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rompelstilchen
Joined: Sep 09, 2013
Posts: 5
Location: bxl
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 Posted: Mon Sep 09, 2013 12:40 pm Post subject:
lm358 fm noise |
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hello
I am trying to use lm358 as a voltage follower for a R-2R DAC as input
here is my actual setup
works great but as i plug the output of the lm358 to any amplifier, i get random radio station interference
i added ad 100k from pin3 to the ground , but i still get radio interference coming in
how could i avoid this ?
thx
[edit : i saw on the web that lm358 aer super old and i was wondering if there were newer/better alternatives (with the same pinout) ] |
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analog_backlash
Joined: Sep 04, 2012
Posts: 393
Location: Aldershot, UK
Audio files: 21
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| Posted: Mon Sep 09, 2013 2:14 pm Post subject:
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Hi rompelstilchen & 
I was looking at a 4-bit R-2R DAC circuit that I built and I used an LM358 with no problems. Then I thought about it a bit and I realised that my output is a control voltage, so doesn't go into an amplifier anyway. What are you planning on using this for? Are you actually putting a digital audio signal into the DAC? If it's just to produce a CV for a VCO etc, I don't think that the radio signal will affect it (unless it's a really strong signal, maybe). This link:
http://electro-music.com/forum/topic-59359.html
deals with a similar problem, but unfortunately it didn't seem to solve it. It might help you though.
Gary |
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rompelstilchen
Joined: Sep 09, 2013
Posts: 5
Location: bxl
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| Posted: Tue Sep 10, 2013 2:49 am Post subject:
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| analog_backlash wrote: | Hi rompelstilchen &
I was looking at a 4-bit R-2R DAC circuit that I built and I used an LM358 with no problems. Then I thought about it a bit and I realised that my output is a control voltage, so doesn't go into an amplifier anyway. What are you planning on using this for? Are you actually putting a digital audio signal into the DAC? If it's just to produce a CV for a VCO etc, I don't think that the radio signal will affect it (unless it's a really strong signal, maybe). This link:
http://electro-music.com/forum/topic-59359.html
deals with a similar problem, but unfortunately it didn't seem to solve it. It might help you though.
Gary |
it is for audio (guitar stompbox) |
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analog_backlash
Joined: Sep 04, 2012
Posts: 393
Location: Aldershot, UK
Audio files: 21
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| Posted: Tue Sep 10, 2013 7:10 am Post subject:
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OK - that's making more sense now. There are lower noise op-amps around (e.g. the TL071/72/74), but that won't help (I don't think) as it just has lower internally generated noise than (say) the LM358. Have you tried any of the ideas on the link that I attached? If not, I think that would be a good starting point.
If it's going to be a foot pedal, is it going into a metal enclosure? This should help. I normally use screened cable to connect to input and output jacks - again this can help (although you might see others using twisted pairs of wires for this).
Hope that some of this helps,
Gary |
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rompelstilchen
Joined: Sep 09, 2013
Posts: 5
Location: bxl
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| Posted: Tue Sep 10, 2013 7:14 am Post subject:
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| analog_backlash wrote: | OK - that's making more sense now. There are lower noise op-amps around (e.g. the TL071/72/74), but that won't help (I don't think) as it just has lower internally generated noise than (say) the LM358. Have you tried any of the ideas on the link that I attached? If not, I think that would be a good starting point.
If it's going to be a foot pedal, is it going into a metal enclosure? This should help. I normally use screened cable to connect to input and output jacks - again this can help (although you might see others using twisted pairs of wires for this).
Hope that some of this helps,
Gary |
yeah i thought about using tl072 but i saw on some forums it was not realy linear, it is to be used as a voltage follower for an r-2r dac
maybe it will work if i add a bias voltage to the input, i'll see...
finding the right opamp is a pain in the ass, not mentioning they are not all in local store, so i would have to order them...
thx for the help |
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JovianPyx
Joined: Nov 20, 2007
Posts: 1988
Location: West Red Spot, Jupiter
Audio files: 224
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| Posted: Tue Sep 10, 2013 8:42 am Post subject:
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Since the project is a guitar stompbox, I'll assume the amplifier is a guitar amp. Such amplifiers have rather high input impedance which makes stray signal pickup easier. I'll also assume that no radio stations are picked up if you plug a guitar straight into the amplifier.
If the ground from the stompbox to the amplifier is not good, you might wind up with an antenna created by the cable. The LM358's low output impedance should (when properly grounded to the amplifier) prevent the antenna effect.
My suggestion is to check the grounding and signal connection between the stompbox and the amplifier.
_________________
FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
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rompelstilchen
Joined: Sep 09, 2013
Posts: 5
Location: bxl
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| Posted: Tue Sep 10, 2013 8:55 am Post subject:
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| JovianPyx wrote: | Since the project is a guitar stompbox, I'll assume the amplifier is a guitar amp. Such amplifiers have rather high input impedance which makes stray signal pickup easier. I'll also assume that no radio stations are picked up if you plug a guitar straight into the amplifier.
If the ground from the stompbox to the amplifier is not good, you might wind up with an antenna created by the cable. The LM358's low output impedance should (when properly grounded to the amplifier) prevent the antenna effect.
My suggestion is to check the grounding and signal connection between the stompbox and the amplifier. |
yes definetely.. i found i had the same with lm386
but all grounds are connected and i dont catch anything if the guitar is plugged directly in the amp
i also use a small mixing table that mix audio from my different computers, i also got antena signal
wicked
it mainly happen if i max the sensitivity knob on the mix table or if i use the disto channel on the amp
so it happens on extremes, ill keep digging |
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JingleJoe
Joined: Nov 10, 2011
Posts: 878
Location: Lancashire, England
Audio files: 14
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| Posted: Tue Sep 17, 2013 1:30 am Post subject:
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have you capacitively coupled the output of your noise circuit? and ground sheilded your cable?
if so, a combination of the two could be acting as a resonant LRC network, so try changing your output capacitor.
_________________
As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"
Green Dungeon Alchemist Laboratories |
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rompelstilchen
Joined: Sep 09, 2013
Posts: 5
Location: bxl
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| Posted: Tue Sep 17, 2013 5:07 am Post subject:
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| JingleJoe wrote: | have you capacitively coupled the output of your noise circuit? and ground sheilded your cable?
if so, a combination of the two could be acting as a resonant LRC network, so try changing your output capacitor. |
i shielded the cable, no more issue  |
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