Sony STR-DE375 Review

The STR-DE375 is a surround sound amplifier made by the Sony Corporation of Japan. This is a 5.1 channel amplifier with built in FM and AM receiver. The amplifier has RDS, and Dolby Pro Logic functions. It does not have a Dolby DTS Decoder, but instead has RCA inputs for individual channel input (DVD 5.1 multi-channels). The Canadian version of this amplifier is 80-watts per channel, and the Malaysian and Singapore versions have 90-watt channels. This is the UK version where all five of the channels are identical providing a respectable 50-watts output into an 8 ohm speaker load. The sound quality is actually very good due to the wonderful Japanese transistors and design. It has the 2SB1588 and 2SD2439 pair of Japanese transistors in the power output stage. The driver chips are the uPC2581V, which make an excellent job of keeping the power stage to 0.7 % THD. It also has the well-known Mitsubishi M61501FP chip, which performs the input function and sound processing. This chip has internal tone and bass circuitry and sounds very much like its predecessor.

Protect Mode Error Message

In this Sony amplifier, the protection mechanism consists of a pair of 0.22-ohm current sensing resistors in series. A pair of 2SA988 transistors keeps a watch over the current flow through the resistors. An additional protector circuitry to flag the main microcontroller consists of ISS133T and a pair of BN1F4M in a Darlington arrangement. This protection mechanism was common on amplifiers of this era. The front display panel will display the PROTECT message, if the speakers terminals are shorted

For the amplifier "experts", you twiddle this and it makes the sound go up and down.

Smooth Operator

This one had a strange problem, which I had never come across before. The amplifier would not start after depressing the power button. Standard practice is to check the standby board, and it was fine because all the necessary voltages were present. The control signals, power, and reset were all fine as well. I pressed the power button, but there was no response. There was no life to the amp, yet all voltage rails were showing the correct power levels. As a last resort, I decided to get the scope out to have a more detailed look at the current and voltage levels. I decided to check the clock signal on the system microcontroller uPD78206 as my starting point to make sure that it was running the firmware. The clock pulse was like nothing I had seen before; it had spikes and was irregular. I have seen enough clock pulses to know that, that was not normal operation. Ideally, it should be a smooth regular pulse and this was not a smooth operator by any means. The only component responsible for the clock was the external 5 MHz crystal (X201) between pin 11 and pin 12. These are normally very reliable, but I figured I had better change it to see if it made any difference. After changing the crystal, the pulse returned to normal. It was a smooth operator again! Moreover, the power button worked, and the amplifier began to work as well.

Price/eBay

The STR-DE375 is an early amplifier in the STR series and a basic specification, however the early ones were made very well. There are no coax or optical digital inputs, but the sound performance is very good. It was mass produced, however over the years, they are starting to get rare as people have moved on to the more modern amps. These occasionally turn up on eBay, and the price varies depending upon the condition. An amplifier in good condition together with its remote control could fetch a tidy sum. If you have a broken or faulty amplifier showing the Protect error then you could still sell it on eBay as spares and repairs, because there are always electronic engineers looking to buy something vintage to mend.

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