# PLLXO high pass

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#### Soulwax

I’m looking to employ a passive 1st order high pass filter between my CD player and integrated amplifier to damp in-room low frequency response a little. The schematic below illustrates this.

An externally hosted image should be here but it was not working when we last tested it.

http://www.t-linespeakers.org/tech/filters/passiveHLxo.html

The input impedance of my amplifier is 50K ohms.

The desired high pass filter frequency is 85Hz.

If the value of C is given by the formula

C = 1 / (2 × pi × R × F)

How do I calculate the value of R? And what is the purpose of parallel R with respect to amplifier input impedance?

Apologies for my novice understanding of electronics!

Thanks.

#### 5th element

The R in this schematic is any value you wish to give it, such as the 5-10kohm they suggest in the web site.

However this is also in parallel with the amplifiers input impedance.

So lets say we choose a 5k resistor and your amp is 50k. We end up here with an input impedance of 4.54kohms.

Thus you use 4540 when calculating the filter. In this case with a 5k resistor you will use a 440nF capacitor.

Atleast I think that is correct.

#### Soulwax

Thanks 5th.

R (in the above schematic) must be determined by some reason or formula – how for instance was the value range of 5k ohms to 10k ohms arrived at?

Further why is the need for a parallel resistor since the amplifier has an input impedance that could (if I understand correctly) be used as the value R in the formula C = 1(2piRF).

This is what I don’t get!

#### BobEllis

using 5-10K generally will not impact your preamp's performance and allows you to use capacitors in the readily available range. For instance, if you just use the 50K rated input impedance, you'd need 3.74nF to get 85 Hz. You may be able to live with the error using standard values, but it is easier to get 440 nF as 5th suggested building up from standard values.

#### Soulwax

I see. Thanks Bob.

#### Soulwax

From a purist standpoint would it be preferable to keep the 50k ohm input impedance of my amplifier thereby retaining an easy load for my CD player?

A 3.3nf capacitor would give minus 3db around 95Hz which is acceptable.

I’m able to source an MKP polypropylene cap (RS 1908680) would this be a good choice for intended application?

#### 5th element

Yup an MKP would be fine. Sure keeping the 50k input impedance would make your CD have an easier ride so why not. It also makes the caps cheaper too.

If I were you I would also buy a few other values aswell just so you can choose a higher or lower frequency should you desire.

I use 4.7nf caps in my active xover at points so low values are not a problem.

What happens though is the lower in frequency you go the bigger a cap gets. Or put differently the higher up you go the smaller they get. You are using a 3.3nf for a f3 of 95hz. I use the 4.7's for around 2000-3000hz.

So if you wanted to use the 50k input imp you would be using rediculously small caps for a frequency like 2000hz. In your situation 3.3's and 50k are fine. If you want to make a higher freq PLLXO then use resistors to lower the input impedance.

#### Soulwax

The fog is lifting 5th! Thanks for your help.

Just checked my calcs again tho and I think 33nf is required for 95Hz not 3.3nf

#### 5th element

Soulwax said:
The fog is lifting 5th! Thanks for your help.

Just checked my calcs again tho and I think 33nf is required for 95Hz not 3.3nf

Hehe yeah you are indeed correct 33nf

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