Review: Underhill Dual Filter Onboard Preamp

[bowthing]

After reading through this thread yesterday I perused your website and I'm intrigued... I wish I had a bass I could put one in! I really like the modular setup.

I was wondering if there's any way to get the variable filter Frequency and Boost controls on a single stacked pot?

Unfortunately not. Frequency pots are hard to get these days, and anything special ordered requires you do, say, $100k/yr business with the manufacturer. Sad. I'm always on the lookout, though. You never know what might pop up.

 

[bowthing]

I’ve used filters with normal pickups and they work just fine in most use cases- what the low impedance pickups offer is a much wider and less peaky natural response. The resonant peak of the Alembic pickups happens above the highest cutoff point of their low-pass filters so there is no ugly point on the sweep with a huge resonance and the filters see a relatively flat frequency response to play with. Typical passive pickups will have a much lower resonant peak so somewhere in the top of the sweep of the filter you may not here much effect and at some point you may get a giant (possibly even slightly clipped depending on the headroom of the preamp) peak where the filter freq meets the resonant freq of the pickups.
I’ve used the ACG and Lusithand filter pres successfully with both passive and active pickups and I think they are awesome and would totally endorse both. I currently have one of the J-Tex filter pres ready to drop into something and hopefully at some point I’ll pick up one of the @bowthing ’s setups as well.

I would recommend grabbing one of the current single filter options with adjustable Q (for me having a significant Q peak available is key) and popping it into an external box to see what it can do for all your basses, then if you dig it you can configure something more versatile for an onboard filter pre (where you can have separate filters per pickup directly interfaced with the pickups).

Good explanation. One thing that I do is set the input impedance to 220k (instead of up to 1M like many preamps). This gives a really nice tone with the filters bypassed, and levels out the resonance of the pickup some, so the filter can bring it back wherever you want.

 

[deepestend]

This seems like a good demo of what a filter preamp can do, esp 1:40-2:10.

 

[Passinwind]

Unfortunately not. Frequency pots are hard to get these days, and anything special ordered requires you do, say, $100k/yr business with the manufacturer. Sad. I'm always on the lookout, though. You never know what might pop up.

The crux of the biscuit is needing to stack two dual pots. Custom Omeg dual pots aren't necessarily prohibitively expensive at all, but they are also quite deep and there's no way a dual-dual stack would fit in anything I'd need it to.

Hoping we'll be seeing another Seattle GTG this fall, I'd love to check out your work!

 

[WG Plum]

Hoping we'll be seeing another Seattle GTG this fall, I'd love to check out your work!

Heck yes!

 

[bowthing]

The crux of the biscuit is needing to stack two dual pots. Custom Omeg dual pots aren't necessarily prohibitively expensive at all, but they are also quite deep and there's no way a dual-dual stack would fit in anything I'd need it to.

Hoping we'll be seeing another Seattle GTG this fall, I'd love to check out your work!

Yup, I hope we can both make it this time!

 

[bowthing]

The crux of the biscuit is needing to stack two dual pots. Custom Omeg dual pots aren't necessarily prohibitively expensive at all, but they are also quite deep and there's no way a dual-dual stack would fit in anything I'd need it to.

I had Omeg make up some pots for me, and they are pretty nice, but like you say quite large. Also, the shafts are plastic and the bushings are 10mm, not ideal.

 

[NKBassman]

@bowthing, I think I'm confused about the FS1 filter description on your website, specifically the part about using the S3 switch.

The FS1 Filter is a resonant low-pass filter with a switchable 6 dB or 11 dB resonant peak. It is essentially an FQ1+FQ2 filter. An S2 or S3 switch is required for each filter. The S3 switch will set the filter to 8 dB in the middle position, and shift the frequency up or down (depending on the order of the cable connections). Like the FQ, an S1, S2, or S3 bypass switch can be added to cut out filtering.

My understanding was that the filter pot adjusted the frequency, and the switch affected the resonance peak Q. With the S3 switch, does it select between 6, 8, and 11 dB resonance?

 

[bowthing]

@bowthing, I think I'm confused about the FS1 filter description on your website, specifically the part about using the S3 switch.



My understanding was that the filter pot adjusted the frequency, and the switch affected the resonance peak Q. With the S3 switch, does it select between 6, 8, and 11 dB resonance?

I should probably not mention using the S3, because it IS confusing! Because of the way the filters work, two capacitors must be switched to keep the frequency the same at different Qs. If you only switch one (the middle position, it's an on-on-on switch) then you shift the frequency a little. The frequency pot still works the same, but the range of frequencies (and the current frequency setting) shifts up or down a little. It's not a huge difference, but it is audible.

 

[NKBassman]

I should probably not mention using the S3, because it IS confusing! Because of the way the filters work, two capacitors must be switched to keep the frequency the same at different Qs. If you only switch one (the middle position, it's an on-on-on switch) then you shift the frequency a little. The frequency pot still works the same, but the range of frequencies (and the current frequency setting) shifts up or down a little. It's not a huge difference, but it is audible.

Gotcha. Thanks

 

[deepestend]

Does anyone know how the Audere Z mode compares to a filter preamp? Is it just a more controlled filter option?

 

[bowthing]

Does anyone know how the Audere Z mode compares to a filter preamp? Is it just a more controlled filter option?

It's a beast all it's own, but remotely similar to dual filters with one rolled way down. To do the things peculiar to a dual preamp, you could put a separate Audere on each pickup and then blend them (that would be pretty wild!).

 

[rutrho]

Following...

Do you have a flavor of preamp that plays nice with single pickup basses?

 

[NKBassman]

@Alex Xander

 

[bowthing]

Following...

Do you have a flavor of preamp that plays nice with single pickup basses?

The PD3 works fine. There are some pics on the support page of a single pickup cavity. It's a bass that has a G&L MFD pickup, and I used the variable filter. I am actually working today on a dedicated single version! The main difference is less parts, so lower power (less tha 1/2), and less cost, uses the same filters, pots, etc.

One thing I tried for fun was running the pickup into BOTH channels of the dual preamp, and also running one coil into the neck channel and one into the bridge channel. Then I got two separate filter tones. It made a pretty good "scooped" sound, but didn't have a lot of unique tones like combining two pickups.

 

[rutrho]

The PD3 works fine. There are some pics on the support page of a single pickup cavity. It's a bass that has a G&L MFD pickup, and I used the variable filter. I am actually working today on a dedicated single version! The main difference is less parts, so lower power (less tha 1/2), and less cost, uses the same filters, pots, etc.

One thing I tried for fun was running the pickup into BOTH channels of the dual preamp, and also running one coil into the neck channel and one into the bridge channel. Then I got two separate filter tones. It made a pretty good "scooped" sound, but didn't have a lot of unique tones like combining two pickups.

Gotcha! Reason I ask is I built one of @Passinwind's preamps as a pedal a while back and I really like what it does with a P bass, so I have wheels turning in my head about doing something similar with a new P I just picked up, but like... onboard. lol.

I'll dig through site and documents a little bit tonight, when i'm not at work.

 

[deepestend]

I think I'm going to order a preamp w/dual filters to check it out at the end of the month. I'm pretty sold on the buffered blend that lets you dial in the gain of each individual pickup... and really curious about what I can do with the filters. It's for a J with a Nordstrand P with the offset pole pieces and a Big Split (soapbar w offset pole pieces).

 

[jfiver]

Installed a PD3 with variable Q filters in my main bass this week - it's an excellent little unit! It works nicely with the bass' Aguilar Superdouble pickups. Unfiltered, the PD3 seems very close to the sound of Aguilar OBP-3 that it replaced; possibly slightly less through the mids and a little more in the treble range?

The filters are are a lot of fun. Tim was kind enough to wire up bypass switches for each filter, which makes it ridiculously versatile. (My current fav is to set the bridge filter low with a moderate Q and the neck filter in the upper-mids with a higher Q.) Flipping between combinations of engaged and bypassed filters can give 4 good, distinct tones without even touching the blend.

Makes me wonder how I'll go back to the 3-band eq on my other active bass, tbh.

 

[deepestend]

@jfiver please post pics of your bass. Would love to see the setup.

thanks for the review. Good to hear more about the “basic” tone of the preamp. I was really wondering what that was like.

 

[jfiver]

@jfiver please post pics of your bass. Would love to see the setup.

thanks for the review. Good to hear more about the “basic” tone of the preamp. I was really wondering what that was like.

Sure thing - here's the before:


And the after:


It's a bit of a rat's nest, but it works. The PD3 board is sitting above the pots because the bridge pickup wires were cut quite short and, without wiring in extensions, I didn't have many placement options. The pots are also at all different orientations because the new pots require a certain amount of clearance for the connectors, and I wanted to avoid any bends or stresses.

(Probably not going to end up sticking with the new knobs, but they're what was easily available. Sourcing specific parts is still a bit hit and miss here in Australia.)

Re. the "basic" tone; my other two observations so far are that:
- the tone is very good. It's nice and clear. The OBP-3 has a slight, but very musical, colouration that the PD3 lacks, but it's nothing that you can't approximate well enough with a tweak on the amp.
- bypassing the filters opens up the top end just a little bit compared to engaging the filters at their highest frequency and minimal Q. It's noticeable when practicing solo, but not when there's other instruments in the mix.