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Versatile and comfortable passive pre-amp

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#102
BillyK, I have just calculated the price of Uriah's offer:
35$ for 4 matched LDRs + 10$ shipping to Germany + 19% VAT = 54$ (= 41€)
This is very much!

I will therefore offer you to match the LDRs myself.
I will ask 25€ from each of you, who calls for my service, order as many LDRs as possible from FARNELL (an LDR cost 2.09€) and match them
using my test-rig (see attached image).
I will match the LDRs as accurately as I can, but will not sit in a hermitically sealed room, will touch the case of the LDRs with my fingers,
smoke a cigar and drink a whisky.

Tell me, if you like my offer and / or want to improve the test-rig.

Best regards - Rudi_Ratlos

P.S. I will check once more, if the resistor-values - as shown - make sense.
 

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#103
Gentlemen, I have just gone downstairs, listened to a CD and adjusted the volume to a level that was pleasant in my ears.
I then switched the amplifier and VCPre off, removed the motorized potentiometer and measured the value of its 2 sections.

I am using a YAMAHA-S300 CD-player that gives an output-voltage of about 2V.
The measurement of the 100K dual potentiometer shows 81K on the series-LDR-section and 9K on the shunt-LDR-section.

I could not believe and reproduced this situation on another LDR-test-rig, feeding it with a 2VDC signal.
The result: the output of the LDR attenuator shows a value of 230mV.
Multiplied with the total gain of my FC-100 amplifier, this amplitude is big enough for pleasant listening.

The usage of the resistor-values shown in my post above is not appropriate then, and I will change and use much higer values, when I match them.

Best regards - Rudi_Ratlos
 

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#105
Take your time, BillyK, I appreciate your assistance very, very much, but please do not publish the BoM, before I have sent
you the EAGLE_ULP_BoM for the auxiliary PCB (HomeTheater-Bypass, DIL-to-SIL adapter, Soft-Power-On - but these are only
a few components) as well.

METAL and I agreed in the meantime to offer you a "simple, but absolutely fail-safe" software- version as a start.
No periodic events, no automatisms (like temperature warning - it is very hard to predict, how the PIC16F886 - µProcessor behaves,
if you press a key on your remote handheld at the very same moment, while the PIC is responding to a temperature alarm):
we will offer you a fail-safe software-version without any automatism and I, myself, feel absolutely sure (refer to my post
regarding the values of the resistors in the matching process as well) that this groupbuy will offer you a "real gem of passive Pre-AMP".

Best regards - Rudi_Ratlos
 
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#115
palmito said:
I'm in for one too, all except the transformer (that was the final decision right, 120v people get an off-board transformer?).
Click to expand...

No AFAIK the PCB was changed to be able to use 120 V transformers. I think you have to find one in the US with the right pinout. Look for a EI38 (3,2 VA ) type with 7,5 V secondary winding.

this is the pinout:

http://www.jesiva.com/pdf_upload/EI38-14 3_0.pdf


Rudi_Ratlos said:
I will match the LDRs as accurately as I can, but will not sit in a hermitically sealed room, will touch the case of the LDRs with my fingers,
smoke a cigar and drink a whisky.
Click to expand...


As long as you connect your self to safety earth and are not involved in certain other physical activities while doing all these things at once it should be fine.
 
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#120
Gentlemen, find attached the current order.

@tobu: maybe your letter weighs more than 500 gr. That is why I have accounted for 7€ shipping in your case.
I will of course refund you the surplus, if not needed.

@palmito: look at how I wired the transformer. I paralleled the input and output to support a transformer like this:
AVB3.2/2/9 - BLOCK - TRAFO, 3.2VA 2X 9V | Farnell Deutschland
having 2 x 115VAC as input and 2 x 9VAC as output. If you know an American transformer brand with EI38 core (have a look at Jean-Paul's PDF)
and 115VAC input and 7-8 VAC output rated at about 400-500 mA, I will remove the dual / paralleled tracks.

Best regards - Rudi_Ratlos
 

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