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Old 20th May 2015, 07:37 PM   #1
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Default A little advice regarding boosted op amp

Hi,

Note: I wasn't entirely sure whether to post this here or in the amplifiers forum, please move if it is more appropriate addressing another forum

Now Ive finished my degree (finally!) in my spare time I have been pondering making a small amplifier using an op amp gain stage or two, and buffer stages.

In particular, I plan to use LM4562/LME49710/OPA2132 for voltage gain, and either BUF634 or LME49600 for current gain.

I am aware that many headphone amplifiers have been made this way, but I am hoping to use several buffer ICs in parallel to achieve higher power, perhaps single digit watts (10 watts would be ideal if it proves feasible).

My first ideas:

Assuming a maximum 0.707 Vrms input, and a voltage gain stage (or stages) to give 12-15 v/v gain.
Assuming unity gain from the buffer stage and limiting the current to 80% of the 250mA claimed limits.

Seeking P=10 W, R=8 ohms:

P=V^2 / R --> V^2=80 --> V=~9 Vrms across laid to give 10 W

I=V / R --> 9\8=1.125 Amps required for 9Vrms with 8 Ohm load.

Gain= 9/0.707= 12.7 V/V

9 Vrms = 12.7 Vpk assuming 0.707 Vrms input, using +/-15 to 18 Volt supplies.

Using 7 buffers in parallel the load share is:

1.125 Amps / 7 = 0.161 A

Also I am planning to use current sharing resistors, but I'm not certain how to size them. I have a load of 0.22 and 0.47R so.ill probably give them a shot.

Firstly, thanks for reading if you made it this far!

Can anyone see any major flaws in my logic? Layout advice? Anything that I haven't properly considered?
Any advice that you can offer in regards to making something that more than just works ��
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Last edited by mondogenerator; 20th May 2015 at 08:16 PM.
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Old 20th May 2015, 08:13 PM   #2
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A quick glance and the maths looks OK. But my concern would be the price of the buffer chips and you want to use 7 of them per channel...
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Old 21st May 2015, 05:19 AM   #3
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Haha yes I may have a rethink and lower my expectations, I have 4 or 5 buf634, so ill probably settle for using 4 and less power output.

The maths look ok, I always have to double check myself and that I've converted RMS to peak etc, so that's a relief (I had been out with a few uni guys 'celebrating' the end of studies, was a little sozzled, and didn't trust my maths skills)

Thanks.
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