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Old 13th November 2011, 11:50 AM   #1
regal is offline regal  United States
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Default clean 11 Vrms for 600ohm load

Odd mission, what topology should I look at for an amplifier that can swing 11Vrms but only needs to deliver ~200mA.

Its an unusual deal, with the 600 ohm load one would think you could find a topology with very low distortion. The problem is that most opamps can't quite get to 11 Vrms, most gainclone/chips increase in distortion directly with voltage not current.

So I guess what I am looking for is a discrete direction where the thd/imd drop significantly with an easy load but still can handle 11vrms.

Any thing like this exist?
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Old 13th November 2011, 11:54 AM   #2
mbar is offline mbar  Poland
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Yeah, parallelled NE5532 with 18 V supply.
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Old 13th November 2011, 12:08 PM   #3
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Consider the LME49600 buffer from National. Will handle 200mA, very low distortion. LME49600 - High Performance, High Fidelity, High Current Audio Buffer
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Old 13th November 2011, 12:43 PM   #4
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All major manufacturers (TI, National, Analog Devices) offer a online selection tool that will guide you to a suitable solution. The proposal of jackinnj will certainly do the job.
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Old 13th November 2011, 02:18 PM   #5
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It would be useful to know a few more parameters, like noise and distortion targets, required voltage gain, desired output impedance, driving impedance, space constraints and budget. Supply noise density, too.

I'd be thinking a unity gain buffer driving an inverting gain stage driving an inverting output buffer of no more than gain 2 with 1 or 2 dual opamps in parallel, all with NE5532s on approx. +/-18 V. 5532s are cheap and have good transfer linearity, and 4 in parallel should do an excellent job on 600 ohm loads. You can easily afford 47R..100R combining resistors.

Last edited by sgrossklass; 13th November 2011 at 02:21 PM.
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Old 14th November 2011, 10:11 AM   #6
djk is offline djk
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11V RMS is 31.1V P-P

A 5532 is only rated at 24V (min) with 15V supplies, even with another 3V on the supplies it's going to be close.

The OPA2604 is capable of driving 600Ω loads with power
supply voltages up to 24V.
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Old 14th November 2011, 10:33 AM   #7
AndrewT is offline AndrewT  Scotland
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The 5532 and 5534 are 44v devices.
They can be run on regulated +-21Vdc supplies.
They will easily give 11Vrms into 10k load. and may still work well into a 2k load.

I would suggest a discrete transistor buffer to get the necessary current capability to drive a 600r load.

You could set the buffer to always remain in ClassA even for peak currents and you can modify the sink/source on the opamp output to convert that output stage to single ended ClassA for all peak currents.
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Old 14th November 2011, 11:21 AM   #8
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The LME49600 would do the job, but it is about ten times the price of a NE5534. The OPA2604 is five times the price of the NE5534. The datasheets gives (short circuit) output current for the LME49600 as 550mA, for the NE5534 as 38mA and for the OPA2604 as 40mA. You pay your money and you make your choice.

Then again - 11V into 600ohm gives 18mA of current AFAIK. You should therefore be OK with the NE5534. For the final choice, other things such as slew rate, noise, distortion etc. will have to be considered as well.
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Old 14th November 2011, 11:26 AM   #9
AndrewT is offline AndrewT  Scotland
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DJK already told us that 11vrms is 31.1Vpp.
That is ~26mApk into a 600r load. Then you have to add in the current requirement to charge/discharge all the capacitances on the output of the opamp.
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Old 14th November 2011, 11:30 AM   #10
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Quote:
Originally Posted by ingenieus View Post
The LME49600 would do the job, but it is about ten times the price of a NE5534.
Testing OpAmps Under Load (Pease)
http://www.national.com/an/AN/AN-1485.pdf

Gain and Linearity Testing (Pease)
http://www.national.com/an/AN/AN-1671.pdf
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