Originally Posted by Junebugvinyl
I have built and enjoyed two B1 units (one balanced) and now want to give the Mez B1 a go. Four questions:
1) I see the boards are still for sale but the SK170 sources seem sparse since the 10 years later board and BOM was issued. Suggestions for sources of well-matched sets for the signal path?
2) I want to build balanced (all the more need for good matching) so is it just a matter of doubling up all parts and the board except using a single tranny that has twice the specified current capacity?
3) I want to integrate a remote controlled relay based ladder type attenuator. See The δ1 relay-based R-2R stereo attenuator
Other than space and power supply what other cautionary notes would you have for me?
4) What is the purpose of the 100 ohm resistor on second set of outputs? I intend to have multiple balanced outputs on my unit.
1) punkydawgs on eBay sells 8X matched which are verified and well near together but not cheap.
You could alternatively transplant JFETs from your older B1 builds if you still got them.
2) Doubling up all parts yes. Including two transformers or one with two completely separate sets of secondary wires. Because the PSU sections are independent on bridges and won't share a single AC source correctly.
3) The input to ground resistor of DCB1 (220k but flexible) should be at least 10x higher than the output impedance of the attenuator.
4) Breaking interaction between parallel connected equipment. You may measure significant extra THD on an output if with an idling component hooked directly in parallel. Depends on its input circuitry characteristics and the interconnects also.