Saturday, 12 May 2012

Getting better sound out of the Marantz SR7500

My goal is to improve the audio fidelity of my Marantz SR-7500 AV receiver. The aim is to reproduce sound in the same way as it was engineered in the recording studio (without spending hundreds of dollars).

1. The Marantz SR7500 has the ability to switch off the video decoding circuitry. Marantz calls it Video OFF. In theory by turning off non-required circuitry will reduce conducted and RF emissions.
In 2 channel stereo mode I couldn't hear any perceived improvement.

2. Upgrading the capacitors in the speaker crossovers, from electrolytic to Metallised Polypropylene film MKP capacitors. This should improve the quality of the output from the speakers. MKP capacitors have much lower distortion. The drawbacks, they  are physically much larger than an electrolytics (for the same capacity and voltage rating), and much more expensive.

Older speakers are more likely to benefit from a capacitor upgrade.

3.Other modifications

  • Replace low voltage rail decoupling capacitors with long life low ESR, 105C units. Mainly United Chemi-con, and Nichicon brands.
  • Removal of muting transistors. These are bipolar transistors (BJT) placed across the audio signal path and ground. Its purpose is to mute the audio output when changing inputs for example. There is some perception that removing these from CD players in the analog path yields noticeable improvement. Some evidence suggest the BJT junction has parasitic capacitance that is frequency dependent. This effect supposedly impairs stereo imaging.I am not sure how much truth there is to this claim. There is a real risk that "thumping" may be heard from the speaker outputs if these are removed, especially when changing input sources for example.
         Higher-end equipment tend to use a signal relay instead of a BJT.
  • Removing all possible AC audio coupling capacitors in the audio path of the amplifier. In theory the design of the circuitry by using split dual power supply rails (-ve and +ve) should help accomplish this. The nature of dual supply rails is that the circuit is naturally biased with no DC offset. On the look for DC offset at the op-amp outputs. Such DC offsets can be found reasonably easy by looking at the incoming point of an AC coupling capacitor. Depending on the amplitude of the DC offset will determine if the capacitor stays in or gets replaced by a piece of link wire. Target candidates are unity gain / buffer op-amp stages.

Update 24/0712 Pre-Amplifier Modifications

Removal of AC coupling (DC blocking) capacitors from the audio signal path. The Marantz SR7500 receiver has plenty of scope for this to happen.

1. The circuit schematic of the INPUT board, has many op-amps in buffer configuration. All of these have AC coupling capacitors on both the input and the output.
These can be safely removed. Replace the capacitor with a tinned copper wire link. A biasing resistor to ground will also have to be removed.

2. Volume board, has a set of AC coupling capacitors and bias resistors that can safely be removed.

3. The muting transistors can be removed. I have found no thumping to occur at the speaker outputs when changing input sources.

4. DSP board. AC coupling capacitors and bias resistors can be removed from differential op-amp interfacing to the CS4392 DAC converter.

Note: Care needs to be taken with op-amps with voltage gains  >1.This is because if any DC offset is present to the op-amp inputs, the output will  be amplified by the gain. (both AC and DC amplitudes) it is possible to run out of headroom into saturation (clipping) of the supply rails, resulting in severe distortion.

The result is quite a different sounding amplifier. In contrast, low fidelity audio could be best described as a monotonic bassy boom. This is far from that, much more dynamic and clear. The low frequency bass response is much more tighter.
In a well designed pre-amplifier there should be no AC coupling capacitors in the audio path.

AC coupling capacitors in the audio path are undesirable as it causes distortion and changes the characteristics of the sound.

The end result is I had removed a staggering 58 AC coupling capacitors and biasing resistors! This could have shaved a few dollars off the manufactured cost of the unit. 
The Marantz SR7500/SR8500 are very good receivers if you have some knowledge of electronics and wish to improve the units sound reproduction.

As always - For new players the author does not recommend attempting to repair  mains powered equipment. Mains equipment requires care, intention and are at lethal voltages.

Disclaimer: The information contained in this post is for educational purposes only. The author accepts no responsibility for any damages or harm that may arise.

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