Friday, 11 March 2011

Madrigal Proceed PCD

Madrigal Audio Laboratories, the company behind the Mark Levinson line of audio equipment, decided in 1989 to enter the CD player market.  Instead of releasing a Mark Levinson CD player they formed a new brand called Proceed.  The brand was discontinued in 2003, generally thought to be due to the disappointing performance of its multi-format PMDT transport, and its remaining product line was rebranded as Mark Levinson.

Proceed's first products were a family of three; the PCD, the PDT and the PDP.  These are essentially the same thing boxed in different ways, the PCD is the one box CD player, the PDT and PDP are a transport and DAC respectively.  All shared an unusual upright form factor.

The PCD uses an unusual combination of parts.  The transport half is straight out of a high end contemporary Philips player (or many other European players), consisting of the CDM1-Mk2 laser mechanism, the TDA5708 / TDA5709 servo chipset, the SAA7210 decoder IC and the SAA7220 digital filter IC.  The SAA7220 is not used as a digital filter in this player, it is only used to output a S/P DIF digital audio signal.  The DAC half on the other hand was more like what would be found in a contemporary mid to high end Japanese or American player, consisting of the Yamaha YM3623 S/P DIF receiver IC, the Nippon Precision Circuits SM5813 digital filter IC and a pair of Burr Brown PCM58 DAC ICs.  The output stage uses a pair of Analog Devices' OP42s for the I/V converter and a quad of AD845s for the buffer, each with a LT1010 current buffer within its feedback loop.  The two halves of the one box player are connected together by S/P DIF, just the same as they would have been in the PDT and PDP separate DAC and transport.

This S/P DIF link is a very poor feature of this player.  S/P DIF is designed as an external single line bus for use between digital audio devices.  By compressing down what is normally carried on three or four lines into one, the S/P DIF format saves on cables, but there is a trade off.  Converting to and from S/P DIF inevitably introduces timing errors, or 'jitter'.  This is detrimental to sound quality, and should have been avoided.  It also doesn't help that the S/P DIF receiver the DAC half uses is one of the worst available, and introduces far more than its fair share of jitter.

I received this particular PCD in non working condition.  During playback the analog outputs produced a horrible, loud noise signal.  This fault had suddenly started in the middle of playing a CD.  I took the case off, and seeing nothing obvious, fired up my oscilloscope and started my examination.

I first noted two things:
  • The horrible noise was present on the digital output, meaning that the fault was almost certainly in the transport half of the player.  It's always good to have a digital receiver, be it a DAC unit or a PC with a S/P DIF input, so that you're able to test a digital output.
  • The player was reading the table of contents of a CD (listing the number of tracks) and changing tracks fine.  In most cases if a CD player does this then the laser mechanism and servo are working fine.
This means that the fault was localized somewhere between the digital output and the decoder.  This put the focus on three functional blocks; the digital filter (which outputs the S/P DIF signal), the decoder (which interprets the output of the laser and converts it to digital audio) and the power supply.  Never forget to check the power supply, nothing works right when it's not fed power correctly.

The power supply was fine, so I looked at the digital bus entering the SAA7220, the digital filter this player uses to create the S/P DIF signal.  The signals were all present, and looked valid, so I removed and replaced this IC.  The fault was not eliminated, so that IC wasn't faulty, it must have been receiving correctly formatted yet corrupted data.

The third item I checked was the decoder functional block.  This block consists of two main items; the SAA7210 decoder IC, which does most of the work, and the MN4264 64kB dynamic RAM IC that forms the frame buffer.  In this player the decoder interprets the signal from the laser into frames consisting of a pair of audio samples.  It then stores these frames in the frame buffer, and then retrieves and outputs them at a pace determined by a clock signal fed to it.  If the frame buffer is near empty the decoder speeds up the spindle motor (the motor that turns the CD), if the frame buffer is nearly full it slows down the spindle motor.

The decoder block was outputting valid looking digital audio, and was controlling the spindle motor correctly (the motor was slowing down as it player tracks further toward the outside edge of a CD), so I didn't suspect the main IC (which performs those functions).  I therefore replaced the RAM IC, and hooray, the problem was fixed, and the player would now output clear, undistorted audio.  The RAM IC was damaged, and had been giving the decoder IC random garbage when it requested a frame.

Now that the PCD is up and running it's time to modify it.  I'm considering doing the following things:

  • Replace the electrolytic capacitors.  This is always something I look at doing.  This type of component has a limited life (usually between 2000 and 4000 hours), after that they won't meet their original specifications.  This can cause a number of bad things depending on what they are used for, including preventing the CD player from reading any discs. 
  • Install a low noise clock.  Almost all CD players will benefit from a low noise clock, but this one will benefit more than most.  I intend to feed a low noise clock signal into both the transport and DAC halves of the CD player directly.  This will bypass the S/P DIF bus and feed the SM5813 a clean, low noise clock signal in place of the noisy recovered clock from the YM3623.
  • Replace the output stage opamps.  Leaps and bounds have been made in the design and manufacture of opamps since the PCD was designed in the late '80s.  It currently uses two OP42s for the I/V converters and two AD845s for the buffers.  At the moment I'm thinking about the OP132 and the LME49710 as replacements for the I/V converter and buffer opamps respectively.
I'll post an update to this entry once I've completed modifying the player and have had a listen.

Update (14 April): How the PCD Sounds

At this stage I have only done the first and last modifications listed above.  I used six OPA134 opamps to replace the original ones.  This choice was due to the OPA134s lower bandwidth compared to many other modern opamps.  The output stage of the PCD uses current buffers within the feedback loop of the output buffer opamps.  When using current buffers this way it is best to select an opamp that has a lower bandwidth than the current buffer itself.  The LT1010 has a bandwidth of 10MHz, while the OPA134 has a bandwidth of 8MHz.  This is not as much of a problem with modern current buffers, as they generally have higher bandwidths (the popular BUF634 has a bandwidth of 180MHz), effectively nullifying this limitation.

The sound quality of the player has much improved, especially in the higher frequencies.  Most of this is down to the opamps, the replacement capacitors were as much for reliability as performance.

The owner and I still intend to install a low phase noise clock into this player.  The design of clock I am currently using is satisfactory, but is too large to install in many players.  Unfortunately when I designed that clock I didn't consider compactness to be a high priority, a large oversight.  I am in the late stages of developing a replacement, when it is ready this player will get the first of these new clocks.

I will post a further update when I have installed the clock, as well as a separate post about the clock and low phase noise clocks in general.