Phono stage FET issue

missak

Member
2016-06-20 12:15 am
Dear, friends.

I own 2 equal Sansui AU-X911DG integrated amps, and I had to replace some parts at the phono stage of one of them - transistors 2SK389 and 2SC1845. The unit with replaced parts now sounds a bit different than the original one. A bit brighter, less body... The original unit sounds undoubtedly better. All caps and resistors are ok on both. I checked everything and I only realized some differences between voltages on the original and the replaced unit (see pic below). It seems that these differences are due to the Idss of the FETs, that are higher at the original amp FETs.

The 2SK389 has 3 Idss classifications:

GR: 2.6 ~ 6.5mA
BL: 6 ~ 12mA
V: 10 ~ 20mA

Only "BL" or "V" grade are recommended at the service manual, I replaced them with BL grade .

Is possible to change some circuit component values (resistors, diode?) to compensate the replaced FETs Idss, changing the current through them, and make it sound better??

Thanks for any help. ;)
 

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anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi missak,
Your current sources are identical, there is some difference between the Idss of the JFETs, but they are within tolerance. Your input signal is typically going to be in the <10 mV range, so compared to .15 volts, I don't see a problem. The only thing you can do is to try another 2SK389, or maybe more before you get it sorted out.

-Chris
 

ticknpop

Member
Paid Member
2005-05-28 9:43 pm
toronto
Yes your just measuring the difference in idss of the 389 or the difference in the components of the current source , but well within component tolerances.

To improve it -

A) Are the resistors 1% metal film?

B) filter caps C3, C6, C32, C33 to Silmic. Bypass each undeneath the pc board with a 0.1 uf Vishay 1837,

C) Replace C2, C5, C7, C8, C13, C50, with polystyrene. Carefull to match and select C7 and C8 because they are the RIAA caps. You can parallel smaller values to get the correct ones if you can't find some values.

D) Use bipolar Muse for C10, C11, and C12- actually replace C11 and C 12 with a single 22uf Bipolar. Bypass each undeneath the pc board with a 0.1 uf Vishay 1837,

E) C1 to 1uf 50V polyester
 
Last edited:
Dear, friends.

I own 2 equal Sansui AU-X911DG integrated amps, and I had to replace some parts at the phono stage of one of them - transistors 2SK389 and 2SC1845. The unit with replaced parts now sounds a bit different than the original one. A bit brighter, less body... The original unit sounds undoubtedly better. All caps and resistors are ok on both. I checked everything and I only realized some differences between voltages on the original and the replaced unit (see pic below). It seems that these differences are due to the Idss of the FETs, that are higher at the original amp FETs.

The 2SK389 has 3 Idss classifications:

GR: 2.6 ~ 6.5mA
BL: 6 ~ 12mA
V: 10 ~ 20mA

Only "BL" or "V" grade are recommended at the service manual, I replaced them with BL grade .

Is possible to change some circuit component values (resistors, diode?) to compensate the replaced FETs Idss, changing the current through them, and make it sound better??

Thanks for any help. ;)

Just wondering, 2SC1845 comes in four grades for gain. Is your replacement compliant with what is specified in the service manual.
 

missak

Member
2016-06-20 12:15 am
D) Use bipolar Muse for C10, C11, and C12- actually replace C11 and C 12 with a single 22uf Bipolar. Bypass each undeneath the pc board with a 0.1 uf Vishay 1837,
Thanks, ticknpop.

All eletrolytic caps on the circuit are Elna Silmic. Do you think that Muse bipolar are better than 2 Silmics "back to back"?


Just wondering, 2SC1845 comes in four grades for gain. Is your replacement compliant with what is specified in the service manual.
Thanks, mjona.

Yes, the 2SC1845 that I used are E grade, but I'm not sure they are originals... Do you think the 2SC1845 is critical to the point of causing worsening of the audio?
 

ticknpop

Member
Paid Member
2005-05-28 9:43 pm
toronto
Yes real bipolar electrolytic Caps have lower distortion than back to back polarized ones - the higher voltage the better ( if it fits)

Check Cryil Batemans 5 articles and test on capacitors

( too bad you can't get Blackgates anymore I like them a lot. You need 2 of 470 uf and 2 of 22uf I have Blackgate NX 6.3 volt series of those sizes in stock if you want for $60 USD total plus shipping- really low price if You check anywhere else
Better check dc voltage at output of opamp to be sure 6.3 volt cap is ok . It should be, but better if you check before thinking about this, and they take several Hundred hours to burn-in. I usually put them in series with tape out jacks on a radio reciver with a 1k load on them for a few weeks to avoid having to play so many records
 
Do you think the 2SC1845 is critical to the point of causing worsening of the audio?
The 2SC1845 is functioning as a constant current source and your supplied voltage measurements (-14.4V, +11V) suggest that it's functioning exactly like the original 2SC1845.
You could substitute the 2SC1845 with any NPN transistor with Vceo >40V and Ic >10mA.

If you're feeling adventurous, swap the phono stage 2SK389s with the control stage or power-amp stage 2SK389s - they're probably the same suffix (BL or V) as the original (missing) 2SK389s.

Good Luck!
 

ticknpop

Member
Paid Member
2005-05-28 9:43 pm
toronto
More low level detail, more smoothness, beter dynamic contrasts.
Your circuit looks like it's working OK so changing passive components likely to make a bigger change than swapping semiconductors, and you don't want to damage the 389's or the circuit board by soldering them more than nessecary
 
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Thanks, ticknpop.
mjona.

Yes, the 2SC1845 that I used are E grade, but I'm not sure they are originals... Do you think the 2SC1845 is critical to the point of causing worsening of the audio?

No, not if you have fitted a genuine part there should be no problem.

I can see a lot of searches on the web for the original part which is a high voltage low noise transistor. Can you post a picture of the replacement- someone watching should be able to judge if you have a genuine part or a fake.

The 2SC 1845 is a high voltage low noise device, low noise being important in this application. For a replacement see https://www.fairchildsemi.com/datasheets/KS/KSC1845.pdf
 
If your amplifier came with Silmic electrolytic caps I see no point in swapping these for some alternative audiophile electrolytic type.

My advice is to stick to your approach of replacing only the parts due to necessity as with the 389. Do you know what caused this to fail and apart from the 1845 were there any other replacement parts.