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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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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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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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.
 
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.
 
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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I love it! All that work on the wood front panel just to test. Interesting about P5, what do you do when you can't hear a difference while adjusting? Where do you adjust it to then?
I'm so glad to hear the noise level is reduced, that my was concern since I'm using an old Hafler preamp.
Let me know your results with the Music Hall 25.3 when you A/B balanced input versus RCA with the tube output. I'm curious if there will be any difference or did Music Hall using the two different outputs to create similar sound quality from each output.
It's been nice to have someone else building this old design preamp. I have the rest of the week and weekend to work on the build when the panels arrive. Nothing but snow, freezing rain and just plain old rain for the next 10 days anyways. What can you do in the rain except get wet?
 
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Is that the Marantz preamp you've been using? I have an old basic Marantz tuner, I love the horizontal tuning wheel.
Have you tried using the tuner section via the preamp into the BZLS yet? 2.0v rms is 2.0v rms, that was what Nelson was talking about with the reference to Holman and THX.
 
Yeah, it’s a 2235 receiver. It’s got preamp out so I had used it to drive the ACAs unbalanced. I also had run the Music Hall 25.3 through the AUX input unbalanced as well as hooking them up directly to the ACAs using the XLRs. I haven’t yet connected the 25.3 to the BZLS using both RCAs and XLRs and switched between them for an A-B comparison.i think both the DAC outputs are active simultaneously, aren’t they? Incidentally, I’m sure you’ve popped the top of your 25.3. That thing’s really packed with circuits, eh?

Regarding P5, I don’t even know what I’m listening for. It’s just a variable resistor since it’s wired using only two contacts, right? But no discernible volume change as I adjust. I guess it’s possible the wiper isn’t making contact and it’s just serving as a 500-ohm resistor, but that would have to be the case for both.

In any event, there is plenty of gain. The 23.5 has a 2v signal, right? This system is in a 20x20x8 family room with tile floor and wood paneling and lots of big windows, so I assume it’s pretty lively. Speakers are 89-dB Boston Acoustics 5HD bookshelves, and with P3 and P4 at maximum I get a comfortable volume at 9 o’clock on P1 and P2. At 12 o’clock it’s LOUD but not noticeably distorted.
 
You did a fantastic job. I’m also finished, with a tweak or two like larger feet. I think I’m going to take the money I saved by drilling my own panels and get some nice printing on the front panel. I’m still fine-tuning a lower output level in my left channel, so I’m glad I stuck with the separate pots so I can balance volume. But I’m very happy with the outcome. Spent about $500, a bit more than expected but reasonable for the result, I think. Listening now to Martha Argerich. Piano music really shows off good systems.
 

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Here's a wiring diagram another member sent to me. I found it very helpful despite the labels in Korean. It departs from the ZBLS specs by using one four-gang pot instead of two two-gang pots to control input and output levels, as well as just one transformer for the power supply. But it includes balanced and unbalanced inputs and outputs so that's a big help as wiring goes.

On the chance anybody out there picks up the wiring diagram, note that it uses the XLR pin 3 as ground, whereas pin 1 is accepted practice.
 
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Thanks for the kind words. First time either of us (jfuquay and I) built a project from "scratch". I now have a balanced preamp to drive the ACA mono blocks. jfuquay - Nice work. I like the on - off switch nice touch. I like the chassis, where did you get it? What a nice match to your ACA's, they look like family.