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Old 30th July 2006, 08:18 PM   #1
steevo is offline steevo  United Kingdom
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Default Help building Hifi preamp using TL074

I have built a using preamp using TL074 on copper strip board, but it as terrible Signal to noise ratio producing lots of what appears to be white noise with bit of hum. I used 1 of randy slone cicuits, well part of randy slone preamp circuit from his book. Has any one buitl a preamp on strip board succesfuly without these problems, can any give me any pointer on good conscruction techniquies or should i try producing a dedicated cicuit board. Are the strips on the board too close for small siganl cicuits. Any help appreciated. I Just need a basic preamp with a gain to about 10.

Thanks

Steevo
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Old 30th July 2006, 08:48 PM   #2
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I am currently building/testing a stereo preamp/amp using a OPA2134 and a LM3875 for each channel. I currently have the unit on the bench and have eliminated the hum. I had two types of hum.

The first was ground loop hum. I used a terminal strip as a common grounding (zero volt) point. I connected separate lines to it from the ground of the filter caps, each opamp ground, each LM3875 ground, each input ground, and each speaker ground. The terminal strip is connected to the safety ground via a ground loop breaker (1 bridge, 100nf, 8R).

The second type was, for lack of a better word, ambient noise (flourescent light, RF, computer switching, wall wart switching, human aura, etc.) To eliminate this, I shielded all input cables with aluminum foil and connected this to the safety ground. The final version will use shielded cable. All metal parts need to be connected to the safety ground, including heatsinks, standoffs, and even the metal plate on the bottom of the ground loop bridge. Check for loose components. The table I was working had a molded plastic top; I noticed this generated some static electricity. Placing the circuits on piece of wood helped.

I am using a Maxim DS1802 Dual Audio Taper Potentiometer with Pushbutton Control IC (DIP20) instead of traditional potentiometers for volume and balance. This is virtually noise free and less expensive (2USD + 5 cheap momentary contact PB switches). I have used both a computer with onboard sound and a portable DC CD player as input. With a nominal input of about 150mv and the volume set at near clipping (20Vrms), and some 95dB loadspeakers, I hear no noise, not even hiss between tracks. With the inputs shorted and the volume all the way up, I can hear a slight hiss when my ear is placed next to the tweeter.

By the way the power supply was built on a 100mm x 160mm stripboard. The preamp/amp was buit on a 75mm x 90mm stripboard. The opamps and DS1802 are socketed. There are 20 jumper wires on the power supply and 30 preamp/amp. Not exactly a low noise layout.
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Old 31st July 2006, 05:12 AM   #3
modder is offline modder  New Zealand
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Default TL074

I once built a parametric equaliser using TL074. It worked fine but the sound was rather naff. It's those op-amps.
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Old 31st July 2006, 01:02 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

it is important to arrange the volume control such that as volume
is reduced noise and hum is reduced. Not done in many simple
circuits, the noise and hum output is constant, not a good thing.

/sreten.
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Old 1st August 2006, 06:06 AM   #5
EC8010 is offline EC8010  United Kingdom
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If you've built something on stripboard and it doesn't hum, then transferring it to a PCB is unlikely to improve it by very much. Hiss is down to the active device used and circuit impedances, not layout. The TL074 would not be anyone's first choice today for a low-noise application; OPA2134 is a much better bet.
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