Listening to the Topping E70V has revealed the need for a balanced connection between the DAC and preamp. Using the unbalanced outputs results in a less than satisfactory sound. Even using an XLR-RCA cable where half of the signal is thrown away and CMRR advantages of balanced interconnects are lost, makes a noticeable improvement.
So the need for a line stage with a balanced input is here. Looking at a few possibilities, the chosen one here is John Broskie's "Unbalancer". The urge to just buy one of his PCBs was strong, but unfortunately, they are out of stock. No reply from Mr. Broskie as to when they may again be available.
A couple of things were concerning about the overall design. Most notably, the input differential amp has an LM334 as the current sink, and it's tied to ground. The LM334 has a maximum current rating of 10mA. Less than the 7-8mA of each half of a 6SN7 type that would likely be used here. Never been comfortable with running a device at 100% of capacity and hoping for longevity. Exploring options and after speaking with Dave Berning, a simple bipolar transistor with pi network filtered zener reference has been chosen. Guy Riffle has been tirelessly simulating different scenarios, and we think that tying the sink to a negative 16-18 volt of so negative supply, using a 6.2 volt zener should work. Even in simulations, there was an improvement in noise performance adding the 1k/100uF filter between the zener and base of the 2N4400 (lots on the shelf).
The other issue with the original design was the unregulated, very basic power supply. Instead, we are using vacuum tube damper diode rectification, all film caps in the B+ supply, a 6Hy 150 ohm choke, a depletion mode MOSFET cascoded current source, then 0A2/0B2 gas tubes for a ~258 volt B+. Finally, two 150mH 2amp chokes followed by 10//0.1uF caps at the circuit loads. Again, the simulations look very good. (Using PSUDII).
Four inputs, reed relay switched, and two outputs, one effectively high pass around 100Hz with my crossover, and the other full range. Two inputs are balanced, two unbalanced.
A 4PDT-CO switch is being used as a phase reversal and mute switch.
Each channel has a separate gain control. They are 100k stereo stepped attenuators (from eBay). Very small and they use surface mount resistors. Should provide very close tracking +/-.
Using a simple MOSFET soft turn on setup with a 150k resistor and 100uF cap at the base of the MOSFET.
Heaters will be DC, but not regulated. Simple Pi network filtering. Negative 18 volt supply will be similar and also unregulated. The 5 volt relays will get power from the heater supply followed by an LM7805.
Using a chassis from an abandoned project: hardwood sides and 1/4" plastic top plate. So, yeah, true to form, it's not pretty!
Basic construction is done, creating circuit boards now: high voltage, low voltage, relay and amplifier.
It'll probably be a few more weeks until complete. Life does get in the way.
Anyway, not many discussions going on here, so figured this might be of some interest.
Stuart
BTW, there is an unfinished Izzy Wizzy phono stage here. It has transformer coupled output and no coupling caps. Seems like finishing that will be a great next project!
Attachments: |
![Back plate.jpg](./download/file.php?id=2357&t=1)
Back plate.jpg [ 290.78 KiB | Viewed 61974 times ]
|
![Top plate.jpg](./download/file.php?id=2356&t=1)
Top plate.jpg [ 371.13 KiB | Viewed 61974 times ]
|
![Switching.png](./download/file.php?id=2355&t=1)
Switching.png [ 80.96 KiB | Viewed 61974 times ]
|
|