gavincurtis
New Head-Fier
I am using my little dot Mk6+ with the ESS SABRE DAC that is built into my LG G7 phone. I added a non-balanced to balanced adapter to my amplifier as shown. Being a noob, I learned that even though my headphones are balanced, when using the RCA inputs, only one side of each amplifier push-pull is used...what a waste!! So I did what the manufacturer should have done.... install the proper converter.
I am using Texas Instrument's DRV134 precision audio balanced driver chip and powering off the +15v AND -36v rails from the amplifier. The DRV134 is actually powered off +12V and -12V rails provided by the regulators as shown. The chip also offers additional gain so the conventional non-balanced RCA signal is raised to that of balanced, allowing me to keep the gain of my Mk6+ to a minimum. I will be adding a DPDT switch so when I use the balanced input, the signal will not back-feed into the output of the DRV134.
After 8 hours of use, my power resistors for the regulated +15v and -36v rails were very discolored. So that part of the faulty design of the amp had to be upgraded to handle the additional current draw of the DRV134.
The 4.2K resistor I replaced with a 10 watt 1.5K resistor and increased my zener diode to a 1.5 watt. The -36V rail resistor I replaced with a 1.2K 10 watt resistor and upgraded to a 5 watt -36V zener. Now I have sufficient power for the DRV134 and more stability for the MK6+namplifier's op amps. I have chassis mount 20W versions to mount on the base plate on order to clean things up.
One problem I am encountering regardless of tubes I install.... my right channel drops all the way to ZERO..... still plays...but voltage drop across the .27 ohm resistor for the mA meters verifies my meter is working. I have bias and it makes no difference. This was a factory problem before my modification. Any ideas? Moving tubes around, even trying my Tung Sols.... nothing changes on the right. It hovers between 0-20mA. The left channel is 60-80 mA. The meters themselves are good, so this is something in the circuitry.
On order are all new .22uF capacitors in case one is leaky... my capacitors in my amp appear to be pulls...the are all different lot numbers, even the WIMA housings vary from unit to unit. They do appear to be genuine WIMA, just a mishmash of manufacturing dates.?. Replacing with new and also installing PPS (Polyphenylene Sulfide) capacitors for the 1uF coupling stage.
Also, being that I am not a tube expert, what are the bias voltages supposed to typically be from the Op Amp to get me in the ballpark? I can go from one extreme to the other on the trimmer and current drain does not change, but at some point the audio does drop out. Tube Noob here and I have yet to see left and right meters agree.
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