Wednesday, 12 November 2014

Active X-overed HiFi ?

In the Netherlands we have this expression about plumbers always having leaky taps in their homes.

Sooo 
While business being on the slow side I decided to upgrade my personal hifi / mediacentre sound system to something with a bit more throw and detail.
Having so much stuff from prior experiments lying around I made it a design restriction to myself not to buy anything new.
This worked out to be a much longer lasting effort than I calculated for.
Three weeks in a row of precious time wasted (?)
Oh well, let's call it educating myself.
Let me try to give you a condensed report..

Design imperative was to build a small system (needed to fit in the bookshelf) with enough throw and definition to listen music at very low levels at night while the family would be asleep.
For that we would need FIR filters and so a Soundweb 9088ii will be at the hart of it all, see my prior posts on the upgrade and repair  of these nice tools.

Next thing would be the speaker cabinets themselves.
Top-end wasn't going to be that time consuming where as I was to recycle my priorly build tractrix waveguides.(also see previous posts)
Loaded with some DH450 drivers from my preferred, and highly appreciated supplier A&D-audio

rearview
frontview

Now why is that resistor in series?

Three reasons: 

First of all, evidently this horn/driver combination will have a lot more gain than the accompanied (hifi-) mid speaker.
So a big attenuation in the digital domain would be needed to overcome a magnitude mismatch.
Rather than choking on the associated loss of digital headroom/resolution, I decided to make a passive attenuator.

Secondly the amplifier will have to make some amplitude swing so distortion introduced by the amp (class AB crossover distortion) would be relatively small.
For some reason (presumably distortion associated) I also like the sound of warmer electronics (warm not hot!).
Ok, ok writing this I detect mumbo-jumbo alert, but believe me there really is a difference.
While I don't have the engineering skills to explain this difference, this doesn't mean I am lacking in, on firm science based, perceptional skills!

Third and not the least:
The driver amplifier combination will now operate in a (partly) current drive mode (again also written about in a prior post).
Naturally we now have to use some (min. phase) EQ to compensate the magnitude differences introduced by the impedance curve of the driver in series with this resistor, but the overall reduction of distortion is quite convincing.


Now for the mid speaker:


left is hifi      -      right is PA

From the maximum total cabinet hight of 28 cm (had to fit in the bookshelf, remember) I now only had 17cm left.

In pro-audio industry more and more smaller speakers are being used in those line arrays you see at every bigger event.
So (being a pro-audio designer) one would say I have plenty of sample speakers lying around to be used as this mid speaker.
Quite disappointing!
From all the samples I tested  (yup, a lot of 'big' names) the only one remotely optional for this purpose is the one shown in the picture.
(This experiment alone actually took a whole working week, so fast forward here ;)
Does this imply that all PA speakers are horrific?
Yes and no, for this purpose they will not be suited in a same way as a hifi speaker will not rock you.

Although this statement seems quite obvious one really has to add in a lot of question marks here..why?..

Actually this is a pledge for a paradigm shift in our industry.
We neither have the tools nor the (scientific) language to quantify the above described statement.
Surely this has something to do with distortion but how, what, why?

Anyway, for the purpose of the story let's continue with this Seas mid speaker.
In the pictures above you can still see an open back.
Actually being a bit too lazy to do a proper cabinet tuning (either closed or reflex) I thought this dipole configuration would be a good idea.
Stupid!
Horn loaded, zero phase shift, directed high's don't combine with room loaded low's..
So in the final cabinet this is a closed box (with luckily the correct volume for a .7 tuning)

Same thing for the subs which I made from some no brand speakers I had lying around.
Nothing special here, only highly over dimensioned for the purpose, in that they all ways will be operated in their linear region only.

soon follow up on electronics and (FIR) filtering

for now here's a preview of the final result:







Friday, 26 September 2014

Bringing Soundweb greens back to live


We are still using quite a lot of  old green Soundwebs 9088 and 9088ii in our soundprojects. The main reason being  that these ancient machines were allready capable of some FIR filtering.
Mind you: there is a bug in the software which makes you have to reverse the filter kernels. Presumably nobody ever noticed using linear phase filters (symmetrical as these will be) ;)

Anyway I had some broken ones lying around so I thought it would be time to do some repairing.
One of the apparent vulnerabilities is the power supply cap marked as C216.
So after replacing these I was able to get some off them back to live.
For all you out there plagued by the dreaded searching.... (at infinitum) modus I will repeat some 'rules':

  1. replace C216 cap, while you are at it maybe it's a good idea to upgrade cap C265 with something beafier as well.
  2. use the spare jumper at the option A position to set your machine in debug modus.
  3. switch on power, you now should get the 'nightrider' row of flashing leds.
  4. use the backup loader.exe to reload the firmware. make shure you have the correct *.a21 file. I learned the hard way do not to make a mistake!
  5. when done switch of power, replace jumper and reboot.
As allways use correct settings of com ports and bitrate (38400 front, 115200 back). Also the FTDI driver / USBtoRS232 works fine on windowsXP (and 7) but on windows 8 it appears to be troublesome.

Now for the fun (and utterly time consuming and by no means commercially viable) part.
Let see if we can do some 'audiophile' modifications.
In the issue 1 model (9088) there are some VCA IC's build in to facilitade volume controle directly after the DAC's.
These suck! ;)
I forgot the type number but in a earlier experiment some years ago I removed these and  shortened the signal path. So no more volme controle on the output and maybe (with high gain power amps) a bit excessive noise.
But hey, who cares for noise?
It's the universe coming through our speakers!
Big plus!


Next thing I thought would be whorthwhile is replacing all these (non-polar) caps in the signal path.
Well this wasn't going to happen.
All my regular dope dealers like Farnell and RS and the like seem to have stopped selling these, hmmm, bummer.
Now what if we replaced them with regular caps back to back with a DC bias.
(This is an idea I actually stole from a very old Midas PRO4 console I once owned).


Big improvement?
Argh euuhh, yes with all my skills I can notice the difference (some more clarity) but I 'm not even shure if this is desirable.
So, nah not going to do that on all our machines...

 ...edit...edit...edit...
Not completily convinced by the dispointing results of the above modification I decided to give it a second try.
Same mod but this time I tested a stereo a/b comparison..wow.. this is some serious shit.. much broader and deeper image and a very convincing musical impact..now why the hell is this???

Sunday, 31 August 2014

LouReed with new carbon cone speakers

This summer the shipment of our brand new carbon fiber speakers finally arrived.
The first listening test worked out quite promising, so we decided to upgrade our LouReed systems with these babies.
Nico is going to use a set of these speakers for some complicated acoustic performances during itgwo , so the design goal of the adjusted filters would have an emphasis  on producing a clear natural sound.
As opposed to rocking your guts with e.g. Rammstein, that is.
Probably Sebastiaan will call this boring (I guess) :)


By looking at the speaker you might expect a really full bottom-end with long excursion bangs of the speaker like 'car' hifi speakers do.
Not so!
Fortunately in this case 'cause that would bring also a lot of intermodulation distortion. Think thumping yourself on the chest while singing. Not looking for that in this case.
So why a rubber surround? 
As is the case with al stiff materials in speaker design, shurely they sound a lot better: stiffer cone -> stiffer piston movement -> less break up distortion.
But when they do breakup (and at at 'our' levels they always do): HORROR.
Remember the 2' titanium vs 2' aluminium driver diafragm 'upgrade' problems?
Or the special sound of that A brand line-array with yellow speakers for everything, when you hit it real hard?
Well here comes the genius of our Chinese friends: the rubber surround will absorb those hard breakup modi! Brilliant!
 
I also tested a other speaker in the same cabinet.
From the front the cone looks like your standard 12' (guitar) speaker.
Soundwise this gives you that nice zzhangg you are looking for in metal guitar a la Rammstein.
Not that pak-pak from a modern plastic coated speaker (think 'french' PA's).
So what's the Chinese ingenuity here? Back Coating! 
Al the advantages of a stiffer cone by coating without the ugly breakup. Again brilliant!



So in fact we now have two recipes for our LouReed systems
Does this mean a different magnitude/phase slopes of the overall response?
No, certainly not, actually the final results show quite simular curves.
Does this mean different EQ needed to match or linearise the responses?
No, also certainly not, actually in both designs zero EQ was applied.
(to the speaker that is)
Shurely, different program material makes me want to apply different micro EQ (no more then 3 dB, Q<2, that is), but I always end up in bypassing them again.


Which will have to bring me to the next topic:

Distortion matching as a design goal in loudspeaker tuning


oh, euh, yes, by the way: all the time we are talking distortion here I mean that very-fine-grained-hardly-noticeble but still quite -important-emotion-wise effect for which I don't know how to quantify else then by using my ears...

Wednesday, 9 July 2014

Valid dual FFT measurement

On several instances last week I have been talking to people how to acquire valid  dual FFT measurements.
It really stroke my how much miss understanding there still floats about, so here we go:
Crash course measurements in KBLsystems-style:

First::
Dual FFT means we can finally get rid of that annoying pink/white noise at terrorising levels.
Just use your favorite music!
As long as it contains enough spectral content it will be a perfect source and you can use both eye and ear in your system tuning.
Keep a keen eye on your coherence tracer to see if your program material works.

Second:
Surely you can use the 'delay finder' function to find the overall time difference between the reference signal and the measurement signal, but be aware that all software will make this snap to the maximum of the impulse response which can be
a) quite challenging when the IR is smeared over time e.a. in a filtered sub respons
b)  will yield the 'wrong' phase response

Let me elaborate the latter:

 

This represents a close mic measurement of mid speaker. Clearly one can see the low end magnitude roll-off and it's associated phase behavior of a small speaker in a closed cabinet.
In the following picture the above graph shows  the IR with the delay finder nicely snapped to the maximum of the IR:


We will now show you why this is wrong.
The high end roll-off in magnitude which is (partly) due to the inductance of the voice coil should also find its counterpart in phase behavior. But in above graph one is to believed that this would not be the case.
Next picture is a simulation I made:


The black ine is the imported measurement and the red line is a constructed model based on this measurement.
The thin red line show the associated (min.) phase behavior.
So the measured phase should look like this. In fact the thin black line all ready shows the 'correct' trend.

Now how do we acquire this?
Simple! 
Adjust the delay finder by hand to the ONSET of the impulse response:


Tada!
Valid measurement!
Now break your heads on what I 'm actually doing here <grin> ;)

Friday, 6 June 2014

business as usual

Doing some 'real' work at the moment:


Small PA set for new venue almost finished.
Used two of the top cabinets I made previously with those home brewn horns and a straight forward QB3 sub.
Amazing amount of bottom end from only two 15's!!
Again hooray for  A&D audio S15G710:





Also got that wireless controle of Soundweb 9088ii (RS232 to USB over ethernet) going, thanks Koos for the idea:


O, and for those interested: here is Soundweb design, yup multiband limiting with FIR!
Funnywise it triggers the DSP overload detect circuit so the little red led is on all the time.
Well some gaffa tape will remidy that ;)

Wednesday, 7 May 2014

Current drive??

So we were on holiday.

Gave me time to rethink my last struggles with passive filters.
For years now we are aware of audible effects of components which do not directly relate to the electrical parameters of that component.
Sonic differences between the blue and the green cap..(green is always better :)
Also a conversation with a real horn loving tekno-head (those guys terrorizing former eastern europe countries with big piles of often home brew PA systems) and his love for Labgruppen PLM20000 amps made me rethink the whole concept of 'damping factor' in power amps.
I also recalled a remarkable experiment with (big) series capacitors on a (to us) well known and often used 10" speaker with my audio companion Sebastiaan.
And last but certainly not least all my experiments over the years with tube amps..

Serendipity made me stumble across:


A very well written book about the fact that the concept of a loudspeaker as a voltage to SPL converter is in fact totally wrong.
The principal of a electro-dynamic loudspeaker is based on:

F=B*i*L
Remember from high school? (and the grins: bil means buttock in dutch).
No V for voltage in this equation!

So today I did this experiment:

Two small hifi -isj cabinets (only the low speaker) one connected directly to the poweramp, the other one with a series resistor of 50 ohms to simulate (a partly) current source. To compensate for the obvious volume difference I made a quick Soundweb file, this gave me also the possibility (try) to equalize the magnitude difference caused by the impedance curve of the speaker in series with that 50 ohm.

Acoustic recording from a Argentinian choir craftly made by our Sebastian, and:

WOW!!
This is something!
Wierd things happening..
More to follow..







Wednesday, 23 April 2014

Line Array


Yup, behold and wonder finally, after the whole pro-audio world is all ready (ab-)using them for years in all kind of matters I finally also start yabbering about them:

Line Array's


Or actually  curve linear arrays or whatever you like to call these boxes on a string..
There has been enough talks about the 'working' of these and actually, there is not much to say since mr Harry Olson did his thingie all ready in 1957 or so.
The one model that is definitely not going to work for me is the pie shaped dispersion nonsense.
So clearly we need a string of small loudspeakers in a very close spacing relatively to the frequencies they have to reproduce.
And furthermore a total array length of say at least 2.5 mtr to do anything at the, for us metal heads , so beloved low-mid frequencies.
That will give us the combination of one 12' and two 6' speakers in a cabinet of 35cm high. Which gives us a array length of more then 2 mtr with 6 units.
Not a big problem, and our FIR filter skills will prevent the comb-filtering in the horizontal plane.

But then it is 2014 so we apparently also need some high end..
Well lets start of with making a 'waveguide' (duhh) for these:


Tada..the first experiment.
A bit difficult to do large scale measurements in a shed from 4 x 5 meters so we will have to resort to some brain action:

 Let 's assume we have a straight array of 2.5 high, suspended from the ceiling with the lowest cabinet at ear height. (totally wrong I know, I know)
At long distance the waves from every unit in our will arrive at more or less the same  times.
If we approach the array to a  distance X, the path length between the highest and lowest unit in the array will start to introduce cancellation.
Take 3440 hz as a crucial frequency, so with 0.05mtr path length difference this will yield to a full cancellation.
Calculate..calculate.. any closer then 62mtr this will give grieve!!
Now pull back and tilt the whole array for b cm.
With b = 10 cm you can move up to 20mtr, and with b = 20 cm up to 12mtr
Conclusion: All ways tilt your array!

(there is a caveat though!, soon more to follow)



Thursday, 27 March 2014

LouReed rises again..

More over one and half year ago one of our 'LouReed' sound systems was stolen from a parked van.
Surprise, surprise  a attentive civilian found this in the woods recently:



Shock and horror, our meticulously crafted cabinets tossed away!
Probably not useful for the people who stole them as they need to be operated with a separate (FIR-filtered) X-over to get anything like alone a decent sound.

Today I have been refurbishing them and to my really big astonishment: the 12" speakers are still full-spec functional. Hooray for mr Simon Leung (A&Daudio) for the excellent manufacturing quality!
The 1' drivers were less fortunate, not that they would be of lesser build quality but of course these are a lot more vulnerable.

So here comes LouReed Phoenix..
While we are at it we will also update the FIR filtering for these cabinets.
For those interested in FIR: Thomas Drugeon (AKA pos) made this excellent utility:
rePhase




And, oh, yes, I also made a quad amplifier using Hypex UCD400 blocks and a switchmode PSU.
If now we could find some decent DSP modules (with FIR!!) we then finally could make our powered cabinets.. (nah, sorry no minidsp, nice but not stable enough for our "pro-audio-abuse").
Might have an other idea though..will get back to ye.




Wednesday, 19 March 2014

SoundCube


Two cabinets finished!
You will just have to believe me: they sound 'awesome'.
Now listening to  a 24/96 live + orchestra registration of Hooverphonic, excellent,  thanks to our Russian friends from rutracker..!


Measurement on +/- 1mtr.
The bumpiness is due to (small) size of my shed, clearly min. phase interaction of reflections.
Mark the straight phase response though, unreal for a passive filtered cabinet!
haha, FIR filters suck my fat one!

Next will be molding the multiband limiter. (Soundweb9088)

Thursday, 27 February 2014

passive filters aarghhh

Working for 3 weeks now to get the passive filter going with this small cabinet I made previously.
Well known 10' speaker frequently used in all kind of boxes (monitors) but this time with a faital horn to get some more throw.
Uptil now no brilliant results... aarggh...

Today 16.00, joepie, breakthrough, replaced the faital with my own constructed horny (see pic) and now we are getting somewhere.