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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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21st Jul 2019, 7:25 pm | #1 |
Nonode
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,571
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Another alternative for Sanyo LD3141
The Akai 4000DS tape deck uses four LD3141 amplifier ICs which are notorious for becoming noisy over time but the IC is now obsolete and any NOS parts are likely to have the same problems.
There have been a number of replacements using discrete components as the IC is just a two transistor amplifier. This alternative uses a TL072 dual op-amp IC to replace two of the original ICs in the replay or record amplifiers. It requires no modification to the tape deck PCB. The op-amp has a dc gain of one and with the + input biased to half supply the voltage on the output pin is approximately 12V, similar to the original. The AC gain is determined by the resistors * and ** and the external feedback network connected between the amplifier output and pin 3 of the original amplifier. The original circuit has a capacitor between pins 2 and 3. This is retained in this alternative and provides dc isolation for the feedback circuit. The gain for the replay and record amplifiers are different meaning different values for the resistors * and ** are needed. Some adjustment of these values may be needed but the values shown are those which were suitable for the 4000DS I was working on. The circuit shown below is powered from the +24V supply on pin 8 of one channel and 0V from pin 4 of the same channel. To avoid any problems with grounding, the ground connection of the ** resistor in the channel 2 amplifier is connected to pin 4 of original channel 2 amplifier. Note that the LD3141 output is on pins 6 and 7 but in the 4000DS only pin 6 is used with pin 7 left unconnected. I built two circuits on a couple of pieces of veroboard and wired them in place of the original ICs as shown in the pictures below. The original IC includes a resistor between pins 8 and 9 which powers the replay pre-amp or the mic/DIN amplifier. To maintain the supply to the pre-amp a 470R resistor has to be fitted between pins 8 and 9 on the main PCB of each amplifier. The LD3141 follows a discrete transistor amplifier for the replay pre-amp or the Mic/DIN input amplifier. These amps use 2SC458 transistors which are also known to go noisy. These should also be replaced with BC549 or similar transistors but note the base connections are different. The output from the tape deck is now noise free. Keith |