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Old 29th November 2006, 12:58 AM   #1
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Default transistors

im repalcing transistors a1491 with 2sa1294, c3855 with 2sc3263, on a nakamichi amplifier one because the left channel has blown and took out two resistors, seems that im slightly uprating the two 15amp transistors do i upgrade the smaller ones, numbered c3421, a1358, if so what to?
and replacing all the capasitors i can with rubycon za's will this make an improvement to sound?

any help would be appreaciated thanks.
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Old 29th November 2006, 06:36 AM   #2
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For heaven's sake! Don't use Rubycon caps! They are by FAR the WORST electrolytics made in Japan in terms of reliability, and not very good sound performance, either. Use Nichicon!
As for the output transistors, I would either use 2SA1492/2SC3856 or 2SA1302/2SC3281. I don't think you will get good reliability with the numbers you mention. The 2SC3421/2SA1358, in this particular circuit, can be replaced with 2SD669/2SB649, but you must add insulators behind them if heatsink mounted.
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Old 29th November 2006, 08:47 AM   #3
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I moved this to SS where it belongs.

Stephen, I disagree completely. The Rubycon ZA and ZLs are some of the best caps out there, and I use loads of them.

As for the transistor replacements, I don't know Japanese types so I can't really comment on that. Jacco, where are you?
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Old 29th November 2006, 09:14 AM   #4
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I'll back up Pinkmouse. Rubycon actually make Blackgate (not that I advocate boutique parts).

It's also bad advice to recommend use of 2SA1302/2SC3281 as these are the worlds most faked transistors, and went obsolete years ago with the advent of their replacement 2SA1943/2SC5200.

You shouldn't need to replace/upgrade the driver transistors as the new output devices probably have higher current gain (hFE). However, if the output devices have blown it's always prudent to replace the drivers as they usually get damaged (but go undetected by simple tests) and may fail later on, taking out everything you just repaired.
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Old 29th November 2006, 09:42 AM   #5
Nexus is offline Nexus  Netherlands
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Hello Guys,

just keep in mind that 2sc5200/2sa1943 have a 5MHz higher ft than the 2sc3281/2sa1302 pair.

If 2sc3281/2sa1302 pair is used, and upgraded to 2sc5200/2sa1943 than check the stability of the amp.

Best Regards:
Nexus
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Old 29th November 2006, 10:15 AM   #6
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Al,

i'm just a second rate geenie, rubbing ís no good, i broke the bottle after i emptied it.

The A1294/C3263 have almost twice the Ft as their obsolete Sanken brothers, 35MHz versus 20MHz.
Stability might be an issue, maybe that's what the term reliability referred to.
The recommended 2SA1492/2SC3856 epitaxials differ in a lower Ft number.
In power terms, A1294/C3263 are not just a lot stronger on the specs. The're the same ones as the big Sankens but in an MT100 case, their datasheet numbers are conservative.
Regular replacement for the broken Sanken numbers would be the 2SA1265/2SC3182, made by Toshiba.

(the 2SA1491/2SC3855 are an oddball couple btw, in Sanken terms they haven't been manufactured that long.)

I don't get the Rubycon/Nichicon advice either.
Rubycon has always been an elite Japanese manufacturer, long before the BG thing started.
My previous encounters with Nichicons put them on my last resort list.
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Old 29th November 2006, 11:10 AM   #7
pooge is offline pooge  United States
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Quote:
Originally posted by Nexus
Hello Guys,

just keep in mind that 2sc5200/2sa1943 have a 5MHz higher ft than the 2sc3281/2sa1302 pair.

If 2sc3281/2sa1302 pair is used, and upgraded to 2sc5200/2sa1943 than check the stability of the amp.

Best Regards:
Nexus

If the amp is compensated for a lower ft transistor, why would substitution of a higher ft transistor cause instability? It should have a higher phase margin.
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Old 29th November 2006, 11:30 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi Pooge,
good question.
If the delay through the final device is smaller (due to higher fT) then the total delay through the whole amp should be less.
This would imply your conclusion
Quote:
It should have a higher phase margin
is correct and yet many say the opposite.
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Old 29th November 2006, 12:03 PM   #9
pooge is offline pooge  United States
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The oscillation problem may arise from parasitic ocsillations. Small base stopper resistors may help this.

Maybe a poorly bypassed power supply could also be a factor.
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Old 29th November 2006, 12:45 PM   #10
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Who said the statement is correct ?

Car talk,..again:

Consider the tires as the output devices.
The size of the car determines the engine horsepower, see the vehicle as the loudspeakers.
The engine torque determines the size of the tires.
The steering response should always be faster than the car, otherwise the car will run off the road.

Going through a corner, the steering will have to turn into the corner more than the direction the car is going to, difference between steering direction and car direction is the phase shift, the phase margin is what is left before the tires start to slide. The nominal contact surface of the tire remains the same, but the effective surface at higher steering versus car direction angles is reduced.
The higher the speed of the car, the greater the difference between steering and driving direction.
At the tire/car critical speed point (angle speed) the gas should be reduced or the brakes hit or the tires loose grip and the car crashes. Maximum allowable phase shift is reached, hitting the brakes reduces the speed=> freq. limiting

Replace the tires with faster ones, and the angle between steering and the driving direction of the car is reduced at the same speed because the raised grip of the tires makes the car respond faster.
=> sustained hFe value at higher frequencies.
Bigger tires allow for a bigger car, but in general bigger tires mean less cornering grip.
Putting racing tires on an Edsel doesn't make it a thoroughbred, but it generally doesn't make the car drive worse. That does not mean the tires will not have problems with the camber and so on of the wheel suspension geometry/type.
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