I’m feeling guilty for not saying anything on this blog for such a long time, so will write something quick now.
I took some time off to decorate the sitting room; laying a parquet floor and a vintage looking HiFi system (Amstrong 521 amp with Bluetooth inserted inside and a pair of Olle Mirsch loudspeaker cabinets with Jordan JX92S drivers with tuned reflex port – I’ll add photos later), then it was Christmas, then my usual 3 winter cold viruses in a row . . .
Then next job was to work on the speakers for my office/workshop. Some couple of years ago I bought a pair of KEF Concertos locally for £80. Nobody else was interested, I think because the owner’s wife doesn’t like the teak wood look so got him to stain them black. Staining varnished wood is always going to be difficult and they looked a mess. Being skint that didn’t put me off and I have tried to strip the old varnish and stain off but not that successfully. I also discovered that one of them has been dropped at some point and the bottom ‘lip’ at the front had been repaired using rubber solution and a stapler.
I’ve decided that I will paint the cabinets white and have chosen a grill cloth to compliment. In the meantime I have replaced all electrolytic capacitors with polypropylene versions (adding around 1Ω for every 10µF to compensate for the lower ESR, otherwise the filters’ cut off points would change) and mounted it on the rear as it now all takes up so much space. I’ll box the crossovers in before painting and that will reduce the ringing of the lower part of the cabinet; knocking with knuckles from bottom to top reveals a lot of bounce in the bottom half. It is stiffer at the top because the cardboard tube used for the B110 midrange’s compartment holds the cabinet rigid at that point.
There are some other little tweaks but I’ll go into them when I put some photos up of that project. The reason for all the work on them? The sound they make is so easy and listenable, they make my Naim SBL’s seem quite dull in comparison! They must be around 50 years old but still sound fantastic. I can totally recommend any speaker that has the KEF B139, B110 and T27 speaker driver combination.
There’s another project hanging around in the background – a friend of mine is into vintage cars and wants a good quality windscreen wiper timer module, so he can delay the wipers on his old Morris Minor. That design is finished and I’m waiting on finding the right cable lengths before assembly and once we are sure all is good will will put it into production for general sale.
Right now I’m back on the job of designing my HiFi. I had a great idea, went down the wrong road, realised and am now back on the right path. I’ll explain . . .
I have a lot of parts that I will no longer be using (due to design change) and we don’t have a HiFi system for our kitchen so I wanted to put these parts together. I wanted to keep it simple – the only inputs would be Bluetooth for ultimate convenience and a built-in DAC for ultimate sound quality (allowing a computer to be the ‘source’ so delivering high-resolution files to the amp). Plus the smallest footprint possible for both amp and speakers. So the design brief; simple, small, great sound quality, reasonable cost. I then came to the conclusion that many other people would want exactly the same thing – a HiFi system for today’s modern way of living.
So that is what I am doing at the moment, laying out my own amp’s design to get the above; simple, small, convenient, superb quality, affordable. Here is a picture of the layout drawing as of this morning. All parts are in place, all MDF panels are mapped out and I am now adding interconnections, thus making it easier to build repeatedly.
Basic Amp Layout Drawing.
You cannot see the dimensions given on this picture but the amplifier will be 141mm high by 307mm wide and 170mm deep (not including another 30mm for the heatsinks and another 15mm for the control knobs). Output will be 12.5WRMS from calculations so I would say a comfortable 10WRMS per channel in the real world.
The design is totally mono-block, in other words two totally separate amplifiers in the same cabinet with only the earthing being common. This means signals from one channel with not affect the other channel and visa versa = improved stereo separation.
There are four transformers:
- one for each channel’s, small signal front end,
- one for each channel’s output transistors.
I’ve looked at Cookson Gold and believe the design can take the cost of using 0.8mm x 5mm Sterling silver ribbon to connect between each set of output transistors’ reservoir capacitors – you may not be able to understand the importance of this if you don’t make your own amp designs. Note again that the output transistors get a power source all to themselves so no feedback into the signal source end.
There will also be 2 sets of phono input connections mostly because the PCB caters for a couple extra; it make the unit more useable and because it won’t add much to the final price. So the full range of inputs to the amp will be:
- Built-in DAC
- 3.5mm Jack
- 6.5mm Jack (so you can plug an electric instrument in and jam)
- Pair of phono sockets
- Pair of phono sockets
The speakers will be small full-range units with the option of sub-woofer stands to increase height of the full-range drivers so they sit above the arms of any sofas. They’ll also add a bit more oomph.
On the drawing above can be seen a large toroid mid-way up on the right. This is a larger mains transformer than used in the 10WRMS design I am currently working on. It will allow for 60WRMS per channel output and that will be the next design after this one goes on sale (hopefully sometime this year).
This picture shows the layout of the front panel:
Front Panel Layout.