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Obsolete parts in older projects Could you please tell me whether any hard-to-get parts are required to build the Constant High-Current Source from June 2002 (siliconchip. com.au/Article/4065) or the 50W DC Electronic Load from September 2002 (siliconchip.com.au/Article/4029)? I realise that you probably don’t have PCBs for these projects. (R. M., Melville, WA) ● For the Constant High-Current Source from June 2002, the heatsinking arrangements might need to be changed to suit available heatsinks. The remaining parts are commonly available. For the 50W DC Electronic Load Advertising Index Altronics.................................37-40 Dave Thompson........................ 111 Digi-Key Electronics...................... 3 Emona Instruments.................. IBC Jaycar.............................. IFC,53-60 Keith Rippon Kit Assembly....... 111 LD Electronics........................... 111 LEDsales................................... 111 Microchip Technology.................. 5 Mouser Electronics..................OBC Ocean Controls............................. 7 PMD Way................................... 111 SC RTV&H on USB...................... 75 SC USB Cable Tester.................. 91 SC Vintage Radio Collection...... 10 Silicon Chip Subscriptions.......... 6 Silicon Chip Shop.................... 101 The Loudspeaker Kit.com............ 9 Tronixlabs.................................. 111 Vintage Radio Repairs.............. 111 Wagner Electronics..................... 87 112 Silicon Chip from September 2002, the STW34NB20 200V, 34A N-channel Mosfet is obsolete, so an alternative will be required. Suitable parts that are currently available include the IRFP240PBF, IRFP250(N)PBF, IRFP260(N/M)PBF and IXTH26P20P. Searching for another discontinued part I am trying to build the Sound Level Meter from your Electronics Test Bench book but I am having difficulty finding a three-position, two-pole switch with the correct pin placement. This project is probably over 20 years old. Is there some way I can mimic what the switch does with jumper pins, perhaps? Failing that, where would I get such a switch? (S. N., Clayton North, Vic) ● You are right that switches with the contact arrangement used in that project are no longer available. Switches are available with a similar layout, but you will have to wire it to the board using flying leads. You could use a DP4T slide switch from Altronics (Cat S2040) and wire the switch terminals to the PCB, with the third and fourth positions wired in parallel. You could also use the Altronics S2033 (4P3T) slide switch and ignore the third pole. It would also be possible to wire up a rotary switch like Altronics S3008 or S3022, or Jaycar SR1212. The PreChamp is an old design I am building several PreChamp pre-amplifiers (July 1994; siliconchip. com.au/Article/5252) to increase the signal output from the line output jack (not the headphone jack) on a TV, and plugging the resulting increased signal into a Bluetooth transmitter then to Bluetooth headphones. It works OK, but I’m not happy with the resulting audio quality when compared to another pair of wireless headphones that I have. Using a signal generator and a Hantek USB scope, I have discovered that the frequency response of the PreChamp is not flat. With a constant input level at all frequencies, I found that at 100Hz the output level was 85mV but at 10kHz, the output level climbed to 200mV, and at 15kHz, the level was 225mV. Australia's electronics magazine I have altered the Preamp’s gain by changing the two resistors to 1500W and 150W using the formula printed in the magazine, giving a gain of approximately 11 times, which is around 21dB. Would this have altered the frequency response of the PreChamp? I suspect not. Can you suggest any components that I can change the value of to get the frequency response flatter? (N. L., Christchurch, NZ) ● The PreChamp is quite an old design and we would not design something like that today. As a result, it has relatively poor frequency response flatness. Still, it should not be behaving in the manner you have described. Our circuit analysis of the original design shows that it has a plateau-type response with -3dB points at around 60Hz and 100kHz, and -1dB points at around 115Hz and 37kHz. So it suffers a fair bit at the lower frequency end, but should be pretty flat at the high end, up to about 20kHz. Changing the gain-setting resistors doesn’t have much effect on the calculated response. Note that we published a new design in January 2013 – the Champion (and Pre-Champion). That circuit has a much flatter frequency response. We even published frequency response and distortion graphs in that article, unlike the original Champ/PreChamp. Still, we aren’t sure why you are getting an increased response at higher frequencies. That points to an increase in feedback impedance with frequency, but the only non-resistive element in the feedback network is the 1.5nF capacitor, which should have the opposite effect. The only explanations we can come up with are that your input coupling capacitor is too low in value or faulty, which would cause lower frequencies to have more attenuation and thus give you a rising response with frequency. It could also be a similar problem with the output coupling capacitor. SC The March 2022 issue is due on sale in newsagents by Monday, February 28th. Expect postal delivery of subscription copies in Australia between February 28th and March 11th. siliconchip.com.au