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Old 6th May 2010, 04:08 AM   #11
sregor is offline sregor  United States
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LME49811 solutions seems like overkill for a 10W PC system. Especially since the spec sheets recommends +-20 or more for supply voltages. Simplest solutions are lm1875 or tda2050, but both work best with split supplies. There is a thread in here 12V DC gainclone? for 12 volt solutions, mostly bridge amps designed for the car market. The most notable is tda152Q class h (most power without separate supply). The questions raised seem to be: How good? How simple?
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Old 6th May 2010, 04:30 AM   #12
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And I thought the question was how much thread drift we could accumulate before the OP gets around to responding.

Is TDA152Q the correct part number? I'm not getting any hits.
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Old 6th May 2010, 04:40 AM   #13
sregor is offline sregor  United States
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TDA1562Q
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Old 6th May 2010, 05:56 AM   #14
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Only 55dB PSRR? Ouch. Between that, the not exactly promising THD specs, the lack of GBP or slew rate data, and the general sparseness of the datasheet on the analog side I'd stick with the 1875 or ADAU1592 unless third party measurements showed good results for the 1562.
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Old 6th May 2010, 06:11 AM   #15
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TPA6021 y can try
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Old 7th May 2010, 06:09 AM   #16
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Default output noise from a 49811 amp

The output noise from a 49811 amp (Compact 49811 with ThermalTrak) is measured. Note that the amp is not placed in an enclosure. The input of the amp is shorted to its input ground. The measured noise level is 32 uV (BW=22 k) or 40 uV (BW=30 k). The FFT is shown below. The gain set resistors are 5.6 k and 240 Ohm. Their thermal noise affect output noise level. Low resistance should be use.

The noise level shown in 49811 data sheet was measured from the Fig 1 circuit. The gain set resistors are not optimized for low noise.
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Old 7th May 2010, 07:03 PM   #17
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Quote:
Originally Posted by panson_hk View Post
Their thermal noise affect output noise level. Low resistance should be use.
Why do you say it's thermal noise and not voltage or current noise from the 49811? The 1.8k input impedance National uses is a pretty reasonable tradeoff between resistor size and preamp drive current requirements, but in my experience that means the dominant noise term's current noise through the 56k. With a 5.6k gain set I'd guess the dominant term is probably still current noise from the 49811, but a 240 ohm input impedance is not a terribly practical configuration due to the input current requirements.

Not to sound like an op amp fanboi, but this is another reason why combining a unity gain stable op amp with output transistors looks attractive for low power amps. Current and voltage noise from the chip is lower and, since all gain set resistors are the same size, it's possible to choose resistors in the 1-2k range where the voltage and current noise terms are comparable and the drive current requirements on the preamp are reasonable.
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Old 8th May 2010, 05:32 AM   #18
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Quote:
Originally Posted by twest820 View Post
Why do you say it's thermal noise and not voltage or current noise from the 49811? The 1.8k input impedance National uses is a pretty reasonable tradeoff between resistor size and preamp drive current requirements, but in my experience that means the dominant noise term's current noise through the 56k. With a 5.6k gain set I'd guess the dominant term is probably still current noise from the 49811, but a 240 ohm input impedance is not a terribly practical configuration due to the input current requirements.

Not to sound like an op amp fanboi, but this is another reason why combining a unity gain stable op amp with output transistors looks attractive for low power amps. Current and voltage noise from the chip is lower and, since all gain set resistors are the same size, it's possible to choose resistors in the 1-2k range where the voltage and current noise terms are comparable and the drive current requirements on the preamp are reasonable.
Unfortunately, the LME498xx data sheets do not provide parameters commonly found in opamp. Without those data, how can we minimize noise? One approach is to use as low resistance in critical parts as possible.

You have been suggesting using unity gain opamp with transistor buffer stage. Where do we get the voltage gain?
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Old 8th May 2010, 06:00 AM   #19
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Hi,

Quote:
Originally Posted by hamedfazelm View Post
hello
i want to build a 3-way active pc speaker and I need simple, low power(
about 10w) and high quality chip amps for amplifying tweeters and mid-ranges.
Well, 10W can be handled using +/-15V rails. The dissipation to provide this as Class A (single ended) into 6 Ohm is also not that great, as we only need 2A peak current.

So I might use whatever op-Amp's sound I like with a simple Darlington Emitter follower on a big heatsink with a 2A CCS and regulated +/-15V Rails, total dissipation only 60W, so not too bad...

Or just use a bunch of Op-Amp's that can provide highish current in parallel, the LM6172 can provide 100mA peak current per channel, so 10pcs in parallel can do 2A peaks at 12V peak voltage, giving around 12W into 6 Ohm.

Using 40pcs bridge/parallel can even give as much as 50W/6Ohm...

Ciao T
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Old 8th May 2010, 06:04 AM   #20
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Originally Posted by panson_hk View Post
Without those data, how can we minimize noise? One approach is to use as low resistance in critical parts as possible.
We're generally in agreement on resistor sizing. But one can also model things. And when I've done that thermal noise is usually one of the smaller terms in the noise figure.

Quote:
Originally Posted by panson_hk View Post
Where do we get the voltage gain?
As Thorsten just reiterated, for a 10W amplifier per the OP's request no voltage gain's needed; parts like the LME49710 and 49990 maintain good linearity to 10V. Reduce that swing by the STD03s' Vbe and you get about 10W RMS into 4 ohms (if you look at my first post in this thread you'll see I ask the OP what impedance they wanted 10W into for precisely this reason). Similarly, an LME49724 based H bridge---or fully differential amplifier, if you prefer to think of it that way---goes to 40W into 4 ohms, also at unity gain from a differential input. For power levels above that the 49811 is the best part I know of. But, as I've observed already, 50+W is not required to make a pretty good racket for home audio purposes.

Last edited by twest820; 8th May 2010 at 06:06 AM.
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