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Old 8th June 2011, 09:48 AM   #81
hesener is offline hesener  Germany
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remember resistors have thermal noise, so if you see a difference between a short, a 1k resistor and a 100k resistor you know your input amplifier has low noise indeed!

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Old 8th June 2011, 11:14 AM   #82
AndrewT is offline AndrewT  Scotland
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Hes has brought up a good topic.

The 100k across the input of your pre-amp should create audible wideband noise. No separate hum component should be audible nor measurable. Develop the casing until you achieve this.
The 1k resistor should not create any audible noise nor any hum components.
The 0r0 resistor should not create any output voltage.
If you can measure the hum free difference between 0r0 and 1k0, then your system is indeed sensitive enough for what you want to do.
If you can read the (hum free) difference for 100r resistor cf. 0r0, then you have a very sensitive system.
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Old 8th June 2011, 12:33 PM   #83
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ok well the problem is certainly with the preamp itself picking up the noise. with the input to the preamp shorted I get a significant bump at 50Hz. With the output to the preamp shorted it disapears and I see the noise floor of the sound card, so it is not the cable from the preamp to the sound card that is picking up the noise.
1st pic left channel output of preamp shorted right channel no short but input shorted.
2nd pic left channel powered right channel no power, both input shorted.

So even without the preamp running it is obvious it is picking up background 50 Hz. Looks like a quality metal box is the next step required

preamp is running off a sealed lead acid battery.

Tony.
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Old 11th June 2011, 12:37 PM   #84
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@wintermute

You need to shield everything. Look at the pictures, even AC adapter plugged to AC socket will radiate 50Hz noise.
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File Type: jpg Spectrum__2.jpg (57.0 KB, 497 views)
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Old 13th June 2011, 12:51 PM   #85
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Quote:
Originally Posted by wintermute View Post
So even without the preamp running it is obvious it is picking up background 50 Hz. Looks like a quality metal box is the next step required

preamp is running off a sealed lead acid battery.

Tony.
The box must then be grounded. From Morrison: Rule 1. An electrostatic shield enclosure, bo be effective, should be connected to the zero-signal reference potential of any circuitry contained within the shield.

Try turning off the lights, even in they're incandescents!

One of the interesting observations from Morrison's book "Grounding and Shielding" is that a room is functionally equivalent to a capacitor, and a person standing in the room influences the distribution of the eelctrostatic field.
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Old 13th June 2011, 01:05 PM   #86
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One of the interesting observations from Morrison's book "Grounding and Shielding" is that a room is functionally equivalent to a capacitor, and a person standing in the room influences the distribution of the eelctrostatic field.
And some of us more than others- I need to go on a diet!

It's probably lesser known these days, but watch out for glass body diodes- they're light sensitive and will pick up modulation even from incandescents and impose it on the circuit!
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Old 13th June 2011, 01:05 PM   #87
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Thanks stormsonic and jackinnj! That difference just with and without the ac adaptor plugged into the wall socket is quite an eye opener!

I actually pulled the wireless card out of my pc, put the sound card at the bottom of the case and put blanking plates into all of the empty slots, This seems to have also helped somewhat (though it could be the 50 Hz wasn't as bad at the time of the tests).

I went to buy the box today but the store was closed (public holiday today) so looks like being next weekend before I can get it. I was actually thinking of putting RCA jacks on, and having them non-insulated from the case, thus making them the connection to the case and effectively having the shield continue from the cable to around the box.

The current plastic box just has a hole in it with the output lead going through and being soldered direct to the pcb. I wasn't sure whether to do the same with the BNC (input) jacks as well or not, figured I might get earth loops if I did.

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Old 13th June 2011, 01:28 PM   #88
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In some cases BNC's are great, in some cases XLR's are better. Of late, I've been making my own XLR's with shielded twisted pair -- making the cable as short as comfortable and possible. I got a crimper for BNC's and have been using RG-174 for these hookups.
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Old 19th June 2011, 01:37 AM   #89
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Well I think that I've made one step in the right direction I got the diecast box yesterday and the results are good. The 50 Hz problem is gone completely. RMAA graph shows left channel with input shorted (old plastic box) vs right channel input shorted new diecast box.

The is a spike at 143Hz that wasn't there before, which could possibly be some very low level oscillation due to the input and output jacks not being isolated from the box, but I think it is probably deep enough in the noise floor to be inconsequential, and it is certainly an odd enough freq to be able to be separated out. I realised that the measurement on the old box I had a 150K resistor to ground on the output, I'm going to try putting in a 56K which should give a corner freq of about 0.2Hz with the 15uF output caps, the current new box measurement has no resistor on the output.

The other thing that was interesting was how significantly the floating input performance improved!

also attached a pic of the new box. The holes were a bit off (I don't have a drill press, and apparently blunt drills and not a sharp enough centre punch!) but it went together ok

It's a great day today for balcony speaker measurements so any of the PS are probably going to be delayed a little. Its not often I get a still day when everyone else is out of the house

Tony.
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Old 12th July 2011, 01:32 PM   #90
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OK so its been a while but I finally did some more measurements I think that the metal case on the preamp gives enough shielding to get some whilst not perfect, usable results.

Note it is still on breadboard, but I think that these comparisons are worthwhile despite that.

The first pic shows that my CRT monitor was contributing to the noise. The preamp was connected to the power supply with no AC connected (ie the PS was effectively just an aerial.

Strangely I could now see 100Hz not the usual 50Hz peak. I changed the refresh rate on the monitor from 100Hz down to 90Hz and the peaks moved.

second pic shows the difference between 4V in to out voltage difference (left channel) and 5V in to out voltage difference (right channel), 4V differential was with 8.5V output voltage and 5V differential achieved by dropping the output voltage down to 8.0V output current obviously slightly lower for the second, but I think the real difference is due to the voltage drop across the reg. current in the case of 8.5V is 151mA and for 8V 142mA

Hopefully before too long I will actually build this circuit on verro board rather than the breadboard

Tony.
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File Type: png 4vdiff_vs_5vdiff.png (33.5 KB, 81 views)
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