alistermorton Posted January 23, 2013 Share Posted January 23, 2013 10% error http://www.blue-room.org.uk/public/style_emoticons/default/rolleyes.gif No. Meter spec...± 0.8% ± 5 Digits So 11v could read between 10.86v and 11.14v But 9.9V is outside that range. Link to comment Share on other sites More sharing options...
Brian Posted January 23, 2013 Share Posted January 23, 2013 But 9.9V is outside that range.That's right. The meter SHOULD read 11.00v but it's reading 9.90v. If the error was down to meter accuracy then it would read between 10.86v and 11.14v. But it doesn't so something else is going on. Link to comment Share on other sites More sharing options...
sauce Posted January 23, 2013 Share Posted January 23, 2013 So it's just my meter which is not accurate ?Because I don't see what's wrong on a breadboard :) To return to the subject why RV2 has not effect on my mic level in the headphones (with the TL081CP) ?What could I check to test this ? Link to comment Share on other sites More sharing options...
dbuckley Posted January 23, 2013 Share Posted January 23, 2013 Getting a reading of 9.9v, instead of 11v, with that meter is correct. Is this an ohms per volt question? Shunting resistance of the meter? The suspicion here is that if you measured from VCC to 1/2 VCC then 1/2 VCC plus 1/2 VCC would not add up to VCC...? Link to comment Share on other sites More sharing options...
alistermorton Posted January 23, 2013 Share Posted January 23, 2013 But 9.9V is outside that range.That's right. The meter SHOULD read 11.00v but it's reading 9.90v. If the error was down to meter accuracy then it would read between 10.86v and 11.14v. But it doesn't so something else is going on. Ah, right - when you said Getting a reading of 9.9v, instead of 11v, with that meter is correct Can you guess why? I thought you were implying the reading of 9.9V was the fault of the meter - that's what confused me. Getting a reading of 9.9v, instead of 11v, with that meter is correct. Is this an ohms per volt question? Shunting resistance of the meter? The suspicion here is that if you measured from VCC to 1/2 VCC then 1/2 VCC plus 1/2 VCC would not add up to VCC...? I'd expect the input resistance of a DVM, even a cheap one, to be of the order of 10M so would hardly affect the 2x220k divider. If it was an Avo, with an input rsistance of 20k I'd have understood it. fx: Scratches forehead 10M in parallel with 220k is about 215k - quite a significant change. Enough to affect the mid point that much? 220:215 ratio gives Voltages of 11.12:10.88 - still not down at 9.9 Link to comment Share on other sites More sharing options...
Brian Posted January 23, 2013 Share Posted January 23, 2013 So it's just my meter which is not accurate ? No, your meter is perfectly accurate, see below... Is this an ohms per volt question? Shunting resistance of the meter? Yes. The meter has an input resistance of just 1M. This means that when measuring the voltage across the lower 220k resistor, the resistance of the meter is in parallel with this giving an effective value of 180k. Which gives the 9.9v figure. This does mean you need to be careful when measuring the voltage across any higher value resistors. To return to the subject why RV2 has not effect on my mic level in the headphones (with the TL081CP)? If you short the audio line to ground it will just leave the sidetone in the headset. What happens now? Link to comment Share on other sites More sharing options...
alistermorton Posted January 23, 2013 Share Posted January 23, 2013 Yes. The meter has an input resistance of just 1M. Ah - that's quite a bit lower than I'd have expected. Yes, 1M in parallel would upset the divider and give 9.91V near enough. Link to comment Share on other sites More sharing options...
sauce Posted January 23, 2013 Share Posted January 23, 2013 If you short the audio line to ground it will just leave the sidetone in the headset. What happens now? Sorry but I don't understand this line.Should I remove the 2k2 resistor from pin 3 to ground of the XLR connector ? Link to comment Share on other sites More sharing options...
Brian Posted January 23, 2013 Share Posted January 23, 2013 Should I remove the 2k2 resistor from pin 3 to ground of the XLR connector ?Replace it with a short circuit. The signal from the mic amp will then go to two places only... 1) Ground via R14 (1k5) and c4 (1uF)2) The headset via R16 (2k2) and RV2 The signal from pin 3 that normally goes to the headset will now be shorted out leaving just the sidetone signal. This step will reduce the number of components that might be causing problems. I wonder if the simple 2k2 is causing a problem? A Clearcom PSU terminates with a 200R, 4k7 and 10uF capacitor. Just having a 2k2 means the mic signal is attenuated onto the line much less. One day I will sit down and plug this circuit into a simulator and see what's what. Link to comment Share on other sites More sharing options...
sauce Posted January 24, 2013 Share Posted January 24, 2013 Could you draw a little diagram to show me the real audio termination ?Thank you. Link to comment Share on other sites More sharing options...
Brian Posted January 24, 2013 Share Posted January 24, 2013 Image stolen from here. Link to comment Share on other sites More sharing options...
sauce Posted January 24, 2013 Share Posted January 24, 2013 ok thanks I found it inside a pdf of clear-com products.Clear-com just use a 220R instead a 200R. Link to comment Share on other sites More sharing options...
Brian Posted January 24, 2013 Share Posted January 24, 2013 Clear-com just use a 220R instead a 200R.Close enough. I haven't done the calculations but what is obvious is that the 1k5 on the output of the mic amp is there to allow multiple packs to mix their signal's together into the termination. With the 220R termination there will be an attenuation of around 18dB and I guess that the following 'hybrid' stage is designed with this in mind. With only 2k2 of termination then the attenuation is only 4dB. Link to comment Share on other sites More sharing options...
sauce Posted January 24, 2013 Share Posted January 24, 2013 Here the results with pin 3 direct to ground : very good ! RV2 is OK and LED doesn't flash anymore when I speak (all test with TL081CP). I'm going to test with 2 intercom daisy chained. You're a great guy you know !All is working now.The sound stay perfect when I connect the 2 units with the good audio termination and LED is OK too.Just a very little issue with RV2 : when the potentiometer is at minimum I can hear my voice a little bit. If I turn a little bit clockwise my voice disappear and if I turn clockwise a bit more I can hear my voice again (louder of course). Thank you so much I am very happy. Just a last thing I have a little bit of hiss (noise I don't know I to say).What is the best way so have a good gound / earthing ?I saw inside clear-com pdf they put capacitor of 100pF between pin 1 of headset to shell of headset too. My power supply will be in a metal box so I think earth go to shell. Should I connect pin 1 of XLR to earth too ?My intercom's boxes will be in a plastic box so nothing to do more here ? There is always an issue with NE5534 so I think both are broken.I will try with new ones. Link to comment Share on other sites More sharing options...
Brian Posted January 24, 2013 Share Posted January 24, 2013 You're a great guy you know! All is working now. Glad to be able to help and excellent news. ... a very little issue with RV2 : when the potentiometer is at minimum I can hear my voice a little bit. If I turn a little bit clockwise my voice disappear and if I turn clockwise a bit more I can hear my voice again (louder of course). I can believe that. It is to do with how well the circuit around U2B rejects the local mic signal from the mixed signal on the audio line. Small variation in component value will affect it. What is the best way so have a good gound / earthing ? I saw inside clear-com pdf they put capacitor of 100pF between pin 1 of headset to shell of headset too. Good idea if you have a metal box. It is there to reject RF interference. My power supply will be in a metal box so I think earth go to shell. Should I connect pin 1 of XLR to earth too? Box to earth - Yes. Shell to earth - Yes. Pin 1 to earth - No. In these systems the ground line should not be connected to earth anywhere. Most fault with these party-line systems is down to an earth connection somewhere. A capacitor, something about 10nF, connected between pin 1 and earth will help with RF interference suppression. Link to comment Share on other sites More sharing options...
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