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Old 28th December 2004, 05:45 PM   #1
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Default I got this schematic of a power amp, would like comments

I will not tell yet where it came from. I got all the parts to duplicate it, and wondering what expert think of this kind of design. The power supply is +37 0 -37.
I got an original working amp made out of this sch (factory, not homemade) and all i can say is that it gives a lot of power for what it is.
Comments please !!
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Old 28th December 2004, 06:07 PM   #2
Steven is offline Steven  Netherlands
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Simple amplifier, uses cheap (and relatively dirty) 4558 opamp as driver. The next stage with Q101, 103 and Q102, 104 is a 10x amp to get the required voltage swing that the opamp is not able to deliver. The output stage is underbiased and will generate crossover distortion. Biasing is via D105...D107 and the bias voltage it makes (only 3 diodes) is too little to create the required idle current in the darlington output stage. Probably done to save an adjustment resistor and adjustment time. On the other hand it will also not drift.
Q113, 114 is for overload protection.
I guess it is a musical instrument amplifier or car audio amplifier in the category "most bang for the buck".

Steven
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The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973
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Old 28th December 2004, 06:30 PM   #3
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Interesting comments. Yes, it is a car audio power amp, rated at an optimistic 250w max. It is bridgeable and can drive 2 ohm loads.
It is indeed a cheap amp, but delivers very high spl's on proper subwoofers.

My goal is to build such an amp to drive 2 TL for my home basement.
Maybe you have some suggestions to improve this design, i would like yo delete the op amps, and use some back to back fets or regulat transistors. I hate op amps

I also have a schematic from a Randal musical instrument amplifier, and is rated as much as 700w at 1ohm load. The circuit is incredibely simple and uses 2n3773 for the outputs. But the voltage rail is too high for now, i only have a toroidal transformer that can deliver 2x 30v ac.
The speakers i have for this project are 4 ohm car subwoofers that can take a lot.

thanks for the inputs.

Pat Allen
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Old 29th December 2004, 10:02 AM   #4
Steven is offline Steven  Netherlands
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If you want to use these amplifiers and not start from scratch, I would just make very small changes on the original PCB, otherwise you could just as well start designing or building a completely different amplifier (even with the same components, well, at least the expensive components).
Replace the opamp with a better quality one and add a bias adjustment to the output stage by replacing the three series connected diodes by a Vbe multiplier, thermally coupled to the heatsink.
Changing the opamp may result in the necessity to change the frequency compensation (C114, C115), though. You will need a scope to check stability. The easiest is by applying a (small signal) square wave and looking at overshoots.

Steven
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The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973
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Old 29th December 2004, 12:33 PM   #5
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Default Hello Pat, are you hearing Charlie?...this one is Steve.

One of our big brains.

Just to inform, in the possibility you have not this information.

For me, low knowledge guy, everything this man say is a law...well .... each one is each one, you can be another big brain too.

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 29th December 2004, 12:38 PM   #6
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Default Yes, i see you respect him to, i will searching you, as you called and Steve appeared

So, as you have luck, i will be searching you, to have chance to hear what he say too.

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 29th December 2004, 12:45 PM   #7
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Altec Lansing and EV use a more simplified version of this in their 9444B, (EV2600), 9446.(EV3200). The power output of the 9444 and 2600 is 200 WPC@8 ohms and 300 a channel at 4 ohms. The 9446 and 3200 put out 600 wpc at 8 ohms. I forgot the 4 ohm rating. The 9446 and 3200 will mono bridge at 1200 watts.

These can be purchased very cheaply on ebay with the going rate at about $100 an amp. I should note that these amps are commercial and will take a balanced or unbalanced input. They are also fan cooled and feature metal case T0-3 case outputs.

I think I could post a schematic since the original company went out of business. I think you would enjoy the simplicity of the design.
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Old 29th December 2004, 01:24 PM   #8
Eva is offline Eva  Spain
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Inside a car, cabin gain boosts LF efficiency by 20 to 30dB. Mid frequencies are also boosted by 3 to 9dB due to corner gain or wall gain depending on driver placement. Also the cabin load effect at LF reduces the cone excursion requirements for a given SPL up to 1/10th in comparison with outdoors

So almost everything sounds loud inside a car [and manufacturers take good care of this fact ], but car audio gear produces frustrating results outside the car [sometimes also inside it...]

This amplifier only produces about 60W at 8 ohms with a standard unregulated supply. This power applied to a small 6" or 8" driver placed in a medium or large room is not going to sound loud at all, even TL loaded. With more power the cone excursion limit of the drivers will be easily reached. Typical 'Car-audio-loudness' is not so easy to obtain in rooms and it's quite hard outdoors...
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Old 29th December 2004, 01:39 PM   #9
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EVA has brought out some very good points here. Another point to consider is the power output of a car amplifier. I spent a lot of time on the bench in the 90's and one memory comes back to me when I think of car power amps. Even with a bank of batteries they don't deliver the advertized rating. My point is...why bother with the thought of designing or building a power amp when the models I have mentioned can be purchased for peanuts and sound good.
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Old 29th December 2004, 01:47 PM   #10
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Have to know that i already use this exact same power amp outside of a car, in bridged mode with a 2 ohm load (two 4 ohm 10 inches subwoofers) and is sounds as loud in my listening room than in my car. So raw calculations gives
P=e2/r, (36v+36v, bridged mode)>2 /2ohm=2592w? rms=1832?
giving a 50% coeficient working number...916w?
I never got the way to calculate power output capability of a power amp...but i know ohm law !!

Whats the thing with hearing Charlie? i dont get it at all.

For the Altec schematic, yes, show it !!!


thanks

Pat Allen
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