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Old 6th May 2007, 02:09 PM   #1761
mikelm is offline mikelm  England
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If your speaker are 96db / Watt / meter then I think u can more or less choose what ever voltage sounds best !

Unless you listen at VERY high levels it is unlikely you will that you will ever get to higher that 20watts in the range that may be 6 ohms

It is more usual for the highest power to be in the deep bass where u sensibly speculate that the impedance rises.

since Peter says the higher voltage sounds better I would for for that.

+ / - 35 would get my vote

that's my half penny worth

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Old 6th May 2007, 03:37 PM   #1762
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Quote:
Originally posted by phatcenter77
I need just one power supply board, and related parts. If I need to order a complete PCB set that's fine.
I will send you replacement rectifier board free of charge, You will also need 8 diodes and two caps, that would be $10 including shipping, e-mail me for payment info.
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Old 6th May 2007, 03:58 PM   #1763
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Default Re: Transformer selection; interaction between speaker impedence/supply voltage/power out

Quote:
Originally posted by autopoiesis
He posted previously that the higher the rail voltage, the better the amp sounds. My speakers are nominally 8 Ohm, 96db/1W, two-way passive. Although I don't have a Z-curve to see how far Z varies, I am aware that it could be well below 8 Ohms in places.

Page 10 of the datasheet for the LM3875 (http://www.national.com/ds.cgi/LM/LM3875.pdf) shows the attached graph of output power vs supply voltage, for R=4,6 and 8 Ohms, but for a purely resistive load.

Now, 2x25V transformer will give ~±35V rail voltage (1.4 x Va, minus diode losses I put at zero).

Let's assume that Z drops to 6 Ohms from, say, 300-1000Hz, but is higher elsewhere - leading to the nominal 8 Ohm rating.

With rail voltage at 35V, one infers from the R=6 Ohm curve that the chip could supply only 20W at those frequencies, but nearly 60W at frequencies where Z was 8 Ohms.

I don't fully understand the interactions between these variables or whether this inference is well founded. (Or if I should be concerned at all - no-one seems to experience this )

And finally, is 300VA overkill for dual monoblocks? My speakers roll off quite high and I have an active mono sub (which won't be powered by a GC, for now). I'm looking for dynamics/detail with the GCs and understand that a lower VA transformer might be more apt if weighty bass is less of any issue...
I wouldn't be much concerned with those graphs published by National. I once tested the amp into a dummy load, and IIRC even for 4ohm load and 34V supply rails I was getting some 36W output power.

I also tested the amp connected to variac where I could change PS voltage and compare how it affects sonic signature. With lower voltage the sound is smoother and more laid back, with higher supply it becomes more "alive" and dynamic, with better presence and involvement factor. It may be a matter of personal preferrence, but my choice goes to higher voltages. Even when I was running GC from batteries with 6ohm speakers I opted for 3 x 12V cells per rail and supply voltage close to 40V DC.

Presently, I choose 2 x 22V secondries on Plitron transformers I'm using. This produces close to 34V DC rails, with 2 x 24VAC secondaries the actual DC voltage would be 37V DC ( taking into account regulation margin and usually higher mains level).

300VA is not really an overkill, considering that Naim uses 800VA for preamps

I wouldn't go with less than 220VA though. The lower power transformer may sound a bit lean, higher power units may be a bit edgy, although good dynamically. I'm using 300VA units in both stereo and monoblock versions and find them as good compromise.
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Old 7th May 2007, 05:36 PM   #1764
flraer is offline flraer  Sweden
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I hope it is okey that I ask this question here.


I have just finished building my Lm4780 and I really like the sound so far. But, one of the Lm4780 gets much warmer than the other one when I'm playing music.

Edit : It also happens when there are no speakers and input devices conected. Is it called idle?
But, even here one of the Lm4780 gets much warmer than the other one. However, I think both Lm4780 gets a little bit too warm.

What could be the problem?

/ fredrik
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Old 7th May 2007, 11:08 PM   #1765
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In what configuration are you using those chips (parallel, balanced, stereo)?
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Old 8th May 2007, 11:54 AM   #1766
flraer is offline flraer  Sweden
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Im running them as parallel.


I noticed one thing when I made som measurments. When I measured the resistance on the speaker-out contacts ( with nothing connected to it) I get two different values.

The coolest ship had 40 R and the warmer one got 135 R.

Perhaps it is a resistor with wrong value?

Should I try too measure with an oscilloscope to se if there are any oscillations?

I feel like a real beginner right now! =)
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Old 8th May 2007, 02:35 PM   #1767
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Disconnect output 0.1R resistors (R8 and R9) from /OUT/ connection and measure the DC offset from each half of the chip separately for both chips.

http://www.audiosector.com/lm4780%20amp.pdf
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Old 8th May 2007, 08:13 PM   #1768
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Man this all looks great guys! I did not know anything about chip amps / do not know much about building amps in general and reading through all of this is very exciting. I dig projects and want to give props to people like Peter making these things a reality for us DIY types. Thanks man!
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Old 17th May 2007, 01:09 AM   #1769
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Default testing the DAC

Hey all.

I've almost finished the DAC board, but since I've yet to get all the parts for the power supply, would I be able to properly test the DAC with a couple of 9V batteries, or in liu of that, a 9V DC powerpack?
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Old 17th May 2007, 03:29 PM   #1770
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Yes, you can test the DAC with batteries, connect them right after 20R resistors.
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