preamble… | |
 | Recently, I’ve been out shopping and noticed those amazingly inexpensive pocket radios on sale. I don't mean the ones for £10.00, I'm talking about the ones that are on sale for 99p or less.I have wondered for a while now how they can be so cheap. I've bought a few just to dissect and find out a bit more about them. It's quite interesting ...Almost all of them seem to use the same chip, the SC1088. |
| The SC1088 is a clone of the Philips semiconductors ic TDA7088. This is a chip made by Silan Microelectronics, a far Eastern manufacturer of IC's and semiconductors since 1997 in Taiwan. If you are interested you can visit their website here. |
| Okay, enough twaddle... what has all this to do with anything? |
My goal was to produce a reliable FM radio receiver with a really nice sound, to my ears anyway. Read on for details on my quest for perfection. Oh, and for all you perfectionists and thermionic valve enthusiasts, I'm not talking about building an audiophile's dream system here, I don't think I could... I did what I set out to do and that was build an tuner/amp with which I was happy with... |
In the Beginning …In the beginning, Woolworths plc closed down ( that was sad, really ) however everything was sold off at cost... I purchased a few FM Broadcast Band Auto Scan Radios for £0.67 each... I took one on holiday to the Philippines, it didn't work that well. I gave some away. I took another to pieces and destroyed the SC1088 chip (idiot). I decided to try and do something constructive with the remaining one... |
| Shown right is a close up of the SC1088 IC on the 1st board. I used a similar a board in my final build. As the sole objective was to have a custom audio amp I took the audio directly from the o/p on pin 2 on the SC1088. In this picture I have an earphone connected to the board, this is an Ariel only. |  |
| The SC1088 has a typical THD of 1% and a s/n ratio of 56dB… not startling but really not bad at all. As you no doubt know these chips were designed for portable use. To quote the datasheet, ‘It is used when a minimum of peripheral components (of small dimensions and low costs) is important’... |
 | This is the board I finally used... the IC is very small and surface mounted and the prospect of desoldering and remounting something this size did not appeal to me so I reused the board. If you have ever reworked a cheap board like this, you will know, it's a nightmare... The tracks lift with even a small amount of heat. I hate to imagine what it would be like building these things for a living... yes, I do know where they come from! Also on this board there was a decoupling cap to allow the headphone lead to act as an Ariel, I removed this. Also, quite strangely there was a single transistor being used to increase signal gain from the Ariel, I also removed this... the result was a more stable reception, especially on weaker signals. it was already beginning to sound better... |
| Shown right is my reworked board, built up to the spec. on the SC1088 datasheet. All unwanted poweramp components have been removed from the board. Now 1.41mA current draw. On the final build I designed my PCB in such a way that this board was ‘piggy-backed’ onto it. |  |
Power Amp (decision time) | |
 | Hmm… small cheap Mono FM receiver IC, and, I can go in any direction I want with the power amp. The shrill sound of a 2 transistor amp from a pocket radio was definitely out. I decided on an IC power amp… something easy to design around and importantly it wouldn't require an exotic power supply. To be honest most of my time on this project was spent evaluating power amp IC’s. I tried several and even constructed the power supplies to accommodate them. In the end I decided on the chip I had evaluated at the start. The humble TBA820M. Class B, 2W @ 12V/8ohm, around 1% THD and frequency response 25 – 20,000 Hz (120pF used instead of 220pF) & no switching cross-over distortion! |
| I deviated quite a bit from the datasheet, mainly with the following: |
| Ripple rejection cap omitted. Gain lowered considerably. Bootstrap cap increased. Frequency compensation cap set at 120pF. Now, this set up suited my source and my ears… I welcome comments on this, both for and against… |  |
| A minutes silence for this lot please, thank – you … |
 | To be honest, I think there is only one TBA820M in this scene… If I remember correctly I dropped 12VDC into the input side, oops! The rest of these are LM741 dual rail Op Amps… they're not so keen on single rail supply, took me a while to get it right. The 1ohm 1Watt resistor was s p e c t a c u l a r … there is a huge hole in the other side… alternative power amp build (TDA2050) apparently it does not like to be kept waiting for an input to be connected. More on these later on… |
Baxandall Active Tone Control … |
| So the Power-Amp design is completed ( although at this stage I'm hoping for improvements when I mount onto a PCB and away from the breadboard)… With the tuner IC powered up and coupled to my power amp the sound quality was surprisingly good. However, I wanted to add an active tone control… I looked online for some ideas and ended up building a discrete based tone control. I admire the designer of this particular take on a classic design, but, I felt that for my needs the circuit was too noisy, producing a ‘tainted’ sound when the bass and treble were turned up fully. The circuit utilised a BC109c on it’s output stage giving an increase or decrease of 6dB. I spent a lot of time deliberating and experimenting on my choice but in the end I opted to use a circuit from an old book I had in my workshop. It uses a 741 Op-Amp and works very well indeed. I did build this circuit using a single rail Op-Amp and while the sound was slightly cleaner (esp. at higher frequencies) I decided to go for the full retro approach and stick with the 741. |
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| Above: Experimenting with the 741… working very well on a single rail supply! | Above is the first tone control I constructed, based on the classic Baxandall design. The finished article was just too noisy… |
| Right: Slightly revised 741 tone control circuit… Sharing the same power supply as the amplifier ( 12v single rail, regulated, 7812 IC). This gives a really nice warm tone to the final sound. Full punchy deep Bass and crisp clear treble… I’m really pleased with the results… Again, I welcome comments on this, both for and against… |  |
Volume Unit Meter, anyone? | |
 | I’m on a retro roll now… I have some LM3915 IC’s on my shelf (they’ve been there a while ) and had always planned to use them for a LED VU display some day. But, I already have an idea for a traditional wooden case for my radio and I’m thinking that the LEDs will be too much. So, with that in mind I have found a really good circuit which uses the CA3140 MOSFET Op-Amp to drive a VU (uA) meter. If you look at this datasheet you will see there is also a +/- 15Db tone control schematic here as well… maybe next time. |
PCB Design & Etching … | |
| So, I wanted everything to be on the same board… except the Power Supply which I purposely made separately. I used Express PCB along with a XPS to image converter. If you try the demo for Express PCB you will see why I used the XPS converter. In the past I used a really good PCB layout program which, I think, was originally made for Win 3.11. Needless to say I could not get it to work with Win7, although it worked well on XP Pro for many years. I like to use iron on etch resist paper.However, since my last project my old HP LaserJet 4 passed away. No problem, I printed using my inkjet onto paper then photocopied directly onto my iron on etch resist paper. It’s not quite so sharp and I did have to clean up a bit around the IC pads but it’s the only way I can still use it without a Laser printer. |  |
 | I usually build up the ground tracks for two reasons. Apart from the obvious it also greatly increases the life of the Ferric Chloride hexahydrate solution. In the picture to the left you can see the board has already been etched. It’s ready to rub down, check then drill about a million holes… There’s 5 8 pin IC’s on here. I'm using sockets, just in case! Recently I've been reading about people using alternatives to the ferric Chloride solution. Less mess and not so toxic. I would be interested to hear of anyone’s in using any other method. |
| Okay, so the board is etched, cleaned up and I’ve drilled all the holes. The large area on the PCB (top left) is where I’m going to mount the small board with the SC1088. I’m planning to ‘piggy-back’ it on here. The 3volt supply will be achieved by using a LM317T regulator IC. I used an online calculator to choose the correct resistors… actual voltage is around 2.78volts. The plan is to have the SC1088 powered up all the time and switch only the Poweramp/ active tone control and VU driver IC. I am adopting this approach the retain the last station listened too. I thought about using a capacitor but a proper one was proving difficult to source. |  |
 | Quickly, here is a picture of my layout for the 12v 1amp regulated power supply board. It uses a 7812 IC and gives around 1.5a @ 12v. The power supply design is based on Aaron Cake’s ‘ fixed voltage 1.5 PSU design… |
Board assembly … | |
| This is always my favourite part of any project… looking at the picture on the right you can see I’ve got all the components mounted… the small heatsink in the far right hand side is the LM317t, fixed at 3v, this is for the SC1088 board. The ‘single shot’ relays are driven by two hefty 2N3053 transistors. The transistors are driven each by two 555 Ic’s. (I did say this was a ‘retro’ project). I used this 555 circuit at Bill Bowdens amazing site. So, that’s my board ready to test… It powered up and worked 100% first time… woo-hoo! I guess that’s down to the fact I spent a long time with most of it on breadboards to start with. |  |
Case/Enclosure construction… | |
 | I enjoy working with MDF. So much so that a few years past I invested in a router… It takes a while to set up, makes a racket and creates a whole lot of dust but the finished results are very good. I used 10mm MDF, from cuttings I had left over from other things. In fact when I think of it, apart from the aluminium volume knob I ordered from China just about every other part has been salvaged or been laying around in my workshop for a good few years… If you don’t already know… the dust from MDF is not good for you, at all. So, I always use dust extraction… Actually it’s an old vacuum cleaner of mine attached to the router dust collector – it works well though. |
| Jumping forward a bit here ( I’m wondering if anyone will ever read this far already) I’ve cut the MDF, used braces inside the case to strengthen it. Assembled using a rapid set adhesive and run over the inside joints with silicone sealant. The speaker cloth is the proper stuff (acoustically transparent). The white panel is fabricated from 6mm plastic wall panelling covered with inkjet printed self adhesive vinyl (single sheet). The button layout was achieved by drilling holes through the plastic after measuring from my only bit of Vero board used in this project… yes, okay… I used Vero board to mount the 4 momentary action miniature pushes… |  |
 | Inside view of the case (prior to minor tidy up)… top right hand shows the recapped SC1088 board finally in place… |
Powered up & sounding GOOD … |
| This is the finished article then… It really has passed all my expectations… I read a lot about case design and the case dimensions I decided on were a compromise between required space and volume required for the Pioneer Speaker I used. |
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| As I said before… Peaky peaks and low lows… ;-) |
| I welcome any comments here regarding this project… here’s a video of the build if you are interested. |
