[Jens]

Currently working on rolling off my mid/treble system, which is powered by my LF55.

Since I want to do this at signal level, I need to know the input impedance of the Lifeforce 55.  I seem to remember having seen something about it a post here, but didn't find anything when doing a search ...

So, can anyone enlighten me?

[AKSA]

Hi Jens,

Trust you are well as your fair city enters summer!

Zin of the LF is 42K, a little higher than the AKSA.

Cheers,

Hugh

[Jens]

Hi Hugh,

Thank you very much! Out of general interest - I suppose this is the same for the LF100?

Yes, moving into summer here in Copenhagen 

[AKSA]

Identical, Sir, they are just the same at the input stage......

Hugh

[Jens]

Thought so 

[andyr]

Currently working on rolling off my mid/treble system, which is powered by my LF55.

Since I want to do this at signal level, I need to know the input impedance of the Lifeforce 55.  I seem to remember having seen something about it a post here, but didn't find anything when doing a search ...

So, can anyone enlighten me?

Hi Jens,

"Since I want to do this at signal level ..."     I assume you are using a PLLC for this, not an active filter?  What slope are you wanting to do - 1st order?

Regards,

Andy

[Jens]

Hi Andy,

PLLC? Sorry, I don't know what that is ....

A simple 1st order filter will be fine, as the midrange driver is also rolling off acoustically at around the same frequency where the bass system is taking over (approx. 250 Hz).

However, in order to prevent my GK-1's output cap from "seeing" the roll-off cap, I plan to insert a resistor of approx. 50 k across the signal wires running from the preamp to my LF55. Then after the resistor, I will insert a .015 uF cap in series with the "hot" input wire. This should give the desired effect - at least that's what a friend knowledgable in electronics has told me 

[andyr]

Hi Andy,

PLLC? Sorry, I don't know what that is ....

A simple 1st order filter will be fine, as the midrange driver is also rolling off acoustically at around the same frequency where the bass system is taking over (approx. 250 Hz).

However, in order to prevent my GK-1's output cap from "seeing" the roll-off cap, I plan to insert a resistor of approx. 50 k across the signal wires running from the preamp to my LF55. Then after the resistor, I will insert a .015 uF cap in series with the "hot" input wire. This should give the desired effect - at least that's what a friend knowledgable in electronics has told me 

Hi Jens,

You said in your original post: "Since I want to do this at signal level, I need to know the input impedance of the Lifeforce 55."

This implies you want to put a cap on the input of the LF55 to form a 6dB highpass filter?  This is a PLLC - Passive Line Level Crossover.  Compared to an active filter in front of the LF55, which doesn't need to know the Zin of the amp.

Regards,

Andy

[Jens]

Hi Andy,

Yep, spot on - just hadn't seen the PLLC abbreviation before 

This is usually easily implemented merely by introducing an appropriately sized cap in the "hot" signal wire running from preamp to power amp.

However, when the preamp is a tube amp, this is slightly more complicated, as tube amps usually have an output cap. If this output cap "sees" the roll-of cap (because they are connected in series), the resulting capacitance will be not be correct.

The "trick" described in my previous post should be able to prevent the output cap from seeing the roll-off cap (at least so I'm told), so that you can control the roll-off merely by calculating the cap value in relation to the power amp input impedance.

Actually the whole thing can be done with very small components, and I intend to attempt implementing the whole thing in the interconnect running from GK-1R to LF55. That way I need not tamper with the LF55.

Some time in the future I intend to go "fully active". I just need to find active filters of sufficiently high quality that will not cost a fortune. A friend of mine is currently in the process of implementing some new, digital crossovers that are supposed to be very, very good whilst still being at reasonable prices. I'm looking forward to his results 

[andyr]

Hi Andy,

This is usually easily implemented merely by introducing an appropriately sized cap in the "hot" signal wire running from preamp to power amp.

Hi Jens,

Yes, a simple series cap in the 'hot' signal wire into the LF55 will give you a 6dB electrical HP slope ... which is theoretically the best in terms of phase screwups.

However, you said you were matching this with the acoustic rolloff of the driver (sorry, I forget whether it was tweeter or mid-range!  )  Have you allowed for the fact that the LF end of a driver's acoustic response typically includes a 4-8dB hump at the driver resonance point ... then followed by a dropoff at lower frequencies.  Are you calculating the uF required to give you the -3dB "filter point" of the electrical slope at the resonance frequency ... or above it ... or below it?   

You seem to be attempting to add the 6dB of the cap rolloff to the natural acoustic rolloff of the driver but if the value of the cap is too large - ie. giving an "elbow frequency" at or below the driver resonant frequency - yes, it will add to the slope of the driver's acoustic rolloff but you will be left with the hump around the resonant frequency.   

Conversely, if you use a smaller cap so that the filter's elbow frequency starts above the driver's resonant frequency, the hump will be tamed but you will no longer be taking much advantage of the driver's inherent acoustic LF rolloff.     So you probably need to implement a higher order filter.

However, when the preamp is a tube amp, this is slightly more complicated, as tube amps usually have an output cap. If this output cap "sees" the roll-of cap (because they are connected in series), the resulting capacitance will be not be correct.

The "trick" described in my previous post should be able to prevent the output cap from seeing the roll-off cap (at least so I'm told), so that you can control the roll-off merely by calculating the cap value in relation to the power amp input impedance.

AIUI, ss amps as well as tube amps typically have an output cap ... only those amps proudly proclaimed as "direct coupled" do away with this.

So maybe your friends "trick" will do the trick (I'm afraid I have no idea but the logic sounds good to moi!    )

Some time in the future I intend to go "fully active". I just need to find active filters of sufficiently high quality that will not cost a fortune. A friend of mine is currently in the process of implementing some new, digital crossovers that are supposed to be very, very good whilst still being at reasonable prices. I'm looking forward to his results 

"Active filters of a sufficiently high quality"!!      

IMO there are 4 kinds of high-class active XOs ... and I use #1 simply bcoz that's the one I started with about 8 years ago and I can't afford a DEQX!! 

1.  Rod Elliott's active XO (www.sound.au.com).  Rod supplies the PCBs and instructions for these nominally 12dB or 24dB L-R XOs ... but you can tweak the circuit to give you 6dB and 18dB as well!    These are opamp-based ... so some purists will automatically dismiss them!  (However, I think Hugh will agree mine don't seem to have any obvious failings!  )

2.  Transistor-based active XOs.  The only ones I know of are those produced by Naim ... the circuits are available on the Net but I have no idea how to tweak them for different filter frequencies or slopes.  In contrast, there is a heap of information on the Web about opamp-based active XOs.

Possibly transistors will sound better than opamps? 

3.  Marchand active XOs - both opamp and tube-based.  These are well-regarded and come in either kit form or fully made up.

4.  The Oz-designed DEQX digital XO.  In fact this is more than just a set of active filters ... it gives you room-correction DSP and driver phase correction too!  A marvel!!   

Regards,

Andy

[Jens]

Hi Andy,

Well, my system setup is like this:

Bass section is electronically controlled, using a Behringer DCX2496 Ultradrive Pro. This is used for room correction, equalisation and roll-off upwards. The eight 8" Peerless HDS drivers are currently powered by a 500W IcePower module (which I'm dreaming of replacing with a couple of LF100s )

Treble/mid section has an ordinary passive crossover. This crossover includes an impedance correction, so the "bump" is just not there

The midrange driver starts rolling off at around 250 Hz, so just to prevent it from getting to much low frequency input, I want to roll it off electrically at around the same frequency. The passive crossover does not include a huge cap array (this would be around 175 uF!) to roll off the midrange driver. I did have this previously, but found that even with a quality bypass cap it still detracted somewhat from sound.

In my previous SS based preamp I used to have the "simple cap" crossover to roll off the midrange, and it worked beautifully (both measured and sounded right ).

Thank you for listing the active crossovers, which I know already. Currently, I'm looking at this one: http://www.groundsound.com/pdfdoc/Ground%20Sound%20-%20DCN23.pdf

When the search for the "right" active crossover is finished, I intend to go fully active - hopefully in a year or so. I have already purchased a second LF55 (not assembled yet) that I may turn into a LF25 for the treble. I'm sure this will be fantastic

I had planned to build the LF25 this winter, but decided to upgrade my GK-1R first. This turned out to be very time-consuming, as I ran into hum problems, which are fortunately solved now.

Next comes considerations on where I should put this extra amp. Currently, I'm considering moving all amps to a position right behind the speakers, thus more or less eliminating speaker cables. In this connection, I'm also considering "stacking" the amps in a "tower" behind the speakers, but since my speakers are not very wide at the rear, this might mean new enclosures for the amps and possibly splitting them into monoblocks to get them narrow enough for things to look good. Well, we'll see - it'll be quite a bit of work, since I insist on making something that looks good! 

[PSP]

Hi Jens,
I assume that that your current setup sounds better than the system I heard a few years ago??!!!     

By now, I imagine that the angels stop by from time to time when they want to hear some real music.  Does your system ever come on in the middle of the night when no one is around?

Peter

[Jens]

Hi Jens,
I assume that that your current setup sounds better than the system I heard a few years ago??!!!     

By now, I imagine that the angels stop by from time to time when they want to hear some real music.  Does your system ever come on in the middle of the night when no one is around?

Peter

Hi Peter,

Yes, it does

The upgrades to speakers alone (new, improved enclosures and high-quality crossovers) has made quite a big difference, but apart from that I now have a much better player, a "Platinum" upgraded GK-1R, improved cabling all around, and last but not least, a LF55.

It's an entirely different system - but focus is still on music!!!

Dunno about angels - although there are times when the sound is almost angelic

How are you Orions coming on?

[andyr]

Hi Andy,

Treble/mid section has an ordinary passive crossover. This crossover includes an impedance correction, so the "bump" is just not there

AIUI, impedance correction is generally used to correct the driver's impedance around the crossover region or the HF end of a driver's range.  It is not designed to "tame" the LF driver resonance peak ... however, I don't have any practical experience of this as my experience is with Maggie drivers which are basically entirely resistive.  I'm sure Hugh can comment.

So the LF resonance "bump" may still be there?  Have you tried feeding a sig-gen through the mid-range driver with no filter circuits and used a meter to measure the innate frequency response from say 100Hz to 5KHz?

Regards,

Andy

[Jens]

Andy, it would not make any sense to make measurements without the (passive) filter components, when some of these are there specifically to remove the "bump". As I said, with the passive filter in place everything measures and sounds great

Also, you are not correct in assuming that impedance correction is only used for HF correction. It is very common to use this for LF correction, and this is exactly what has been done in my speakers.

Measurements were made with a professional speaker measurement system (including both acoustic and electrical data), when the passive filter was designed. The process for doing this is initially putting the driver in the box without any kind of filtering at all and measuring. You then get electrical and acoustic data from which you can design a first passive filter. After inserting this filter, you measure again and correct the filter. One continues this process until measurements look the way one wants them to look.

From this also follows that no matter what kind of filtering is inserted at signal level, it will not affect the power amp/speaker interface (i.e. the passive filter with any impedance corrections that may be needed), and the midrange driver's behaviour in relation to the power amp will not change one bit.

[SamL]

The eight 8" Peerless HDS drivers are currently powered by a 500W IcePower module.

Wau! 8 x 8" HDS. This is one large speaker.   

There was an active crossover board group buy over diyaudio a few months back by Bob Ellis. It was so popular that it was rerun 2x. The last I heard it was all sold out but as interest are still high, there maybe another rerun.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=74420&perpage=10&pagenumber=1

Edit:
Ops! Just found that the group buy was rerun 4x... and all sold out. Guess there must be 1000+ board sold.
http://www.diyaudio.com/wiki/index.php?page=Active+Filter+Boards+

[Jens]

Well, well - 8 x 8" is for the whole system. Each speaker has 4 x 8" HDS equalised to a straight line more or less down to 20 Hz (in-room, that is )

Thanks for the link to the active filter thread - will have a look at it 

[AKSA]

Folks,

While it may seem astonishing to hear the details of Jens system, it should come as no surprise because his family name, in the original Norse (I presume??) means

SON OF THOR   

Cheers,

Hugh

[andyr]

As I said, with the passive filter in place everything measures and sounds great

Sorry, Jens - you've lost me.  If everything measures and sounds great ... why do you feel you need a cap in series with the mid driver? 

Regards,

Andy

[Jens]

Folks,

While it may seem astonishing to hear the details of Jens system, it should come as no surprise because his family name, in the original Norse (I presume??) means

SON OF THOR   

Cheers,

Hugh

Hehe, true - and though I'm not sure it's actually old Norse as such, it's certainly derived from that  aa