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Everything posted by roger777

  1. I recently replaced the LM833 op amp in my phono pre amp with an LM4562. The new op amp presents none of the issues I found with the LM833, which clearly presented audible signal degradation for which the only solution I found was to lower the closed loop gain by about 7dB by changing the ground resistor in the feedback circuit from 390 ohm to 1K ohm. The circuit design of this pre amp is straight out of the data sheets of both ICs, with some additional RF filtering applied on the input. With the LM4562 installed I was able to reinstate the recommended RIAA equalization (that is restoring the
  2. Thanks for your advice Mike. RS also has a dedicated NZ site. Apparently I could obtain them from there cheaply and rapidly. Unfortunately for me I had already rushed ahead and used "the Bay" to order some National Semiconductor branded LM4562NA/NOPB units from an outfit in Florida at significantly greater cost. With partial hindsight, I am now concerned about them being National Semiconductor branded whereas I assume contemporary units should be branded TI.
  3. Thanks Alan. I understand that the issue with the Tantalums is both an age thing and it is possible they were not as well manufactured in 1981. I am sure many of the modern devices with tantalum capacitors (phones etc.) will be completely useless even ten years from now for reasons other than capacitor failure. I used plain electrolytics for my fix. Suffice to say I did it in a hurry. My reason for opening it up in the first place was actually that in storage it had been sprayed by a cat! Thanks for your link to the Series 5000 pre-amp discussion thread. There are some very useful in
  4. Thanks Matt. Much appreciated. I don't need an A50k pot at this time. Cheers.
  5. Thanks for the information. I confess I wasn't even aware of the existence of LM4562 op-amps. I've been using one of those Jaycar kit phono pre-amps that uses an LM833. The RIAA gain circuit is clearly straight off the LM833 datasheet. I found there to be too much distortion and to fix it I increased the ground resistor in the RIAA gain circuit from 390 ohm to 1k ohm. The disadvantage of this was is a 7dB reduction in gain at 1kHz. It seems the equalisation is insignificantly affected. I think the problem is that the open loop gain of LM833 may be too low above 1kHz, per the spec sheet
  6. Thanks for your helpful offer Matt. Google was no help in locating those parts from anywhere. Right after posting my introduction above it occurred to me to try Ebay Australia. To my pleasant surprise it has numerous suppliers of those parts. I placed orders last night. The parts I was looking for were a 2 pole six position rotary switch and a B100K dual gang mini potentiometer, both of the spline shaft type as used to be sold by DSE, but not Jaycar and Parts Express. It seems they are only manufactured in China. I also found a few on Alibaba just now, but its search function is not as go
  7. Hi, I've been constructing amplifiers and speakers for my own home use as a hobby for decades. Back in the day I even used my EA Series 2 mosfet amp and two pairs of my speakers for a dance in the student hall of residence I was living in at the time. That amplifier was barely powerful enough, but it was all that was available on that particular night. During the last three months I replaced the drivers as an upgrade in those same two pairs of cabinets (again) and very carefully designed the crossovers using the free software available online. Using this software certainly beats usin
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