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QuantAsylum QA400 and QA401
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Old 27th August 2016, 08:02 AM   #2161
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by jan.didden View Post
Hi Duke,

That's why you make the feedback network from multiple identical resistors, one to ground, all the others in series. When you calculate, you will see that all thermals cancel.

Jan
I argued that case in another Thread. AKSA rubbished that low dissipation view. It still remains my opinion.
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Old 27th August 2016, 08:28 AM   #2162
jan.didden is offline jan.didden  Europe
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Has nothing to do wit low dissipation. If you have say 1k to ground and 9 x 1k in series, you have 1/10 the signal at the junction.

Now because of heating and/or signal heating modulation, since all Rs are in series, ALL get the same variations in value. Because of that, the ratio at the junction STILL remains 1/10 even when the individual Rs vary all over the place!

Jan
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Old 27th August 2016, 08:35 AM   #2163
AndrewT is offline AndrewT  Scotland
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and all see the same dissipation, so the same deltaT
Whereas using one resistor with a ten times higher value sees ten times the dissipation and a very different deltaT.

Then maybe it's my lack of eloquence that loses the point.
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Old 27th August 2016, 08:35 AM   #2164
jan.didden is offline jan.didden  Europe
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As promised some shots of the autoranger main PCB.

All SMDs are 0805 or 1206. I soldered this with no special tools, just thin solder and a fine iron point.
There's one exception: a few dual protection diodes in 3-pin SOT, see for instance D7 top right. These are a bit smaller but still easy enough for my almost 7 decades old eyes!
This is my testing prototype and the final will be slightly different but not much.

The small control PCB has also some SMDs, plus of course the buttons, on/off switch and display on the front. DIL/header connectors, the DIL28 controller and some TH resistors on the back.

And please, no comments on my soldering proficiency or lack thereof

Jan
Attached Images
File Type: jpg ar box from front.jpg (534.0 KB, 526 views)
File Type: jpg ar bench display.jpg (583.9 KB, 513 views)
File Type: jpg proto ar resized.jpg (395.2 KB, 519 views)
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Old 27th August 2016, 08:44 AM   #2165
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by jan.didden View Post
Hi Bob,

I use this: https://linearaudio.nl/silentswitcher

Actually this thing is a fall-out of my search for a very low noise, mains-independent supply that I wanted for the Autoranger. Turns out it is a great supply for lots of small-signal things like phono pre's, preamps, DACs, that sort of thing.

Jan
Thanks Jan,

This seems like a really good approach.

Best,
Bob
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Old 27th August 2016, 08:45 AM   #2166
MarkS is offline MarkS  United Kingdom
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QuantAsylum QA400 and QA401
For those interested in the oven approach there's always these items, which at those prices makes bodging a toaster seem hardly worth the effort...
T962 Reflow Oven Exhaust Fan Included Preheat Mode W/ Windowed Drawer Economical | eBay

Also, iirc, there was a Elektor project a while back about converting a small oven for smd cooking...
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Old 27th August 2016, 09:40 AM   #2167
MarkS is offline MarkS  United Kingdom
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QuantAsylum QA400 and QA401
Thanks for the photographs Jan.

I don't see any issues soldering those smd parts, there's not that many and they're not crowded together...

Fwiw, this was my first effort at smd on a parallel dac board from Ebay specifically purchased to ascertain the difficulty of manual smd soldering.

Soldering was done using a Weller WSP-80 pencil with a 1.2mm bent chisel tip, a RS mini fluxer pen and x10 magnification optics. I did initially try a Weller needle tip but found it couldn't deliver enough heat reliably at the point of contact so I went back to the 1.2mm, which I use for most pcb soldering.

Hth and encourages those who haven't been down the manual smd path
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File Type: jpg smd first.jpg (475.2 KB, 495 views)
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Old 27th August 2016, 03:05 PM   #2168
davada is offline davada  Canada
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Quote:
Originally Posted by jan.didden View Post
Has nothing to do wit low dissipation. If you have say 1k to ground and 9 x 1k in series, you have 1/10 the signal at the junction.

Now because of heating and/or signal heating modulation, since all Rs are in series, ALL get the same variations in value. Because of that, the ratio at the junction STILL remains 1/10 even when the individual Rs vary all over the place!

Jan
I have to think about this.

Is it the varying resistance that causes a rise in distortion?
I understand the thermal modulation part but is this the mechanism?

I found some 3900 ppm SMT resistors. I thought they might work similarly to a lamp but with better linearity.
The distortion was through the roof. So I guess not.
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Last edited by davada; 27th August 2016 at 03:08 PM.
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Old 27th August 2016, 03:11 PM   #2169
davada is offline davada  Canada
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Quote:
Originally Posted by jan.didden View Post
As promised some shots of the autoranger main PCB.

All SMDs are 0805 or 1206. I soldered this with no special tools, just thin solder and a fine iron point.
There's one exception: a few dual protection diodes in 3-pin SOT, see for instance D7 top right. These are a bit smaller but still easy enough for my almost 7 decades old eyes!
This is my testing prototype and the final will be slightly different but not much.

The small control PCB has also some SMDs, plus of course the buttons, on/off switch and display on the front. DIL/header connectors, the DIL28 controller and some TH resistors on the back.

And please, no comments on my soldering proficiency or lack thereof

Jan

Sorry Jan I can't resist.

I thought the soldering was excellent on the sample I got from you.
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Old 27th August 2016, 03:24 PM   #2170
davada is offline davada  Canada
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Quote:
Originally Posted by MarkS View Post
For those interested in the oven approach there's always these items, which at those prices makes bodging a toaster seem hardly worth the effort...
T962 Reflow Oven Exhaust Fan Included Preheat Mode W/ Windowed Drawer Economical | eBay

Also, iirc, there was a Elektor project a while back about converting a small oven for smd cooking...
Quite cool.

I wonder if induction would work or would that destroy the parts.
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