lazarus preamp hums even with volume down and no components connected. - diyAudio
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Old 16th June 2011, 04:04 PM   #1
240z4u is offline 240z4u  United States
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Default lazarus preamp hums even with volume down and no components connected.

I spent a very good sum of money getting this preamp repaired from a tech, which he did, but it still hums and it's way too loud for me to live with. It hums with volume all the way down, on any source, and without any components connected besides an amp. There is no ground prong on the power cord of the pre or the power amp.

You guys have any thoughts? It looks like he replaced some diodes in the main power supply of this thing and he twisted 3 small ones together in parallel to apparently match the required rating of the original single diode.

I do have a scope, which I don't really know how to use.

I could take it back to him again, but he had the preamp for a LONG time (just over a year) and there is no way I am dealing with him again. Are there any obvious sources to check for hum? This uses an outboard PSU and it's well laid out and easy to get to the components.

I would like to rule out anything easy or obvious (such as diodes). It seems to be 60hz hum and it does it even if I swap tubes.

Thanks guys.
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Old 16th June 2011, 04:36 PM   #2
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Is any of your two prong devices non-polarized? if so, may I suggest to you to turn around the plug.

on tube gear, I try and encourage to use non-polarized plugs. The reasons behind this is: when I get a hum on something that did not do it on my isolation transformer/variac about 60% of the time the 3rd prong was the culprit. Sometimes, I'll incorporate a ground lift switch on the 3rd prong. 40% of those instances is power leakage from the transformer on the ground causing the hum. Turning around the plug fixes these.

Can you post some pics of the componate side of the power supply chassis and pre amp chassis?
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Old 16th June 2011, 04:37 PM   #3
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1 paralleling diodes is BAADDDD. They are not resistors and do not auto load balance.


2. for the hum, the easiest thing is to lift the heater voltage above ground. The cap is optional but recommended, or use 2 small mkv caps on both sides of the heater as steve does it.


http://i573.photobucket.com/albums/s...refvoltage.gif

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Old 16th June 2011, 04:41 PM   #4
240z4u is offline 240z4u  United States
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Well, the lazarus preamp plug is polarized if memory serves. I can plug it into my variac to see if it still hums though.

I will take some good close photos of the component sides of the boards. In the meanwhile here are a couple of not close enough images that are old. These were taken before going to the tech and don't show the janky diode work.

Click the image to open in full size.

Click the image to open in full size.
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Old 16th June 2011, 04:43 PM   #5
240z4u is offline 240z4u  United States
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Quote:
Originally Posted by nightanole View Post
1 paralleling diodes is BAADDDD. They are not resistors and do not auto load balance.


2. for the hum, the easiest thing is to lift the heater voltage above ground. The cap is optional but recommended, or use 2 small mkv caps on both sides of the heater as steve does it.


http://i573.photobucket.com/albums/s...refvoltage.gif

DCPP Amp
That's interesting about the diode thing. I wonder if that's the problem. There is no way this preamp hummed when it was new. The heater thing is a little beyond my knowledgebase and I am afraid I would blow everything up.
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Old 16th June 2011, 05:12 PM   #6
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in the mean time of getting approved for audio karma dot org,
(it would have been better to directly upload pics on this forum)

Sometimes the power supply's magnetic field is induced into pcb runs, tube elements, and signal transformers (if used) especially if they are directly stacked.
Try spacing out the power supply chassis from the pre chassis to see if this is the case.

is this power supply a mod? If memory serves me right, it was all in one unit, the B+ was linear regulator as well as DC heaters with regulators. parallel diodes is bad call on his part. should use a 6A10 for a simple power diode. The stock pre from memory was two small bridge rectifiers located near the back-left of the pre amp.

Last edited by DavesNotHere; 16th June 2011 at 05:20 PM.
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Old 16th June 2011, 05:19 PM   #7
240z4u is offline 240z4u  United States
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Sorry about that! I thought I had hosted that on my photobucket. I will get them up so you can see them without joining. It is a great site though!

I did attempt to move the PSU further from the main chassis with no change.
Attached Images
File Type: jpg laz1.jpg (80.7 KB, 134 views)
File Type: jpg laz2.jpg (96.3 KB, 134 views)
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Old 16th June 2011, 05:46 PM   #8
240z4u is offline 240z4u  United States
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The outboard PSU is stock for this preamp. It's a very rare bird and I haven't found any information whatsoever on the internet. It's called the "Lazarus balanced preamp" with balanced outputs.
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Old 16th June 2011, 06:47 PM   #9
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Um 2 resistors is too diy for you? Ok put the multimeter on dc. While the unit is on measure from 1 heater to the high voltage lead of one of the tubes. Then measure from the high voltage lead of the tube to ground. If they are the same voltage then your heaters are not lifted above ground.
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Old 16th June 2011, 06:48 PM   #10
240z4u is offline 240z4u  United States
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Okay, I can do that. I looked at the schematic and wasn't sure how you would implement that. Thanks!
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