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Old 7th May 2012, 01:39 AM   #51
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Oh dear. I forgot to mention a couple of things with that point to point.

If you use 60/40 solder, the amp may self destruct in approximately a year (sooner or later). You'll need to paint contact points with Gel Flux every time and use an odd numbered electronics solder that doesn't turn dull when re-heated or jostled. The deal is that soldering point to point stuff that's already soldered (multiple connections) needs good electronics solder for firm, lasting connections (not 60/40).

Aluminum wire between transformer and rectifier is crucial for low offset, since copper wire near the transformer will make confounding offset discrepancies in center tap and dual secondaries transformers.

Generally, for non-inverting chip amplifier, a 10k input load resistor (range 12k to 10k or smaller value) is excellent for extra low DC offset every time, regardless of the feedback resistor value. Although not great for everything, a metal film resistor is great for input load.

NFB cap size is about 220uF to 330uF (when feedback shunt resistor is from 2.2k to 2.7k range--for example a gain setting with 62k/2.7k or any nearby values). However, if you had used a datasheet example with 680R (or 1k) for feedback shunt resistor, then factor UP how much inconveniently bigger your NFB cap will need to be so that extra low bass can pass. The size of this component regulates the low bass versus mid bass balance. You can choose warm or cold bass or a nice balance of both.

I mentioned the NFB cap size because I assume you're pushing some big woofers with that Quad Parallel TDA7293, the on-chip limiter cuts biggest signals first and that datasheet has illustrated a bass blocker in ST's erroneous schematics. I think you'll want to adjust that. Unlike the datasheet, the chip itself is capable of excellent clear powerful bass.
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Old 7th May 2012, 01:45 PM   #52
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Quote:
Originally Posted by danielwritesbac View Post
Aluminum wire between transformer and rectifier is crucial for low offset, since copper wire near the transformer will make confounding offset discrepancies in center tap and dual secondaries transformers.
Daniel
I have never heard this before, what attributes does the aluminum wire bring to this.
I would think aluminum wire would be very difficult to attach, cause a severe reliability loss, and possibly be even dangerous.
If some resistance is desired I would think that there are other possibilities or materials including smaller gauge copper.

Thanks
Antonio
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Old 7th May 2012, 02:22 PM   #53
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Quote:
Originally Posted by magnoman View Post
Daniel
I have never heard this before, what attributes does the aluminum wire bring to this.
I would think aluminum wire would be very difficult to attach, cause a severe reliability loss, and possibly be even dangerous.
If some resistance is desired I would think that there are other possibilities or materials including smaller gauge copper.
Thanks
Antonio
I should have said alloy. Fortunately, a nice alloy wire comes free/attached with most transformers. Other transformers have connection tabs at the top and copper wire is okay there (just not draped/tangled near the transformer coil's field).

But, if one were to alter a center tap to a dual secondaries by cutting the link and attaching wires, there could be a ~1v discrepancy that leads to DC offset. This will happen with a single bobbin transformer. Usually there's enough of the factory provided cable to get the job done. But if somebody threw that cable out, it would be a mistake.

As you can see, the problem does not happen often. Most people who need a dual secondaries transformer will simply buy it. And then, no problem.
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Old 7th May 2012, 04:31 PM   #54
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Don't believe the bits of Daniel's posts that you can understand. And ignore the bits you can't understand.
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Old 8th May 2012, 03:35 AM   #55
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. . . And ignore the bits you can't understand.
What is the correct name of that all silver color alloy wire that comes attached as the lead wires for transfomers?
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Old 8th May 2012, 09:29 AM   #56
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It could be tin (Sn) coating, or it could be solder (Sn+Pb) coating, or it could be leadless solder coating (lot's of formulations).

The wire should be copper in all normal applications.
Aluminium could be used where low weight is critical.
Silver could be used where extra cost is advantageous.

Have you ever looked further up the wire to see what is coating the lead outs beyond the tinned ends?
Have you ever scraped the leadouts to find what colour is under the tinning?
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Last edited by AndrewT; 8th May 2012 at 09:33 AM.
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Old 8th May 2012, 11:09 AM   #57
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Well, I checked 3 hammond, 2 antek and 2 stancor.
Stripping the wires back, the color is the same as the tinning.
When scraping, the color under the tinning is the same as the tinning.
Cutting into the wires, the inside color is the same as the tinning.
The majority of this alloy material is not copper.
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Old 16th May 2012, 01:18 AM   #58
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Default Big Current Buffer??????

The most interesting section of this thread starts with Panson at post 12 and then Abraxalito at post 18.
I wonder if any TDA7293's have been employed as a current buffer for an external op-amp or maybe even a tube or discrete? I may be off the mark, but it looks like you can paste the TDA7293 current buffer section onto the output of your favorite headphone amp and then proceed to drive speakers with it. Has anyone made such a composite?
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Old 22nd July 2013, 07:18 AM   #59
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It has been a long time since my last post of this thread. In this summer time, I want to continue this project. I will evaluate an opam + TDA7293 buffer in a unity-gain configuration as sketched in the attached figure.
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File Type: jpg Unity Opamp + TDA7293 Output.jpg (50.0 KB, 210 views)

Last edited by panson_hk; 22nd July 2013 at 07:31 AM.
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Old 23rd July 2013, 04:14 PM   #60
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Hi Panson!

Glad to see that you are back at this project. I'm very interested in seeing if the external driver gives better performance. I assume you can still parallel several TDA7293s in this configuration?

-Charlie
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