Bonsai’s X-Altra MC/MM Phono Preamp

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#482
Bonsai said:
Chris,

Important note: if the preamp is picking up noise, the readings will be all over the place. For your measurements, leave the MC input OPEN and SHORT the MM inputs right at the input connector. This will ensure the preamp willbe quiet enough for good DC readings.
Click to expand...

For shorting plugs what you you recommend? Shorting with just wire or a 470 ohm resistor between signal I put and ground in the plug
 
#484
For MM, short the inputs, but for MC leave them open or, connect a 47 Ohm resistor across the inputs under the connector. If you dont want to use a short wire, you can use shorting plugs like this from Amazon

https://www.amazon.co.uk/Tertullus-...ords=RCA+shorting+plugs&qid=1688571113&sr=8-3

1688571233117.png


Reason is on MC, because its a current input common base circuit, if you short them the gain will be very high (gain is dependent upon cart resistance).
 
#485
Mouser has stock of the ZTX 851 here: Mouser Pt# 522-ZTX851STZ and the ZTX951 here: Mouser Pt# 522-ZTX951

If you've been stalled on your preamp builds, suggest you hurry up and order the devices above (you will need 2 off each to build an X-Altra MC/MM) - they only have c. 300 of the one type.
 
#490
Just checked again today and they are now showing 323 of the 851 and 7800 of the 951. I ordered 25 of each yesterday.

They also do SOT23 SMD versions, but I am checking to see if these can be directly mounted on the board (underside). I suspect these will generally be easier to get in the future. I’ll come back with an update in the next few days.

If you are after really good MC noise performance there isn’t much out there other than the 851 and 951 devices.
 
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#494
Bonsai said:
Quick update re the LSK389B devices. Some builders have struggled to secure these as they have not been generally avaiablle at the major distributors.

TI recently introduced the JFE2140 dual JFET which has been positioned as an alternative to the LSK389/489 devices.
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Yes, I see JFE2140 has slightly less noise than LSK389 and is much cheaper - https://www.patreon.com/posts/84327298
 
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#495
Hello there,
I received my board (V1.0 July 2020 green pcb) a few weeks ago and started soldering resistors and capacitors.
I noticed some discrepancies between the BOM and the silkscreen on the board.
Can any of you confirm that the following values which are from the silkscreen are right ?
If so I‘ll need to order these parts since they are not in the latest BOM.

R61-26-25-10-9 1k ohm
R7-28 120 ohm
R81-85 110k ohm
R79-84 150 k ohm.

C79-86 220 pf
Thank you guys !
 
#497
I've just been on Mouser, and they have large stocks of the LSK389B in SMD8A and TO-71.
You can also use TI JFE2140 Mouser Pt# 595-JFE2140DR

Mouser has Stock of the ZTX951 (few thousand) and RS has some small stock of ZTX851

🙂
 
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#500
Hello, i will build the Preamp too.

The component MUR820G is not available on mouser , is it possible to replace it by an other rectifier ?

It seems that it is not possible in Switzerland to place an order with RS as private, it is reserved for businesses. How did you do for order components from them?
He asks me for proof that I am a professional!
 
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