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Old 16th April 2011, 11:52 AM   #1
lineup is offline lineup  Sweden
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Default Hifi Combo with OPA445AP + OPA541AP

OPA541AP can use +/-35 volt supply. Gain bandwidth 1.8 MHz
It delivers 50 Watt into 8 Ohm at gain=20
THD is low. 0.002%
But upper bandwidth we get is 'only' 90 kHz

OPA445AP is a DIL8 package opamp that can take +/-45 volt supply

If we use OPA445AP at gain=20 and attach OPA541AP as buffer output
we get a combo with very good performance. Tiny THD distortion at 50 Watt.

Image 1. Shows a compare of THD at 50 Watt RMS into 8 Ohm
Image 2. Shows compare of AC, Combo vs. single OPA541
The upper bandwidth is ~500kHz vs. ~90kHz


It is possible to do like this with a normal opamp,
but then with dual supplies. One for buffer and one for opamp.
This combo uses only one supply +/- 35 VDC
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File Type: png compare.png (16.7 KB, 426 views)
File Type: png compare_ac.png (6.2 KB, 412 views)
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Old 16th April 2011, 11:58 AM   #2
lineup is offline lineup  Sweden
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Here is the combo circuit.
It is simple with very few parts.
Easy way to get hifi 50 Watt.
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File Type: png opa445_opa541_combo.png (5.1 KB, 427 views)
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Old 17th April 2011, 09:45 AM   #3
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Well, this looks very simple and models well.
Have you built one, and can you comment on its real world performance?

Rgds,
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Old 17th April 2011, 04:35 PM   #4
lineup is offline lineup  Sweden
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Yes, it is very simple and give a good performance.
No. I have not built it.
I have a situation where I can not do the nessesary work.
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Old 17th April 2011, 05:47 PM   #5
jcx is offline jcx  United States
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2nd low V supply lets you use better input op amps, at only a few 10 mA you can use Zener shunt supplies for the +/-12-15 V input op amp supply - and by sub regulating you get better psrr too

more op amp choices, better perfromance only pennies added cost, or savings if lower V op amp is cheaper than opa445 (digikey wants US$11.8 in singles) - I think I'd go for "less simple"

Last edited by jcx; 17th April 2011 at 06:06 PM.
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Old 18th April 2011, 02:38 AM   #6
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One assumes the expensive OPA541AP could be replaced by the pedestrian but cheap LM3886 for example?
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Old 18th April 2011, 07:05 AM   #7
qusp is offline qusp  Australia
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probably yes, but you couldnt expect the same performance
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Old 18th April 2011, 08:10 AM   #8
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Quote:
Originally Posted by qusp View Post
probably yes, but you couldnt expect the same performance
OK. Question from ignorance here.

I have not previously run across the OPA541AP I downloaded a data sheet and, at first glance, it doesn't seem to be anything special. It costs AU$26.90 at Futurelec, a good supplier in OZ.
At the same supplier an LM3886 is AU$4.20.

The only standout spec is the THD which is quite a bit lower.
Is this why you suggest that the LM3886 will give inferior results?
Would make sense.

Has anybody ever used the OPA541AP as a stand alone gainclone?
How much better than a 3886 does it sound?

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Old 18th April 2011, 08:44 AM   #9
jcx is offline jcx  United States
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unity gain stable internal compensation also makes it easier to use 541 in a multiloop than the audio power "chip amp" LM3886/TDA7293 class parts which are only stable for gains >10-20
and despite the decompensation they have only ~2X higher GBW

for cost effectiveness it is probably worth dealing with the decompensated gain and making up any performance lack by paralleling and Class AB biasing the cheaper chip amps - more complexity for the performance even if all of the parts could be cheaper - spend money on extra heatsinking and power supply

A chip-amp to rival Hi-End - design advice mentions some techniques to improve chip amp performance

Last edited by jcx; 18th April 2011 at 09:08 AM.
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Old 18th April 2011, 11:36 AM   #10
qusp is offline qusp  Australia
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its not the only standout, as jcx mentions its far better suited for lower gain, thd as you say is considerably better and the national part has more than 10 x the input offset. theres more as well, but i dont have the time to analyze it, its obvious to someone who knows what they are looking at and really they are quite different parts, much more standalone power and higher supply voltage for the national part, where the 541 would be better suited to input driver duties and mid level power applications.

@jcx, agreed better performance for marginally more complexity could be had using a fully differential opamp like ths4131 etc, or crazy low noise unity gain stable chips like te lme49990, but i think the point here was simple power supply, simple cct, great performance
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