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Old 16th June 2003, 11:20 PM   #1
matth is offline matth  United Kingdom
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Default Help with +VE clipping

Hi All,

I need some advice on where to start looking for a problem.
I am in the middle of constructing a four channel PA amp using the Maplin 100w mosfet cct (see schematic).
Click the image to open in full size.
Nearly complete.
Click the image to open in full size.
The problem is that when tested under load (7.5Ohm dummy load) the output on each amp shows some clipping on the positive cycle (see trace):
Click the image to open in full size.
Top trace is input (1V/div)
Bottom trace is o/p into load (20v/div)
Sorry for the poor trace quality but this was the best I could do.
As you can see the top of the positive cycle is clipping (at about 28v) but it should be hitting almost 40v. I have checked the supply and it is putting out a equal +/- 57v under load.
This cct was originaly supplied with 55v max due to the 2SJ50/2SK135 o/p pair, however my amps are using the Exicon 10n20 and 10p20 pair which are rated at 200v so should have no problems with the extra couple of volts.
I am uncertain of the suitability of the second stage 2SB716, could this be the cause of the problems?

Sorry for the long first post, I just wanted to cover everything.
Thanks in advance for any help you can give.

Matt.
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Old 17th June 2003, 09:59 AM   #2
matth is offline matth  United Kingdom
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Any ideas / advice?
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Old 18th June 2003, 03:48 PM   #3
dimitri is offline dimitri  United States
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Are you sure that everything OK with values? Are BJTs soldered correctly? Then disconnect the load, both of output FETs and connect right node of R7 to the connection R12 & RV1. Nou you can check by the oscilloscope in the joint point of R7, R12, RV1 if your driver works correctly /symmetrically clips.
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Old 19th June 2003, 07:19 PM   #4
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Can you measure the voltage at the base of TR5? also at collector and emittor. Maybe the current through R8 is too high!

HB.
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Old 19th June 2003, 07:57 PM   #5
matth is offline matth  United Kingdom
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Hi,

Thanks for the replies, I haven't got time to check the cct until Saturday. When I do I'll get back to you with the results.

Cheers.
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Old 20th June 2003, 07:40 PM   #6
matth is offline matth  United Kingdom
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Hi Hugobross,

Here are the results of some measurements of TR5.

Input = None:

+VE = 59.7v
Emitter = 58.4v
Collector = -1.7v
Base = 58v

Input = 1v P-P 1Khz sine (amp not clipping):

+VE = 58.8v
Emitter = 57.6v
Collector = -5.5v
Base = 57.3v

Input = 2v P-P 1Khz sine (amp clipping on positive cycle):

+VE = 58.3
Emitter = 57.4v
Collector = +3.4v !!
Base = 56.8v


HB, it is worth noting that the rated input on this amp is 2.83v P-P and it should have a gain of 33 with the standard o/p mosfets.

Dimitri, the amp does not clip without a load attached. Is it still worth doing the mod you suggest?
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Old 21st June 2003, 07:06 AM   #7
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What is your positive supply voltage with- and without load ?

Regards

Charles
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Old 21st June 2003, 08:03 AM   #8
dimitri is offline dimitri  United States
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Dimitri, the amp does not clip without a load attached. Is it still worth doing the mod you suggest?
------------
Then it looks like integral protection Zener functioning. Could you replace 10n20 or use 15 Ohm load? What will be in this case? Check R13 value.
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