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Old 29th October 2011, 12:46 AM   #11
Leolo is offline Leolo  Canada
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Quote:
Originally Posted by benb View Post
But you can put a high-power (20 watts or more, depending on the output power this thing will give out) 8 ohm resistor,
I don't have one of those. But I do have the heating element from a cloths dryer. So over heating won't be a problem.

[runs to check resistance]

DVM says 7.7 ohms. But it didn't read it right away, but went from 0 to 7.7 ohms and stayed there. Usable as a dummy load?
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Old 29th October 2011, 12:51 AM   #12
Leolo is offline Leolo  Canada
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Quote:
Originally Posted by AndrewT View Post
I have not managed to locate any twisted wire pairs.
Do you believe in twisted pairs for low loop area?
Do you believe that low loop area is irrelevant to performance?
I don't really know how to answer these questions. I have a lot of cat5e, which is unshielded twisted pair. But I doubt that answers your first question.

As to the other 2... what do beliefs have to do with Hi-Fi amplification? At best, we need reproducable falsifiable phenomena, at worst, subjective interpretation of those phenomena. Belief doesn't enter into it.
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Old 29th October 2011, 01:18 AM   #13
benb is offline benb  United States
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RF blocking: Yeah, I looked and don't see anything either. That C2 isn't it, that's "DC blocking" to reduce the DC gain down to 1 to minimize DC offset on the output. The LM3886 data sheet does mention this, though:
http://www.national.com/ds/LM/LM3886.pdf
The schematic on page 6, Figure 2, shows Cc, 220pF, across the + and - inputs to reduce possible RF ("high frequency") problems, as described for Cc on page 8 (and I don't know why they only mention fluorescent lams as a source of electrical noise). Not sure if it would do the exact same thing with the LM1875, maybe others could give an opinion. Putting a 220pF cap across R2 in the circuit you posted in #10 would help reduce RF.

Yes, a clothes dryer heating element of the right resistance (7.7 is plenty close enough) will make a great dummy load for anything under 1,000 watts or so.

For AndrewT's questions, making wires that carry equal-but-opposite currents (such as the transformer primary wires, and each transformer secondary winding) into twisted pairs reduces the magnetic fields (the "loop area") they generate (or for a low-level signal, it reduces the sensitivity to external fields), so there's solid reason to do it (this can reduce some low-level 50/60Hz hum induced by the wiring). His questions may be a bit oblique, but the other posts I've seen from him show a solid knowledge of electronics. I suppose his questions were just an indirect way of pointing these things out.

Last edited by benb; 29th October 2011 at 01:20 AM.
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Old 29th October 2011, 01:44 AM   #14
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re: LM1875 RF blocking
I think he means this, from pg 6 of the datasheet:
"Occasionally, current in the output leads (which function as antennas) can be coupled through the air to the amplifier input, resulting in high-frequency oscillation. This normally happens when the source impedance is high or the input leads are long. The problem can be eliminated by placing a small capacitor (on the order of 50 pF to 500 pF) across the circuit input."
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Old 29th October 2011, 03:02 AM   #15
Leolo is offline Leolo  Canada
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I can hear zero hiss or hum from the amp when powered on, no music. Given that the LM1875 has -92dB ripple rejection, I live in a small village and I'm doing my tests in the basement, this isn't suprising.

However, given how solicitous you folks seem about interference, I'll dust off my 'scope and or get a better pair of ears and check.
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Old 29th October 2011, 11:19 AM   #16
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use your DMM set to 199.9mVdc and set to 199.9mVac to measure the output noises when the input signal is shorted to input ground/return.

Then attach your source (switched off) and remeasure your output noises.
Finally turn on your source but with output signal turned down to max attenuation and remeasure the output noises.
If all of these are <0.1mVdc and <0.1mVac then you do indeed have a quiet system for ordinary sensitivity speakers.
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Old 29th October 2011, 11:37 PM   #17
Leolo is offline Leolo  Canada
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My test source is a battery powered Sandisc MP3 player.
If I stick my ear right up to the speaker, I can hear a very faint hum.


Input shorted to ground: -0.9mVdc 8mVac
Source turned off: -0.8mVdc 9mVac
Source turned on, max volume: -0.8mVdc 9mVac

So it seems I do have an antenna.

Note that I haven't finished my case yet. Far from it. As you can see in the picture in the first post, I only have the back and bottom done.
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Old 30th October 2011, 11:27 AM   #18
AndrewT is offline AndrewT  Scotland
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Your DC values are good and show that the amp is unaffected by the source.
The AC values show a consistent problem.
8mVac is at least 30dB too noisy. It could be improved, when all is corrected, by between 38dB and 40dB. Yes, a total noise target measured with a DMM can be <0.1mVac
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Old 2nd November 2011, 09:18 PM   #19
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Quote:
Originally Posted by danielwritesbac View Post

Your photo doesn't show heatsinks, but it does have nice big heat spreaders. The missing item is conduction of the heat into the air. Perhaps some U-channel added vertically or some holes added, could increase the effective surface area. Air is not particularly conductive, so a "slick wall" doesn't make much of a heatsink.
I disagree, there is no difference between a "heat sink" and a "heat spreader", they are synonymous for the same thing. The conduction of heat from a solid into air is always a challenging problem, regardless of the shape of your heat sink/spreader. What is really important is that you have surface area for the heat transfer to take place. Using fins or some other clever way to increase surface area will increase the rate at which heat is able to be conducted to the air. Unless you are going to mount some sort of fan, this heat transfer will be driven by natural convection and buoyant forces. The amount of heat transfer that takes place is proportional to the difference in temperature between the two mediums. If the air inside your box has no way out, then the air temp will increase along with the heat sink temp and your whole system will eventually overheat. To avoid this, it is critical that you provide some way for cool air to enter the cavity, and hot air to escape. ie, holes above and below the heatsink.

Louis
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Old 3rd November 2011, 04:45 AM   #20
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Quote:
Originally Posted by Leolo View Post
. . .IIRC, there's an LM1875 variant that has a isolated tab. While it might not get the heat off the die as well as the LM1875, surely this is more then made up for by not having to use the little T washers, a thermal pad and 2 layers of thermal gunk.. . .
You can mount the LM1875 directly to a thick piece of solid aluminum bar stock as a heat spreader and then use the Kapton, Thermal Paste, and Shoulder Washers to electrically insulate the heat spreader away from the heat sink. There is the terminology for a higher performance thermal interface.
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