Post by thread:[EE]: Inter-stage audio coupling

Hi: First, understand that my knowledge of matters digital is considerably greater than my knowledge of matters analog, so the following may be an incredibly dumb question. However, here goes. I am using a Dallas Semi digital pot (DS1801) to drive a LM386 audio amp. Generally it works quite well. The data sheets seem to suggest coupling the two stages (as well as input and output) with capacitors that have values of at least a few mf in order to improve frequency response (I'm not dealing with high quality audio here... more like the quality you would get from an AM radio). When I do this the entire circuit works quite well except that when I first turn it on, there is a delay (I presume as the capacitors charge) before any audio appears. The greater the value of the capacitors the longer the delay (it can be as long as a second or two). When I use .1 mf caps, the delay goes away, but the frequency response, as expected, is not as good as with higher value caps. Is there an easy way around this, or is this just something I should live with? Thanks, John Hansen -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

No, you shouldn't have to live with it. You're right, it is an interstage cap charging up, but that's where your analog circuit design is failing. Not sure with the info you have given exactly where the problem is without seeing the circuit. Obviously it is a bias voltage in one of the stages that's the problem (going linear). Impedance matching or buffering a stage may correct the problem. Sometimes an electrolytic in backwards will cause this problem. You state that the circuit functions well without the coupling caps. Just leave them out if the audio is clear and undistorted. Let's see the circuit if at all possible. Rick John Hansen wrote: {Quote hidden}> Hi: > > First, understand that my knowledge of matters digital is considerably > greater than my knowledge of matters analog, so the following may be an > incredibly dumb question. However, here goes. I am using a Dallas Semi > digital pot (DS1801) to drive a LM386 audio amp. Generally it works quite > well. The data sheets seem to suggest coupling the two stages (as well as > input and output) with capacitors that have values of at least a few mf in > order to improve frequency response (I'm not dealing with high quality > audio here... more like the quality you would get from an AM radio). When > I do this the entire circuit works quite well except that when I first turn > it on, there is a delay (I presume as the capacitors charge) before any > audio appears. The greater the value of the capacitors the longer the delay > (it can be as long as a second or two). When I use .1 mf caps, the delay > goes away, but the frequency response, as expected, is not as good as with > higher value caps. Is there an easy way around this, or is this just > something I should live with? > > Thanks, > > John Hansen > > -- > http://www.piclist.com hint: To leave the PICList > .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu

I'd first look at the circuit to see if you need the capacitors at all. These are typically DC Blocking or coupling capacitors. If there's no DC, or if both sides of the circuit are at the same DC voltage (for example, an op amp output and the input of the next stage both being at Vcc/2), no capacitor is needed. Finally, if you do need a capacitor, the low frequency cutoff will be 1/(2*pi*R*C) where R is the resistance "seen" by the capacitor (likely the series combination of the output resistance of the previous stage and the input resistance of the next stage). You can choose the capacitor value to give the required low frequency response while still giving a reasonable bias stabalization time. Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com Reach broadcasters, engineers, manufacturers, compliance labs, and attorneys. Advertise at http://www.hallikainen.com/FccRules/ . ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/web/. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads