Matching hFE Grades with Substitute Parts

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Tejaus

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Joined 2005
#1
I might have to substitute some Fairchild 2SC3503/KSC3503 pairs for some Sanyo 2SA1210/2SC2912 since the latter is getting tougher to find.

The hFE grade on my original Sanyo's is "S" which is classified 140 to 280
according to the spec sheet. Mouser has only the Fairchild 2SC3503/KSC3503 grades of "D 60 ~120" and "F 160 ~ 320". Which would work best as a replacement on a Adcom GFA-555II Input Board Predriver?

http://www.fairchildsemi.com/ds/KS/KSC3503.pdf

2SC2912 pdf, 2SC2912 description, 2SC2912 datasheets, 2SC2912 view ::: ALLDATASHEET :::
 
#2
in general I believe if you have too much gain you can back it off by increasing negative feedback, or bleeding off some input with a b-e resistor. If you don't have enough gain you are stuck. I only did this once, others will have to analyze your circuit.
 
#3
One must know that Hfe ratings are a guideline only. When you purchase a transistor you are not guaranteed you will receive one with any exact gains. Temperature and the current they are operated under have a wide effect on the device. All transistors are a little different. When you see a grade like “S” for the BJT’s your mentioned you have a window only (140~280). My experience is when you’re looking for a target Hfe you will need to obtain a number of devices just to get a few matched pair. The original Sanyo’s in the Adcom’s I’ve seen range in the area of 140~180. Performing testing using a jig made to test under the conditions the device is running under is the ideal choice. The Fairchild devices you asked about, the number are not a “set”. The KSC3503 and the KSA1381 are NPN/PNP models. The “E” grades are 100~200 and the “F” are 160~320. You can order the “E” grade from Mouser, they do not stock the “F” grade as you suggest at this time. You can order several of each to gather a few matched sets.
 
#4
Depending on the exact function of the transistor, the actual gain might be of a lesser importance. For example, transistors making up current sources, current sinks or unity gain buffer stage. As very well researched and written down by Douglas Self, there is an advantage to select very high hFE transistors for the differential input and VAS stages to minimise distortion due to non-lineair characteristics. Finding closed matched transistors, linearity and noise might be of higher importance then their actual gain. Hence, what is the exact application of your trannies?
 
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#5
He’s working on an Adcom GFA-555MKII I believe. The 1210 and the 2912 are used as current source. He has another tread asking for advice on his repair. I have tried to give him some help and offered the original BJT’s. The Fairchild parts are a lot less expensive that’s’ why I believe he is asking about them. I agree for his purpose the gain of the part plays a less critical role than matching the parts. That is unless he is looking to replace the differential pair where I have found that higher gain and very closely matched parts make a drastic effect on the end result in this amp. The end result being less total THD/SIAND numbers.
 
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