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Richard Lee's Ultra low Noise MC Head Amp
Richard Lee's Ultra low Noise MC Head Amp
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Old 27th October 2019, 11:03 PM   #1711
Hans Polak is offline Hans Polak  Netherlands
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Dimitri,
This posting shows a possible solution for your pollution problem with seemingly great effect.
My version of the G = 1000 low noise measurement amp (for Ikoflexer).

Hans
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Old 27th October 2019, 11:51 PM   #1712
stocktrader200 is offline stocktrader200  Canada
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this is all on the AD797. Same theory as my 2 stage phono pre with low noise npn transistor up front. what is the function of the opa2134?
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Old 28th October 2019, 12:10 AM   #1713
syn08 is offline syn08  Canada
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Quote:
Originally Posted by Hans Polak View Post
Dimitri,
This posting shows a possible solution for your pollution problem with seemingly great effect.
My version of the G = 1000 low noise measurement amp (for Ikoflexer).

Hans
Box in box is a classic, been there, done that, if you take a look at that quoted mumetal shielding it also implements this concept (two concentric layers, only much more effective due to the material nature).

To my experience double boxing in what are actually "Duraglit" boxes (thin steel sheet) is less effective than Faraday shielding.
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Old 31st October 2019, 02:47 PM   #1714
Hans Polak is offline Hans Polak  Netherlands
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Wanting to satisfy my curiosity to what extent a Differential topology has the ability to suppress EMF radiation, I used the set-up in the image below with my MC cart connected, but also with a 1R short instead of the Cart.
The upper line is full SE and the other line fully Differential before being converted to SE.
The Diff to SE converter with high precision resistors has ca. 60dB CMRR.
Connection from HA to MA was through a RCA interlink cut in two 45cm halves, both open ends soldered to the HA.
RCA connectors at the other side plugged into the MA, thus giving exactly the same conditions for both lines.
MA connected to the scope with 2 equal 50R coax cables.


Now spectra generated through both topologies could be recorded simultaneously, taking into account that the SE line has 6dB less gain.
Electro Magnetic pollution was generated with a Power Drilling machine at relatively close distance to the to the HA. When increasing the distance to the HA, pollution decreased, showing that it was indeed EMF pollution.

What I measured with this relatively simple test, was a rejection of pollution of between 10dB and 15dB in favour of the Differential setup.

Hans

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Old 31st October 2019, 03:49 PM   #1715
Hans Polak is offline Hans Polak  Netherlands
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P.S. I expect the Electro Magnetic pollution field that Syn08 and Dimitri are experiencing, to be spatially more homogeneous around the DUT than just a simple drilling machine.
If true, a differential topology could even do a better CMRR job.


Hans
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Old 7th November 2019, 10:28 AM   #1716
Bonsai is offline Bonsai  Europe
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Iíve had one or two messages about powering the battery versions of the head amps off other than a 1.5 or 3 V battery.

Here is a circuit concept you can try. No details - you will need to do that yourself but Iím sure there are others here who can help with that.

Use an opamp with a common mode input range that includes the -ve rail. If not, use a Schottky diode in the opamp -ve rail as shown.

Make sure you mount the PSU and the battery in a shielded box to prevent capacitive coupling getting onto the PSU rails. Importantly, when the head amp is active, the battery charging circuit must be completely disconnected from the battery - use a switch that can break both the -ve and +ve rails between the charger and the battery (DPDT type).
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