I bought a 5.1 home theatre system years ago which include this main amp, this cost me around a 100 USD. It plays music but I hear no hifi since it does'nt fit my listening preference even i had replaced the speakers with other pair and player its just the same.
Last month it broke down frying the ouput pair on one channel. I had replaced the output and measure the offset voltage which is to high about 400mV. So I measure the good channel and to my shock its around a volt offset. So i tried to play music, upon power on musicseems okay but after 30 seconds the sound of the busted channel is going low and distorted other is just fine, So i shut it off and investigate why.
Upon re-engineering the schematic I found out this Schematics. Attached)....
I am wandering why at first it sounds but just a little bit harsh and now totally no good.
What's your comment and suggestion for improvement?
Last month it broke down frying the ouput pair on one channel. I had replaced the output and measure the offset voltage which is to high about 400mV. So I measure the good channel and to my shock its around a volt offset. So i tried to play music, upon power on musicseems okay but after 30 seconds the sound of the busted channel is going low and distorted other is just fine, So i shut it off and investigate why.
Upon re-engineering the schematic I found out this Schematics. Attached)....
I am wandering why at first it sounds but just a little bit harsh and now totally no good.
What's your comment and suggestion for improvement?
That has got to be the sloppiest implementation of the balanced VAS I have ever seen. NO WONDER it has a volt offset. It must sound truely horrible.
Q9 should be NPN , C7 is reversed ,diode is reversed , general circuit is overcompensated (100pf caps) with very low VAS current (output droop) cheap low gain transistors. Is there a Vbias adjustment ?? (near R14) very low input pair bias . .5ma ??
Below is how it should be done..
OS
Q9 should be NPN , C7 is reversed ,diode is reversed , general circuit is overcompensated (100pf caps) with very low VAS current (output droop) cheap low gain transistors. Is there a Vbias adjustment ?? (near R14) very low input pair bias . .5ma ??
Below is how it should be done..
OS
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Did you measure the idle current in the outputs and/ or the drivers? (measure voltage over R16/R17 and over R15)
...because if C7 really is the wrong way it could shortcircuit the bias-circuit, leaving the outputs in pure class B.
What about the D1... does any current go through it? (measure voltage over R12+ R15)
Or you could just reverse C7 and D1 and see what happens.
It might just be an assembly error ( which is bad enough) and after you have fixed that, the circuit might have a tolerable sound quality.
Edit: the C7 could allready be damaged. Try pulling it completely out of the circuit ( no big difference in function) and if it works you could refit a new cap.
...because if C7 really is the wrong way it could shortcircuit the bias-circuit, leaving the outputs in pure class B.
What about the D1... does any current go through it? (measure voltage over R12+ R15)
Or you could just reverse C7 and D1 and see what happens.
It might just be an assembly error ( which is bad enough) and after you have fixed that, the circuit might have a tolerable sound quality.
Edit: the C7 could allready be damaged. Try pulling it completely out of the circuit ( no big difference in function) and if it works you could refit a new cap.
Hi OStripper and Nrik....
Yes I've already reversed the diode and replaced C7 and it works. I already replaced Q1 and Q2 matched so offset now is at minimal around 20mV. Also reduced C3 and C4 to 47pF.
As for idle current i can't adjust it since no trimmer around. my voltage reading is almost zero on R16 so i wired a 5K trimmer in parallel with R13 and adjust to to around 0.013V or at least 50ma on R16 since my heatsink will not get hot.
Does quality control applied on this during production?..I was really disappointed since it"s considered costly here at around 1000 Usd. (45,000PHp)
Yes I've already reversed the diode and replaced C7 and it works. I already replaced Q1 and Q2 matched so offset now is at minimal around 20mV. Also reduced C3 and C4 to 47pF.
As for idle current i can't adjust it since no trimmer around. my voltage reading is almost zero on R16 so i wired a 5K trimmer in parallel with R13 and adjust to to around 0.013V or at least 50ma on R16 since my heatsink will not get hot.
Does quality control applied on this during production?..I was really disappointed since it"s considered costly here at around 1000 Usd. (45,000PHp)
1k$$$ I could build 2 complete 250w/2 channel amps with 4-500 watt peak capacity ... chassis and all for 1000$$
20mv is pretty good for no offset trimmer. I am glad you got it working and even happier that I build my own amps.. I did not mean to degrade the quality of your amp.. but the transistor choices are 20th century , sort of like MJE340/350's
.OS
You're right OSTRIPPER... very dissapointed that's why I am now using my assembled amp 15yrs ago during my college as project far better and more powerfull sound.
I have improved the amp a little bit but what i want is totally change the transistors from input to output. what good choices you have.. i am also planning to upgrade the rectifier and filter caps however i have limited space to work on since the casing is compact.
I have improved the amp a little bit but what i want is totally change the transistors from input to output. what good choices you have.. i am also planning to upgrade the rectifier and filter caps however i have limited space to work on since the casing is compact.
Thanks OS,
I will try to look for those parts in local store here. In my old amp I had already used C1845 and A992, D699 and b649 and c2922 and A1216 since these are locally available. Will these still be good now?
Since I cannot use MT200 type but it will not fit in so I thought of using c5200 and A1943 but most of the local stocks are FAKES. NJW's and MJ are hard to find here.
I will try to look for those parts in local store here. In my old amp I had already used C1845 and A992, D699 and b649 and c2922 and A1216 since these are locally available. Will these still be good now?
Since I cannot use MT200 type but it will not fit in so I thought of using c5200 and A1943 but most of the local stocks are FAKES. NJW's and MJ are hard to find here.
Using the same board and with reference to the shematics, will there be good modification on schematics so that only a minimal wiring will be done other than changing the all emiconductors and putting bias trimmer.
How about recalculating the values of components used?
Can somebody help me out?
How about recalculating the values of components used?
Can somebody help me out?
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