Posted 22nd November 2013 at 12:45 AM byrjm (RJM Audio Blog)
Updated 5th January 2014 at 09:17 AM byrjm(update schematic to 20f4)
Update: I've ordered parts for small number of Sapphire 2.0 kits. The normal price will be $125, but as an introductory offer the first batch will be available for $100. Kit includes a set of boards and all the parts for the board. You need to supply the transformers and diodes, as well as a volume control, and the chassis hardware.
November. That time of year for finally getting around to advancing some of my audio projects a little.
The Sapphire has remained in "rev 1+" for some time now, partly because of time constraints, partly because of the lack of popularity, and partly because it was already a re-spin of the beta version and worked just fine.
There were a few housekeeping things I wanted to add though, which have been included in the 2.0 revision.
- added a dedicated ground (GND) pad to connect to chassis...
Posted 10th November 2013 at 12:00 PM byrjm (RJM Audio Blog)
Updated 10th November 2013 at 02:11 PM byrjm
Last one for today.
I was having problems getting this to work, the key seems to be adjusting R6 to null the voltage offset. Works fine now, but this is just a rough reverse engineer of the diagram, the parts values and the type of transistors are essentially placeholders.
The input jFET is shown as a dual package. The output bias current is most likely much lower than the 100 mA I configured, so the class A output power is proportionately smaller. For the rest of the currents and the types of transistors used, your guess is as good as mine.
edit: R7 should most likely be closer to 47 ohms, while another 5p capacitor should go in parallel with the feedback resistor, R17.
Posted 10th November 2013 at 12:42 AM byrjm (RJM Audio Blog)
Browsing through an old issue of MJ (No. 1076), I found some sketches of the Marantz SA 11S3 SACD player analog circuitry which seemed interesting.
The are two discrete op amp "modules", one is used for the I/V converter and low pass filter, the other design is applied to both the line output and headphone outputs.
The parts and values are not given, so the circuit below is just a working mock up in LTSpice based on the published diagrams.
The output stage from the headphone / line driver op amp is attached below. There's a JFET input and BJT gain stage as well, and the whole thing is wrapped up in a feedback loop. The buffer itself, being unity gain, works just fine as a standalone circuit element.