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Old 8th August 2012, 01:00 AM   #141
janusz is offline janusz  Australia
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Join Date: Oct 2004
OK, back from holidays.
I have removed all VAS transistors, checked them and they are all OK!!!. All the other components are OK. Bias transistor is OK (I have even repalced it with a new one), all the resistors in bias citcuit are fine. LTP is fine. Why do I get 1.3V across C15???
- while you get less voltage drop in the same circuit.
Even if the circuit oscillated that DC V drop across C15 should be within limits.

I can increase values of R17 and R20 from 47k to 120k or 240k as these together with the remainder of VAS circuit also affect voltage drop across C15.
Any suggestions???

cheers,
PS I'll be using 2sa1370/2sc3467 in VAS as I have enough of these and these are more linear and have even lower Cob. I also could replace Q9 and Q13 with jfets: 2sk170 and 2sj74. In simulation these give much lower phase shift but that would affect mainly sonics not voltage drop across C15 by 50%.

Last edited by janusz; 8th August 2012 at 01:05 AM.
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Old 27th November 2012, 05:56 AM   #142
spurlte is offline spurlte  United States
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Default voltage regulators

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Originally Posted by ARTISAND View Post
I
May I ask, where did you get your voltage regulators I think I see in the middle of you case.
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Old 14th December 2012, 06:41 AM   #143
spurlte is offline spurlte  United States
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Join Date: Aug 2011
Default Are you still working on amp?

Quote:
Originally Posted by janusz View Post
OK, back from holidays.
I have removed all VAS transistors, checked them and they are all OK!!!. All the other components are OK. Bias transistor is OK (I have even repalced it with a new one), all the resistors in bias citcuit are fine. LTP is fine. Why do I get 1.3V across C15???
- while you get less voltage drop in the same circuit.
Even if the circuit oscillated that DC V drop across C15 should be within limits.

I can increase values of R17 and R20 from 47k to 120k or 240k as these together with the remainder of VAS circuit also affect voltage drop across C15.
Any suggestions???

cheers,
PS I'll be using 2sa1370/2sc3467 in VAS as I have enough of these and these are more linear and have even lower Cob. I also could replace Q9 and Q13 with jfets: 2sk170 and 2sj74. In simulation these give much lower phase shift but that would affect mainly sonics not voltage drop across C15 by 50%.
I am wondering if you have given up on your amplifier? I was going to give it a try.
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Old 14th December 2012, 07:30 AM   #144
janusz is offline janusz  Australia
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Well, I might give it one more try. Actually, I have two different modification versions of it but haven't tried them yet. Apparently many people had problems with it. In simulation it easily turns into a current sink (that's what happened to my amp) or it oscillates - it's very sensitive to the quality of Iput and VAS PS.

cheers,
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Old 14th December 2012, 05:15 PM   #145
spurlte is offline spurlte  United States
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Default Fake Transistors

Quote:
Originally Posted by janusz View Post
Well, I might give it one more try. Actually, I have two different modification versions of it but haven't tried them yet. Apparently many people had problems with it. In simulation it easily turns into a current sink (that's what happened to my amp) or it oscillates - it's very sensitive to the quality of Iput and VAS PS.

cheers,
I have been working on Jim's Audio KSA-100 MKII. Here is the thread:
http://www.diyaudio.com/forums/solid...o-ebay-36.html
I am 'spurlte', and had problems with fake components with vintage components especially transistors. Many jumped up to help. It is totally amazing the kindness of others here at diyaudio.com. I read and will look to find where later to post here, that many components that fail lot testing are not just thrown away as they should, are illegally sold to those who pass these on as workable devices. I finally, with the help of others, got good parts to complete my boards. There are some crooks who are also taking old transistors and passing them on as newer updated devices. So, specifications could be all over the place. Someone said that you learn more when things are not working as they suppose to be working. Forcing you to investigate and I am learning alot. Check out page 25/quote #243 on Jim's Audio KSA 100. But in your case, could you have components out of spec?
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Old 14th December 2012, 05:42 PM   #146
spurlte is offline spurlte  United States
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Also, you will see how Kaplaars completed his Krell KSA-100 MKII Clone and is totally enjoying his effort. Kaplaars found a couple of major uncorrect art work, component markings and a faulty circuit passage. It is all details at different pages thoughout the thread. One example is what was found and explained by Kaplaars on page 15, quote #141. So attention while compairing schematics to PCB clone's circuitry passages is extremely important.
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Old 17th December 2012, 04:07 AM   #147
janusz is offline janusz  Australia
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Join Date: Oct 2004
I did not have problems with components. I tested each one and transistors at different currents. I did not check everything but most of each board for consistency with schematics ane these seemed to be OK. Both of my boards seemed to display exactly the same fault: voltage drop across C15 was too high so up to 2-3mA bias current everything seemed to work well. Only exceeding that bias current made it rise quickly.

cheers,
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Old 25th April 2015, 09:55 AM   #148
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Location: Bangalore
I have done some experimentation on E305 amp & have similar experience as janusz ie; the amp running hot similar to Class A.

Circuit diagram & parts list of the amplifier I did:
Click the image to open in full size.

Bare PCB:
Click the image to open in full size.

Assembled & testing with 28v-0-28v 2A transformer & Peerless 12" 400w 4Ω subwoofer.
Click the image to open in full size.

Amplifier is rated 250w 4Ω which is lower than than the connected load.
Output stage replacement transistors are NJW0302 & NJW0281. Rest all transistors are the same as per the the parts list. Driver transistors are 2SC5171 & 2SA1930. FET transistors 2SJ103 & 2SK246 were not available locally hence had to import them.

At 28v -0- 28v AC =~ +39v -0- -39v DC (after rectification) 2A even with lowest forward biasing set with the variable resistor, the heatsink is running hot. I guess the size of the heat sink is inadequate. Increase the biasing of the driver transistors the heatsink will become quite hot. This characteristic would fit this amp's description in a conjugate Class AB-A model.

The 400w speaker excursion is quite good due to the input stage at the above mentioned VA. However, the current draw for this design is so much that the transformer core also becomes too hot within few minutes.

Last edited by penjoseph; 25th April 2015 at 09:59 AM.
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Old 25th April 2015, 10:10 AM   #149
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by penjoseph View Post
.................... However, the current draw for this design is so much that the transformer core also becomes too hot within few minutes.
This seems like a wiring error.
Did the bulb tester light up?
Did you use a bulb tester?
Is the bulb tester still lying on the shelf?
__________________
regards Andrew T.
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Old 25th April 2015, 10:20 AM   #150
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Initially I connected a 20w 8Ω speaker with 20Ω 2W resistor in series. The amp runs fine with this low load & very less heat. Remove & connect the subwoofer, the equation changes.

With the subwoofer connected & fingertip on the input for 60 seconds the transformer is as 'fresh from the grill' barbeque chicken.

Last edited by penjoseph; 25th April 2015 at 10:23 AM.
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