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Old 13th September 2005, 10:19 AM   #1
Nordic is offline Nordic  South Africa
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Default Decibel Dungeon GC w/LM3875

Hi there

I have settled for building the BASIC ICG circuit from Decibel Dungeon.


I am now at the ordering parts stage
Ordered 6 LM3875 chips which should be here by next Tuesday.
And this is the problem I have right now, makeing sure I get the right parts.

Would someone please look over the circuit diagram and check or suggest components.
I need to be sure of;
which are critical components (which need to be best quality)
Minimum voltage rateing
Minimum wattage rateing
capasitor types (i.e. electrolitic / ceramic), preffered brands?

There is a component I'm not sure of
0.22ufd cap at bottom right, not sure which voltage /type to use

Is there a way to replace the 56k resistor with a suitable potentiometer?

P.S. I will have more questions, no doubt...
Especialy when it comes to the powersupply part, as this version is a little diffirent from other GC PSU's I have seen...

I just have to think my questions through properly, and then translate it to english...
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Old 13th September 2005, 10:45 AM   #2
evo is offline evo  England
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Hi Nordic
I think the cap is this one:
http://www.maplin.co.uk/Module.aspx?...ce=23&doy=13m9

so its probably rated at 240v- maybe someone else could double-check
The BG cap should be 4.7uf

atb
evo
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Old 13th September 2005, 11:15 AM   #3
Nordic is offline Nordic  South Africa
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I saw when going through the GC site they reference it to BX72X... is the X part important, because I see Maplin only show the BX72K one...?

Why do you suggest 4.7ufd in stead of th 47ufd in the original diagram?
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Old 13th September 2005, 12:00 PM   #4
evo is offline evo  England
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Hi Nordic,
I think they're typo's (mistakes) in his spreadsheet he lists the BG as a 4.7 and i think he's put X instead of K

http://myweb.tiscali.co.uk/nuukspot/...inclonesl.html


HTH
evo
( i would wait for someone else to confirm this though)
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Old 13th September 2005, 12:06 PM   #5
Nordic is offline Nordic  South Africa
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On the other hand isn't it supposed to balance each other out?
i.e. the 47ufd cap + 10k resistor to pin 8 VS 56K resistor between input and ground.
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Old 13th September 2005, 01:41 PM   #6
Nordic is offline Nordic  South Africa
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DOh!!!

OK found an article on the CyClone
http://www.platenspeler.com/diy/amps/uk_cyclone_1.html
Click the image to open in full size.

Quote:
The inverted configuration is said to be even better sounding, so therefore I'm very much interested in hearing an inverted clone. If we suppose a gain which is equal to the gain of my GeenKloon 20x (26dB) then the following formulas describe the amp.

Gain = (R2/R1) = 200,000/10,000=20 times, which equals 26dB.

The best value for R3 can be determined using the following formula:
R3 = (R1*R2)/(R1+R2) which equals 9.5kOhm.

In order to preserve low frequencies the value for capacitor C1 must be chosen sufficiently high. 2.2uF may be on the low side, 4.7uF is used more often.
Based on this the 4.7uf value is probably the correct one...

but the R3 = (R1*R2)/(R1+R2) rule trows me a bit, as in stead of the 9.5K the formula would suggest, the IGC uses a 216K load (the line that looks like this - - - - ) would the 9.5k resistor be better suited?
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Old 14th September 2005, 09:44 AM   #7
Nordic is offline Nordic  South Africa
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Click the image to open in full size.
Provisional layout

Bump questions regarding potentiometer, and also the resistor from pin 7's value....

Please excuse repeated posts, but I'm using this thread as an ongoing journal for my benefit and other noobs' too. So I will occasionally just post stuff here to remember as I go along.
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Old 14th September 2005, 11:45 AM   #8
Mick_F is offline Mick_F  Germany
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What's that additional line from the input to the star ground (more or less parallel to the input grounding R?
No Zobel?
No separation of signal and power ground?

Mick
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Old 14th September 2005, 11:55 AM   #9
Nordic is offline Nordic  South Africa
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Hi Mick are you talking about the black line to the RCA socket?
that connects to the negative pole, I suppose I could just do the resitor directly between the two poles as oposed to the the ground star...

Would it make a difirence haveing only 1 ground?
Aren't the seperate grounds connected anyway?
i.e. Click the image to open in full size.

Circuit for reference
Click the image to open in full size.
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Old 14th September 2005, 12:04 PM   #10
Mick_F is offline Mick_F  Germany
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It is common practice to separate signal and power ground and as far as I know it is also recommended by National in one or the other data sheet. Have, for example, a look at the LM4780 data sheet and the reference board described in it. Also, have a look at decibel dungeon concering grounding practice, the separation is discussed there.

I understand the black line now. Yes, you could to the resistor directly between the poles of the RCA socket and that point would be your signal star then. Look at Decdun.....

Mick
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