PMC LB1 a historic investigation in legacy sound, part 2

 Part 2

 ..on wards..

So now what will we do? Upgrade that filter with audiophile components?  Redesign it with what we know now? Or even use some DSP to make an active system?

NON of the above.

Audio reproduction is a construct so if we want to know why that box was so successful in those days we have to thread really careful!

So what I did was replacing those burned resistors with beefier ones of the original value. However, I did get rid of that mystery non working 100nF to make some room. Purists forgive me if you hear a change in the 200kHz region.

I also replaced that electrolytic cap with some MKT  I had lying around. We all agree that (old) electrolytics are a pain, right?

They serve as impedance-equalizing for the woofer so not in the 'signal path' anyway.

So how does it sound?

At first I was a bit underwhelmed, so let's look at the measurement to see if I perhaps made a mistake by swapping polarity of one of the components . (note to self, why don't you ever mark down how things where connected?)

No, totally correct. As expected: perfect all-pass behavior. Also the inverted tweeter experiment gives a deep null.

It does sound that way too, like a well balanced loudspeaker. Albeit it bit dull and boring perhaps to much zero-phase-fir-filtering for me lately?

puzzled...

 

..So let's turn it up and see what happens...

AHA !! Euphonics !! I can do euphonics, after all I have been a live sound engineer for many years. So I need a bigger amp, as I was testing with one of my whimsy yet brilliant sounding gain-clone-single-chip amps. 

Unfortunately I don't have some spare Bryston lying around but I do have some class A amp from a prior experiment.

Now stuff starts to be clear why people have blown these speakers into smithereens: they just say more more more more more..

More off what? More of that fantastic low-end! That tweeter I do know: not the best 7kHz I ever did hear, but that transmission line is real fun!

Here you go: measurement very close to the port. You can see the smoothly playing 60Hz (such nice frequency)

But it does look a bit worrisome around 220Hz, right? That's a general problem with transmission lines, maybe I can repair that a bit with some more experimental stuffing

Now will this loudspeaker work as a generic monitor loudspeaker in 2023 ?

We would first have to define what a monitor speaker is. 

Contrary to some big names in loudspeaker-design-industry (not studio people), I have the feeling that working as a mixing engineer is also a part of the creational phase. Not re-production as in playing back the end result.

So a studio monitor is not there as a reference of the end result but to make you do your creative part in a stimulating way. And press you to reach further, higher, better, newer.

These monitors came into fashion when we (in live sound) started working with band pass (6th order) subs..and hey: let's try some Portishead / Massive attack and drum 'n bass (Propellerheads!)

...woohaaaaa... this is the shit..damn!

How revealing is that!

Now does recent music like Billie Eilish, Whispering Sons, (modern) classical music work? nah, unless you really enjoy that '90's sauce on everything.

That being said: some of these sound system blokes are experiment a lot with 1/4 wave sub, as if it is something new.. So who knows what will come into fashion again, after all everything seems to go in circles.

 

And another day of listening + experimenting: Red line is port measurement without any additional stuffing, whitish line is with some extra 'sheep wool' stuffing. Sure: low end is cleaner + sounds more tight with the stuffing but the 'fun' is gone.. 

 

More research is required: I will use the extra parts to make a home brew TL and in the meantime get a better (DSP) crossover because the groupdelay of that conventional filter is starting to annoy me. The restored boxes will serve as a reference (in my memory database) to that.

Keep checking!

PMC LB1 a historic investigation in legacy sound, part 3

 Part3

 

As I mentioned in the previous post: I was getting annoyed by the overall group delay as it is introduced by the text book IIR (LR 24dB/oct) filtering.

So here's the experiment: home brew TL with a very cheap 4" speaker. I totally forgot to take pictures of the making, not that interesting at all: it is a standard 1/4 lambda backloaded TL with some folding. 

Very similar to the PMC design

The big difference will be the filtering: I used one off my DSP boards to make a overall zero phase shift X-over using FIR filtering. How to do so will be a topic for a different series of post. Soon. Maybe.

 

 

While presenting them to my audio peers it became clear to us that there's no way back to IIR filtering once you have tasted the reverberant field as it is presented by a zero phase system..

But that wasn't the topic of this experiment. We where investigating the sonic properties of a Transmission Line. Funny coincidence: right at this moment the Amsterdam Dance Event is happening and all newspapers are full with interviews with the current stars of EDM. Accompanied illustrations show pictures of studios with big (PMC) transmission line monitors. 

So this is hot stuff at the moment.

 Now how does a TL sonically compare to a different approach?

 

Very same speaker, different hornloaded tweeter but we can ignore that. Literally. We have been practising listening for some time.

In this small cabinet I used a digital implementation of the Linkwitz Transform to get some LF response squized out of that wee speaker.

In the STM32 part of this blog I did talk a bit about the intricacies of this.

So we now have two things to compare AFTER each other. As always when listening and evaluating audio stuff: you have to remember it for a few seconds. Which isn't simple and definitely needs some training..

 

 

How do they compare?

Purple line is close to the port of the TL (transmission line) while the green line is close to the speaker of the LT (linkwitz transform) closed box.

Clearly the TL has quite some lower response around 60 hz (again, such nice frequency) all though at the cost of a bit funky phase response.

Forget the resonances at 180, 300 (hey..odd harmonics..hmmm) they certainly spoil the fun in this experiment and are the main reason that some music really shines while other sounds horrific. Missing fundamental psycho-acoustics at play here.

But we are not finished yet.

 

I totally like that 60hz. From such a small box. So I made another, bit bigger TL with a 10" speaker. Right at this moment it is playing at my feet under my desk. Also at actual quiet levels it really does bring an extra dimension to the genre I am currently digging in:

Hauntology 

(EDM is too boring for me)

So you can get an idea of the sonic landscape.

Now what about those pipe harmonics in this setup?

Well, very steep FIR filtering at 80 Hz proved to solve that.

How? By using down sampling to a sample frequency that doesn't take zillions of coefficients to get the desired resolution of the linear phase filter.

Naturally (a lot of) delay has to be added to the (high end) desktop speakers. 

And now things get really interesting: 

I never was a fan of 'separate' sub woofers. Also in live sound. The setting of delays from subs to mains or vice versa always is giving you grief when they are some distance apart. Most certainly if you understand what phase-alignment involves.

Why would that be as wave lengths around those frequencies are in meters? So how could some centimeters make a difference? Well maybe the (much higher in frequency) harmonic distortion produced by subs have to be in time with your main?

With the above very steep filtering I could, at least at lower levels, get rid of that distortion and now the sub is behaving as it should: it seems as if my wee desktop speakers are producing a tremendous amount of accurate bass. Most certainly with instruments I know well (like bass guitar)

I also tried a similar setup with a conventional cabinet, but that doesn't give you the same results: a TL has a lot more efficiency at the freq. of interest, meaning the cone has to move less and thus produces less distortion compared to an (augmented) closed box.

So that leaves only the quirky phase response (at 60-70hz) from a TL to be evaluated against program material. (I could differentiate it to get the groupdelay to get a better insight)

Let's see..

..Audio reproduction is a construct..

Making a DI box

Over the past decades making a DI box has been one of my recurring activities.

From repurposing scavenged  transformers into make shift 'passive' boxes to  experiments with unbalanced to balanced chips.

The problem is phantom supply. 

All though rated at 48volts it's purpose as a PSU for any circuitry is really limited, because the way it is implemented in almost every mic pre-amplifier:

The 48 volts is fed through 2 resistors of each 6k8 in each leg of the symmetrical input, thus giving an impedance of 3k4. Which means that if your circuit draws say 10mA of current only 14 volt of that supply is left to begin with.

Your own circuit will have some resistors in the supply lines too, to unload the output from your circuit. So the voltage to work with and thus the headroom of your DI will even drop more.

Hence the really limited headroom of a lot of modern, commercial DI boxes

Old skool '70's electronics to the rescue: 

 

This circuit is no rocket science and well known in every pro-audio circle, mostly credited to Bo Hansen..

Quite a few mods though, just as a reminder for myself a picture of the early limited (as in only 20 pcs of that specific transformer in my very old stock) prototype:

 

 

 

 

By popular demand we decided to try to do a reissue fairly recently.

The problem was finding a nice affordable transformer: these Swedish ones are really nice, but man, do they charge..Pure by coincidence i stumbled across OEP. Now those i do remember from the classic BSS AR116 DI. (Which has nothing to do with the modern AR133 !!) And with some googling I found them to be used by modern Telefunken as well (speaking of brand name marketing!!)

 

So here we go a batch of class A, current driven, transformer balanced DIs in a stainless steel housing. 

Yes, stainless steel. 

Every DI gets abused once and awhile as a stage weight, box tilt, or even as a hammer so to prevent them from changing into ugly lumps of corroded, dented metal, we folded and water jet cut some stainless steel sheets:

(of course I didn't do it by myself: credits to Koos Roadservice)

 

 

 

 

 

 

Still not totally satisfied by the end result I learned myself how to powder coat (not difficult at all) 

 

So here it is..open for orders now..hahahaha...

KBLsystems class A transformer balanced DI