Evens C-500 amplifier ....opinions? - Page 2 - diyAudio
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Old 5th March 2010, 08:25 PM   #11
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I have no problem with 2sa1943 2sc5200 trannies except that there are many fakes of these, also there are many second source manufactured ones which dont perform half as good, thats why I stay away from them except if I can be 100 percent sure they are original toshibas.

All 3 transistor examples I gave for bias transistor are used by high end amps, the very costly ones, there are another 2 or 3 I can mention but availability might be a problem, These give superior performance to the usaual types used in this position.
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Old 5th March 2010, 09:48 PM   #12
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That 2SC4495 is really hard to find ...only found 1 shop that sells it for 9..a lot ...i think is a bit to much for a bias transistor.
I checked Profusion but they don't have it.
Anyway thanks for advice...I'll try bd139 and if its working fine then no need for fancy trannie.
Also on the website it says something about matching some transistors...other than the outputs....you have any idea which pairs ?
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Old 5th March 2010, 09:52 PM   #13
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Found 2sd1508 for cheap !!
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Old 13th March 2010, 11:33 PM   #14
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Finished prototype....working ok!!
2 Problems
1. 60 mv dc offset on output
2. I can't measure the mv on the 5w resistor from the output trannie emitor to set bias.
For test I used BD911/912 and +/- 18 cc rails.
Bias transistor is 2sd2012....replacement for 2sd2058 apparently...
All seems ok so far...need further testing at higher rail.
Amp is easy to build..readily available components...nothing fancy.
Can someone help me fix the offset please? Any input much appreciated!!!

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Old 14th March 2010, 09:35 AM   #16
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Here is a quick guide how to go about testing.
60-80W Power Amplifier

The evens amp page also has some instructions.

If you still have offset at the operating voltage which is high then try to match the input transistors Q6 and Q7 and see if it helps. It would aslo help if the current mirror transistors are closely matched.
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Old 17th March 2010, 02:51 PM   #17
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which ones are the current mirror transistors?
I managed to burn the amplifier...i wanted to see how far can i go...only thing that burned was the outputs and Q10, Q5.....I think they are to low power for their job in the schematic.
I upped the voltage to +/- 40 volts and the BD911/912 BURNED....replaced with 2n355 and mj2955, replaced Q10 and Q5 all works fine.
Upping the voltage made the amplifier more silent....the small hum disappeared...only small hiss now...which is normal.
After replacing Q10, Q5 the dc offset increased from 60 mv to 85 mv.
I still cannot set the bias...on the 0.47 ohm resistor i read 0 mv without signal and when i apply signal it goes up according to the signal as it should be.
I think i can fix the offset by fitting a 100 ohm trimpot between R9, R10, R12......as I've seen on most similar chematics.
Any ideas how to fix the bias problem?
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Old 17th March 2010, 05:26 PM   #18
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Strange the bd911 should be able to survive 100 volts. I think you may have a mistake somewhere.

When you measure the v between the resistor are you measering DC value. Make sure that your resistor values are correct according to schematics in these positions.

When you say you blew the amp did it blow playing loud music or did it go just by itself, youll have to check if the amp is not oscilating.

Check that you have the bias transistor connected correctly, meaning the the base collector emmiter legs are not swapped somehow.

The trimpot to fix the offset is a way to go but did you try to match the input transistors first, secondly Q8 and Q9 and also Q3 and Q4 which is the current mirror.

Do a little measurement, what is the voltage between the collector and emitter leg of O11.

Attached I show picture of a trimpot setup to change offset but I would do some matching first as most of the time you should be able to get very low offset this way and not really need this adjustment.
Attached Images
File Type: jpg Untitled.jpg (23.8 KB, 185 views)

Last edited by homemodder; 17th March 2010 at 05:41 PM.
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Old 17th March 2010, 06:13 PM   #19
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Ok, all sorted now!!!
Fixing the dc offset works fitting a 100 ohm trimpot between R9, R10, R12, managed to get 2 mv dc output without matching the input transistors.
Bias fixed also. The problem was the bias transistor and the trimpot. I used BD139 and 500 ohm trimpot and i managed to adjust the bias. With 2SD2012 and 100 ohm trimpot I couldn't up the bias at all. With bd139 and 500R pot I managed to get 60 ma idle current. I left the amplifier at maximum bias for about 20 minutes and the output transistors barely warmed up, so did the preoutput transistors mje15032,15033.
All this was done with a +/- 40 V supply and 2n3055 mj2955 transistors.
I need to test the amplifier at higher rails and with several pairs of output transistors. I will try then to set the bias with 2sd2012 transistor to see if it works....maybe is suitable just for higher voltage.
At the moment I am playing some FLAC classical music and i can say the amplifier sounds very good. Mids and highs are crisp, clear and without coloration. Bass is punchy but I think this amplifier needs a low pass filter. Sometimes i noticed some really low frequency driver movements. I think is called subsonic filter. I will try to reduce the input cap from 10 mfd to something like 4,7 or 2,2. Can someone comment on the bandwidth of this amplifier? I know that C10 limits the high frequencies...in the original design it was 33pf but on the latest design which I received from author it is replaced with 470 pf.
Anny thoughts?
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Old 17th March 2010, 06:29 PM   #20
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Thank you for the help homemodder!
I checked all the components and the board. All fitted ok ...no shorts on the pcb.
I measured voltage across Q11(BD139 now) collector and emitter and i got 2.4 volts with bias set to 30 ma.
When I blew up the amplifier I played music as loud as possible....after 3-4 seconds one of the outputs smoked and a fuse blew up.
What is weird is that all this happened on the positive side of the amplifier. The NPN transistor burned and the +v fuse blew up. And Q10, Q5 burned as well...I think Q10 burned first and Q5 when I re powered the amplifier to see if is alive .
Something weird. The preouts are getting slightly warm but Q12 on the positive rail is warmer than Q13....just a bit tho....maybe is just my imagination...I will try with a thermometer.
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