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Old 17th February 2012, 04:37 PM   #1
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Default Decware Zbox - looking for Tube buffer mods to reduce buzz

Recently got a used Decware Zbox. Unit has a buzz in both channels. Checked out all the parts and all parts are good. The internal wiring is a rats nest and may windup redone. But before going there, looking for some basic "quiet" mods that might help. Some of the items I am thinking about

- two 100 ohm resistors to ground each leg of the heater power supply
- reversing the phasing of the heater supply
- adding a choke to power supply
- adding more capacitance to power supply
- converting the AC heater circuit to DC
- adding an OC2 tube to power supply

I'll post some schematics of the Zkit4, which looks very similar to the parts and layout I have been testing. So from a concept model, any mods to make this circuit quieter, should work on the Zbox (no schematic available).

The schematic with the OC2 is an upgrade that is not in place, but certainly worth considering.
Attached Images
File Type: gif zkit4schematic.gif (5.2 KB, 233 views)
File Type: gif zkit5schematica.gif (6.9 KB, 232 views)
File Type: gif zkit5schematicb.gif (6.9 KB, 231 views)
File Type: jpg zboxreva.JPG (117.5 KB, 239 views)
File Type: jpg Picture 15.jpg (138.9 KB, 241 views)

Last edited by speakerfritz; 17th February 2012 at 04:56 PM.
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Old 17th February 2012, 08:00 PM   #2
adason is offline adason  United States
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first thing I would do is dc on the filament
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Old 17th February 2012, 08:05 PM   #3
adason is offline adason  United States
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then elevate it to 1/3rd of supply voltage
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Old 17th February 2012, 08:32 PM   #4
DF96 is offline DF96  England
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The wiring in the last picture seems designed to cause buzz by having large loops carrying charging pulses. Not sure of this is your one.
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Old 17th February 2012, 09:50 PM   #5
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Quote:
Originally Posted by DF96 View Post
The wiring in the last picture seems designed to cause buzz by having large loops carrying charging pulses. Not sure of this is your one.
The last picture is the stock unit. Which is what mine looks like. The picture to the left is one from another forum where the OP "fixed" his unit, but really didn't get into details of what he did.
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Old 17th February 2012, 09:52 PM   #6
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Quote:
Originally Posted by adason View Post
first thing I would do is dc on the filament
I'm thinking that as well. I have a 2"X2" board that will regulate from 1.5 to 24 volts at 1.5 amps and can be adjusted to 10 volts which is what the AC heater voltage currently is. The heater supply looks like it's .3 amps so it's not problem for the board. Not sure if going ac to dc is one to one on voltage.
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Old 17th February 2012, 10:14 PM   #7
DF96 is offline DF96  England
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I would sort out the wiring first. I am astonished that such poor quality building is being commercially sold, even from China. Was it a kit? Mind you, the daft circuit design is perhaps a warning.

First step: identify the wires from the transformer to the rectifier bridge. Ensure that they follow the same route and are at least roughly twisted together. Keep them away from all other wires.

Step two: identify the wires from the rectifier to the first (reservoir) capacitor. Do the same for them.

As currently wired, the transformer wires (red?) look reasonably OK but the cap wires (grey/pink?) are appalling. They don't follow the same route, and it looks like the negative wire is twisted around a grounding wire. Awful! The ground wire should connect to the cap, not the rectifier, and be kept away from the rectifier-cap wires as much as possible except where it actually joins.

The aim is twofold: reduce the area of loops carrying charging pulses, and stop injecting buzz into the ground connection.

Make that change and report back. It is likely that the unit has other wiring problems too.

PS just noticed that the Decware website claims that the units are handmade in the USA. Do you have a genuine unit or a chinese clone?

Last edited by DF96; 17th February 2012 at 10:28 PM. Reason: add PS
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Old 17th February 2012, 11:31 PM   #8
scott17 is offline scott17  United States
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The grid stoppers in the second photo do not look like 22k. They look more like 47K. Is that correct? Hard to tell, they may be 470R which is probably OK. I would think 1K should be sufficient. I agree with DF96. The PS wiring needs to be corrected. The wiring is abysmal. I'm surprised that work would come out of Decware, but I don't have any direct experience with their products. That circuit should work fine with an AC filament supply, but you would certainly want a tighter twist that what is shown, and routed more like the first photo. I also notice that the first photo in/outs have been changed to shielded which may help.

Last edited by scott17; 17th February 2012 at 11:40 PM. Reason: stopper value comment
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Old 18th February 2012, 03:28 AM   #9
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Thanks for the wiring walk thru..will give that a go first. My unit is a Decware made unit. I've seen a lot of photos of this box and they follow the rats nest lay out.

One of the thoughts I had, and verified, is that the hum/buzz would probably be less noticeable on the flea powered amps Decware sells. So I tried the Zbox on a 4 WPC SE tube amp and the hum/buzz was barely noticeable. I'm trying to use this box on a few tube amps that range from 50WPC to 350WPC. Seems like th higher up in power you go, the more noticeable the noise is. Same problem with SS amps . So maybe the craftsmanship is good enough for flea power amps. Certianly theres much to be improved. I go thru the wiring this weekend.
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Old 18th February 2012, 10:59 AM   #10
DF96 is offline DF96  England
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It is quite amusing to compare the wonderful prose on the Decware website, and their prices, with the rat's nest of wiring in one of their simpler products. A newbie DIYer would be ashamed of that. Doubtless it has glowing reviews, although I remain quite baffled as to why people will spend 100's of dollars on a simple (and badly made) cathode follower.
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