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Old 30th April 2008, 11:44 AM   #101
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Quote:
Originally posted by Peter Daniel
If we consider minimized circuit as in attachment, the DC offset will depend on input impedance and a chip itself. With 22k shunt resistor, the offset is usually below 80mV.

I measured at least 800 chips and offset variation (in that particular circuit) can be from 0mV to 120mV; the latter occuring for less than 2% of the chips.

If you have a potentiometer at the amp or use a preamp, the combined shunt resistance will be less than 22k and the offset goes further down, usually to less than 20mV.

So if you are certain that the source does not produce any DC offset (as it will be amplified by a factor of 30), you don't really need Ci.

Thanks for the info!

What do you think of the following:

Input star ground fed by 4R

NFB 27k, and then 820 ohms

Input 22k, and then input filter cap
passive pre of, 470R, and then 1M+Alpa A20K

This is a wild guess on my part, but the intention is the following advantages:
The very good volume pot is just $1
The 470r is a compensation for that input filter cap, then. . .
The input filter cap can can be practically anything seemly from the rummage box or hobby shoppe.
The gain of 34 will work for weaker modern sources.
The resistor values come in most economy variety packs like Radio Shack's #271-312A
The 1m load at the amp board acts as a slight divider upon the input cable that leads to the pot.

This may be "too much thinking" on my part, but, if it works right, then it could skip the majority of the usual costs and troubles.
What do you think? Is it "hit" or "miss"?
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Old 30th April 2008, 11:59 AM   #102
gychang is offline gychang  United States
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Quote:
Originally posted by Bluto

From there he can make the thing simple enough for us newbees to build with limited instruction. I've agreed to write up and photograph whatever he comes up as I build it if Gychang likes the idea.

I appreciate the input of true Pro's on this as it will only help Dan design an excellent beginner Amp.

Thanks - Bluto

This is great, Bluto and Dan, please lead us into the world of Gainclone for the dummies, or dummy me.

gychang
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Old 30th April 2008, 12:15 PM   #103
Nuuk is offline Nuuk  United Kingdom
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Quote:
While guys like me and Gychang are near to totally lost when we listen to You, Peter, Dan and John speak Dan still has a handle on what we're after as do you . He can understand you guys and thats whats important .
I really am keen to help so could you point to something (anything?) that I have said that has 'lost' you?
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Old 30th April 2008, 01:27 PM   #104
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Quote:
Originally posted by Nuuk
I really am keen to help so could you point to something (anything?) that I have said that has 'lost' you?
Well, my mistake, not yet corrected, is that I didn't create an itinerary. You know, like a road map. The great example that you and others have given me (post#77) leads to the idea. . . that its like some friends left for a journey, two days ago, and an itinerary is needed to catch up.

I was thinking of a web-based assembly manual, mostly photos, with very little chatter. On the web, the photos could be made click-able, which would lead to a more complete text reference with details only for the item pictured.

There's also the operation of tools. There are some tools that don't visually appear to be tools, like a jar of flux, the capacitor values chart, the light bulb test device, a ball of #0000 steel wool (cleans flux off iron), the cap drainer, the desoldering bulb, and a really bright light for the workspace. And, who would expect that tools from non-electronics fields, like hemostats, would be so useful?

I think its that, even though the materials are simple, they do visually appear to be complex.

EDIT: Here's a frequent example of a visual complexity: The chipamp.com power supply has a simple and elegant schematic. . . however, the beginner is often thoroughly confused during its assembly and hookup.
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Old 30th April 2008, 03:13 PM   #105
Nuuk is offline Nuuk  United Kingdom
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From what Daniel tells us, it seems a lot of the 'missing' information is available for the want of a quick Google! For instance, this is a great place to learn about flux etc.
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Old 30th April 2008, 03:57 PM   #106
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I think that the beginner would enjoy soldering if flux gel were painted onto the surfaces to be soldered--before picking up the soldering iron.

That makes soldering easy and fast. And, its so much better than repeated tries or roasting components.

After finishing (in record time) the mess of flux can be removed in a few seconds with alcohol and an old toothbrush.

Here is the tool:
(electronics grade gel/paste flux)
Attached Images
File Type: jpg gelflux.jpg (42.8 KB, 535 views)
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Old 30th April 2008, 05:31 PM   #107
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Default Zero offset

Referring to post 97 and post 101

After adding the input filter cap (so there's no worries about dc from the source), I measured the DC offset, which had gone up to 127mv. That's no good.

Then I realized. . .
The 50k pot and the 22k resistor (in Peter Daniel's schematic of post 97) makes a load less than 22k. Forehead slap.
So, the load on the input would actually correspond to common resistor values of either 15k or 18k, and 15k is zero dc offset (this time).

Great!

Now, I've got a power thump.
Watching the analog multimeter (on DC scale), after pulling the power plug, the needle slowly pulls down lower than zero, the woofer slowly pulls inwards, and then releases with a "thunk" noise. Ah darn.
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Old 30th April 2008, 05:38 PM   #108
Colin is offline Colin  United Kingdom
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Quote:
Originally posted by Nuuk
From what Daniel tells us, it seems a lot of the 'missing' information is available for the want of a quick Google! For instance, this is a great place to learn about flux etc.

Good page.

Babani Books have some helpful stuff. They do a whole book on the art of soldering (good bedtime reading).
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Old 30th April 2008, 06:13 PM   #109
Bluto is offline Bluto  United States
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Hi Nuuk -

You communicate excellently.

Anything that you have said that confuses me personally is only because of my lack of electronics knowledge. It's not you.

Someday, hopefully, I'll understand that language as well.

Bluto
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Old 30th April 2008, 07:51 PM   #110
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Default Now transformers

Good tips and links!

The thread is moving forward in the right way.

I ordered the parts in Farnell.com and in the meantime I got on ebay a couple of toroidal transformers as part of unfinished active speaker amps. They were advertised as 120V 18-0-18. However, the label says:

Pri: 230V 50Hz
Sec 18-0-18 White - Black- White
0V-11.5v RED-RED
33-0-33 Green- Blue- Green
3.5-0-3.5 Blue-Black- Blue
2-0-2 Yellow-Black-Yellow

I have not found answers for these questions:

Why was it advertised as 120V? Why it does not say VAC anywere. Are they in fact 230 VAC?

As you can see there are many cables coming out from this things. Will I use only the 18-0-18 ones? Can I use the low V ones to feed a switch with an on-off led.

More questions will follow after I start making the connections to the diodes, but in the meantime I will be grateful if someone can clarify the above.

Thanks

Regards

Antonio
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