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Old 11th January 2006, 08:48 PM   #11
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ok, i have added the output stage to the main scheatic now. anyone see any mistakes? i will start adding the component values soon.
Thanks,
Owen
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Old 11th January 2006, 10:31 PM   #12
jaycee is offline jaycee  United Kingdom
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Instead of using two diodes, use an LED as your voltage reference for the current source. They are more thermally stable and lower noise than diodes.

Add a resistor in the base leg of the current source that drives the NPN half of the amplifier.

C1 looks incorrect to me. You should have a resistor + capacitor to ground on the -VE side of the LTP, and a resistor from output to the -VE side.

AS someone already pasted a link to Doug Self's Blameless amplifier, that is a good reference point.
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Old 11th January 2006, 10:52 PM   #13
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Ok, thanks,
I will make those changes, i thought C1 looked incorrect aswell but wasnt sure.

not quite sure what you mean by "Add a resistor in the base leg of the current source that drives the NPN half of the amplifier." if you mean in the output stage it will be hard because the output stages are salvaged from an old amplifier and on their own little PCB. whole point in this is that i am tryign to reuse them. mabey i should just bite the bullet and strip those boards for parts and mount all the components on a single PCB.. might be better.. i just liked how the little PCB's with the power transistors and the bias transistor mounted to the heatsink..lol..

atached is a picture of the output boards salvaged from an old fisher amplifier.

I'll go and read the up on the blameless amplifier again and put some vales into my schematic.
Many thanks,
Owen
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Old 11th January 2006, 11:08 PM   #14
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ok, i think i understood what you meant about the resistor after lookign at the blameless again. i have used the values for the resistors, etc as shown on the blameless amp not sure if they will be ok in this amp though so any sugestions for changes would be welcome. updated schematics with values atached.
I'm new to solid state amplifiers so this is like being back in school, we didnt really cover them much in my HND.
Still would liek sugestions for what transistors to use and if any of the ones i have here are suitable for anything?
how about useing 2n3906 for Q3 and Q4?
Many thanks for all your help,
owen

EDIT: forgot to atach schematic
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Old 11th January 2006, 11:50 PM   #15
CBS240 is offline CBS240  United States
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Quote:
Originally posted by DoomPixie
Ok, i have added a Vas stage and corrected a couple of errors i found.

CBS240: i made the changes you sugested as i understood them but im not 100% sure if thats what you meant?

anyone have any sugestions for transistors to use? i will make a list of what i have later but if they are no good i can always order more..

many thanks,
Owen

Hi Owen,
You got it almost right. The +in resistor is correct. The input capacitor goes to the base of this transistor, and this will set the input impeadence (Zin) to the value of that resistor (R5).

For the -input transistor, there should be a resistor from output to the base of this transistor. Then place another resistor from that base to a DC blocking capacitor to ground. This forms a voltage divider that determains the closed loop gain. To AC signals, the capacitor will conduct, but to DC it is an open circuit. This means that any DC drift will affect the diff bias greatly and it will correct for it. If you use a 47K from output to base of -in, and a 1K in series with a 47uf electrolytic Non-Polar to ground, then the AC closed loop gain will be about 47. Don't forget to place a 0.1uf mylar cap in parralell with the electrolytic because electrolytic caps have inductance as well as capacitance and this will affect audio quality.
because it could affect the bias of Q5.


Edit>>I just read post 14 and you have it right for the -in.
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Old 12th January 2006, 12:00 AM   #16
jaycee is offline jaycee  United Kingdom
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You want to use small signal transistors that are fairly high Ft, and a decent Vceo. BC546/556 or even 2N5401/5551 are suitable.

VAS and it's current source can be BD139/140 or MJE340/350.

I'd go with +/-35v rails.

Add an input coupling capacitor, 4.7uF should do
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Old 12th January 2006, 12:10 AM   #17
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hi,
thanks for the replies, i think the input rails on the original were around +/- 35 or 40V because the power supply caps were rated for 50V. i'll measure the transformers supply rails tommorow.

thank's for the suggestions, i will probably use BD139/140 for the Vas and current source as i already have a load of BD140 transistors and can get the BD139's fairly cheap. But i have a lot of "lucky bag" style selections in the post so mabey i'll be lucky and have some BD139's in them.

Hopefully my new power supply capacitors will be here tommorow and then i will make the power supply PCB and build that. then i can get crackign on the rest of it.
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Old 12th January 2006, 03:23 PM   #18
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power supply capacitors arrived today! started to make a pcb for the power supply now, will go and measure the voltages the transformer is kicking out so i know what the rails will be. will post back with my findings soon pic of the cap's atached. they are 63V 4700uF Cap's to replace the 50V 2200uF that were orignally used..
thanks,
Owen

EDIT: forgot to say they are "for audio power supplies" dont know if it makes a diffrence or not but hopefully they are ok, they were pretty cheap.
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Old 12th January 2006, 07:02 PM   #19
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spent the last 2 hours etching the pcb for the power supply.. (dont know why it took so long)
painted the tracks on freehand with acrylic modeling paint after i decided i wanted to make them bigger than the toner transfer i had made. didnt work out too badly all in all, looks a lil tatty if you look closley but i checked it out with a multimeter and it all tests good.
drilled holes for the two cap's and then my drill's battery ran flat.
anyway, a picture speaks a thousand words. so here you are, cap's arnt soldered in , just there to show size compared to the PCB and the Mug.. looking good i think.

Thanks,
Owen
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Old 12th January 2006, 08:30 PM   #20
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and.. now it's finished.. same basic psu as the original amp had but have replaced the two 2200uF capacitors with 4700uF capacitors.. hopefully that will help with the problems the old amp used to have with power led dimming under heavy bass...
Thanks,
Owen
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