Waveform distortion on Aleph 30! - diyAudio
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Old 13th May 2003, 06:44 AM   #1
panos29 is offline panos29  Greece
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Angry Waveform distortion on Aleph 30!

Dear all,

Its been 2 years now that I am very happy with my aleph 30 clone based at exactly the published schematic. Only difference is that my power supply is at +/-28VDC biased at 1,6A per channel, also current gain is exactly 50%. Although i am satisfied with the overall sound i decided to take some measurements and i did.
My surprise was very big though to see that the amplifier is not behaving normally(at least to my understanding). To explain that I have to say that I found 18Vrms on 7,5 Ohm at 1Khz with excellent sinus and square. But at 10Khz I only have 13Vrms and very bad sinus as well as square, so bad that both look identical! At 20Khz the waveforms are funny, to say the least, and there is only 9Vrms available with tha same input (1,8Vrms). What I wanted to know is if this behaviour is typical for this topology or my amp got somekind of problem. Forgot to say that after the measurements I added 15pf extra at the feedback loop (25p total) and things got slightly better but certainly not good at all! All input and output mosfets are matched to within 10mV as well as all 0,47 Ohm resistors, offset is 15mV at start and 3mV after 10minutes, heatsink temp is 60 degrees celsius with 25 abient. Any answer will be greately appreciasted as am in the proccess of building my aleph X, but if these are typical I will quit this project as my gainclones reproduce all aveforms excellent at least to 30Khz and alot cooler and also I am not always able to say which is which in a blind testing! (pictures of the waveforms will follow asap). Terribly sorry for the long post, but I really had to tell about it!

Sincerely Panos
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Old 13th May 2003, 07:31 AM   #2
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I also noticed a bump on the waveform at frequencies > 10 kHz, the shape was very similar to the one shown by Algar Emi in Aleph30 thread.

See
Aleph 30


Raising the compensation caps to 33pF Silver-Mica things got much better, altough gain at 20 kHz is slightly lower than at 1 kHz, but this is a minor concern.

I also read that current balance in the diff pair might be related to such behaviour.

Cheers

Andrea
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Old 15th May 2003, 09:09 AM   #3
Raka is offline Raka  Europe
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Default Did you manage to solve it?

Hi Panos,

How is this going? I have bought the mosfet for my A30, but your problem fears me! I wouldn't know how to fix your problem, so maybe I wait for this solution before try to machining something.
Besides, gainclone is so cute and cheap... is tempting me... you devil... oh,... it's sooo cheap....hmmmm... come to me .... I'm falling... embrace me, oh chip.....
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Old 15th May 2003, 09:42 AM   #4
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Hi Panoz,

is this behaviour only shortly before the amp clips or also when you measure at 2 or 4 volts rms?

I measured my Aleph5 up to 200 kHz and found a -3dB point of around 100kHz and no dependency on frequency. Square wave was also ok at 10 and 20kHz.

Did you also have a look at the input? Often the signal generators have a very high output impedance and do funny things at higher frequencies into a low impedance/capacitive input.

william
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Old 15th May 2003, 10:31 AM   #5
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Quote:
Originally posted by panos29

Only difference is that my power supply is at +/-28VDC biased at 1,6A per channel...
I believe that you have read this:
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Old 15th May 2003, 11:36 AM   #6
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I have approximately 2.5 A bias with almost 29V rails but the problem was only solved with the introdiction of higher value (and higher quality) compensation caps.
AC current gain is about 56%

Cheers

Andrea / my new toys arrived.. I'm eager to play with them
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Old 15th May 2003, 12:46 PM   #7
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whow... those look mighty cool
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Old 15th May 2003, 01:01 PM   #8
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Indeed they are... but I didn't think they were so HUGE before seeing them with my eyes.. they measure about 10x15 cm and are quite heavy, more than 1 kg each.

I hope they really sound "as clear as light"

Cheers

Andrea

PS Hope I didn't hijacked the thread too much..
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Old 15th May 2003, 01:03 PM   #9
Raka is offline Raka  Europe
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Ey!

what are those thingis?
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Old 15th May 2003, 01:09 PM   #10
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Raka, you forgot this

They are vintage ESS Heil Air Motion Tweeters, the smaller brothers of the worldwide famous AMT1 (oh AMT-Freak, where art thou?).
Those buddies have a lot of fans around the world... very hard to get them at a reasonable price even if used!

Cheers

Andrea
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