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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 7th October 2018, 04:58 AM   #3171
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
The two pots output impedance will add up. That impedance is source to the preamp and augments the R element of its RC input filter. The more RC slows down the slew rate response and narrows the HF -3dB frequency response point. You would want the total to stay within the range of the 20K-50K recommendation for DCG3. The article's Fig.7 standard configuration that says below it "BAL = 2.5 VOL. For example: VOL = 10k log, BAL = 25k linear" should be like a 35K standalone pot for output impedance.
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Old 7th October 2018, 10:53 PM   #3172
Atom666 is offline Atom666  Canada
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Thanks Salas. I actually meant a 20K log for volume and a 50K linear with the added resistor for balance with a switch to short out the balance control until needed.
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Old 11th October 2018, 10:00 PM   #3173
Salas is offline Salas  Greece
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Set up that pots system alone first and measure its resistance from input to output when rotating so to find the max. See if it falls in the recommended range or to change pot values. The equivalent max for series type 20k-50k nominal pots is 5k-12.5k when at half point value travel.
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Old 18th October 2018, 07:09 AM   #3174
quan is offline quan  Australia
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Hi Nick, any advice re reducing noise level on the DCG3 headphone amp?. I can hear the noise on the back ground noise btw skipping cd tracks. . Thank you .
Quan
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Old 18th October 2018, 12:10 PM   #3175
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
What kind of noise with what kind of headphones? Your ATH? Some hum? Because I can't hear any hiss or hum with a gamut of headphones on my build
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Old 18th October 2018, 01:35 PM   #3176
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
In any case, if its not about some low level hum to solve but its hiss, you can reduce the gain to x2.33 with R7=750R. That's when the standard x3 is inadequately loud for your system and your headphones 115dB/V sensitivity regarding useful volume pot range and/or background noise creeping up.

Maybe its an opportunity to test with the alternative BC327-40 Q1 Q2 also. You may put female header pins for those components to can swap them with the BC560C originals and compare. The DC offset setting up procedure with VR1 must be repeated when changing Q1 Q2.
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Old 21st October 2018, 12:58 AM   #3177
quan is offline quan  Australia
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Thanks Nick, i will try the Bc327 in position of Q1 and 2. BTW with the DTSB psu my 325mA fuses popped twice on start up already. The voltage mseasured steady at 18V . May have to raised the value a little bit.
Quan
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Old 21st October 2018, 01:18 AM   #3178
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
At T350mA (time lag type) they usually don't, but you may put T400mA ones also

Like those sold in Australia: https://au.element14.com/littelfuse/...0ma/dp/1354601
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Old 21st October 2018, 07:31 AM   #3179
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Ok , thanks Nick.
Quan
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Old 25th October 2018, 04:39 PM   #3180
Johnny Canuck is offline Johnny Canuck  Canada
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Default 0.25W Resistors

Just doing some final checks before powering up my DCG3 build, and found that I may have made a mistake - can someone please advise?
I have used Takman REY metal film, but through lack of attention have installed 0.25W resistors in about 50% of the positions, rather that 0.5W, which is probably closer in characteristics to the CMF55 recommended in the BOM.
R3 is a 1W resistor.
Question: is it okay to leave the 0.25W pieces in the circuit, or, where is it better to have 0.5W (like R6 perhaps)?
I expect to bias at about 110-120 mA, and gain of about 3.4x.

-Keith-
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