Voltages all high on one C4S board [resolved]

[carlsor]

-  The differential outputs of my single board DDDAC are terminated to 133 ohm resistors which provide signals about 2.7V above ground. My buffer uses BF862 JFETS which present virtually no load to the PCM1794 DAC outputs but more forcefully drive the differential signals through Cinemag 15/15B transformers. The single ended Cinemag outputs are terminated with a 10K LDR resistor and 150pf WIMA which gave the best SQ. Top quality power supplies for the digital and analog sides of the NOS DDDAC have a lot to do with the analog sound. The 2SK208O JFET on pin 20 is a must in place of the 6K resistor.
- Presently, my DDDAC is connected to a "Truth" buffered before and after LDR volume controller which also has outputs to my speaker amps and is my source selector. The Truth is so transparent and uncolored - it is easier to just leave it in the system. Connecting the Cinemag outputs directly to the mainline would require an output resistor that would end up with a combined load of about 10K at the normal listening volume of the mainline.

- A buffer tube unbalancer should sound great with a single board DDDAC, but I have no experience with this.
- Using an output transformer without a buffer would require a stack of at least 4 DDDAC boards. Some persons have gone 12 high!!??
- No one reports being totally happy with capacitor coupled outputs.
- Direct coupling won't work with the Mainline because of the 2.7V offset.

[carlsor]

Update,

I received some metal can 2N2222A transistors and replaced the defective one. All voltages now within reason and output to coupling cap adjusted to 145V. I tested the Mainline with all stock parts which sounded better than expected.

I replaced each 10uf Dayton coupling cap with two 3.9up Mundorf silver/oil bypassed with a 0.1uf TFTF V-Cap for a total of 7.9uf on each side. I doubt that I can hear the difference between a low frequency cutoff of -3db at 5Hz versus 4Hz. I also read that trying caps larger than 10Hz made no audible difference. Besides, these were caps I had lying around from earlier projects and they filled all the allowable space. No shortage of bass!

I also replaced the 270 ohm resistor in the PS with a 270 ohm 5H choke because it's part of my religion to do that with power supplies for tube amps. Didn't do a before and after test, but for $15 it can't do any harm.

After 30-40 hours of break-in with music - wow! Very musical, detailed, dynamic, tight and flowing. Best instrument separation and soundstage placement I have ever heard from headphones. No temptation to screw anymore with internal parts. There are better headphone cables, power cords, and interconnects to try.

Ross

[Paul Birkeland]

Just an FYI on the choke thing:  The hybrid shunt regulator functions like a 30,000H choke isolating the power supply from the signal carrying components.  In a circuit without this radical decoupling, using a choke will lower power supply noise quite a bit more than a resistor with the same DC resistance, but these benefits are diminished when using the regulator.

[Tom-s]

Ross, about the capacitors you put in. Maybe just using one mundorf yields better results.

I haven't ever heard a coupling capacitor bypass that I thought did anything positive. 

[carlsor]

After the break-in period is complete I will remove the V-Cap Teflon bypass caps and listen for any change in the sound.

I don't see the problem with putting two identical coupling caps in parallel in order to reduce the bass cutoff frequency. There should be no odd interaction between them. I would like to know if there is any theory or experience that indicated a problem with this.

[carlsor]

I notice that the high voltage goes to the three tubes at the same time the filaments start heating up.
My other tube projects used tube rectifiers or a time delay-on relay to allow the heaters to warm up before the tubes were slammed with high voltage. Supposedly this delay extends tube life.

Any downside to adding a delay to my Mainline?

[Paul Birkeland]

Putting two identical caps in parallel isn't really bypassing.  Bypassing refers to putting a very small capacitor across a much larger one.

A time delay-on relay could potentially be added to the Mainline, but you'd probably also need to add a power transformer to power it.  I haven't yet been able to exhaust a pair of 6C45 tubes, so it's quite difficult to make any real statement about their life expectancy and how that might change with various modifications. 

You can populate the A side of each high current C4S board with a cap and a resistor to more slowly apply B+.  We haven't tested this, and I'm not 100% sure the 10,000uF caps will fit all that well if larger parafeed caps are installed, but it's an inexpensive experiment that doesn't involve drilling holes in the chassis or figuring out if your time delay relay is going to work with a voltage doubler. 

[carlsor]

Thanks Paul.

I recommend 12-rocketman in ebay as a source of 6C45P tubes. Price is $10.90ea with reasonable shipping from Russia.
Tubes sounded great too.

carlsor