have the schematic for that?

I installed the 3 band Para Noll for someone, and it's a real nice sounding preamp.

I don't have a schematic for the True Blue, but I do have the SWR SM-series schematics which had 4 semi-parametric bands. I'd almost bet money that Steve Rabe tweaked that design and added a 5th band to make the True Blue. Whatever I come up with won't be an exact copy of the True Blue, but it would be similar.

The real trick, and what makes or break it, IMO, is setting the Q of each band to something useful. I believe that Q will change with frequency, though, so maybe it's not such a trick.

Another thing to consider is that Rabe used Gyrators in place of inductors. Nothing wrong with that, but it might also have a little more character (if you want that sort of thing) using real inductors - like the NTMB uses in the mid control. But then you'd have to use something like a Grayhill 12pos switch for each frequency control to switch out LC pairs.

Do you recall how you got the Noll and about what the preamp cost (didn't see any US dealers)? I might be interested in that for a Turner Fretless I have with a HUGE frequency response and very little ability to control it precisely.

[SGD Lutherie]

Originally Posted by lowfreqgeek

Do you recall how you got the Noll and about what the preamp cost (didn't see any US dealers)? I might be interested in that for a Turner Fretless I have with a HUGE frequency response and very little ability to control it precisely.

It was brought to me with the bass. The owner tried to install it and it wasn't working, so I had to clean that up. it made installing an look OBP-3 easy! lol actually most of it was prewired, but the blend control and output jack were weird.

I emailed Noll for a wiring diagram and got one super quick. Then it was easy.

[FunkMetalBass]

The only place I've ever found that sells one is this German company:

Noll electronics - BassParts.de (the site is translated in english)

Unfortunately, you're at the mercy of poor exchange rates and international shipping costs.