ORTOFON PPA 600

Ortofon was quite famous in the vinyl days for manufacturing quality phono cartridges. In 1988 or thereabouts, they decided to introduce a power amplifier, the PPA 600. This was a well made product utilizing Toshiba 2SK405 and 2SJ115 power mosfets. Her rated power output was 350W into 4Ω and 225W into 8Ω. She was beautifully designed in appearance as well as in engineering. She would be right at home in recording studios, even for domestic HiFi.

Strangely, she never gained traction in either market even though she’s an excellent amplifier. Perhaps it has to do with the business side of things like marketing, distribution, pricing, etc. Let’s take a closer look at what the PPA 600 is all about.

ORTOFON PPA 600Fig 1 – ORTOFON PPA 600 Amplifier Schematic

Fig 1 is the schematic of the ORTOFON PPA 600. There are a total of eight power mosfets at the output, four 2SK405 and four complementary 2SJ115 running off +- 75Vdc rails. Note that no source resistors are used for current sharing. She is totally dependent on the mosfets internal Ron. Those were the early years of mosfets and engineers were sold on the idea that source resistors were not needed unlike in BJT. Personally, I do not subscribe to that.

The Red line is the signal path in the PPA 600.The typology is mirror image, that is the top and bottom halves are the same. The input differentials standing current are determined by Constant Current Sources. Collector resistor is 6.81kΩ, rather high for my liking. She then connects to the VAS for more amplification.

Their choice of using BF423/422 in this stage is rather odd. Sound wise, they are not the best match with the 2SC2240/2SA970 at the inputs. Same as the drivers BF471/472. These are like the MJE340/350. When I rework this amplifier, I will definitely replace them with modern ones.

As for the Toshiba 2SK405/2SJ115, they are no longer in production. Mine should be working but for other PPA 600 with damaged outputs, all is not lost. They can easily be replaced with 2SK1058/2SJ162 and the amplifier tweaked accordingly.

Fig 2 – Inside the ORTOFON PPA 600

Fig 2 is the PPA 600 with the top removed.This is my preferred method of cooling. Two large heatsinks forming a tunnel for forced air cooling. A large pcb stretches along the heatsink end to end. I don’t know whether the Toshiba power mosfets underneath are on a separate pcb. I usually hard-wire them but this approach is not very practical in manufacturing.

Summary

The Ortofon PPA 600 is a fine power amplifier, even by today’s standards. I believe Ortofon only came out with this one amplifier and she vanished in the market a few years later. I am fortunate enough to have this rare PPA 600 in my collection. Condition is mint. I’m just waiting for free time to restore her.