RIAA preamplifier may need more gain

My reason for writing concerns the magnetic cartridge preamplifier design published in the March 2002 issue of SILICON CHIP. In the article, it is suggested that the output of a magnetic cartridge is typically 5-10mV at centre frequencies. However, this is really only the case for lower-quality cartridges, which tend to have poor tracking and high distortion.

High quality cartridges, such as those made by Shure and Ortofon, are more likely to have an output of 2-3mV. As I recall, even the Stanton 68x series of cartridges, which were considered to have higher-than-average output, gave a nominal 4mV.

The issue is important because an RIAA preamplifier using a single stage of IC amplification is limited to a gain of about 40dB (ie, x100) at centre frequencies. This is because almost an extra 20dB of gain is required to provide the low-frequency boost. For good quality audio reproduction, a gain after feedback of 60dB is about the limit for good-quality ICs, whether they be single (TL071, NE5534 etc) or dual (LM833, NE5532 etc). The technical reasons for this limit are well-explained in Analog Digital's application notes for one of their high-performance linear ICs (type OP27, I think).

With a cartridge delivering 2-3mV of output and a 40dB RIAA preamplifier, the output level will be 200-300mV, considerably less than the norm for modern peripherals such as CD players, tuners, VCRs and cassette decks, most of which have a nominal output of 1V or more. This discrepancy can have some uncomfortable, and even damaging, consequences if one forgets to adjust the volume control before switching from the record player to some other input.

The simplest solution to this problem is to add a buffer stage (x4 is about right) after the RIAA preamplifier. However, I seem to recall that lower overall noise can be obtained by dividing the required x400 or x500 amplification about equally between two IC stages. A modified version of the 2-stage RIAA preamplifier designed by David Tilbrook (ETI September 1981) does this job quite nicely and delivers an output level which is more consistent with modern peripherals.

I was pleased to see the use of a PC-mounted toroidal transformer (available from Altronics and specialist suppliers) in the RIAA preamplifier design. I have used these transformers in a number of my projects and have been impressed by their construction, quietness and low external field when used in sensitive audio equipment.

Brian Knight,
Evandale, SA.

Comment: you are right in that some cartridges may need an additional stage of amplification to bring them up to par. However, there is no particular benefit in splitting the gain evenly between two stages. The reason for this is that the noise performance of the whole preamplifier is largely determined by the input stage of the first op amp.

Solar panels not worthwhile in NZ Very gratifying to see both Leo Simpson and Ross Tester, in the editorial and article in the March 2002 issue, showing a healthy dose of common sense about the usefulness of solar power at this stage of development. Often I have cringed when I have heard some "greenies" proclaim their short-sighted vision of how much better the world would be if only we would let them have things their way. The greenies have a noble attitude but bless their well-meaning hearts, I think their reason and their logic sometimes let them down. In NZ where I live, I am always amused when I see environmentalists buying solar panels to get "free, non-polluting" electricity. NZ's power is generated by the environment itself (how friendly can it get?) using hydroelectric and geothermal methods and these sources of energy will probably be provided freely by Mother Nature until Judgement Day! I might have a blinkered view but I still think it's a bit loony for environmentalists in NZ especially to waste good money buying expensive, inefficient solar gear which has already harmed the environment by its manufacture. SILICON CHIP's reportage of solar energy should be compulsory reading for all "fad" environmentalists. Keep up the good work. Stan Hood, Christchurch, NZ.

Are photovoltaic cells really green?

Thank you for your March feature article "Solar Power for All". Given the current environmental "Solar is Green and Green is Good" hype, it is pleasing to see that someone is willing to scrutinise the claims made by manufactures and others on the performance of photovoltaic cells.

However, there is one question I have regarding photovoltaic cells when ever their performance/efficiency is discussed. Does a photovoltaic cell produce more energy in its lifetime than it takes to manufacture it? I think this question is more fundamental in the debate of claims that photovoltaic cells are clean, green and non-polluting than cost/efficiency issues.

I suspect not, however I don't have any conclusive proof of this. My argument here is what's the point in expending energy making photovoltaic cells (and in the process producing CO₂ as nearly all Australian electricity production does) if they don't return an equivalent amount plus interest.

Don't get me wrong, I do believe that photovoltaic cells have their place in power production (eg, remote areas where there is no power grid). However, as an "alternative" power source in urban areas the energy equation just does not stack up.

Glenn Mayall,
Gosford, NSW.

Tip on making PC boards Further to the excellent article by Heath Young in the February 2001 issue on making PC boards by toner transfer, your readers might like to know that using a modern family iron can be less than successful for a number of reasons, including steam holes, not hot enough and not heavy enough. I discovered that if the iron can't make the paper go brown - it is not hot enough. I bought an ancient 700W Hecla electric iron (heavy, no steam holes, no thermostat!) for $5 from a local market and it works great. Rob Clark, via email.

BassLink should be done properly

Wow! You certainly nailed your colours to the mast. I hope you haven't hoist your petard there by mistake.

I am referring to your editorial on the new Lucas Heights reactor in the May 2002 issue of SILICON CHIP. I want to support you and I think your "We were here first" argument has much merit. I enjoy remembering that when Tullamarine airport was being built, the Government tried to do the right thing by erecting many large signs proclaiming, "Warning! Airports are noisy neighbours!".

Controversial projects usually become controversial for good reason; they have benefits and liabilities. It isn't good science and it isn't good engineering to shut our eyes to the liabilities, and it isn't smart to ridicule people who remind us of the liabilities and who seek to minimise the harm done by those liabilities.

Before complaining of a "low level of scientific knowledge" it is useful to remember that science includes much more than physics and electricity. It is useful to remember that green and environmentalist groups are usually populated by people of above average education and that we technologists probably should welcome their input when they remind us of design criteria that we might otherwise forget.

The BassLink project is a particularly unfortunate example. The opposition to the project isn't the irrational "in there and against it" you imply. The opposition is to doing the job badly. The environmentalists want what we technologists should want - a project of which we can be proud, not ashamed.

Keith Anderson,
Kingston, Tas.

USB light not a new idea

In your March 2002 edition, your article entitled "The Itsy-Bitsy USB Lamp" is a great idea and I'm sure a lot of readers will be making one. There is one problem with the article though, and I quote from paragraph 4: "It is such a delightfully simple idea we're wondering why no-one ever thought of it before."

Actually, there is a similar commercial product that I believe I saw advertised or in shops 12 months ago. Just to check my sanity I did a quick check on www.google.com (key words USB and LIGHT) and came up with a number of results, the most relevant being: www.kensington.com/products/pro_cas_d1334.html

This device has a flexible shaft, it stays in the shape it's bent to, plugs into a USB port and uses a white LED to provide light, so it's not exactly the same.

Mark Grover,
Adelaide, SA.

Fuel cells not necessarily clean and green Thank you for the article on fuel cells in the May 2002 issue. It is most interesting and informative. However, I wish to correct some common misconceptions regarding these devices as power sources. Firstly, they are not "emission free". As the diagram on the opening page of the article shows, their exhaust is water (H2O) which is "emitted" from the cell. However, I will concede that they do not emit carbon dioxide and I suppose we should say they are "non-Greenhouse contributing" rather than "emission-free". However, the source of their fuel, hydrogen, is possibly not "non-Greenhouse contributing." Pages 12 and 13 list a number of makers of fuel cells in the USA and show that most operate on hydrogen from an unnamed source. If this source is (for example) an electrolytic process powered by electricity from natural-gas, oil or coal-fired power stations, it may be that the use of the cells on this fuel will contribute more CO2 to the Greenhouse effect than would the use of conventional fuels in our cars. The DaimlerChrysler cells derive their hydrogen from sodium borohydride, which is derived from borax. My Macquarie Dictionary tells me that borax, sodium borate, is a substance "occurring naturally or prepared artificially". What is the energy consumption involved in preparing the hydrogen fuel from borax? Is the borax naturally occurring or if not, what is the energy consumption required to obtain it? What happens to the waste products from these processes? A number of the cells are claimed to operate on ethanol as a fuel. Ethanol, C2H5OH, contains carbon. What happens to this carbon? The carbon in ethanol represents a little over half the energy contained. If we allow that this energy is "lost", the overall efficiency is much lower, roughly 33-40%. And how much energy is used in extracting the hydrogen from the ethanol so that the fuel-cell can use it? UTC's petrol-powered fuel cell and Suzuki's natural-gas cell have the same problem. I am not sure how much of the total energy in petrol is contributed by the carbon; for propane, a major constituent of natural gas, it is about 50%. Again, this is apparently not utilised by the fuel-cell, nor are we told how much energy is wasted in extracting the hydrogen. Additionally, these two petroleum-based fuels are sources of Greenhouse-contributing CO2. Ethanol is not considered as such, since it is usually derived from growing plant matter and is part of a "local" carbon cycle. Petroleum fuels, on the other hand, are generally held to contain the carbon which was extracted from our atmosphere as it changed from CO2-rich to O2-rich, aeons ago. It is not considered desirable to release this carbon back into the atmosphere. While I see fuel-cells as a desirable energy source to replace the internal combustion engine, I feel we should beware of the "hype" extolling them as the ultimate cure for our Greenhouse problems. We must realise that a lot of the information we are being given is aimed at selling the cells, rather than at fully informing the public. Greg Mayman, via email. Comment: we have not yet finished the fuel-cell story. Ultimately, the hydrogen for solar cells will have to be produced directly from water by solar power if the whole process is to be really clean. And we're not talking about using solar cells for electrolysis of water - stay tuned!

Bosch ignition schematic wanted

I have a V8 VS Commodore and I need a schematic diagram for the ignition module. Bosch only gave me the pinouts of the module and this is all they can help me with. Any help would be appreciated.

Mark Sully,
PO Box 274,
Niddrie, Vic 3042.
marksully@piarc.com.au
Fax (03) 9366 6872.

LP Doctor is a great project

After six months and hundreds of LPs, I can only say of your "LP Doctor" that it's one of the best projects you've ever described.

The "LP Doctor" has given a whole new life to my record collection. On some discs, the click detection LED flashes continually, yet the music sounds clean and not at all disturbed by the short interruptions as the clicks are removed. It's a great project and one that I use every day.

One thing though: what are the parameters for the treble filter? I can't detect any difference with the filter in or out! Does that mean that I have an inbuilt organic filter? Which components should I change to increase the slope of the filter?

In the May 2002 issue, you described a "32 LED Knightrider". I particularly liked the idea of a programmable stop light for vehicles. I've seen something like that before but it wasn't as versatile as your project. This was quite interesting, although I probably won't build one at the moment.

On the other hand, I can see me building a slightly different project. I envisage a multi-LED panel that spells out STOP when I hit the brakes but can also be reprogrammed to spell out BACK OFF when I am too closely approached by one of those characters who like to attach their bonnet to one's exhaust pipe! It would probably take more than 32 LEDs but I am sure there would be plenty of readers who would appreciate the chance to warn off those inconsiderate and impatient drivers one meets every day.

Jim Lawler,
Hobart. Tas.

Comment: the treble filter is very gentle, only -3dB at 10kHz and -12dB/octave above that. The reason you cannot hear any difference is that your own inbuilt filter probably cuts in well below 10kHz, perhaps as low as 5kHz or 6kHz. And it will be a "brick wall" filter rather than -12dB/octave.

Having said that, the treble filter could be made to have a much more apparent effect by increasing the 560pF capacitor at pin 6 of IC5b (& IC7b) to .001μF and the 150pF feedback capacitor to 270pF. This will drop the -3dB point to 5.6kHz.

5.25-inch floppies revisited Here's a novel recycling tip which has a mental challenge as well. 5.25-inch floppy disks still have a use - for CD storage from multi-disk CD-R spindle packs! Cut them right along the base near the opening and remove the magnetic disk. Slide in your CD-R and it's now in its own padded storage packet. Put the 5.25-inch floppy with the CD inside back into the paper jacket and voila, instant transport case! Brad Sheargold, via email.