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Old 16th June 2010, 11:43 AM   #1
NVD33 is offline NVD33  France
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Default Mosfet Citation 12 - Negative swing clip first

Hi all,

I have browsed the forum but did not find answer to my concern. I just have finished one channel for a Citation 12 (Mosfet). Everything is OK I am very happy with the sound.

I played with it by making some investigations with a scope and found that the clipping is not symetrical; negative swing clips first at 28 Volts; the positive swing clips at 30 volts.
- Is this situation normal? Both supply rails are perfectly symetrical (36 Volts)

- I have at output a DC offset about 40 mV, can I reduce it by better matching of output FETs? or matching input symetrical pair ?

Thanks for your help,

Vince
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Old 16th June 2010, 07:51 PM   #2
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I think that's normal.

For offset, I would adjust the value of the resistor off the
Collector of the input transistor which drives the base of the
npn voltage gain transistor.

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Old 16th June 2010, 08:58 PM   #3
NVD33 is offline NVD33  France
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Hi Mr. Pass,

I am very pleased to receive a reply from N.P himself

So there is no way to make the clipping symmetrical on both positive and negative swing. I will make a try to reduce the offset.

Thank you for your help and also all your actions for DIYers.

I have all the parts but could not find yet heatsink to build an Aleph 5. Parts are in a box somewhere in my garage for many years. Your reply encourages me to start a new search for heatsink and build this Aleph.

All my best,
Vince
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Old 18th June 2010, 09:46 PM   #4
NVD33 is offline NVD33  France
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Hi Mr. Pass,

As you said, I adjusted the value of the resistor of the
Collector of the input transistor which drives the base of the
npn voltage gain transistor ( 680 R replaced by 560 R) and the offset is null now.

About the clipping, I have noticed that when I reduced the bias current (thru IRFP 240) down to 30 mA the clipping is more symmetrical (without load).
In your document related to Mosfet Citation 12 it is said that "D3 ensures symmetrical clipping" (see attached screen capture).
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File Type: jpg Citation12_clipping.JPG (105.1 KB, 398 views)
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Old 18th June 2010, 10:44 PM   #5
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Quote:
Originally Posted by NVD33 View Post
In your document related to Mosfet Citation 12 it is said that "D3 ensures symmetrical clipping" (see attached screen capture).
It does so by limiting the positive drive voltage.
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Old 19th June 2010, 06:12 AM   #6
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Hi.I want to drive Citation 12 amp with +-44 volts.Is it possible?thx
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Old 20th June 2010, 02:36 AM   #7
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You would have to consider adjusting the bias resistors of the
1st and 2nd gain stages.

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Old 20th June 2010, 06:36 AM   #8
NVD33 is offline NVD33  France
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Hi NP,

That's what I did and still trying to do because I replaced MPSL51 by BC55C and MPSL01 by BD139, those transistors are easy to find overhere. The amp is singing but I am not sure that I didn't mess the bias somewhere. In my case, must I concentrate my effort in adjusting the bias resistors of the 1st and 2nd gain stages only?

By the way I use for output devices IRFP240 and I have increased supply voltage up to +-40 Volts without seing .

Vince
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Old 20th June 2010, 02:14 PM   #9
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Offhand, yes.

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Old 25th June 2010, 09:35 PM   #10
NVD33 is offline NVD33  France
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Hi NP,

I worked around the amp, it is doing fine I am very very happy with the sound.

I have a question about output devices bias current. I set arbitrary bias current at 100 mA about 10 mn after power up . After couple of hours of operation I measured the bias current (heatsink is warm not really hot) it was reduced to 60 mA. I think that the situation is normal and I wonder why not set the bias current directly at 60 mA at power up because it will stabilize itself at 60 mA after a while. At 100 mA bias or 60 mA bias the negative swing always clips first at the same voltage, I noticed no difference in sound or amp behavior.

Thanks a lot for your info,
Vince
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