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Old 21st April 2003, 11:58 AM   #1
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Default Need advise on this amp...

I've been thinking on building this amp, but before I do, I'd like to hear your comments, suggestions, mods, etc.

Thanks in advance.
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Old 22nd April 2003, 07:07 AM   #2
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Hey,
I like the schematics of this amp, however I would add a 100K pot for DC adjustment on the inverting input of the opamp and doubble up no. of output power transistors.

What supply voltage do you entend to use?
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Old 22nd April 2003, 09:10 AM   #3
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Default Power supply

ACD,

The suggested power supply source was a 48-0-48/5A that would give around +/-67 volts. However, I have a transformer from a former project that has a secondary voltage of 44-0-44/5A that would give a rectified voltage of +/-62 volts. The thing that strikes me most about the circuit is it looks so simple to build.


Can you show me how to put a pot at the input of the opamp for dc adjustment?

Any other comment about this amp would be helpful.

Thanks,
Jojo
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Old 22nd April 2003, 09:59 AM   #4
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Often it is the most simple designs that works best...
I have build several amps much similar to the one that you are interested in. The design looks very similar to famous older amps like Phase Linear and GAS etc.

Connect the 100K pot between the positive and negative supply to the opamp, and connect the center pin of the pot via 160K resistor to the connection between R4 and R5.

My current amp uses +/- 63VDC, and produces app. 160W/channel into 8 ohm.
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Old 22nd April 2003, 10:08 AM   #5
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I just made this schematic for you regarding DC-adjustment.
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File Type: jpg dc-adjust.jpg (16.0 KB, 887 views)
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Old 22nd April 2003, 11:06 AM   #6
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Default Mods

ACD,

Thanks for the mod on the schematic. So this pot will now be used to adjust DC in the output of the amp right?

The suggested output were 2sc2922/2sa1216 transistors, can I substitute it with 2sc3281/2sa1302 but double the number of output xsistors? That would be 8 transistors per channel.

Jojo
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Old 22nd April 2003, 11:46 AM   #7
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Yes! This will adjust the DC-offset on the output.
Let the amplifier warm up for 15 minuttes and then adjust it
(offset better than +/- 0,2 mV is possible).

If you use 4 x MJ15003 and 4 x MJ15004 output power transistors per channel, then you don't have to worry about a
protection circuit (I'm talking by experience ).
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Old 22nd April 2003, 12:09 PM   #8
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Default Motorola

ACD,

The MJ15003/4 pairs are still available at a local shop here in my country. They are still stomped "Motorola" and the shop said that these are old stocks and not counterfeit.

Maybe I'll make a prototype pcb and test different output transistors.

Do you have a schematic of the amp your using now?

Jojo
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Old 22nd April 2003, 12:21 PM   #9
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this is almost identical to the op-amp based amp I am going to built, except that in my case, I will be using some mosfet in the ops.

It is essentially a CF ops without a va stage. Pretty simple, as you had pointed out.

The only thing that differs from traditional cf ops is the grounding of the emitters of q2 and q3. Does anyone know why?
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Old 22nd April 2003, 12:39 PM   #10
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Default Another amp

Just want to share a schematic of an amp I used to build. It uses cheap and locally available parts.

The book where it came from said that it can deliver 250W/8ohms and 500W/4ohms. I don't know if that is true because I have no way of measuring the power outputI have successfully built 2 stereo units and we use it for pa purposes. This amp may not be the clearest amp I've heard but it sure is loud! It requires a 35-0-35/25A transformer that is so huge and heavy (22Kg) that the locally available chassis needs reinforcements.

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Is there a simple/crude way for me to measure an amps power output? I saw a dummy load at a shop once, I can also borrow a signal injector and a digital voltmeter or whatever they call it. Can I measure an amps output power with these? How?

Jojo
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