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Old 21st October 2005, 04:10 AM   #351
jleaman is offline jleaman  Belgium
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Default Re: Flip it over....

Quote:
Originally posted by brentw
Jason... Take a close look at the underside of the board to see them. I haven't changed anything on it yet. In the days when I only had one DAC I was afraid I would have none if I messed with it. Next time it finds its way back to my desk I will bring it up to date and also give it a USB front-end

You are not allowed to put a usb front end on it..
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Old 21st October 2005, 07:51 AM   #352
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Quote:
Originally posted by Peter Daniel
Here's the schematic. I was working on that design with another forum member, Veteran, and he provided the layout's concept, I made the final trimmings. Schematic is also drawn by him.

I've been using that buffer in my preamp for last two years, and I'm very happy with a performance. Combining that with an input gain stage and volume control circuit, may create quite attractive line stage.

Sorry for being a pain in the neck but without an input cap the servo may possibly want to adjust offset voltage from the stage before the preamp/buffer when things are DC coupled. Just a note to those that may want to try this schematic.
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Old 23rd October 2005, 06:13 PM   #353
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That is a good suggestion and care have to be taken that previous stage provides proper offset. As the name implies, this is an output buffer, not a stand alone circuit.
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Old 23rd October 2005, 10:15 PM   #354
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Default 5 Channel

Peter,

I have on order an LM3875 Dual Mono Premium Kit, but it's not hear yet so I haven't been able to look it over and play with it. I'm planning to use it with an M3, your Green Board DAC (also on order), and CD transport to build a little dedicated CD unit suitable for headphones and small speakers.

But I have read several posts dealing with questions of bridging and multi channel applications, and it seems you have given these boards an amazing amount of flexibility. This has given me the idea to possibly build a home theater amp also.

My question is, for a 5 channel application, would the LM3875 or LM4780 be the better choice. What I am considering is a bridged pair for the front 2 channels for higher power (2 boards?), and a board and a half for the center and rear channels......or what the heck, just go for six channels....2 more boards.

Do both kits have this flexibility? Would one work better than the other? You mentioned the LM3875 would probably have slightly better sound quality, and that is important to me.

Thanks,

Robert
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Old 23rd October 2005, 11:11 PM   #355
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Hi Robert,

Your package shipped October 14th, I will forward the tracking info.

I was using for extended time both LM4780 and LM3875 in bridged mode, and it's not easy to choose preferrence here. The smaller chip seems to be more delicate in perfomance and shows more immediacy, maybe a bit more resolution and overall better control of highs and lows, but not by a large margin. The bigger chip, OTOH, has better drive and a bit fuller sound, sounds more like your typical high power amp, not a chip amp. I'd say, if you have efficient speakers, which are easy to drive go with LM3875, probably in a single ended configuration. If more power is required, go for LM4780, it won't disappoint you either.

I'm presently using LM3875 in bridged mode, but I'm still seriously contemplating going back to a single chip, and the decision is not that easy.

For implementation choices on both chips check those links:
http://www.diyaudio.com/forums/showt...556#post636556
http://www.diyaudio.com/forums/showt...319#post584319

The LM4780 has more flexibility, as two chips in one package can be implemented at least in 3 different ways.

The bridged application requires that you provide balanced signal to the amp, as the amp does not perform single ended -> balanced conversion. The bridged amp also runs much warmer than a single chip (if pushed, the temperature is closed to what I was getting with Alephs)
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Old 26th October 2005, 05:11 AM   #356
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Default 4780 configurations and power supply

Okay, I'm still a confused newbie that needs help figuring out the power requirements.

I'm going to build the 4780 Dual Mono kit (I hope you received my order Peter), and I expect to need it to handle a 4-ohm load.

So, how best to configure that? Parallel? How much output would that provide?

And what transformer would that require? I've read the data sheet until my brain starting smoking and it's not sinking in.

Nearest I can figure is a pair of 20-0-20 150VA toroids?

Am I on the right track? Or?


..Todd J.
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Old 26th October 2005, 05:42 AM   #357
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Look at the page 13 of datasheet: http://www.national.com/ds/LM/LM4780.pdf

It is recommended to use parallel config into 4 ohms. With supply +/-35V you'll be getting approx 120W.

I would go with transformer having 2 x 22V secondaries and 300VA per channel. Such transformer will produce 32-34V DC and you'll be still around 100W power output into 4 ohms.

I received your order and it will ship this week.
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Old 26th October 2005, 03:43 PM   #358
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>>Originally posted by Peter Daniel
>>Look at the page 13 of datasheet: >>http://www.national.com/ds/LM/LM4780.pdf

Thanks, that enlighted me a bit.

>>I would go with transformer having 2 x 22V secondaries and 300VA
>>per channel. Such transformer will produce 32-34V DC and you'll be
>>still around 100W power output into 4 ohms.

I'm curious about the calculation. I thought we multiply RMS output by 1.414 then subtract the voltage dropped by the rectifier stuff, which is about 1.4 volts? Wouldn't 2x22 result in 29.7 V. ? 2x24v. would provide 32.5 wouldn't it? Or am I missing something? (Remember, I am a newbie.)

>>I received your order and it will ship this week.

Cool! Thanks.

..Todd
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Old 26th October 2005, 03:57 PM   #359
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Also... (and please excuse the dumb questions)

>>>I would go with transformer having 2 x 22V secondaries and
>>>300VA per channel. Such transformer will produce 32-34V DC and
>>>you'll be still around 100W power output into 4 ohms.

300 VA/channel? Wouldn't that allow about 10 amps per channel? Sounds a bit umm... overzealous isn't it? Even a pair of 225VA toroids pushed to full throttle would probably blow a standard 15A breaker.

I'm not sure city council would let me push it that hard.

Am I missing something there too?

..Todd
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Old 26th October 2005, 04:20 PM   #360
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If you want, you can go with 60 VA trafo's, I'm just telling you what sounds best If 300VA seems too big, 225 might be more appropriate then, if you worry about city council.

Transormers have voltage regulation margin, which allows specified voltage at full current draw. With normal amp use (or bigger than neccessary transformer) it will never happen, that's why higher voltage than you calculate using formulas.

I'm using 300VA Plitrons on regular basis and this is what I measure on the amps.
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