Kef Egg HTS2001

These are good surround speakers but as standard the treble is mushy, and the mid-bass resonates a little too much. I’ve also tried the drivers in MDF cabinets and realised I don’t  like the way the metal enclosure colours the sound. The frequency response charts below give a very accurate representation of the sound – a very uneven bass response below 350hz, a ‘small speaker echo’ at 1.5khz in the mid-range and a little too much high end. The driver has a standard 4″ mount so can be easily replaced or moved into better cabinets. The aperture is 96mm and the raised mount edge outer diameter is 102mm. The mounting points are 115mm apart diagonally and 82mm horizontally. These pics are from a Russian site with some more info about the Eggs:


My project:

The driver is SP1498. It has a second order crossover with the drivers connected in reverse polarity, and a zobel on the mid-woofer. The crossover mounts directly onto the woofer terminals. The prominent mid-bass can be tamed with acoustic damping foam, cut to shape, and inserted at both ends of the reflex tube. For my taste, the foam is 2cm deep at the internal input end and 4cm deep at the output end. The mushy treble is fixed by adding MKP1837 or Relcap RTE bypasses on the electrolytic caps.


The tweeter has a 2.2uF series cap and a 0.4mH shunted inductor. This works as a Linkwitz Riley high pass for a 6.8ohm driver at 5400Hz. The mid-woofer has 0.68mH series inductor with a 12uF shunted cap, and a zobel shunt of 8.2R + 15uF. This works as a Linkwitz Riley low pass for 3.8ohms at 1800Hz. It’s not unusual and should be relatively easy to upgrade to bi-amping. The issue with the actual crossover is the need to change the components – the main inductor for the mid-woofer is a ferrite core and all the caps are electrolytics, most with a +/-20% tolerance. I measured them and only one had the correct value – all the others were at the limit of their tolerance. That’s a huge variation and means the two speakers sound similar but not the same.

So after fitting bypasses, sometime later, I went back and replaced all the electros with Wima poly caps and the ferrite core inductor with a Solen 16awg air core 0.62mH inductor. This sounded much more revealing and got me wondering how good/bad the cast aluminium egg shell enclosure is. So a few years later, I mounted the drivers in large 5 litre MDF enclosures tuned to 70Hz, and got an even better sound, even with the crossover. I like the looks of the metal eggs but it seems they are not the best use of the drivers. Besides, the new egg boxes look good too, and because the sound is much improved, this is how they are staying.


Anyway, back tracking, the main plan was to run them wirelessly from the source with an active subwoofer, and bi-amp them. (Later, I ditched the wireless due to the latency delay).

The bi-amp was very successful. The circuit is a second order like the original, built with 2.4k resistors, sweet sounding Ero MKP 22nF caps (matched to under 1%) and LME49720HA op amps. As it turned out, the resistors values had to change to 1.2k on the high-pass and 4.8k on the low-pass to match the original sound :


The sub amplifier was as cheap as the transformer on its own so I didn’t expect much. And the original amp didn’t take long to blow up so I replaced it with a mosfet “L7”. It sounds great – very clean, smooth, enjoyable tonal quality.


“L7” LJM Design. Output power 150W @ 8R DC +/-56V, 300W @ 4R.
Frequency response range 4HZ -3DB, 350K HZ -3DB
THD <0.01% 100W 1K Hz, Slew Rate = 38V / uS, SNR> 99DB
CFP complementary feedback input stage. Floating Mosfet output.
Class AB. Output stage using IRFP240, IRFP9240, MKP coupling capacitor.

L7 LJM amplifier.JPG


The sub-woofer filter section had only 2 nasty op amps but spaces for 3 to create a phase inverter. So they were replaced with OPA2132 and I added Elna Silmic and ceramic bypass caps on the power supply rails for each op amp. There were no bypasses at all before. I replaced the cheap ceramic caps in the audio path with polypropylene.


Besides a lack of bypasses on the power supply, it didn’t have much of a regulated power supply for the op amps either. It used a simple resistor + 15v zener to drop the 56v rails. So, on each rail, I used two 12v 1 watt zeners to boost this to 24v and used 420r 5w resistor to create a basic pre-regulator before the smallest cheap regulator I could find – a small 317/337 psu. It just fits.

The 317/337 uses low noise 15V zeners at the base to set the output voltage and lower the output impedance and noise. You can read about how that works on this site. It elevates the humble cheap part into something quite decent.

I added the 2nd order crossover for the Eggs so the psu was running 5 op amps and the psu itself: about 60mA on each rail. This is about the limit of the pre-regulator.


The amp thumped at on/off, and needed a speaker protection kit based on the uPC1237 to finish it off. That uses 8mA at startup and 45mA when the relay latches. This is too much to add to the op amp circuit so another circuit is needed to drop the +56V rail to under 25V (the limit of the circuit’s caps). So a 20k 1/2 watt potential divider shunts 2.8mA to ground and the centre tap of the divider is adjusted to feed 34V to the base of a Tip41c NPN pass transistor. 2mA is enough current at the base so the hfe gain & base-emitter voltage drop will enable a 33.3V 80mA output to a 7824 regulator, which in turn feeds the speaker protector circuit. The NPN pass transistor gets hot so it needs a small heatsink but the 7824 stays cool so it runs naked. And free. Ahem. Finally, I changed the connectors so there isn’t a speaker-level input anymore, and the empty holes are used for bi-amp outputs.

And I’m pleased to say the outputs are clean, clear with no audible noise. It looks a mess but in fact works very well.



So with the active sub done, it was time to bi-amp the Eggs. I like Tripath amps so I got some very good value SKN TA2021 amps to try out – just US$20 each with air core inductors, good decoupling, output relays, etc. After burning in for a few days, they sound great but have a little background noise with an ear against the tweeter – as expected. So I didn’t think they needed any modifications at all, and the only annoyance were the tiny speaker terminals, making them tricky to tighten, so I had to use speaker plugs. I made a regulator from an AMB Sigma11 and 80VA toroidal transformer and fitted it into an old ATX psu case. 80VA is the minimum – with the crossover at 3Khz the tweeter will draw about 3W when the woofer is drawing the maximum 20W – it’s a 15/85 split @ 3Khz. So the amps will output 46W and they are >80% efficient so that’s a maximum 60W load on the regulator/toroid. In addition, these are surround speakers so I won’t be using them anywhere near their maximum.


I hooked it up to listen to an original Egg vs the bi-amped and the bi-amped speaker had far too much output over the crossover range no matter whether the tweeter was phase inverted or not, and it sounded like the tweeter output was too much. I replaced the 2.4K resistors in one channel of the active crossover with trimpots and adjusted them until I got a reasonably flat response. I did side-by-side listening with an original Egg and adjusted until the sound was very similar to the original. Then I removed the pots and measured them – the low pass used 5kR and the high pass 1.2KR – so it seems that doubling/halving the calculated values is necessary. I fitted 4.8K and 1.2K resistors and that worked like a charm. I had also dropped the tweeter output with 2R 5W resistor in series and added a zobel on the mid – 9.4uF +8.2R. The zobel caps were later upgraded to Wima MKS 4.7uF with Vishay MKP1837 10nF bypasses. The bi-amped speaker sound is very similar to the original Egg (just a little warmer with less prominent upper mids), and is a little cleaner (less distortion) and more natural (less obvious resonance) so I’m very pleased, and considering it’s cheap to do, I think it’s well worth the effort too.

While modifying the Eggs, I tried out different wadding to replace/add to the foam in the originals. I found that a thin, back layer of thin strand poly fibre seemed to knock back the upper mid range resonance, and replacing the foam with thick strand fibre damping helped relieve the “small speaker” sound in the lower mid range. For bi-amping, I needed new speaker terminals so I drilled a 4mm hole in the rear egg shell to screw on some new terminals. The cable output holes were sealed with Blu-tack. All done and sounding great!

p1070744p1070747p1070746p1070748The sub is a little unusual – I had to fit one under a sofa so I got two 4″ inch subwoofer drivers from Taobao and my first effort was a push pull design with drivers at each end of a pipe. However, the drivers were too far apart and this created strong nodes and anti-nodes. So next I mounted each on a 90-degree, 100mm diameter pipe bend, at the end of two 2-metre long pvc pipes – the pipes have a quarter wave resonance of 43Hz at 23 degC. This makes two transmission line speakers to maximise the drivers’ efficiency. The open ends of the pipe are slash cut at 45deg because this stops the pipe booming – Google Helmholtz resonator and edge tones of a pipe for more on that.

While the speakers fit under the sofa, the amp did not, so I made another using a Tripath TK2050 for the Eggs and a mono TPA3116 in PBTL that has an adjustable sallen-key subwoofer crossover filter.  I also put the Egg drivers into bigger cabinets (they sound soooo much better) and fitted a pair of cheap 4″ full range drivers into the Eggs – these were far too warm so this was fixed with 100uF and 8.2R connected together in parallel and wired in series with each driver. This knocks of 8.5dB at the warm end while leaving the high treble unaffected – it re-balances the sound, but sadly not well enough to be keepers. They are clearly not full range despite having a whizzer in the centre and also, they don’t like the egg shell enclosure. Here is the 4″ replacement:

4inch driver

I found some very pleasant sounding inexpensive 3″ coaxial drivers that work well in a small enclosure so I made some MDF adapters to fit them into the egg shells. They only need one capacitor as a crossover, 2.2uF to 3.3uF depending on taste, because the drivers are so well matched – both are 83dB and the woofer falls off above 5kHz for a -6dB crossover point around 6.5kHz. They sound very good in the eggs’ metal boxes – a decent bass extension, a little on the warm side with 2.2uF, but not unpleasantly so, with no obvious peaks or troughs in the response in the mids or treble. Great little drivers!

3inch coaxial

The brand is Suowei / Sovox and the model number for 8 ohm woofer and tweeter is  SVF078WR-88-068-098 for square and SVF090 for round. There are also various 4 and 8 ohm versions.

3inch coaxial freq reesponse