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Review: Underhill Dual Filter Onboard Preamp

Deano Destructo

Stingray & Serek addict. Hasn't slept since 1979.
Supporting Member
Dec 10, 2000
1,315
1,169
New Orleans, LA.
Hi Everybody!
Let me start off by saying I really hate doing reviews, but I feel compelled to write one on the Underhill PD-2 Filter Onboard Preamp because I was really surpised and am now a huge believer in this little bit of tech. Let me start off by saying I'm not the most knowledgable player when it comes to technology, but I do appretiate innovation and gear that just works solidly ever time. Plus, for me to personally take a moment and write a review about a new product I've come across, it'd have to be something that I felt not only sounded amazing, but filled a nitch in the bass gear market that was severly lacking and take a big step forward from a design perspective from that of any similiar product available on the market. This filter preamp easily definitely ticks all those boxes in my opinion.

First a bit of backstory, I was recently in a position where I'd aquired a new bass (a Wal Mk.2 clone) with a Lusithand preamp installed. Unfortunately, within weeks of accepting shipment, the Lusithand failed on me. It seems one of the push/pull pots which are wired directly into the circuit board just popped free of the housing and ceased working. It'd just spin freely within it's housing and not engage the filter any longer. Following this, I reached out to the owner of Lusithand in the UK where they're located and they were very nice in offering for me to send it to them directly for replacement. Unfortunately, the turnaround time to do so would leave me without a working instrument for quite some time. While I contemplated this I thought I'd do a little research and see what if any domestic builders were availible on this side of the pond currently building onboard filter preamps. That's when I first came a cross a local company called Underhill basses (https://www.underhillbass.com/) and it's owner Tim who just recently began building his own dual filter based preamps inhouse and I'm sooooo glad I did.

Sound-
While I truly love the sound of the Lusithand NFP-Special and an ACG I recently had the chance to try, I gotta give Tim over at Underhill huge props for taking his own spin on a sound of which Wal has set the standard and not only making it his own but improving upon it significantly in my opinion. I use to own an old '84 Wal mk.1 and neither the Lusithand or ACG really duplicated that rich tone I remember. The Underhill PD-2 on the other hand offers something the other two significantly lack. The two filter knobs not only seem to offer a wider sweep across their respective bands, but also contain more usuable tones as well. The PD-2 I currently have installed is also much quieter overall as well and since I picked up this bass mostly to use in recording, that's a huge bonus. This preamp through the stock pickups allows for deep dub like tones to the Wal like growl someone like Justin Chancellor of Tool is known for. Deep dub to aggressive growl and everything in between with just a simple twist of a filter knob. Plus, in the PD-2 model which I have, there are two tiny internal trim pots which all you to control the individual gain of each pickup as they're filtered by the preamp. You can dial these to balance volume or boost the filtered gain as needed. In my case, this has allowed me to take a bass with low output impedence pickups and balance it out to closely match the volume of other basses in my arsenal. :thumbsup:

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Construction-
Now this is where I was most pleasantly surprised by the Underhill preamp. Where as most Wal based filter preamps on the market utilize 6mm potentiometers wired directly to the circuit boards, meaning that should a part short out, break or be in need of replacement, the dual preamp's designed by Underhill allow for an array of swappable replacements. Not only are the wiring and pots used top notch, with the latter just feeling more robust and built of the long haul, but can you easily preplace parts or swap out the filter's layout/design from bass to bass with just a few changes.Not to mention if you find yourself in a similar position like I did, you don't need to replace an entire preamp should it just be a pot that fails. Now the pot can just be swapped out and then your up and playing again in no time. This design in my opinion also allows for a huge degree in flexibility should you ever want to remove the preamp from your bass and use it in an other instrument with a different control layout. Also, by not having your pots wired straight into the circuit boards it allows you to place the preamp wherever within the control cavity which makes sense to you, instead of having the place the boards in relation to where the holes for the pots are located. Within some basses real estate can be tight and the Underhill design does away with this concern as you can see below.

IMG_2475.jpg



Bottom Line -
What initially turned out to be an unfortunate circumstance for me with my Lusithand preamp breaking, became a blessing as it opened the doorway to me finding a much better designed and sounding solution in my opinion located right here in the US. After nearly 30 years of playing, the number of times I've come across a product that I felt really pushed or innovated the design of something already existing forward have been few and far between. However, given the few options available for purchase aftermarket for this type of product, I really can't say enought great things about not only the sound and design but the materials used as well.

That said, I do still plan on sending my original Lusithand preamp back for repair but given it's set in the box ready to ship for weeks and I still haven't sent it off since acquiring the above, that really tells me all I need to know about how pleased I've been with this product. Not to mention Tim with Underhill's customer service has been top notch every step of the way and he's made himself available for any and all question I've had has just been the icing on the cake. I really can't recommend this product enough and can honestly say his knowledge of electronics is astounding. :thumbsup:

So in summation, if you're like me and in the market for a filter based preamp or just trying to snag that "Wal sound" at a fraction of the price take my word of it gang, this thing is IT! :bassist:
 
It looks nicely made and as an EE that's designed a lot of products to go to market, I like the idea of non PCB mounted pots for exactly the reason you state. I like gear to be easily serviceable and as the era of the small electronics repairer has just about gone, most people could replace the pots or have them done.
 
It looks nicely made and as an EE that's designed a lot of products to go to market, I like the idea of non PCB mounted pots for exactly the reason you state. I like gear to be easily serviceable and as the era of the small electronics repairer has just about gone, most people could replace the pots or have them done.

Agreed, Snert.
A design like Underhill’s is long overdue in my opinion, especially for a filter based preamp which as a product has been around forever. When I started looking into this as an option, I literally asked myself for the reasons you’ve stated “why has no one thought to do this before?”. :thumbsup:
 
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When I started looking into this as an option, I literally asked myself for the reasons you’ve stated “why has no one thought to do this before?”.
I've done it the same way on my builds in the past. I've got a couple of other basses with preamps with the pots on the PCB that have given some issues as well I've had to repair and module plus separate pots is the only way to do it for me. However, if I were to offer boards to people who's skillset I don't know, something like EMG, or East (built on a J control plate) etc would be the way I'd do it.
 
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I just learned of Underhill over the weekend and am glad to see a review. I'm considering the buffer and filter for a PJ project I'm building.

EDIT: not expecting Wal tones, just trying to make each bass in my arsenal unique. This build will be a 32" scale PJ. I emailed Tim to ask if his system works with alternative pickup switching schemes since I hate blend controls and have a spare Dingwall 4-way pickup selector that I'd love to put in this bass.
 
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Thanks for the review, this info helped me make a decision.
I just placed an order with Tim and I would like to say that his customer service is outstanding.
Answered all my silly questions respectfully and concisely.
Told him what I’m seeking and he put together a package for me.
This pre is going to be installed in a build I recently finished and am looking for something different and unique while attempting to capture the tone I’m after.
I’ll probably do a review as well after I’ve had a chance to use it.
 
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It looks like the "boost" (is that the Q?) is fixed on the filter - you can adjust the frequency but not the size of the spike.
Also is there an HPF as well? Does not look like it. I leave the one on my ACG preamp full freq and unboosted, but I'm sure there's a use for it...
 
It looks like the "boost" (is that the Q?) is fixed on the filter - you can adjust the frequency but not the size of the spike.
Also is there an HPF as well? Does not look like it. I leave the one on my ACG preamp full freq and unboosted, but I'm sure there's a use for it...
The ACG HPF becomes necessary (IMO) because both of the LPFs in the ACG EQ-01 only go up to 3K or so (IIRC)- though without it boosted you essentially have it shut off. I believe the Wal and definitely the Alembic filters (as well as the Lusithand) all go up higher on the LPFs and so that extra control to add highs isn’t really needed in a 2-pickup + 2 filter arrangement (set one pickup with a higher LPF frequency for highs and the other with a lower LPF frequency for punch + character).
I await the newer version of the Underhill that is shown as coming soon that will have adjustable boost at the cutoff
 
The filters go up to 3.5 kHz which is in the same ball park as the Wal the Alembic goes higher as did the original 01 . The high pass is there to allow you address frequencies above the cut of the LPF filter. Originally the LPF went higher but you lose resolution if the sweep is to great. Having more control over the mids is important. So we cut the LPF which we can do because we have the HPF which also overlaps the the LPF adding more sound shaping. The HPF also allowed you to get close to the pic attack feature of the Wal which is not possible with just the LPF unless they go up to around 7 khz.
I change the HPF to a sweepable mid/treble in the DFM because a lot of people did not understand the HPF and what it can be used for. The DFM has the pic attack on a push/pull along with various other features that the 01 does not have.
 
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New Preamp Day ; Underhill Preamp

The particular set up I chose is the following:
Vol-Blend(11 position rotary)-Neck tone-Bridge tone- 3 way switch.
3 way switch ~ 1st pos. tone pots bypassed / 2nd ~ Neck tone on & bridge tone off
3rd ~ Both tone pots on.

Let me say this first; I never imagined that I could get such a varied amount of tones from my bass with this pre.
This bass went from a one trick pony to an all around performer.
This is a parts bass that I absolutely love for various reasons and is now my number 1 player.
Yes, if all you have experience with is the Non-filter based preamps then you will probably need to take the time to experiment and get used to all the options.
Each pickup tone control varies from full bright to full fat (no mud) and anything in between. This is not your standard capacitor type tone control, it is all based on low/high pass filters. I am not savy on all the technical aspects of the filter based pre’s but it just works and it works very well !
The quality build of the Underhill pre is top notch professional. The pots are high quality and so smooth and the frequency transition is smooth and predictable.
The preamp is extremely quiet - the lowest noise floor I've ever experienced in an onboard preamp.
The system is designed to be modular so Tim can put together an option that suits your needs.
Tim’s customer service is stellar too. He took the time to answer all my silly questions and when I explained my tone goals, he put together a package for me. I received the product quickly and the install only took 30minutes.
I have used and tried all manner of 2-3-4 band cut/boost type preamps and while they are well suited for many applications, I am now a convert to the world of Filter-Based pre’s.
Check out Tim’s web site and give him a call.
Money well spent. I am very satisfied with the results.(as you can tell)
Thanks
 

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The ACG HPF becomes necessary (IMO) because both of the LPFs in the ACG EQ-01 only go up to 3K or so (IIRC)- though without it boosted you essentially have it shut off. I believe the Wal and definitely the Alembic filters (as well as the Lusithand) all go up higher on the LPFs and so that extra control to add highs isn’t really needed in a 2-pickup + 2 filter arrangement (set one pickup with a higher LPF frequency for highs and the other with a lower LPF frequency for punch + character).
I await the newer version of the Underhill that is shown as coming soon that will have adjustable boost at the cutoff
The variable Q filter is about 5 weeks out (test boards will be here Monday). There are some technical advantages to the fixed Q (lower noise, more headroom), but having a variable is pretty cool. Options...
 
A variable filter works by using gain to vary the resonance. A fixed filter can be a unity gain design (the caps set the resonance). So depending on which variable design you use, you either have to attenuate the signal at some point to handle the increase in DC gain (not the resonance gain, but the base gain), or let the gain get really big, and tap the unity gain signal afterward with a buffer. In the first approach, which is what ACG uses (and I will probably use), it is unavoidable to add noise, because you have attenuation and then re-amplification to balance the gain of the filter amp. In the second approach, noise is lower because you have one less gain stage, but internally you add the DC gain to the resonance gain, and it gets quite high for a 9V battery. Works great with 18V. With the fixed (unity gain) filter, you never exceed the resonant peak gain internally, and you only need one op amp, with no gain. Signal-wise there's no way to match that with a variable filter. It maybe could be done with a variable capacitor or inductor, but neither would fit in a bass!

I am doing extensive testing with both designs to decide which I like the best. There's always a trade-off with every engineering decision!