What is your opinion on this schematic for OPA-541

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Hi everyone,

What do you think about this sch. ?
It was applied from Inverted LM-3875 sch.

:D
 

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Hello lineup, thanks for a nice link.

I just tried it out with no Rcl at pin 8 ( not connected, just leave it ) and use 100k pot. 24-0-24 transformer with UR1620/1620R give +/-35 v to the chip.

I'm facing the opposite problem, sound come out too loud.
dont' know why :whazzat:
When listening at volume ~10% I feed lack of bass, need to turn the volume about 25% way to get sufficient bass but over all sound come out too loud.

I'm not sure if the gain is too high or not ?
 
100db said:
Hello lineup, thanks for a nice link.

I just tried it out with no Rcl at pin 8 ( not connected, just leave it ) and use 100k pot.
24-0-24 transformer with UR1620/1620R give +/-35 v to the chip.

I'm facing the opposite problem, sound come out too loud.
dont' know why :whazzat:
When listening at volume ~10% I feed lack of bass, need to turn the volume about 25% way to get sufficient bass but over all sound come out too loud.

I'm not sure if the gain is too high or not ?

Here is an app note for OPA541, about how using Current Limit.
http://www.ti.com/sc/docs/psheets/abstract/apps/sboa057.htm

The pin 8, should be conneted to output, if you do not use Rcl, resistor for Current Limit.

If you use 4.7uF - 10 kOhm at input, there shouldnt be a problem with bass.
And gain of 22 should be a very good value with signal from CD-player.

You should add a zoble network from output(pin5+7) to ground.
One resistor 4.7 Ohm (2/5 Watt) in series with 100nF capacitor will be good.
Will make your amp a bit more safe.
And avoid high frequency oscillation, which not always you can discover and know is there.

Looking at THD+noise diagram, and at 1 kHZ
0.07 % at 50 Watt is very good!
0.11 % at 5 Watt is strange, but still okay.

Normally I would expect lower distortion at lower power output.
Maybe somebody can explain ?
 
you should only use gain up to 10 with opa541 and opa549 they dont sound too well with higher gain but sound awesome at gain of 10. lower the gain and u should hear beautiful music out of your opa541



also use lower value resistors (1K and 10K for gain setting works great and sounds great but 470 ohm and 4.7K resistors on my 2 ch opa541 chipamp sounds the best so far)
 
Every one,
thanks for your input. I try to correct my sch. follow your advice, is this seems to be ok ?

After changing from a cheap pot to a stepped atten', output that used to be too loud become normal. I have to turn the volume up to 50-60 % at normal listening level.

I found this opa-541 (20 hrs used ) give a tighter bass but the old 3875 ( 200 hrs used) with a basic sch. as above give better detail. :rolleyes:

What is your experience on this 2 chips ?
:)
 

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The 4.7uF input cap and 10k resistor sets the rolloff of low frequency.
It is a high pass RC filter.
The filter has a roll off frequency, and passes frequencies higher than this roll off frequency.
If we call this freq 'f'

Formula:
f = 1 / (2 x pi x R x C )

pi = 3.14
Using 10k and 4.7uF we get:
R = 10.000
C = 0.0000047

f = 3.38 Hz
At this freq bass is -3dB


10 x 4.7 = 47 (kOhm x uF)
If you keep 4.7uF cap and makes 10k resistor 1k, we get
1 x 4.7 = 4.7 (kOhm x uF)

As this value is 10 times lower, the frequency,
according to formula f will be 10 times higher.
So you get roll off at 33.8 Hz.
This filter will pass frequencies higher than 33.8 HZ
but begin to block lower freq.

I personally can accept a roll off at f=10 Hertz.
If RC =
15k x 1uF = 15 (kOhm x uF)
This makes f = 10 Hz, almost
1.5k x 10uF = 15 gives the same.


To keep your f, now 3.38 Hz
and use 470 Ohm resistor, you will have to increase Cap
to keep RxC the same.
As R is 20 times smaller ( 10.000/470 )
your cap need to be 100uF. (20x 4.7)

This is the drawback of using too small resistor, instead of 10k.
You need a very high value cap.
And as we want to use film cap (polypropylene, 9/10 people agree film cap should be used in input)
this will be a little problem.

10uF you might find (or parallel two 4.7uF) and if you make resistor 1.5k
you will have roll off: f=10 Hz

I would not recommend lower feedback resistors,
than 1k-2.2k and the other 10 times higher
for example 2.2k and 22k
 
Hi Nordic,
I still "need" the attenuation because I still listen to different kind of music from vocal / jazz up to punk and power metal some time.
I also use this amp to listen at night, and I can't turn it too loud.
:D

Question,
If I need the attenuation, is it better to use as a power amp and connect with pre-amp or the stepped atten' is better ?
 
Nordic said:
I have been wondering about that, this week...
Why do we attenuate the amp, beloew the range of what our speakers can produce? Are we not makeng the amp do unneccesary work?

This answer does not apply to systems with subwoofer.
I assume a two way loudspeaker system with 6.5" or 8" woofer and a 1" tweeter.

Say LSP has a -3dB rolloff at 50 Hz. This rolloff is made by the LSP box.
The box with its size and other quality is a high pass filter.

If we make RC input filter at amplifier have same rolloff f=50 Hz, -3dB.
We get a total slope of -6dB ( 3dB +3dB ) at 50 Hz.

If we set RC filter f=20 Hz. Means -3dB at 20 Hz.
The slope will be maybe only -3.5dB ( 3 + 0.5 ).

================

I would say, if amplifier have frequency response at 5-10 times lower
the addition of rolloff coming from amplifier will be of no practical significance
compared to loudspeaker rolloff.

This should mean, if LSP -3dB at f=50,
I will use RC input filer at max 10 Hertz
maybe as low as 5 Hertz.


Same with upper amplifier rolloff.
If your loudspeaker lowpass rolloff is -3dB at 15.000 Hz
you can do well with an amplifier 5-10 times higher frequency rolloff.
In this case 75.000 - 150.000 Hertz


If I am correct, a normal (medium cost) 2-way loudspeaker response
will practically not be effected
if amplifier system has at least a frequency response of 10-75.000 Hertz.

And most amplifier have this.


:cool:
 
lineup said:


This answer does not apply to systems with subwoofer.
I assume a two way loudspeaker system with 6.5" or 8" woofer and a 1" tweeter.

Say LSP has a -3dB rolloff at 50 Hz. This rolloff is made by the LSP box.
The box with its size and other quality is a high pass filter.

Id we make RC input filter at amplifier have same rolloff f=50 Hz, -3dB.
We get a total slope of -6dB ( 3dB +3dB ) at 50 Hz.

If we set RC filter f=20 Hz. Means -3dB at 20 Hz.
the slope will be maybe only -3.5dB ( 3 + 0.5 ).

================

I would say, if amplifier have frequency response at 5-10 times lower
the addition of rolloff coming from amplifier will be of no practical significance
compared to loudspeaker rolloff.

This should mean, if LSP -3dB at f=50,
I will use RC input filer at max 10 Hertz
maybe as low as 5 Hertz.


Same at upper ampllifier rolloff.
If your loudspeaker lowpass rolloff is -3dB at 15.000 Hz
you can do well with an amplifier 5-10 times higher frequency rolloff.
In this case 75.000 - 150.000 Hertz


If I am correct a normal (medium cost) 2-way loudspeaker response
will practically not be effected
if amplifier system has at least a frequency response of 10-75.000 Hertz.
And most amplifier have this.


:cool:
 
Nordic said:
I'm not qualified to answer your question, but I think you misunderstood me... what I wanted to know is why we make our amps roll off frequencies so much lower than our speakers'.


odysseybmx414 said:
did you try with lower gain? if so plz tell how it sounded. i dont use input caps on my gc i just use ci to ground to keep unity gain at dc

Sorry for my misunderstanding :rolleyes: ,
and I think you already got your answer from pink mouse and lineup :)
 
Hi Again,

My OPA-541AP was short out by my mistake before it pass burn-in period. :hot:

I'm thinking to make either (another)one of inverting OPA541 or Bridged inverting LM3875, I already search & read some thread talking about this chip but couldn't find exact answer for me. Could you help me on this ?

1. Sound charactor compare to LM-3875 ( Positive / Negative ).
See many people using this chip to power sup-woofer, sound quality is not good enough for the main speakers ?

2. Out put power of OPA 541 when use with 8 and 4 Ohm speakers, +/- 35v power supply. and a gain of 10

3. Any different in sound quality of plastic and TO3 package ?

thanks :D
 
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