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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Bob Cordell's VinylTrak
Bob Cordell's VinylTrak
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Old 4th July 2013, 06:20 AM   #41
padamiecki is offline padamiecki  Poland
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thank you Bob!
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Old 29th May 2014, 09:58 PM   #42
jackinnj is offline jackinnj  United States
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Bob Cordell's VinylTrak
Just getting around to building this -- the MM version only.
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Old 30th May 2014, 10:43 PM   #43
jackinnj is offline jackinnj  United States
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Bob Cordell's VinylTrak
i simulated the 75uS response -- i don't think i will use a relay, just a molex jumper:
Click the image to open in full size.

The simulation, fwiw, used the new transistors bob recommended, and the updated current mirror.
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Old 2nd June 2014, 09:50 PM   #44
mlloyd1 is offline mlloyd1  United States
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jackinnj:

thanks for sharing and keep posting.
i'm gonna build a version of this too (also mm only), as soon as i can get around to to it.

mlloyd1
(and yes, i already have a bunch of "roundtoits"; they didn't help ...
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Old 28th January 2015, 12:09 PM   #45
padamiecki is offline padamiecki  Poland
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Hi
Please alight me,
Damping mode mm means that passive 75us in the preamp is swiched off (D1,2 off)
and the cartridge is loaded with extra resistors to get the compensation for cartridge inductance from 200 to 700mH.
What value resistors connected parallel to input 100k resistors R1,2 (fig6 in LinearAudio vol 4) will get right damping for 200, 250, 300, 400, 500, 700mH?
Or damping was created electronicaly by adding inverting opamp with the gain 9 and via the resistors (470k) to Vin+ and Vin-?
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Old 28th January 2015, 01:05 PM   #46
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by padamiecki View Post
Hi
Please alight me,
Damping mode mm means that passive 75us in the preamp is swiched off (D1,2 off)
and the cartridge is loaded with extra resistors to get the compensation for cartridge inductance from 200 to 700mH.
What value resistors connected parallel to input 100k resistors R1,2 (fig6 in LinearAudio vol 4) will get right damping for 200, 250, 300, 400, 500, 700mH?
Or damping was created electronicaly by adding inverting opamp with the gain 9 and via the resistors (470k) to Vin+ and Vin-?
It's been quite awhile since I've looked at that, and most of the information on calculating those loading (damping resistors) should be in the original LA article. In a nutshell, in damping mode, the 75us rolloff in the preamp is "stopped" at 8kHz, flattening the electrical response from there on up to higher frequencies. This means that the passive 75us is not switched off, but instead the relay is engaged to alter the 75us response so that it stops falling at frequencies above 8kHz. The damping resistor is chosen so that it creates a pole with the cartridge inductance at 8kHz. The value of the damping resistor should equal the reactance of the cartridge inductance at 8kHz.

Cheers,
Bob
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Old 28th January 2015, 01:36 PM   #47
padamiecki is offline padamiecki  Poland
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Dear Bob
thank you,
1. how should I calculate the resistor value for AT 440MLa with the parameters:
DCR 776 Ohm
impedance@1kHz 3.2 kOhm
inductance@1kHz 490mH
2. calculated resistor value should be divided by 2 and putted instead of grounding input 100k resistors R1,2 (parallel with ~100p cables capacitance)?
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Old 30th January 2015, 11:02 AM   #48
padamiecki is offline padamiecki  Poland
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I could be more specific, maybe my questions were too chaotic?
Anyway I do not understand about cartridge loading.

I read a text about VinylTrak @LAvol4 again and there was nothing about how loading selector is built, there was only that the selector sets the inductance: 200, 300, 400, 500, 700mH. There was no formula too.

Please explain me what values of the resistances and capacitances should be used in this selector?

thank you in advance!
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Old 30th January 2015, 01:14 PM   #49
jan.didden is offline jan.didden  Europe
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I think Bob said it:

'The value of the damping resistor should equal the reactance of the cartridge inductance at 8kHz.' There's your formula in plain English.

Jan
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Old 30th January 2015, 03:19 PM   #50
jackinnj is offline jackinnj  United States
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Bob Cordell's VinylTrak
Z = 2 x PI() x f x L
In the case of the Shure 15 Mk V L=370mH
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