Restoring Vintage Audio Equipment

Phoenix HiFi

Marantz 33 Preamp Overhaul

This unit has been in the stable for about 10 years.  I had performed some experiments on it along the way to make it DC coupled as much as possible: I tried eliminating coupling capacitors, I tried subbing an opamp in for the gain stage, I bypassed caps in the power supply regulators, etc. 

It came time to restore the original circuitry, so a complete recapping of all electrolytics.  Where form factor was a concern, I found suitable replacement caps.  Look at the power supply board in the lower right corner.  It has the same virtual look as the original as far as caps go.  But I retained some film bypass caps in key positions across electrolytics.  Also note the filter amp board (the one with all the inductors - yes, they made the tone filters with inductors and capacitors, the old fashioned way!) has the red Wima MKP film caps as bypasses across electrolytics.  There are several on the main boards (the big ones) as well. So once all the boards were re-populated and cleaned up, it was time to try to fit them all back into the chassis.  It took a couple of long sessions, but all the wires ended up in the right position!  Of course switches and controls were cleaned really really well before I tried soldered all those wires.

I did put in one major modification for use in my system: I added a balanced output. I'll get a better picture of the back panel installation posted soon, but you can see the added on board and its attachment to the new XLR connectors in the final assembly photo below.  I used the THAT Corp 1646 balanced line driver chips, themselves driven (per the data sheet) by a National/TI LM4562 dual op amp.  This addition really cleans up the sound in my main system.  The balanced output is sent across the room (~25' of cable) to the DEQX crossover/equalizer, which features a balanced analog input.  Since I'm running only the rejuvenated Fisher XP15 speakers at this time, it is used strictly as an equalizer.  The DEQX outputs send  unbalanced outputs to a pair of Lundahal line transformers which send a balanced signal to the pair of McIntosh MC275's (yes, with upgraded caps...) operated in mono, providing about 150 tube watts per speaker.  So the balanced line driver is a welcome addition to the sound.  Controlled and well damped.

The Marantz was subjected to several bench measurement with the Sound Technology 1700A Distortion Analyzer.  It did not make the full 10Vrms of output that is specified.  It got to about 9+V before clipping.  This I attributed to the regulators not quite making it to +/- 15V, but only to about +/-13.5.  I did not pursue it further, but I suspect that I ran into regulator "dropout" sooner than originally designed.  I'm guessing that trying different pass transistors might fix that. 

Total Distortion + Noise  measurements were quite good, well below the spec.  What was cool was that the 400Hz and 80kHz filters had very little effect on the measurements.

Soundwise, I'm pretty happy with the preamp.  Headphones were the first audition when the system had only been powered on for several minutes.  It didn't sound good and looking at the scope trace of the distortion, there were signs of crossover distortion. That would be a good area to research for an upgrade since there are so many good headphone amps available now.

After a couple days of break-in, however, the preamp outputs sound pretty good operated in the system.  It provides that controlled sound of well damped solid state units, with good tight bass.  Surprisingly, it does not assault you with that solid state glare.  It does provide nice soundstaging including decent depth.  Not holographic, but believable.  In that sense it retains the laid back Marantz voicing: not 1st row center, more like 10th row center.