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Old 26th July 2010, 04:58 AM   #221
yatgai is offline yatgai  India
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Quote:
Originally Posted by AndrewT View Post
we are getting there.
Now add the High pass filter at the input.
This will convert the amplifier to AC coupled.

I like the differentiation between GNDa and GND.
What about the speaker return?

I would expect pin5 to require decoupling.
National do not expressly say which or if both supplies need decoupling.
Thanks Andrew for the reply

I have added high pass filter at the input in ZR & ZC both with values ????, I think I have got the formula for calculation of corner frequecy wrong, can you suggest the values for ZR & ZC for lowest possible Fz

in both the boards LM3886 & LM1875 speaker ground returns to Power Ground, and power ground is joined to signal ground via 10R/2W resistor in case of LM1875 PCB and 10R/2W parallel with 0.1uF/63V on LM3886 board.


I have not seen any PCB with LM3886 decoupled with 0.1uF capacitor, if need be I will mount them under the PCB on legs of IC

Regards

Yatin
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Old 26th July 2010, 10:23 AM   #222
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by yatgai View Post
I have not seen any PCB with LM3886 decoupled with 0.1uF capacitor, if need be I will mount them under the PCB on legs of IC
pins 1 & 5 are shown commoned to V+ with Cs as decoupling. National specify Cs as including 100nF.
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Old 28th July 2010, 04:59 AM   #223
yatgai is offline yatgai  India
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Thanks Andrew

I need to rephrase my last line in post 221. It should be read as

"I have not seen LM3886 leg 5 separately decoupled with 0.1uF capacitor"

as leg 1&5 are jumpered and deocoupled with 0.1uf Capacitor, I think it will be OK.

as for the question on the R & C values for input filter, can you suggest the values, or the formula for calucation of the corner frequency? I am using Fo = 1/(2 x pi x R x C) both R & C values in Ohm and Farads respectively.

Thanks

Yatin
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Old 28th July 2010, 10:25 AM   #224
AndrewT is offline AndrewT  Scotland
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I have found that F-3dB about ten times lower than the lowest frequency required from that amplifier gives good bass.
I have also found that this ten times applies when a reduced bandwidth is required for a Mid or Treble driver amplifier.

If your lowest audio is 20Hz then F-3dB should be 2Hz, if you follow the above.
I do the same for the HF end of the audio spectrum.
F=1/2PiRC is the correct equation.

Setting the input filters for F-3dB to 2Hz and 200kHz results in +0-1dB bandwidth of 4Hz to 100kHz.
There is much flexibility in choosing these frequencies to suit your equipment and your ears.
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Old 28th July 2010, 01:06 PM   #225
yatgai is offline yatgai  India
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Thanks Andrew

Proposed to use ZR=47K ohm & ZC=0.47uF this will give F-3db at 7Hz, and this will filter out any frequency below 70Hz, inlcuding 50~60Hz AC power hum ? (India the AC power is 220V at 60Hz)

Thanks & Regards

Yatin
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Old 28th July 2010, 03:02 PM   #226
AndrewT is offline AndrewT  Scotland
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Originally Posted by yatgai View Post
Proposed to use ZR=47K ohm & ZC=0.47uF this will give F-3db at 7Hz, and this will filter out any frequency below 70Hz, inlcuding 50~60Hz AC power hum ?
no.
Setting F-3dB to 7Hz will allow frequencies of 70Hz and above to pass with almost no attenuation.
60Hz will be attenuated by a tiny amount.

If you want to attenuate 60Hz then F-3dB should be set to >=600Hz, but that is preposterous for an audio amplifier.
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Old 29th July 2010, 01:28 PM   #227
yatgai is offline yatgai  India
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Thanks Andrew

Then it raises another point, whether ZR ZC filter is necessary at the input

Is it a electrical requirement or sonic requriement

Pardon my ignorance

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Yatin
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Old 30th July 2010, 09:30 AM   #228
AndrewT is offline AndrewT  Scotland
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yes, it is required.
It is the DC block and it is the high pass filter.
These are needed for electrical performance and for audio performance.
The one capacitor does a variety of jobs.

It's disadvantage is that it interferes with the wanted audio signal passing through the amplifier. This interference must be minimised for optimum audio quality.
Omitting this cap is an option, but the design of correction and protection systems must be followed through to prevent bad audio transmission and equipment damage.
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Old 30th July 2010, 01:43 PM   #229
yatgai is offline yatgai  India
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Thanks Anderw

Got the point

Regards

Yatin
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Old 12th August 2010, 05:00 AM   #230
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Some interesting tricks for LM1875 Active Speaker:
10k input load and 15k* feedback resistor
Overbuilt power supply or regulators
NFB bandwidth about an octave lower than input bandwidth
The bass roll off of the input and NFB can be aimed at the corner frequency of the woofer to do less of what the woofer can't do, thus causing it to do more of what it can (corner frequency manipulation).
In this example, the feedback resistor could be 10k for most bass or 18k for most treble or anywhere in-between, with the small caveat that changing the feedback resistor will cause you to have to re-calculate gain settings and also NFB cap bandwidth.

However, some speakers don't respond to this. Full range speakers like Lowther, Fostex and similar seem to require vacuum tubes. But they don't. What they require is current drive in order to achieve their advertised speaker driver frequency response. One-third of the way down this page --> Dial-A-Tube <-- is Lenard Audio's Variable Current drive application for power op-amps. There's also a power supply that exploits larger value resistors in the PI filter section of the power supply, making a form of soft clip (hoping I read that right). So, theoretically, you could make an LM1875 "Dial-A-Tube" preamp (class A or AB), headphone amp, (class A or AB), or power amp (class AB).

There are other designs possible, such as class ABC symmetry (armstrong boost) for 100w or class H/G for 500w or 1kw. However, at some point, it would become more cost effective to purchase either additional speakers (along with additional LM1875's too) or more-efficient speakers.

I just wanted to point out that LM1875 can be used in many different designs.
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