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Old 16th March 2009, 11:29 AM   #3291
ashaw is offline ashaw  Australia
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I have an interest in designing a SMT version of the F5 in the hope that Linear systems comes out with the long awaited LSK74

I was wondering if these two transistors could be substituted for Q5 and Q6 ie ZTX450/ZTX550 and if so what changes would be needed.

FZT792A
FZT692B
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Old 16th March 2009, 11:56 AM   #3292
AndrewT is online now AndrewT  Scotland
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Hi SLR,
running a bridged pair of F5 into 4r0 is the same as using a pair of single F5 to drive 2r0.

The ClassA power of an F5 is limited by the ClassA bias current.
I begin to wonder why you want to use a rather nice ClassA amplifier to drive bass only speakers, when for much of the time the amps will be running in ClassAB.
Is your decision to use F5 for the bass valid? Or more a view through rose tinted specs?

There are other ways to use your balanced signal to drive your speakers.
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Old 16th March 2009, 12:02 PM   #3293
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Quote:
Originally posted by ashaw

... what changes would be needed.



You don't need to change anything as long as the NPN substitution has the base-emitter on voltage of about+0.6V while the PNP of about -0.6V.

If the base-emitter on voltages are quite different from the value of 0.6, you need to adjust the size of R19 and R20.

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Old 16th March 2009, 12:07 PM   #3294
ashaw is offline ashaw  Australia
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-0.75 + 0.9.
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Old 16th March 2009, 12:09 PM   #3295
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by ashaw
-0.75 + 0.9.
are these the base emitter voltages of the FZT substitutes?
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Old 16th March 2009, 12:17 PM   #3296
ashaw is offline ashaw  Australia
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yes the PNP part FZT792A has a

VBE(on) of -0.75 V at IC=-1A, VCE=-2V

the NPN part FZT692B has a

VBE(on) 0.9 V IC=1A, VCE=2V
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Old 16th March 2009, 01:01 PM   #3297
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ashaw,

I could say that both have Vbe(on) of about +0.6 and -0.6V.
Refer to the attached data sheet of FZT692B. So, you could use them as good substitutions without adjustment any.


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Old 16th March 2009, 01:07 PM   #3298
ashaw is offline ashaw  Australia
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cool they are the only smt parts I thought might be similar enough.

I was thinking of making up some test boards to send to someone with these parts, to compare them to the original if anyone would be interested.
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Old 16th March 2009, 01:20 PM   #3299
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by ashaw
I have an interest in designing a SMT version of the F5...........
I was wondering if these two transistors could be substituted for Q5 and Q6 ie ZTX450/ZTX550
comparisons should be done using as close as possible the range of operating currents expected in the completed amplifiers.
1A @ 2Vce is preposterous.
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Old 16th March 2009, 01:26 PM   #3300
ashaw is offline ashaw  Australia
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yes I do unserstand that, that is why I was asking if I could send some parts to someone better qualified so that they could test them.
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