Wednesday, 12 November 2014

Active X-overed HiFi ?

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

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


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.
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: