Moving Coil Phono Head Amp, Zetex AN-23, March 1996.

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mediatechnology
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Moving Coil Phono Head Amp, Zetex AN-23, March 1996.

Post by mediatechnology » Sat Aug 03, 2019 11:59 am

This is a three transistor Moving Coil head amp published by Zetex in 1996.

The NPN ZTX851 and PNP ZTX951 would also work very well in this application.
Mouser stock both the ZTX689 and ZTX789 which have higher current gain.
I haven't built or measured this circuit to see if the ZTX689/789 have as low rbb' as the ZTX851/951.


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Moving Coil Phono Head Amp Zetex AN-23 March 1996.

"Moving Coil Phono Head Amp," Zetex AN-23, March 1996.

PDF Download: https://proaudiodesignforum.com/images/ ... h_1996.pdf

ZTX689B Data Sheet https://proaudiodesignforum.com/images/pdf/ZTX689B.pdf
ZTX789A Data Sheet https://proaudiodesignforum.com/images/pdf/ZTX789A.pdf
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Re: Moving Coil Phono Head Amp, Zetex AN-23, March 1996.

Post by mediatechnology » Sat Nov 09, 2019 3:15 pm

I just built this circuit with a +26dB flat post-amp to see how the ZTX689 and ZTX789 fare.
The total gain with the post amp measures 59.5 dB.
I'm having to deal with a lot of hum contamination but it looks to be in the 1 to 1.2 nV√Hz range based on looking at the noise floor between hum peaks and comparing it to the balanced MC preamp's noise under similar conditions and gain.

The ZTX851 in the same circuit measures about 3 dB lower noise.

The ZTX689 and ZTX789 have much higher gain however.
Apparently their rbb' is a little higher but their bias current would be 3-4 times lower than the ZTX851.

For direct comparison I should probably try the ZTX689 in the MC preamp.
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Re: Moving Coil Phono Head Amp, Zetex AN-23, March 1996.

Post by mediatechnology » Sat Nov 09, 2019 5:04 pm

Made some measurements with less hum interference using the test circuit in the OP plus +26 dB gain post-amp.
Total gain is 59.5 dB measured.

10Ω R source

ZTX689 Ein -134.93 dBu; 0.986 nV√Hz; NF 7.75 dB
ZTX851 Ein -136.1 dBu; 0.85 nV√Hz; NF 6.6 dB

10Ω R source A-weighted

ZTX689 Ein -137.3 dBu-A
ZTX851 Ein -138.8 dBu-A

30Ω R source

ZTX689 Ein -133.8 dBu; 1.1 nV√Hz; NF 4.1 dB
ZTX851 Ein -134.5 dBu; 1 nV√Hz; NF 3.4 dB

In a quieter circuit my guestimate is that the ZTX 689's rbb' may be in the 3-4Ω range, certainly <<10Ω.
They're certainly pretty decent though not quite as good as the ZTX851.
The gain however is 3-4 times higher; the ZTX689B's hfe is 500 minimum (0.1A Ic) and the Cob around 16 pF vs the ZTX851s hfe of 100 minimum and a Cob of 45 pF.
Both the ZTX851 and ZTX689 have very good 1/f noise performance.
The '851's are very consistent; I haven't had enough experience with the 689/789 to know about them for sure.
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