Review of new pre-amp/high-pass filter

drurb · #1 07-09-2007, 09:47 AM

One of those simple little products for which many of us wish is finally available. A little over a year ago, our own fdeck and I had a dialogue about the design of a useful pre-amp/adjustable high-pass filter (pre/HPF). I bench-tested a pre-production version and after a tweak here and there by fdeck, I must say that this is one sweet little circuit from the standpoint of both laboratory measurements and months of use with my DB and rig (RS-2 pickup, EA iamp200, Wizzy 12).

The pre/HPF incorporates a 10M-ohm input and a 12-dB/oct high-pass filter with the corner frequency adjustable (via a knob on the outside) from about 40 Hz to 110 Hz. Only discrete components (transistors) are employed. There are no op-amps in the circuit. The latest production versions also include a phase-reversal switch. All of this is encased in a palm-sized box and the circuit is powered by a 9-volt battery that is connected only when a plug is inserted into the output jack. The battery should provide well over 100 hours of use.

My own measurements showed the frequency response of the pre/HPF to be essentially ruler-flat above the high-pass cutoff at least out to any frequency that could imaginably be relevant to a DB player. That is, the device is not "voiced" in any way. Distortion and noise are acceptably low even when the pre/HPF output is over 1 volt. I'll leave the geek-talk there.

Measurements aside, from my months of actual use of the pre/HPF, I can say that it sounds great. More to the point, it has no "sound." It just does its job. I have never once had an issue with clipping, distortion, or noise of any kind. It effectively solves the "infrasonics problem" about which I so often speak here. I would not want to be without it.

If you're interested, I suggest you send fdeck a PM. In the spirit of full disclosure, I want to explicitly state that I have no financial interest in this device at all. In return for my assistance in testing and measurement, fdeck let me keep the prototype he sent to me.

P.S.-- My understanding is that the price is $50 plus shipping.

Uncletoad · #2 07-09-2007, 10:08 AM

Nice.

This could be just the thing for Walter Woods amps that don't have these two needed features.

**Freddels** · #3 07-09-2007, 02:09 PM

Is there something to alert the user when the battery is low?

drurb · #4 07-09-2007, 02:22 PM

Quote:

Originally Posted by Freddels

Is there something to alert the user when the battery is low?

Yes, distortion.

No, there is no battery-level indicator but as the battery voltage sags, the sound would deteriorate. I'd say if you gigged 3 hr/week and changed the battery twice a year, you'd be fine. Alternatively, you can ask fdeck about running the pre-amp with a "wall-wart" power supply.

A note on wall-warts. They are usually unregulated supplies. This means that the output voltage will be at the rated value only when the load draws the current that is specified along with the voltage. If you take, say a 9-volt wall-wart, and connect it to a volt-meter (which hardly loads it at all), you might see something like 14 or 15 volts.

The bottom line is that fdeck should be able to tell you what type of outboard power supply is safe to use. That would solve the battery issue. I use mine with the battery. For me, it makes life easy.

Phil Redman · #5 07-09-2007, 03:47 PM

Quote:

Originally Posted by drurb

All of this is encased in a palm-sized box and the circuit is powered by a 9-volt battery that is connected only when a plug is inserted into the output jack.

Pix would be helpful.

fdeck · #6 07-09-2007, 05:22 PM

Thanks, drurb, for the kind words. Being careful to observe the commercial user policy, I will just mention that I should have a "brochure" at my web page in a few days.

Having sold small electronic gadgets in the past, albeit not music related, my modus operandi is to offer an open design. In keeping with this practice, I will soon upload a "white paper" which documents my design and test results.

Greg Clinkingbeard · #7 07-09-2007, 05:40 PM

If this unit is half as good as your 12" 17lb speaker, it will be a winner. I like the little box so much I just made another.

**Adrian Cho** · #8 07-25-2007, 12:24 PM

fdeck, still looking for that brochure on your website...

fdeck · #9 07-25-2007, 08:20 PM

At long last, I have some info at my little web page.

Many thanks to drurb for advice and encouragement, and to all TB'ers for excellent discussion and ideas.

B String · #10 07-26-2007, 05:38 PM

This sounds like a very useful high quality piece of gear.
I have a question though:
shouldn't the 10m ohm buffer and phase switch be in front
of the preamp, but the high pass filter be between the
preamp and power amp? How does it work putting the
high pass filter between the bass and the preamp?

fdeck · #11 07-26-2007, 10:16 PM

Quote:

Originally Posted by B String

This sounds like a very useful high quality piece of gear.
I have a question though:
shouldn't the 10m ohm buffer and phase switch be in front
of the preamp, but the high pass filter be between the
preamp and power amp? How does it work putting the
high pass filter between the bass and the preamp?

Good question. Assuming everything is linear (a decent assumption for upright bass amplification), the filter should be able to go anywhere with little or no difference in behavior. That's because at any given frequency, each stage of the system has a multiplying effect, and putting the stages in any order is analogous to multiplying numbers in any order. There are ramifications to signal-to-noise at higher frequencies (the all important "gain staging" of preamps), but these are minimal at the low frequencies where the hi-pass filter operates.

Then there's the practical issue from my side of wanting everything to be in a single box. But if you decide to DIY the circuit, you could split up the sections with impunity by adding a 10 uF capacitor after the phase switch.

Just don't put it after the speaker.

**Francois Blais** · #12 07-27-2007, 06:10 AM

Sent you an email, Francis.

Do you plan to add a balanced XLR out sometime?

Regards,
François

drurb · #13 07-27-2007, 07:17 AM

Quote:

Originally Posted by francois

Sent you an email, Francis.

Do you plan to add a balanced XLR out sometime?

Regards,
François

I'm sure fdeck will correct me if I'm wrong but, as I understand it, while he could certainly hang an XLR connector on that box, it would require a re-design for the unit to actually supply a balanced (differential) output. Thus, one could achieve the same effect by buying the appropriate one of these.

**Francois Blais** · #14 07-27-2007, 09:39 AM

Thanks for the link.
But these adapters wouldn't provide a true balanced signal, yes?

My other question is: but is it important?

drurb · #15 07-27-2007, 10:04 AM

Quote:

Originally Posted by francois

Thanks for the link.
But these adapters wouldn't provide a true balanced signal, yes?

My other question is: but is it important?

You are quite correct. As I mentioned, in order to achieve a true balanced (differential) signal, fdeck would have to change the circuit of his device. As designed, the circuit provides an unbalanced output. That is, the signal is with reference to ground. So, if he didn't re-design his circuit, incorporating an XLR connector on his box would be electrically identical to using an adapter of the sort I suggested.

Whether it is important that you have a balanced output depends upon whether you experience problems with noise. Given the relatively high signal levels produced by piezos, their impedance, and the fact that they are not followed in the circuit (your amplifier) by a high gain stage (as microphones are), I suspect you will have no problem at all with the unbalanced design.

By the way, the link I provided was just an example. It was the quickest picture I could find. I don't know that the link even shows a desirable place to buy the adapters.

Scott McArron · #16 07-27-2007, 03:18 PM

here's question for ya: I know this pre/HPF was built for uprights and piezo p/ups, but how well would it work with EB? And would there be any issues of using it in front of a pre that doesn't have an effects loop?

Phil Redman · #17 08-01-2007, 08:02 AM

Quote:

Originally Posted by Scott McArron

...and would there be any issues of using it in front of a pre that doesn't have an effects loop?

I've got the same question more or less. I'm currently running a Fishman Pro Platinum into a power amp (QSC PLX) but am thinking about upgrading to either an Avalon U5 or a GT Brick, neither of which have an HP filter. If fdeck's box were in front of the preamp would there be any adverse tonal effects or would it nullify the benefits of upgrading the preamp in any way? What about inserting the HP after the preamp? I noticed in the spec that it can handle up to 4 volts p-to-p.

Thanks,
Phil

tornadobass · #18 08-01-2007, 08:11 AM

I'm thinking about this little guy for recording purposes. Last night, my acoustic blues trio did a little bit of recording and I ran my BugBass into the Boss BR-1180 via the HP filter and line out on my Clarus. Without the HP filter, I was concerned about killing my monitors because we were going direct and listening through the monitors as we played.

The BR-1180 required some compromises to get two instruments at once, and in this case I had to run two line level inputs. That meant using the effects send on the Clarus, which didn't really had to be nearly cranked at the input to have enough output.

I imagine the preamp/HP box could be put ahead of a little ART preamp and get the right level. And would be a much less expensive device than using a Clarus mainly for its HP filter.

Just curious...for those of you recording via a piezo (What else? Mic a solid body EUB!), are you using a HP filter into the board?

drurb · #19 08-01-2007, 08:17 AM

Quote:

Originally Posted by Phil Redman

I've got the same question more or less. I'm currently running a Fishman Pro Platinum into a power amp (QSC PLX) but am thinking about upgrading to either an Avalon U5 or a GT Brick, neither of which have an HP filter. If fdeck's box were in front of the preamp would there be any adverse tonal effects or would it nullify the benefits of upgrading the preamp in any way? What about inserting the HP after the preamp? I noticed in the spec that it can handle up to 4 volts p-to-p.

Thanks,
Phil

The Fishmans have a 10 M-ohm input impedance. If that is what follows your EB and you are happy with the sound you get when your pickup is loaded by that impedance then you should be fine placing fdeck's device between your EB and just about anything else. By the way the Fishman Platinum Pro Bass pre-amp does have a HPF. I assume that you are not using that model. Nothing will, necessarily, be nullified but one could set the various tone controls in a manner that could cancel effects across devices.

Phil Redman · #20 08-01-2007, 08:25 AM

Quote:

Originally Posted by drurb

The Fishmans have a 10 M-ohm input impedance. If that is what follows your EB and you are happy with the sound you get when your pickup is loaded by that impedance then you should be fine placing fdeck's device between your EB and just about anything else. By the way the Fishman Platinum Pro Bass pre-amp does have a HPF. I assume that you are not using that model. Nothing will, necessarily, be nullified but one could set the various tone controls in a manner that could cancel effects across devices.

I'm using the current Fishman with the HPF (which I rely upon heavily, usually setting the EQ flat), but if I ditch the Fishman I'll need an HPF to go with whatever replaces it. The Fishman is a nice piece a gear with all of the right facilities but I'm trying to optimize tone by going for a higher end preamp like the U5 or Brick. Also the Fishman lacks enough gain to drive my power amp optimally. I've played around with using an FMR RNP instead of the Fishman and it was a noticeable improvement in tone and has gain to spare, but again, no HP filter.

Either way it sounds like placing the fdeck HPF in front of any of these pre's would work fine.

Thanks,
Phil