EEVblog #1162 - Little British Monitors

[EEVblog]

Review and teardown of Little British Monitors, a small compact bluetooth monitor available as a kit on Kickstarter:
https://www.kickstarter.com/projects/1439311797/little-british-monitor-mkii-reference-micro-speake

[NiHaoMike]

Get out your spectrum analyzer and do a comparison against some $50 stereo speakers.

I would have liked to see an option for wired digital input (S/PDIF and/or USB) in order to eliminate the need for a decent DAC. The piece of wire attached to the Bluetooth module looks like a tack on with no RF knowledge so it would be interesting to see if removing it actually improves RF performance - yet another use for the spectrum analyzer. As for the heatsink being positioned with the fins horizontal, that's a design fail - it would perform much better with the fins vertical. (I'm in the process of evaluating a Silverstone passive cooled PSU with the same design mistake...)

I also note that you didn't comment on the PCB designed with an unlicensed copy of DaveCAD, and there doesn't seem to be an AvE reference I saw in other videos about that speaker.

[darik]

When they don't have XLR I feel like they should be called minitors. Or monitettes.

[johnlsenchak]

To bad that the amplifier  board  didn't  have discrete  transistor  driver  and   output power  stages  for each channel , instead of one of those Class A/B  modules  which  have a lot more T.H.D.  when driven hard


T.H.D.=  Total Harmonic  Distortion

[ecat]

been a while

The vid prompted me to check my memory of this little speakers spiritual great grandmother the ls3/5a, currently selling 2nd hand for >£1000 !!

On the way I found the following thread discussing why the BBC went in search of thin walled cases and why they apparently work so well.

https://www.harbeth.co.uk/usergroup/forum/the-science-of-audio/speaker-design/215-bbc-style-thin-wall-cabinets-why-so-special

enjoy.

 

[flash2b]

Around 22m there is a 'bug' in the video and some remarks about the Weller no fuse problems are audible. Look like some editing go wrong.

[AmmoJammo]

Yeah, that's really not a transmission line....

The 4x50watt power claim is interesting as well...

[inOr]

The low frequency response of those woofers doesn't look like anything to write home about - falls off abruptly below ~100Hz.  Do recording studios really rely on such as these?  Or do they add a big woofer third channel?

[AmmoJammo]

The low frequency response of those woofers doesn't look like anything to write home about - falls off abruptly below ~100Hz. 

Looks more like 65Hz to me.
Still not ideal, but for most "pop" music, people aren't really going to care.

[oPossum]

I saw the title and expected to see small CRTs from Sir Clive.

[b_force]

Yeah, that's really not a transmission line....

The 4x50watt power claim is interesting as well...

It is not, I just modeled and simulated the speakers and for this little volume you just need a pretty long bas-reflex port.
Which "magically" is exactly the length of the port in the design.
These little Dayton woofers are actually pretty nice.

300 pounds for these things is a big scam though.
The PCB is dirt cheap, no DSP is being used, only simple opamps with some of these Class-AB audio chips.
The woofer is a Dayton Audio ND91-4 which can be ordered very cheap in higher number (100-500 pcs), the tweeter isn't acutally a tweeter but a full range driver from Tymphany, looks like either a PMT20 or PMT30. Those little buggers were optimized for budget fullrange solutions for TV sets (or those cheap computer speakers).
Go for around 3-5 bucks a piece (you can find them on digikey)

Put some costs for some simple plywood and stuff in there and you're done.

Anyone who is only slightly involved in acoustics and audio knows this ain't the right price at all (and can probably guess the used materials, parts, speaker units, since they are VERY common)

[b_force]

To bad that the amplifier  board  didn't  have discrete  transistor  driver  and   output power  stages  for each channel , instead of one of those Class A/B  modules  which  have a lot more T.H.D.  when driven hard


T.H.D.=  Total Harmonic  Distortion

Well, that really depends how good (or bad) your discrete design is.
But technically you could make a composite (or nested) amplifier to get a VERY low THD (if that is what you're looking for)

[retrolefty]

OK Im old, but what I see is one volume control for both speakers. So how would
one adjust the "balance" for proper separation which is  dependent on listener location and room response?

[Tom45]

OK Im old, but what I see is one volume control for both speakers. So how would
one adjust the "balance" for proper separation which is  dependent on listener location and room response?

I would think adjusting balance is a job for the preamp that is feeding the speakers. Are you aware of powered speakers that have a balance control? I've never seen any with that feature.

[Fungus]

Around 22m there is a 'bug' in the video and some remarks about the Weller no fuse problems are audible. Look like some editing go wrong.

 

[Fungus]

Here's a video of a girl building a pair:

[b_force]

Here's a video of a girl building a pair:

A pair of rubber tires  , jeez

[HKJ]

Here's a video of a girl building a pair:

Definitely a male attraction dress she wears, but if see does everything by herself she is fairly good at building stuff.

[bsdphk]

I can't see why they would not be classified as transmission-line speakers?

TL speakers are a rarity, which is probably why Dave seems to wonder if it is even "a thing", and it is certainly both unusual and probably hard to get right in such a small enclosure.

Their rarity is due to not many woofers having what it takes (see below) and because there is (still!) no way to create a good TL design, except trial and error.

The best-selling TL speaker design ever was probably Bo Hanssons "Rauna Njord", which were made from concrete and therefore weigh 35kg each. (https://vintagehifiproducts.wordpress.com/2014/06/19/rauna-of-sweden-the-story-continues/)

TL designs often have high cross-over frequency.  The physical requirements of a bass unit to drive a transmission line are almost the same as for full-tone units in horn-enclosures, and if you have a full-tone unit as bass anyway, you can get the cross-over out of the midrange by pushing it up, instead of down.  The Rauna Njord's cross-over is 3.5 kHz and the tweeter is mounted coaxially on the lower of the two bass units.

TL designs and some horns can be used with current-drive, where the amplifier delivers a current rather than a voltage proportional to the signal.  At least in theory this is better, because the magnetic field in the speakers coil depends on the current, not the voltage.  In practice it takes a lot of experimentation.  See also Nelson Pass' F1 and his writings about it.

[Fungus]

Here's a video of a girl building a pair:

Definitely a male attraction dress see wears, but if see does everything by herself see is fairly good at building stuff.

Her FAQ is a refreshing read: https://pastebin.com/V3474kYs

[AmmoJammo]

I can't see why they would not be classified as transmission-line speakers?

A 20cm long port, does not a transmission line make...

[b_force]

I can't see why they would not be classified as transmission-line speakers?

A 20cm long port, does not a transmission line make...

That's correct.

Although there is a grey area, but in general a transmission line for around 65Hz would be a lot longer.
Also the placement of the woofer and the "entrance" of the line would be different, as well as that most transmission lines are tapered.
(technically you can also get a straight transmission line)

Like I already said before, I simulated this speaker as a ported design and basically came to the same port length.

[retrolefty]

OK Im old, but what I see is one volume control for both speakers. So how would
one adjust the "balance" for proper separation which is  dependent on listener location and room response?

I would think adjusting balance is a job for the preamp that is feeding the speakers. Are you aware of powered speakers that have a balance control? I've never seen any with that feature.

I am old school and prefer passive speakers and separate electronics.

I have seen powered speakers where each speaker has it's own volume control, making balancing possible.

[bsdphk]

It certainly does, the question is only question is at what frequency and how that sounds.

First of all, I judge the actual transmission line to be approx 12+15+5 = 32cm long.  It goes along the bottom, up behind the partition and down in front of the partition to the woofer.

The line is also very heavily damped, so the speed of sound is severely reduced, probably as much as 40%, and that brings us to a plausible TL frequency of (40% 340m/s / 32cm ~= 425Hz)

I'm pretty sure the high aspect ration of the TL, something like 1:7, will also increase the wavelength, but no idea by how much.  (Ask any microwave plumber:  There is a reason wave-guides have low aspect ratio)

The TL port only needs to be N+½ wavelengths, where N is allowed to be zero, which improves the impulse response, but there are trade-offs.

The bottom octave of a TL design is delivered by the port, and from around 60Hz these speakers drop dead in precisely the way TL designs do at their bottom.

If the port kicks in around 120-180Hz and a half wavelength of the port below 210Hz is well within the uncertainty here.

All that said:  A speaker which goes silent below 65Hz, can be wonderful for speech, lieder and chamber music, but a bass-reflex design of the same outher dimensions, would at least pretend that the double- and electric bass were invented.

[AmmoJammo]

Hey, if you wanna call it a transmission line, go for it!