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Old 31st January 2007, 06:42 AM   #1
bemoth is offline bemoth  Sweden
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Default NooB questions! (chill-amp)

Hi
This is my firs post and my first build, and the amp i am planing to build is feddes Chill-Amp ( http://jwg.student.utwente.nl/fedde/chill-amp.html )
So time to spill those stupid noob questions: Im not planing on using a pot bacaus its gona work as a power amp in my system, so i guess that i shuld use the 10K resistor thats in the datasheet but its now this question takes a tur for te worse wtf does it mean when(in the schematic) you have a arow pointing to the center of the resistor? should i have one vire from in to cap and from that wire one to the resistor then to ground?
oh and btw, the caps on pos sup and neg sup, do the have to dc caps? (i can get all the caps for free but non of them are dc)

//Learning diyer
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Old 31st January 2007, 09:36 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
omit the passive attenuator in front of the power amp.

Just connect the C1 & C2 junction to the input RCA centre pin.
Connect the isolated RCA barrel (ground/return) to the S-GND beside the P-GND using the screen of the input lead.

By the way, I see the input impedance is set to 22k (R1). You can get this amp to work well down to low frequency with just 4.7uF of DC blocking capacitor.

Is your source/pre-amp fitted with a DC blocking capacitor? If yes then you only need blocking at one location. If both are fitted then one has to calculate the effective series capacitance to find the frequency that DC block will start to roll-off at.

I also see no DC blocking capacitor on the Negative Feedback (NFB) lower leg. This amplifier will generate a large DC output offset due to mixing DC and AC coupling in these two locations (Input & NFB).
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Old 31st January 2007, 11:02 AM   #3
bemoth is offline bemoth  Sweden
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"By the way, I see the input impedance is set to 22k (R1). You can get this amp to work well down to low frequency with just 4.7uF of DC blocking capacitor."

do you mean to remove tke 22k and just go with the DC vlock caps? and the dc blocking caps, can it be anny electorlytic caps?

"Is your source/pre-amp fitted with a DC blocking capacitor? If yes then you only need blocking at one location. If both are fitted then one has to calculate the effective series capacitance to find the frequency that DC block will start to roll-off at"

I dont know, but i can check the schematic when i get home...or someone maby knows, Its an Sonab R4000.2

"I also see no DC blocking capacitor on the Negative Feedback (NFB) lower leg. This amplifier will generate a large DC output offset due to mixing DC and AC coupling in these two locations (Input & NFB)."

Should i put a capacitator there? if yes, what value should that capacitaror be?

Thanks for tha awnser so far
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Old 31st January 2007, 03:22 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
keep the 22k at the input. It sets the amp input inpedance and also allows the input bias current a route to ground. It is this bias current times Zin that generates the DC output offset voltage.

If your Sonab has a DC blocking cap then you can omit the DC block on the power amp. ,even better would be two inputs. One through the DC blocker and one direct to the RF filter. Two pins @ 0.1inch centre can be shorted out with a removable plug to bypass the cap. But it's there for other sources.

I do not believe in mixed AC & DC coupling but many chipampers do this.
I will leave it to them to advise you.

If your Sonab has a servo to minimise DC output offset and no DC blocking cap then you can omit the DC block at both the input and at the NFB lower leg and the resistances seen by the two inputs(inverting and non-inverting) will be nearly equal and thus your output offset will be near zero. This would be your ideal situation.
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Old 31st January 2007, 04:34 PM   #5
bemoth is offline bemoth  Sweden
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ok, now it turns out that i cant read schematics as good as i taught i am putting a link that goes the carlsonplanet foum, a few posts down is a schematic, can someone look at it and tell me it has dc blocking caps or a one of dose servos?

http://www.carlssonplanet.com/forum/...980065b08aac47
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Old 31st January 2007, 04:41 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
the schematic is a Sonab Tuner.
Is that what you have as a source?
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Old 31st January 2007, 05:39 PM   #7
bemoth is offline bemoth  Sweden
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i know, its a reciver, but it can run as a pre-amp
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Old 31st January 2007, 07:37 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
I just wanted to be sure that I was reading the correct schematic.

the output impedance looks like it is a 4.7k resistor and the DC blocking cap looks like it says 1uF.
This is designed for feeding ultra short cables and an amp with Zin>=50k. Notice the following power amp has r703? set to 47k.

It will be a pretty poor match for your chipamp with normal length interconnects.
The treble will be rolled off very early, the bass will be rolled off early and the filters will be affected by the position of the balance control.
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Old 31st January 2007, 07:50 PM   #9
bemoth is offline bemoth  Sweden
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Hmm... that dont sound good but i realy dont want to have the amp as anything other then a power amp, i only lissen to vinyl records so if i make as an "integrated" amp i have to make a riaa to
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Old 31st January 2007, 08:01 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
if you really like the phono and tuner then consider adding a buffer after the volume control. The mono/stereo switch may be a suitable tapping point.
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