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Cascading diamond buffers - a cheap low THD 10W amp with TIP41C
Cascading diamond buffers - a cheap low THD 10W amp with TIP41C
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Old Yesterday, 11:06 PM   #191
suzyj is offline suzyj  Australia
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Originally Posted by xrk971 View Post
Suzyj,
Very nice compact design. Those are some very nice THD figures you have and I would love to see the harmonic distortion profile to see where the H2/H3/H4/H5 etc distribution is. Can I ask why you did not use SOT23 for the TO92's and perhaps SOT223 for the TO247's? It would have been a slick flat SMT package then. You could even make it on metal core PCB and bolt the whole thing to a heatsink for higher power operation.
Attached incredibly early not well calibrated dodgy (did I mention they're early) measurements of 1KHz THD into a 47 ohm load with no heatsink.

I'm badly noise limited, which is making the measurements harder than they could be. Also I'm finding that unless I run my oscillator at the point where I'm about to run out of loop gain, the oscillator harmonics dominate.

Not that I'm complaining, mind. Hard to measure is my goal

Attached piccie of my spreadsheet, plus plot of 2.5V RMS into my spec-an, both with and without 1KHz notch, which knocks my fundamental down ~50dB.

If I reduce loading on the amp (increase load resistor from 47 Ohms to 1K) the harmonics drop into the noise floor. I'm thinking (given that it's predominantly H2) that I'm suffering due to the output transistors not being in thermal contact with one another.

So it needs a heatsink, or at least some metalwork to ensure the output transistors are at the same temp.

Like I said, early, dodgy measurements. I'm waiting on boards for a new source that will hopefully make this process easier.

I've gone off SOT23s a bit of late, as I find they don't save as much PCB real estate as I'd like. TO-92's are quick and easy to deal with, plus they include a via for every pin.

My goals for this amp are something that's easy to build as well as performing well, as I'll probably be making a fair number for line arrays and such forth.
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File Type: png 5W Cascaded diamond 1KHz 47 Ohm THD - No heatsink.png (123.5 KB, 45 views)
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Last edited by suzyj; Yesterday at 11:14 PM.
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Old Yesterday, 11:31 PM   #192
suzyj is offline suzyj  Australia
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Originally Posted by Earfanatic View Post
About cooling, I saw this heatsink I planned to use before, maybe it can hold 2 boards, has a T profile in. Maybe too big. Im not sure if it is widely available, because I couldn't find the maker.]
I've been running a pair of SB12PFC-4 (12cm 4 ohm mid-woofer in my noiseUnit speakers) for a few days playing music in my study with these, with no heatsink, to see how they fare. Haven't blown them up yet, despite the occasional complaint about the volume from my husband.

I'm wondering if perhaps given that transistors are cheaper than heatsinks, the solution is simply to have many output drivers and no heatsink.
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Old Yesterday, 11:41 PM   #193
Markw4 is offline Markw4
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Originally Posted by suzyj View Post
I'm wondering if perhaps given that transistors are cheaper than heatsinks, the solution is simply to have many output drivers and no heatsink.
If net efficiency stays the same, then you should produce the same amount of heat with more transistors. Where would the heat go without a heat sink?
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Old Today, 12:07 AM   #194
suzyj is offline suzyj  Australia
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Originally Posted by Markw4 View Post
If net efficiency stays the same, then you should produce the same amount of heat with more transistors. Where would the heat go without a heat sink?
Output drive current capability for this board is essentially down to output transistor gain, number of output stages and quiescent current per output stage. If I double the number of output stages I can halve the quiescent current per stage for the same output current capability. The heat associated with that quiescent current is now dissipated across twice as many devices

overall heat dissipation under load is independent of number of transistors, so by increasing the number of transistors you're decreasing the heat each one needs to dissipate.

Essentially more transistors = more heatsinking area - the transistors themselves are little heatsinks.
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Old Today, 12:10 AM   #195
globalplayer is online now globalplayer  Germany
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Originally Posted by suzyj View Post
Essentially more transistors = more heatsinking area - the transistors themselves are little heatsinks.
Won`t this raise THD?
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Old Today, 12:15 AM   #196
Markw4 is offline Markw4
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Transistor plastic cases probably don't conduct heat as well as aluminum fins, or have as much thermal mass to possibly help handle transient heat loads. Also, they would need to have sufficient airflow similar to fins. With a heat sink it could be exposed on the back of a speaker cabinet, say, something people sometimes do.

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Old Today, 12:52 AM   #197
suzyj is offline suzyj  Australia
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Yes, very good point. I think the rationalle behind using a heatsink vs using more transistors depends on what overall power level we're wanting to do. At 5W it's unclear as to what's optimum.

Regarding THD with more drivers, adding more drivers increases capacitance, putting more demands on the stage driving it. Slew rate will decrease, all other things being equal. Whether that increases THD will depend on whether it's the slew rate that's dictating THD.
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Old Today, 01:06 AM   #198
xrk971 is offline xrk971  United States
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Cascading diamond buffers - a cheap low THD 10W amp with TIP41C
Densely packed SOT223s on a metal core board bolted to a sink would work very well.
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Old Today, 04:15 AM   #199
anatech is offline anatech  Canada
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Cascading diamond buffers - a cheap low THD 10W amp with TIP41C
Hi Suzi,
Could be. But then, what does the capacitance look like on the larger devices? Forced air cooling would help, but then you've created a small vacuum cleaner. Filter bags anyone?

Using a metal cored PCB gets away from DIY a bit. You could use a bunch of TO-126 cases (or similar) lined up in two rows that would fit a square section of aluminium tube down each side. Cheap heat sink while retaining the multiple output transistors. Higher power.

-Chris
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