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Balanced Zen Line Stage Preamp
Balanced Zen Line Stage Preamp
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Old 13th February 2019, 11:31 PM   #21
jfuquay is offline jfuquay  United States
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Balanced Zen Line Stage Preamp
I drilled my back plate and while it's not perfect, it looks fine (for a back plate). I decided to jump into the inputs wiring and everything went well, AFAIK, using 24awg twisted pairs stripped from cat5 cable. Thank goodness you've got four solid colors and four striped. Otherwise it would be hopeless keeping them organized. I still need to ground the XLR shields and cases.

I decided not to run a shaft extension to the back for the select switch because it was so tight. My chassis has two fairly large three-sided channels running the length of both sides, and I thought I'd tuck the input wires inside to keep them out of the way and maybe get a little shielding. I could also maybe run them between the perforated base and the steel bottom cover? The select switch lugs were about as expected, and the two input ALPS pots with pins instead of lugs were easier than expected.

I decided to install the p3 and p4 output pots inside the chassis for adjustments. I think ultimately I'll settle on a resistor, however. My XLR cables arrived from Blue Jeans Cables, so maybe I'm not that far from hearing it.
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Old 14th February 2019, 01:00 AM   #22
elwood625 is offline elwood625  United States
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You are getting close! I like the side channels where you ran the input wires, hidden and shielded(?). I'm hoping adjustment of p3 & 4 is like setting bias on the ACA's, once or twice and done.
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Old 16th February 2019, 04:41 PM   #23
jfuquay is offline jfuquay  United States
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Balanced Zen Line Stage Preamp
OK, this is pretty close. I'm nervous about getting the front panel right so I cut a mockup out of scrap wood. The 10mm front I selected requires a good deal of countersinking on the front to expose the mounting shaft threads. But my step bits can manage it, and I'm using a friend's drill press to keep it clean.

I moved P3 and P4 to the back panel for accessibility. Still, I anticipate using the inputs as volume controls. (I also realized I left off R18 and R19, so need to amend that.)

One question: I'm still a little uncertain about the XLR and RCA outputs. N.P. says "XLR pin 1 is attached to the grounds of both RCA connectors. These grounds are isolated from chassis ground." (So far so good. But then ...). "The case of the XLR connector, if metal (mine is) and shield are attached to chassis ground."

I thought Pin 1 is the shield. How can the grounds be isolated from chassis ground if you attach the shield to chassis ground?
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Old 16th February 2019, 05:26 PM   #24
twitchie is offline twitchie  Canada
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Balanced Zen Line Stage Preamp
It's all about the context in which NP was writing it. He is saying the grounds on the RCA connectors are isolated from chassis ground - they go to pin 1 of the XLR connector. The XLR connector case and shield (pin 1) attach to ground. He is saying don't connect the RCA grounds to anything except for Pin 1. He is not saying the XLR pin 1 doesn't go to chassis ground. That's how I read it but I am often wrong. If in doubt, err in the side of safety and then only go looking for ground loops if you have hum. If you don't then go look for recreational beverage of choice.
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Old 16th February 2019, 06:21 PM   #25
elwood625 is offline elwood625  United States
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1 - If your using two wires PLUS a shield for the XLR's, then the shield goes to PIN 1 as a ground along with RCA negative (ground) and then to PC board ground. That way the grounds "float" with no connection to chassis ground.

2 - If your running three wires PLUS a shield, then the three wires go to PINS 1, 2, & 3, and the shield then goes to the XLR case ground. The wire for PIN 1 will go to PC board ground and PINS 2 & 3 to the selector switch, volume control and then PC board input. That way the XLR case and the shield for the three wires goes to chassis ground. PIN 1 is the ground that "floats" on the PC board.


The only items going to chassis ground should be the IEC ground, the power supply PC board for the thermistor and the shield's for the XLR connectors.



IMHO the XLR connectors shield to ground is optional and should be disconnected if you ever have a ground loop or hum.



Either way you do not want chassis ground connection to either the input or output PC board grounds, they "float".


Wow, you are getting close, my front and rear panels won't arrive until Thursday. I did get all of the parts from DH Labs to make XLR cables, so I'll spend the next couple of days making cables.


Not to mention watching the Daytona 500 tomorrow, day of rest.
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Old 16th February 2019, 06:25 PM   #26
elwood625 is offline elwood625  United States
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The XLR interconnects only have two wires and a shield, shield to pin 1, pins 2 & 3 to the two wires. The XLR interconnect has no provision for XLR case ground, so why use it inside the preamp?
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Old 16th February 2019, 07:02 PM   #27
jfuquay is offline jfuquay  United States
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Balanced Zen Line Stage Preamp
My XLR cases have a tab for a ground. I bought Neutrik "D" Series Panel Mount XLR Connectors from Parts Express. I'll pop the tops on my Music Hall DAC and an ACA out of curiosity to see if they have.

But thanks to both responders for jumping in. I have been running the ZBLS last night and today, and it seems very stable as far as the voltages. My unregulated rail is still high -- 88 v instead of 80 -- but maybe that's 'cause it's not regulated?

I'm going to add the R18 and R19 resistors and hook it up all balanced: iMac TOSlink=>Music Hall=>ZBLS=>ACAs.

I've been using a Marantz 2230 preamp out to drive the ACAs so far using RCAs. I use the receiver's FM radio. I can't use the preamp out as a signal source for FM, can I? Isn't it already a boosted signal from the receiver's preamp? Seems like a no-no.
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Old 16th February 2019, 09:06 PM   #28
jfuquay is offline jfuquay  United States
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When I looked at R18 and R19, the schematic seems to show them on the pot's out pin, or coming straight into the circuit. Elwood625, I think you have said those resistors go on the wiper pin, the center, and you've been right on other matters. What am I seeing?
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Old 17th February 2019, 02:31 PM   #29
elwood625 is offline elwood625  United States
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I have R18 & R19 connected to the output side of the selector switch going to the input of P1 & P2. The wiper of P1 & P2 then goes to the PC board inputs.
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Old 19th February 2019, 12:03 AM   #30
jfuquay is offline jfuquay  United States
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Balanced Zen Line Stage Preamp
Default It's alive!!

Up and running. Everything is wired except for the power switch, which I hooked up with clips temporarily since it needs to run through the front panel. Running fully balanced fed by the Music Hall 25.3 and driving the two ACA mono blocks. Iit sounds very good. No noise or hum at all.

The only thing I'm not completely sure of is P5. I somehow ended with 10-turn Bourns cermet trimpots on both channels and I honestly can't hear any difference as I turn (and turn, and turn) the adjustment screws. I may snip them off and just go with Nelson's recommendation of 430 ohms.

elwood625, thanks for your patient advice. I appreciate it. Hope your panels arrive soon and you can proceed.
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