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Old 8th November 2004, 01:07 PM   #1
dqswim is offline dqswim  United States
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Angry 3+ weeks and still humming

Three weeks and still I have not figured out how to get rid of hum in my x'ed bosoz.

Once I get some better results I'll post some better pictures. Here a few of what i have know.

Parmetal chassis- with victoria magnetics transformers. I have 1500-3-1500 crc follwed by voltage regulators given two sets of +-58v and for a crossover that has yet to be built the power supply chassis also has two sets of +-31v and I have another chassis and a set of mox crossover boards.

http://pubweb.northwestern.edu/~dqa7.../power_box.jpg

Here is the back of the bosoz chassis with 7 lead input from the power box and the coarse input volume control with 6 ~6db steps. The switches are all ebay salvaged- old daven (shallco) switches.

http://pubweb.northwestern.edu/~dqa7...bosoz_back.jpg

And a poor view of the inside showing the works.

http://pubweb.northwestern.edu/~dqa7...ack_and_in.jpg

And the difficulties. I do not have an oscilloscope but using the computer as a frequency generator the 60hz tone matched up with the hum and the 120hz did not. So I'm thinking ground loop. I installed high power bridge rectifiers in the power supply chassis to isolate the grounds ala zen v4 with no luck. Added a nice vibration pad to the transformers, a little less mechanical noise but still hum. Shorted the ps wires in the bosoz once and blew apart a 1rf610, replaced and still got the hum.

Both channels hum and with one power supply disconnected in the power box I still get the hum so no looping through the two channels. With one channel of input and output connected, still hum.

Power supply, seems good with a voltmeter with no noticed dc change on the rails (<10mv) loaded and ~50mv using the ac function. The voltage on the irf610's is also stable. I'm also using a ccs with 1rf610 pulling 80ma for both channels and that also checks fine.

Unless I am mistaken the only way to get this hum is through a ground loop.

So the gounding. Chassis is isolated through a bridge rectifier. 7 wire connector to bosoz with seperate grounds and a chassis connection. Signal path has input grounds (3 rca and 1 balanced) daisy chained together for each channel connected to ouput ground at the input volume control, which has a wire to the bosoz board which has 1 ground point, were the leads from the psu come in.

No loops that i know of.... just scratching my head and wondering what to do with these expensive funny looking paper weights. It just seems so simple on paper. Thanks
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Old 8th November 2004, 01:23 PM   #2
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Is the hum coming through the speakers? I assume the input to the BOZ is via RCA connectors that are isolated from the chassis? I assume that earth is connected to the chassis in the power supply box? Are the two chassis (Power supply and preamp) isolated from each other? Are you using balanced outs from the preamp? If you are using RCA connectors are they isolated from the chassis? Are all the grounds in the preamp tied to one point and isolated from the chassis? Do you get the hum without anything connected to the Boz? These are just a few questions to help you get started in solving this problem.
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Old 8th November 2004, 02:00 PM   #3
dqswim is offline dqswim  United States
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Hum is coming through the speakers without the input connected. I don't have a measured value for the hum but it is about medium-low music listening level.

I am using rca connectors that are isolated from the chassis. Left and right grounds are isolated from each other and connected to earth through a bridge rectifier in the power supply unit.

Almost all signal grounds are connected to one point on the perf board which connects to the psu ground. The exception is the input and output grounds which connect together at the input volume ground before connecting to the "single ground point". I do not think this creates a loop because they just come together before reaching the single ground point but maybe it could be the issue?

Also would I be better of switching the input grounds which I am currently not doing but is possible with my input switch?
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Old 9th November 2004, 06:25 PM   #4
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You have to short the inputs to ground, otherwise they
will pick up noise.
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Old 10th November 2004, 02:47 AM   #5
dqswim is offline dqswim  United States
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Thanks nelson, I got all excited this afternoon becuase on my most recent iteration of this preamp I didn't have the negative input grounded.

But no luck. I grounded the - input and still hum. I then grounded both inputs and still hum. It makes me wonder if I could have oscillations from something, but on 2 channels it must be a design or build error.

Here is what I have built. The output volume is a shunt between channels. Changing the volume does not effect the hum I get on my speakers with no inputs.

If there is something obviously wrong here I would be glad to know about it.

Click the image to open in full size.
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Old 11th November 2004, 07:32 AM   #6
Taco is offline Taco  Netherlands
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Can you make a drawing (e.g., in paint) with all the details of how the seperate pcb's are connected and how the signal ground is organized. It looks like everything is oke, but maybe you find some error while making this drawing, or somebody else can spot something odd.

For me it looks like a grounding problem of the inputs/outputs and volume controls.
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Old 11th November 2004, 08:54 AM   #7
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dqswim,

do you really have 50mV AC on the rail when loaded? This is way too much!!!! Should be around ZERO for a regulated supply.

Check you PS.

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Old 12th November 2004, 12:22 AM   #8
dqswim is offline dqswim  United States
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thanks for the replies

I am not sure if the 50mv ac on the loaded psu is correct. That is what the ac function of the multimeter is at, but holding the leads together for a similar time and Il get 10mv. Either way the dc voltage changes only about 1 unit in the last decimal place of the multimeter or 10mv. I don't know how to get a better reading with what I have. I imagine that an oscilloscope would help.

I have attached a figure of my ground wiring. This is one of the few times I hope that I got something totally wrong.


I'm still holding out for a happy ending....
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File Type: jpg bosoz_grounding.jpg (42.3 KB, 531 views)
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Old 12th November 2004, 03:33 AM   #9
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dqswim

This grounding principle always works, giving me no hum problem.

Good luck!
Attached Images
File Type: jpg grounding.jpg (28.4 KB, 644 views)
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Old 12th November 2004, 06:42 AM   #10
Taco is offline Taco  Netherlands
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Have you connected AC to your PSU ground?

I should try the following (take thicker wire for ground connections):

rectifiers -> star ground on chasis
powersupply board gnd -> star ground on chasis
IEC inlet -> star ground on chasis

cable with seperate grounds from psu board en + and - rails to other case.

Ground volume switch, rca/ xlr directly to circuit ground of the appropriate channel.

I see jh6you has a made a clear picture and also suggests to connect rca/ xlr directly to the star point. That's even better.
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