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Old 16th November 2001, 03:39 PM   #11
Koy is offline Koy  Czech Republic
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I made some simulations.Changig ( rising) the colector resistor you do only one - increase the bias current. Moving the 47k up you did the same. I am afraid the problem is in the BC. But... It works now, as you posted. This is the essential. Nice sound!
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Old 17th November 2001, 08:37 PM   #12
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Yesterday I finished the second channel of the ALPEH3 with the ALEPH5 mod for the current source. Fired it up and it was immediatly perfect.
I had a short tryout with the pair, and I am very happy.
Now I will start the housing and defintive supply. I will try to get some picturess posted in a few weeks so you all can see the fruits of my labor.

Koy,
I like to hear which simulation software you used. I have tried a few, but either I make a lot of mistakes using these packages or the results are not always real world reflections. In short I hate them so far. Of course this can change over time.
For now, I will stick to my method of building, trying, and optimizing a test circuit first, and second start a simulation to confirm (or not) the real world results.
Used this method on power supplys and was very succesfull. Tried simulations on even the simplest amps and had mixed results.(most where hopeless)
Maybe we should start a new thread on this subject?
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Old 19th November 2001, 09:40 AM   #13
Koy is offline Koy  Czech Republic
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I used Circuitmaker. Of course simulation differs from reality. I know it. Bud it can suggest solution.
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Old 3rd December 2001, 06:13 AM   #14
Sud is offline Sud
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rtirion, I owe you a huge thanks!

I was suffering distortion at low power levels with the Aleph30 I'm building. I read this thread, and noticed that the Aleph30 also uses 1.5K and 47K in the active current source. I changed these values to 4.75K and 221K and the problem disappeared. THANKS!

Of course, this means that my Aleph 30 has 3.8A of bias current. I'm driving a MTM setup with Focal midranges and Accuton tweeter - the impedance dips to about 4 Ohms, so I figure the extra bias current helps with the lower impedance, too.

Sud
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Old 3rd December 2001, 11:18 AM   #15
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Sud,

Happy to hear this thread helped. If your heathsinks can handle the extra dissipation, I would say enjoy your ALEPH30.
Just out of curiousity:
What type of output device did you use?
Did you use MPSA18 or something else for the BJT's?

Regards
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Old 3rd December 2001, 01:13 PM   #16
Koy is offline Koy  Czech Republic
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I am going to move the bias up.I did it by rising 47k to cca 200k ( cca 1.2A per IRFP). I dont know - why do you changed the 1.5k to 4.7k? It lowers the bias.Value ( 4k7) used in other Alephs and correlates with higher supply voltage of this amps.
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Old 4th December 2001, 09:42 AM   #17
Sud is offline Sud
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rtirion,

> What type of output device did you use?

I used IRF244s for the output devices, in 3 pairs.

>Did you use MPSA18 or something else for the BJT's?

I originally used MPSA18, then tried ZTX450. There was no difference in bias current. I couldn't comment on whether there was a difference in sound, because I've only applied your resistor changes with the ZTX450 installed.

Cheers,
Sud
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Old 4th December 2001, 08:20 PM   #18
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Koy,

Changed the 1.5k to 4.7k because getting the value for 47k up did not solve everything! You can read this in my earlier replys to this thread.
I do think the most important resistor to set the quiescent current is not the R111 1.5K/4.7K or the R113 47K/221K. I think 0.47 Ohm source resistors are the most important ones. I regard the resistors R111,R112,R113 as the construction to get a Vgs that is needed for proper/sufficient conduction of the Q106.

I did not (software) simulate this in any way but actually build different setups and took measurments for each setup.
Painstakingly, Time consuming? Yes but at least I got some real world values. And I like the end result.

Sud,
Happy listening and thanks for your reply

Regards
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Old 5th December 2001, 06:40 AM   #19
Koy is offline Koy  Czech Republic
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I agree. I noticed the same.With combination of 1k5/4k7 and 220k I am not able to get higher than to 1.2A per IRFP.Only solution is to change 0.47 to lower value.

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Old 5th December 2001, 03:05 PM   #20
Koy is offline Koy  Czech Republic
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Sorry but I donīt see one think.A3 uses the same resistors ( 47k,1k5,0.47)as A30. Only one transistor less.How can I get with the same values about 2times higher bias current?I am not able to go higher than to 400mV per 0.47.
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