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 Forum index » DIY Hardware and Software
lm358 fm noise
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rompelstilchen



Joined: Sep 09, 2013
Posts: 5
Location: bxl

PostPosted: Mon Sep 09, 2013 12:40 pm    Post subject: lm358 fm noise Reply with quote  Mark this post and the followings unread

hello

I am trying to use lm358 as a voltage follower for a R-2R DAC as input

Posted Image, might have been reduced in size. Click Image to view fullscreen.

here is my actual setup

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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: 371
Location: Aldershot, UK
Audio files: 21

PostPosted: Mon Sep 09, 2013 2:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi rompelstilchen & welcome

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

PostPosted: Tue Sep 10, 2013 2:49 am    Post subject: Reply with quote  Mark this post and the followings unread

analog_backlash wrote:
Hi rompelstilchen & welcome

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



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Posts: 371
Location: Aldershot, UK
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PostPosted: Tue Sep 10, 2013 7:10 am    Post subject: Reply with quote  Mark this post and the followings unread

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

PostPosted: Tue Sep 10, 2013 7:14 am    Post subject: Reply with quote  Mark this post and the followings unread

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
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PostPosted: Tue Sep 10, 2013 8:42 am    Post subject: Reply with quote  Mark this post and the followings unread

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.

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rompelstilchen



Joined: Sep 09, 2013
Posts: 5
Location: bxl

PostPosted: Tue Sep 10, 2013 8:55 am    Post subject: Reply with quote  Mark this post and the followings unread

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: 876
Location: Lancashire, England
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PostPosted: Tue Sep 17, 2013 1:30 am    Post subject: Reply with quote  Mark this post and the followings unread

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.

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rompelstilchen



Joined: Sep 09, 2013
Posts: 5
Location: bxl

PostPosted: Tue Sep 17, 2013 5:07 am    Post subject: Reply with quote  Mark this post and the followings unread

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 Smile
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