JoaMat

A few words regarding filament supplies I’m using with 2A3 and 300B tubes. 9 VAC => unregulated 11 VDC => XL4015 DCtoDC step down converter... ... with trimmers for output voltage and current limiter. It’s easy to set voltage to correct voltage depending of tube and also to trim current to achieve a sort of soft start. So far it works ok.

No output transformer. There are output electrolytics to headphone. Schematically it looks something like this. Reason for three KSC2690A in CCS is to distribute the heat, so no heat sink needed. At the moment I only use two transistors in CCS due to insufficient transformers. I don’t know how to optimize the circuit, but I’m happy with the result.

Aerial picture of a balanced amplifier for dynamic headphones. Only light from LEDs and tubes. ISO-25600, F4, 1/13 sek. And in day light. 4 x 2A3, 4 x 12BH7, 8 x switch mode filament supplies, 2 x GRHV and 4 x transformers. DIY is still fun.

Happy Birthday!

Happy Birthday!

So, I got myself QuantAsylum QA-403 Audio Analyzer. Now I can make some simple analyses of amplifiers I build. Screenshot – my CFA3smd something with HD800 headphone, 2Vrms into headphone. Probably room for improvements. But I’m satisfied and I’m not competent enough to make it better.

Not much (nothing) has happened since my post 11 months ago. Here is a schematic, original to the left and lsk389 version to the right. My idea was to replace 6922 tubes with lsk389 and just make necessary changes and else keep the board unchanged. This is a draft - more work is needed. Of course you can use SOIC lsk389, but I prefer TO-71 since I think it fits better in the layout.

Nice, great news! Did this in 2019 and it has been in sleep mode since then. Maybe UHV-POWER bricks will wake it up. Power transistors plus a few other components on one side and the rest on the other side.

Happy Birthday!

Like this? Think I got inputs to op amp right.

Happy Birthday!

Have had a new bias servo working with my CFA3smd something for a few days. Schematically it looks like this, and the new thing is in red square. Now I get a good reference point to the servos. Don’t understand why I didn’t figure this out earlier – slow brain perhaps. All four servos are fed from the same point. So, one trimmer and full control over the BIAS of all channels. Diagram for new servo - 6 minutes from cold. and diagram for old servo (in mA for the first 40 minutes from power on) Please have in mind that the graph for no servo is for my CFAsmd something. I guess the original CFA3 have a different and better behavior – where all tripple Darlington and vbe transistors are mounted on heat sink.

What dual op amp are you using?

happy Birthday!

Happy Birthday!!!