Thanks, I'm going to try it tonight. Would it hurt to raise the value above 22uf?
Thanks
OK, further recommendations. First, Andrew's suggestion to use RF bypassing at the input is a good one, a 220pF NPO/COG ceramic disc across the + and (-) chip inputs will do the trick. The 1k resistor at the + input of the chip will help minimize popping by limiting current into that pin when turning power on and off. And while Th3 uN1qU3 is technically correct about corner frequency (Fc) and using a 22uF cap for DC blocking the feedback network, there are very good reasons for using a much larger value there. A cap that large in value is almost certainly going to be a polarized electrolytic for physical size considerations, and that being the case, using a much larger value will limit the voltage drop across the cap because of the very low corner frequency (the cap behaves like a dead short for frequencies above Fc so "sees" minimal AC voltage across itself), thereby keeping distortion low and extending the life of the cap. Also it's important to make sure that Fc for the feedback network is at least two to three times lower than Fc at the input because you don't want to feed frequencies into the amp that the feedback network can't control. The 100 nF bypass caps on the power supply pins need to have low inductance to be effective in preventing oscillation, use NPO/COG 100 volt caps soldered directly to the chip pins, the under side of the PC board is a good place to put them. The 4.7 ohm resistor and 100nF cap on the output will help prevent high frequency osciillation, and the 10 ohm resistor and .7 uH coil will counter possible issues with excesive capacitance on the output. The resistor/coil can be made by winding about 10 turns of 18 gauge wire around a 1 watt, non-magnetic resistor (carbon composition works well if you can find them). Finally, as always, good circuit layout practice is important, keep high current paths away from low current input signals as much as possible, use separate conductors to common ground as shown on the schematic, and commom current send/return conductors like power supply leads, input leads, output leads, and so on, should be twisted together to keep stray circuit inductances low.
Mike
Mike
How does an asymmetrical power supply help an audio amp? I'm not saying it can't be done but why would you? I've used asymmetrical supplies in control systems where the control range wasn't hampered by the supplies but I'm at a loss to see how that would improve an audio amp.
It strikes me the same as trying to draw rectangular coordinates on polar coordinate system. It can be done but what a pain.
G²
I am showing what can be done.
I made no suggestion that it was better or less good to adopt any of my examples, simply that they exist.
Your control example shows where it is an advantage (having different output Vpk in the +ve & -ve polarities) A similar argument can be applied to a single ended amplifier, where equal value dual polarity supplies result in maximum Vpk in either direction being very different.
First he needs to get it to work THEN he can think of more exotic stuff. You have to remember that a big coupling cap will behave almost the same as no cap: there will still be thumps at power on and power off.
Th3 uN1qU3, a large value cap in the feedback network isn't "more exotic stuff", it's good design practice, reasons why were listed in my post #23. A 10 Hz feedback Fc would mean using a Fc of at least 20 to 30 Hz for the input filter which will affect the audible low end response of the amp, along with the other issues listed. And you're correct that it probably won't do much to fix the thumping issue, but why not get the design and implementation as good as possible from the beginning rather than having to go back and re-do stuff later?
Mike
Mike
Audible range... he never mentioned what he's using this amp on. Contrary to what people like to believe, a higher -3dB frequency makes small speakers sound better. No point in trying to push deeper lows than the speakers can manage - at best you're going to end up with intermodulation distortion, at worst you'll mechanically damage the speakers.
Hello,
Yes, I am still using the preamp, EQ, and FM radio as well as the original connections. Essentially all I have done is replace the power amplifier circuit with the LM3875s.
I'm a big fan of the tone controls on these old Realistic amplifiers. That and they look clean, nice wood finish. Not to mention that they were hand made in America.
Anyways, I've been too busy to work on it much. The more I listen to it, the more problems I notice. I think that I'm going to take the amps out, desolder the components, and use a PCB. Also, I want to use a better schematic...
Thanks for all of the advice. I have a much better understanding of what I'm doing now...
Th3 uN1qU3, I don't know what kind of speakers Nige838 is using and niether do you, and while the point you make about limiting low frequency response for small speakers may be valid, it's a bad idea to limit frequency response in the feedback network to achieve that end, it should be done at the input of the amp.
Mike
Mike
It's been said many times on this forum that National's data sheets are not well witten in many instances, and thier chip amp data sheets are no exception. They are quite flawed with respect to the designed high pass roll-offs for input and feedback networks. The inputs have Fc set at .13 Hz, and the feedback Fc is set at 15.9 Hz, so if a sub-15 Hz signal finds its way to the input, the feedback network won't work as expected, resulting in instability and distortion. The main point I'm trying to make here is that it makes more sense to do it correctly in the beginning rather than do it wrong and then go back and fix it after the fact. Besides, what's the problem with putting in a 100uF cap instead of the 22uF on the data sheet? It won't cost much more and size won't be too much larger.
Mike
Mike
Nige838 I would bet my bottom dollar the loud pop that you hear
is caused by the preamp section. Before you dismantle it disconect the input signals to the power amp and short its inputs to ground.
Then switch the amplifer on and then off there is a very good chance that
there will be a big reduction to the level of the pop you hear.
Would any of the thread hijackers like to comment and help provide suggestions to a solution.
Regards Ian
Many thanks Mad Tecchy. This makes sense to me. I will try this tomorrow. For tonight, this amp is pushing the jams for my birthday.
Much Appreciated.
I'm researching it..
Nige838, after you've gotten your amp re-built correctly and have figured out that Madtecchy is right about the source of the popping problem, you might want to use this or something like it. The circuit is a speaker controller that will delay connecting the speakers to the amp output at power-on, immediatly disconnect speakers at power-off, and protect them from amplifier faults.
View attachment Speaker Protect BOM.txt
Mike
View attachment Speaker Protect BOM.txt
Mike
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