Hello All,
I'm hoping someone can give me a bit of advice regarding my first DIY amp project
I've made a couple of cmoy before now and have been fairly pleased with myself so decided to move on to an amplifier to drive some speakers on my desk.
For this, i copied the datasheet example application circuit for single supply (powered by a 24v wall brick) and i've been fairly happy with the results, its small, has a mute switch and provides a perfectly suitable amplification for my little desk speakers, i added an RCA connection so i can play music from my phone or laptop.
schematic at the bottom of page 2:
http://www.st.com/content/ccc/resource/technical/document/datasheet/20/6b/75/ae/1b/fa/48/cb/CD00000145.pdf/files/CD00000145.pdf/jcr:content/translations/en.CD00000145.pdf
However yesterday i decided i would try and be a bit clever and overcome one small problem which is that when the input cable is not connected, the amp gives out some terrible noise out to my speakers, i guess that this is noise being picked up on the cable.
Now i didn't have this problem with my cmoy amps so i can only assume this is because the cmoy circuit features a resistor (100k) between the input cap (100nf) and ground to form a HPF at the input with a cutoff frequency around 17hz.
My TDA7265 circuit has the 1uF input caps C3 and C4 so i decided to add a 100k resistor to ground after the cap to form a HPF with a cutoff of 1.7hz in the hope that it would also suppress any noise coming in from the cable, acting like a pull-down resistor.
but now i got nothing, no sound coming through the amp whatsoever, so i guess the resistors did their job but perhaps too well.
Can someone help me out and explain where i am going wrong and what i can do about it
I'm hoping someone can give me a bit of advice regarding my first DIY amp project
I've made a couple of cmoy before now and have been fairly pleased with myself so decided to move on to an amplifier to drive some speakers on my desk.
For this, i copied the datasheet example application circuit for single supply (powered by a 24v wall brick) and i've been fairly happy with the results, its small, has a mute switch and provides a perfectly suitable amplification for my little desk speakers, i added an RCA connection so i can play music from my phone or laptop.
schematic at the bottom of page 2:
http://www.st.com/content/ccc/resource/technical/document/datasheet/20/6b/75/ae/1b/fa/48/cb/CD00000145.pdf/files/CD00000145.pdf/jcr:content/translations/en.CD00000145.pdf
However yesterday i decided i would try and be a bit clever and overcome one small problem which is that when the input cable is not connected, the amp gives out some terrible noise out to my speakers, i guess that this is noise being picked up on the cable.
Now i didn't have this problem with my cmoy amps so i can only assume this is because the cmoy circuit features a resistor (100k) between the input cap (100nf) and ground to form a HPF at the input with a cutoff frequency around 17hz.
My TDA7265 circuit has the 1uF input caps C3 and C4 so i decided to add a 100k resistor to ground after the cap to form a HPF with a cutoff of 1.7hz in the hope that it would also suppress any noise coming in from the cable, acting like a pull-down resistor.
but now i got nothing, no sound coming through the amp whatsoever, so i guess the resistors did their job but perhaps too well.
Can someone help me out and explain where i am going wrong and what i can do about it
You need to remove those resistors you added, you are probably upsetting the bias voltage present on the input stage.
The data sheet is not that detailed but in general on a single supply you can expect the internal bias network to bias the input circuitry to half of the supply voltage. Your resistors have upset this and probably forced the amplifier into shutdown.
You should not be running the amplifier with the input cables disconnected from the source.
You can place a pair of 10K - 100K resistors before the caps (at the input jacks) if you wish. This will provide a dc path for the input caps, and lower input impedance if desired.
The data sheet is not that detailed but in general on a single supply you can expect the internal bias network to bias the input circuitry to half of the supply voltage. Your resistors have upset this and probably forced the amplifier into shutdown.
You should not be running the amplifier with the input cables disconnected from the source.
You can place a pair of 10K - 100K resistors before the caps (at the input jacks) if you wish. This will provide a dc path for the input caps, and lower input impedance if desired.
Thanks for the advice!
i removed the 100k resistors and put them before the cap as suggested. The amp is now working fine but still with the irritation of picking up noise when not plugged into the source. it's something i can live with but id love to find a proper solution.
How is this achieved in commercial products?
i removed the 100k resistors and put them before the cap as suggested. The amp is now working fine but still with the irritation of picking up noise when not plugged into the source. it's something i can live with but id love to find a proper solution.
How is this achieved in commercial products?
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