# Hi pass filter

Discussion in 'Amps, Mics & Pickups [DB]' started by quee, Oct 28, 2006.

1. ### quee

Jul 28, 2005
Hi there
Does anyone out there have a good circuit design for a hipass filter for cutting subsonics?
I seem to remeber an old thread on this but I cant find it now.
cheers Quee

2. ### drurbOracle, Ancient Order of Rass Hattur; Mem. #1, EPC

Apr 17, 2004

Well, they are correctly called infrasonics (subsonic means below the speed of sound in the medium, infrasonic means below the frequency of sound). You might want to PM fdeck. By the way, 110 Hz as the "corner" frequency is at least an octave too high. The fundamental of the low E is around 43 Hz. Whatsmore, the circuit shown in the post above is a single-pole filter and will only provide a roll-off of 6 dB/octave. That's why it has a corner at 110 Hz. The roll-off is so shallow that you must place the corner up high so that the frequencies you really want to cut (40 Hz and below) are actually attenuated appreciably. To explain, 110 Hz will be 3 dB down and then you will drop 6 dB with every octave so that 55 Hz is 9 dB down, 27.5 Hz is 15 dB down, and so on.

If you employ a 12 dB/octave rolloff, you can move the corner down. This has the advantage of not cutting into the desired passband (the frequencies you want to keep). For example, with a rolloff of 12 dB/octave, if the corner were at 55 Hz, that frequency would be 3 dB down, while 27.5 Hz would be 15 dB down and so on.

Oct 10, 2006
Seattle
4. ### drurbOracle, Ancient Order of Rass Hattur; Mem. #1, EPC

Apr 17, 2004
That would be passive, single-pole filter that will suffer from insertion loss. See my post above for the drawbacks of a single-pole filter. The insertion loss means that you will lose output. An active HPF is the way to go.

5. ### quee

Jul 28, 2005
Thanks for the info on the hipass I've been using one a lot in mixdown when recording on the computer, but it would be great to have one for live work too.
Do you know anyone who would make and sell an active hipass filter?
cheers Quee

Apr 17, 2004

As above:

7. ### bloodyjack

Oct 10, 2006
Seattle
Fishman platinum pro eq bass has one as well as lots of other stuff,
including phase reverse, compression and a 5 band graphic tailored to the URB.

Oct 10, 2006
Seattle
9. ### robgrow

May 1, 2004
Cincinnati, Ohio
While this circuit will certainly work, I'd keep in mind that using a garden variety IC opamp in a textbook circuit like this one is not going to perform (read sound) like a more sophisticated design, especially one using discrete components (not IC based). Those who own units such as the Avalon U5 or Summit Audio TD-100, will probably appreciate this distinction.

10. ### bloodyjack

Oct 10, 2006
Seattle
I took the time to reverse engineer the high pass filter in the Fishman platinum Pro bass pre amp (one of the better double bass pre-amps) and it uses the same schematic for its Depth control. I have been a design engineer for mixing board companies for the last 30 years and can safely say you are talking rubbish.

11. ### quee

Jul 28, 2005
A bit of a difference of opinion on ICs there!

Thanks for another circuit design bloodyjack, that's useful to try out and a lot cheaper that an Avalon U5.

Robgrow, I'm sure the U5 and summit TD-100 are great preamps and if I get the chance I'll compare their Hipass filters with the DIY IC circuit and see if I can tell the difference!

cheers Quee

12. ### robgrow

May 1, 2004
Cincinnati, Ohio
The fact remains filters have come a long way since the textbook circuit was first published. Designers such as the late Deane Jensen have published much improved filter designs at least as far back as the 1970's. If you don't think that it's possible to design audibly superior circuits using discrete components vs. common IC opamps, there are many people in the industry who would disagree such as the folks at Summit Audio and Avalon. IC opamps are used because they're inexpensive and easy to use, not because they are the best devices available.

13. ### drurbOracle, Ancient Order of Rass Hattur; Mem. #1, EPC

Apr 17, 2004

Well, of course, the manufacturers who use discrete components would disagree! In addition, it is, of course, possible to design circuits using discrete components that are superior to those designed with ordinary op-maps. That's not saying much. Most ordinary op-amps are quite poor. They have poor gain-bandwidth products, poor noise floors, poor slew rates, and high distortion. Let us not, however, confuse practice with principle. In truth, it can be easier and more efficient to build a quality circuit with a discrete component than with a very high grade op-amp. It is the case, however, that if one designs a filter circuit with a high-end op-amp, it can result in a circuit that measures and sounds superior to many discrete component circuits. I use low-noise, laboratory grade op amps every day in the most demanding of applications-- some in circuits I designed. I know of what I speak. By the way, the HPF I use with my DB rig employs discrete components. I would not recommend the circuit posted by bloodyjack as one can do far better. Still, I appreciate him posting it.

14. ### bloodyjack

Oct 10, 2006
Seattle
Lets put this into perspective most music gear manufacturers use op-amps and it would be difficult to not amplify your bass without going through op-amps. Nearly every manufacture of Heads and Amps use op-amps for there EQ controls. Your sound IS GOING TO GO THROUGH OP-AMPS unless you have an old amp with very little control of the EQ

15. ### PB+J

Mar 9, 2000
arlington va
This is interesting--I have both a fishman proplatinumII for bass and an Avalon U5. The Avalon is hands down the better sounding unit. The fishman is well designed and effective and sounds fine; but the Avalon just sounds much much better--fuller lows, sweeter highs. i have no idea why this is, as I'm not an electronics designer or an audio engineer. But to my ears it's unmistakeable

16. ### bloodyjack

Oct 10, 2006
Seattle
I had to go check out the Avalon U5 its a nice piece of gear for the studio, but the eq is very limited its a 6 pole switch with set eq curves hardly ideal to tame feedback and adjust to room dynamics for live performances and its \$650!!!
Still you can allways adjust the op-amp eq on your amp or the mixing desk in the studio

17. ### PB+J

Mar 9, 2000
arlington va
Yeah, it's interesting--I've never really had a situation where i couldn't get a good sound out of the U5. Sometimes I used one of the Eq presets, but mostly not. The presets are well designed and work well. You can find it for less than that--I paid 420 for one, and used it for two years of gigging at least once a week, with upright and electric

18. ### fdeckSupporting MemberCommercial User

Mar 20, 2004
HPF Technology LLC
I found this article to be reasonably well thought out, on the use of discrete op amps:

http://www.dself.dsl.pipex.com/ampins/discrete/discrop.htm

Of course, IC op amps continue to improve, resulting in an ever shrinking range of applications where discretes still reign supreme.

There are also cases where an op amp is actually too much -- circuits that don't need the full functionality of an op amp, and can be done in a simpler fashion with discretes. Coming up with such a circuit that is not already enshrined as a "classic" is a rare treat.

I would say that music gear uses IC op amps for good reasons, and cheap op amps for cost reasons.

19. ### bloodyjack

Oct 10, 2006
Seattle
For a modern variable EQ stage opamps are the only viable option for a product.
Turns out Walter Woods amps are using opamps too. I found this description on the talkbass forum from Robgrow himself

"The earliest WW amps used conventional linear audio circuitry with all discrete transistor Class A preamps and Class AB power amps. The preamps are particularly interesting in that they were based on a Class A FET and bipolar transistor gain block, which helped give the early amps a nice warm tube-like sound. These preamps had relatively high distortion just like most vacuum tube based guitar amps have. Fortunately the distortion products are of the "good kind" that lend themselves to a warm, musical sound. Later models dropped the discrete transistor Class A preamp and instead use preamps designed around IC opamps (chips). (All the amps with "VARI" controls are IC opamp based I believe.) While IC opamps are used as a cost savings in many commercial products, Walter switched to them in order to design more sophisticated preamps with lower distortion."

20. ### robgrow

May 1, 2004
Cincinnati, Ohio
That is exactly what Walter told me.