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Old 29th September 2012, 01:47 PM   #41
AndrewT is offline AndrewT  Scotland
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Have you ever measured the actual signals sent to your speakers?

I have measured my signals, often.

I have stated quite a few times that my average listening levels are well below 1W.
Typically I see average levels of around 0.5Vac to 2Vac.

I can convert that to SPL if I want, but in the context of this discussion I don't need SPL.
I am trying to point out that the peak to average ratio of most music types (~12dB to >30dB are frequently posted/published) is way above the 30W to 5W ratio (<8dB) you have mentioned.

If you re-play music at average levels of ~5W with a 30W amplifier then I can virtually guarantee that your are listening to clipped signals. Low distortion of the signal whether due to good amplifier design or good source material does not enter the discussion if you habitually clip your signal before you send it to your speaker/s.
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Old 3rd October 2012, 04:56 PM   #42
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Default maybe fixed

Hi All..
Thankyou for your help so far ..
Well i managed to find some time to hopefully fix the brokem scem i posted .. would someone please look over it to see if i have made a pigs ear of it . i will re post the original so you wont have to keep going back to the first post..

Kind Regards Mark PS please ignore the changed location numbers
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Old 4th October 2012, 09:46 AM   #43
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You need a bias resistor to ground for transistors T1 & T3, usually the same value as R21
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Old 4th October 2012, 01:18 PM   #44
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by AndrewT View Post
Have you ever measured the actual signals sent to your speakers?
Sure

Now I understand what you are talking about. I believe that you also understand what I was talking about
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Old 4th October 2012, 02:14 PM   #45
wahab is offline wahab  Algeria
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The front end is not accurately calculated and as such it wont
perform correctly.

Particularly , there will be big VAS current variation with temperature
wich will render the output stage quiescent current variable as well but
with even larger amplitude that may well turn in thermal runaway.
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Old 4th October 2012, 05:53 PM   #46
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Default As is working

i have spent some time and checked the scematic against a working amplifier this is the outcome.. How do you work the gain out on this type of topology
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Old 4th October 2012, 06:00 PM   #47
wahab is offline wahab  Algeria
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(R21 + R6 )/R6 on the first schematic post 42.

The capacitor C2 that goes to ground will define
the low frequency cut off.
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Old 4th October 2012, 06:08 PM   #48
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I get nears as dam it 67.6 that seems very high . the amplifier does seem to have quite a high gain but im not sure it is that high. Also you mention thermal runaway The 5551 used as bias transistor is between output devices on the main heatsink will that help!! if i worked it out corectley the cutoff frequency is very close to 1Hz.. if we devide the 67.6 by 2 we get near enuff to 33 that sounds more like it to me . however still a little higher than i would like

Last edited by madtecchy; 4th October 2012 at 06:23 PM.
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Old 4th October 2012, 06:29 PM   #49
wahab is offline wahab  Algeria
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Q7/Q8 are not correctly biaised , hence open loop gain is low ,
R2 and R4 should be increased to 3.3K to allow enough current
flowing through theses transistors to adequatly drive the output
stage.
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Old 4th October 2012, 06:42 PM   #50
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Thankyou Whab..
I have just checked the board again and r2 and r 4 are 1k5 for sure . if i increase them will i have to adjust values in other places ??
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