High pass - Low pass 80hz filter between preamp and power amp

[Pegwill]

Hi
I read a couple of articles regarding putting a high pass filter at around 80hz to relieve the main speakers of frequencies and diverting them to a subwoofer. It’s supposed to be better for the main speakers. One article also suggested putting a low pass 80hz filter to the subwoofer even though it might have a built in filter.

I would like to try this out by putting the filters between my tube preamp and tube power amp. Try as I may I could not find any information on the sizes of the capacitors or details of how to calculate them and any associated components required.

I was just looking to try it out as it seems a logical approach, but no information seems to be available.

Like all these things I suspect it’s a lot more difficult than just soldering a cap in place.

Many thanks

[Ali Tait]

A simple and quick way would be to use one of these-

https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4

[Pegwill]

Hi
I was hoping to keep everything analogue rather than introduce op amps etc. Just hoping to do it with caps and resistors.

Perhaps it may be to difficult.

[Pegwill]

Hi Guys

After a good deal of scanning different forums and such for those that are interested I managed to find this link that explains all.

http://www.troelsgravesen.dk/High-Pass-Filter.htm

If I was to construct a low pass filter this is usually shown as the resistor first and the capacitor second in parallel. My question is would or does it make any difference if the capacitor is first in the circuit then the resistor - I would then use the amplifier input impedance as the resistor. In effect only using one component the capacitor in parallel across the the the input connection (obviously one for each channel)

Many thanks

[Alex66]

When I set up my main workstation I can send links but there is a thread on DIY audio about doing this, it is something along the line of FAST it is in the full range forum. The basic notion there is using a good full range driver with a helper woofer with a passive line level crossover. What you will need is the input impedance of your power amps or will end up if it is too low having to put a buffer in there. It does get complicated if you want more than a second order filter, then you will end up having to use buffers and cascading stages. I am in the middle of moving so cant provide links as of yet.

[Made in 1968]

I use a processor between DAC from PC and amp, also one from CD player to amp.. Here is what the lit says, sounds great & tbh i dont know what id do without them..

This unit combines 2 processors to align the phase and focus of bass frequencies, improve the response of mid-high frequencies and add harmonic content to any "dull" sounding audio. The surround processor utilises the phase alignment engine to give a greater perceived width to the stereo field.

Dual/stereo multi-band processor
Process variable between enhancer and exciter
Dedicated bass processors
Surround processor to add width to stereo field
Auto noise reduction circuitry
Bypass switch for processed/unprocessed comparison
Balanced and unbalanced inputs and outputs

[Barry]

What is the processor you use Andre?

[Made in 1968]

https://i.imgur.com/ft3aclT.jpg

[Mr.Ian]

Sounds like you want a, harrison fmod, search on the bay about £30 cheap to try

[dave2010]

A simple and quick way would be to use one of these-

https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4That's an interesting idea, but doesn't it mean using yet another amp to drive the speakers which handle the low frequencies? This is hinted at in msg 1, but there it is suggested "just" as a test project. I can't quite see the point of this, unless there is an eventual intention to either use additional power amplifiers for the low frequency parts of the signal, or the OP is using active speakers for the low end frequencies.

Assessing a signal stripped of high frequency components - though not perhaps at the full power of a speaker system, could be done with either DSPs or software systems though headphones. I just don't see how this could be terribly useful, unless the filtered signal really is used to drive speakers within a complete system. Listening to individual filtered parts of a complex signal is not necessarily going to give a good impression of what the end result is going to sound like, IMO. Maybe the objective is simply to test technical feasibility?

Another reason - using speakers for the test - might be to test room resonances in the intended listening room. I suspect that "shaping" the low frequency response badly can give some rather unpleasant effects - just too much booming. A sophisticated DSP could use notch filters to reduce/avoid such effects.

An additional complication would be if only one mono channel were needed in the final installation for the low frequencies. Is that the ultimate intention? That would avoid having to use a stereo amplifier for the low end. There might not be much point in using two speakers for low frequency components - but that would really depend on what the OP has in mind. Can that DSP be configured to produce a mono output from a stereo input (rather than simply dropping one - either the left or right channel)?

[Ali Tait]

Good points. I’ve used the miniDSP kit before very successfully to my ears- though for fully active OB’s. I started with the linked box and then moved to a 4x10HD, which was considerably better overall albeit at a cost. My thoughts here are given the desired 80hz pass, whether DSP is used or not would not be an audible issue, if you are of the opinion that you can hear a difference between doing it analogue or digitally, which frankly to my ears isn’t an issue.