RE: [Elecraft] KX1 speaker output with superimposed oscillation
Hi Bob, Finally, I got it !. It is related with the RC network connected between the output pin 5 and ground, required to keep the amplifier stable at high frequencies at different loads, as you mention on your previous post. I checked the RC network and I even installed a new one: same behavior. If I removed this RC network completely, the oscillation now started with lower volumes !. Using the technique try and error, the problem was solved with a capacitor of 68 nF in parallel with this RC network, that is, just from pin 5 to ground. No more high frequency oscillation with different loads !. By the way, the DC voltage at pin 5 was half the supply: 3 V. I even installed a short leads 10 uF tantalum across the supply of U4, without positive results. I don't know the reason of this behaviour, maybe, as you say on your post, is due to the high gain of this stage (46 dB). Who knows... . Many thanks Bob and have a good day. 73, Paco _ Descarga gratis la Barra de Herramientas de MSN http://www.msn.es/usuario/busqueda/barra?XAPID=2031DI=1055SU=http%3A//www.hotmail.comHL=LINKTAG1OPENINGTEXT_MSNBH ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] KX1 speaker output with superimposed oscillation
Hi Paco, Great detective work, glad to read that you tamed the beast. Hi Hi I am in the process of trying to model the LM386 circuits using LT spice ( program free from Linear Technologies web site SWCAD III) to try to understand the feedback mechanism that results in these amplifier's becoming unstable and turning into an oscillator. Many times I have had success through trial and error getting circuits to work properly but with the help of some engineer friends, I have been encouraged to try circuit analysis. It is strange that placing a capacitor at the output to ground solved your problem because it can cause a phase shift to result acting with the output resistance internal to the IC. Perhaps the phase shift placed the gain-phase margin outside of the area in a Bode plot to stabilize your amplifier These amplifiers have a problem. Before they run out of gain at high frequencies, if there is 360 degree's of phase shift from input to output of the amplifier it turns into an oscillator. 73, Bob Johansen WB2SRF On Sun, 09 Sep 2007 16:28:43 + Francisco Barros [EMAIL PROTECTED] writes: Hi Bob, Finally, I got it !. It is related with the RC network connected between the output pin 5 and ground, required to keep the amplifier stable at high frequencies at different loads, as you mention on your previous post. I checked the RC network and I even installed a new one: same behavior. If I removed this RC network completely, the oscillation now started with lower volumes !. Using the technique try and error, the problem was solved with a capacitor of 68 nF in parallel with this RC network, that is, just from pin 5 to ground. No more high frequency oscillation with different loads !. By the way, the DC voltage at pin 5 was half the supply: 3 V. I even installed a short leads 10 uF tantalum across the supply of U4, without positive results. I don't know the reason of this behaviour, maybe, as you say on your post, is due to the high gain of this stage (46 dB). Who knows... . Many thanks Bob and have a good day. 73, Paco _ Descarga gratis la Barra de Herramientas de MSN http://www.msn.es/usuario/busqueda/barra?XAPID=2031DI=1055SU=http%3A//w ww.hotmail.comHL=LINKTAG1OPENINGTEXT_MSNBH ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] KX1 speaker output with superimposed oscillation
Paco, Thanks for posting the solution to this problem. I feel sure there are a number of us who read about it with interest but did not comment. Good to know the answer to the problem!! 73 de dave ab9ca Francisco Barros wrote: Hi Bob, Finally, I got it !. It is related with the RC network connected between the output pin 5 and ground, required to keep the amplifier stable at high frequencies at different loads, as you mention on your previous post. I checked the RC network and I even installed a new one: same behavior. If I removed this RC network completely, the oscillation now started with lower volumes !. Using the technique try and error, the problem was solved with a capacitor of 68 nF in parallel with this RC network, that is, just from pin 5 to ground. No more high frequency oscillation with different loads !. By the way, the DC voltage at pin 5 was half the supply: 3 V. I even installed a short leads 10 uF tantalum across the supply of U4, without positive results. I don't know the reason of this behaviour, maybe, as you say on your post, is due to the high gain of this stage (46 dB). Who knows... . Many thanks Bob and have a good day. 73, Paco _ Descarga gratis la Barra de Herramientas de MSN http://www.msn.es/usuario/busqueda/barra?XAPID=2031DI=1055SU=http%3A//www.hotmail.comHL=LINKTAG1OPENINGTEXT_MSNBH ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] KX1 speaker output with superimposed oscillation
Hi Dave, Thanks to you. IHope Bob can tell us, with the modeling of the LM386, why this strange behaviour. From the knowledge point of view, the technique try and error is not the best way, we would like to know more... . Now, my KX1, at the LM386's speaker output, pin 5, has the original RC network (10 Ohm + 100 nF) to ground plus one 68 nF to ground also. I even tried to put the 68 nF in parallel with the original 100 nF but it did not worked. Neither with a value of 100 nF, so finally I installed the 68 nF capacitor directly to the output and ground, allways with short leads (I learned this after several years working with RF techniques :)). 73, Paco _ Grandes éxitos, superhéroes, imitaciones, cine y TV... http://es.msn.kiwee.com/ Lo mejor para tu móvil. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] KX1 speaker output with superimposed oscillation
Hi Bob, Hate to say it, but it has the same behaviour with the new value of 4.7k for R21: the audio signal shows the parasitic oscillation just on the negative peaks, even with a 10 uV RF level. In any case, many thanks !. 73, Paco _ Acepta el reto MSN Premium: Protección para tus hijos en internet. Descárgalo y pruébalo 2 meses gratis. http://join.msn.com?XAPID=1697DI=1055HL=Footer_mailsenviados_proteccioninfantil ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
[Elecraft] KX1 speaker output with superimposed oscillation
Hi Paco, Sorry to read that it did not work for you. LM386 amplifiers can do strange things when over driven. There is a lot of available gain. If the amplifier is operating properly, the Idle DC voltage with no audio signal on the output pin 5 should be half of the DC voltage measured on the power supply pin 4 if not you may have a leaky coupling capacitor on the input circuit or a bad IC internal circuit throwing off the bias. I have also found at times from building my own equipment that a de coupling capacitor with short leads is needed across the power supply pins 4 and 6 to keep it stable at high output levels especially if the impedance of the power supply is not low enough. I use a 10uF tantalum capacitor when connecting it is very important to observe the correct polarity. Also the RC network connected between the output pin 5 and ground is required to keep the amplifier stable at high frequencies at different loads. GL 73, Bob Johansen WB2SRF ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] KX1 speaker output with superimposed oscillation
Paco, That certainly sounds like the strong station is overdriving the LM386, and that the overdrive point changes depending on the load to the LM386. Try reducing the RF gain to see if the condition is improved. The LM386 characteristics are different than the LM380 AF Amp used in the K2, so the fact that your headphones and external speaker work fine with the K2 is not really a good indication. The load to the AF Amp in the KX1 changes as the AF Gain is changed, so the fact that you notice the distortion only at the max settings is not an indication of a problem. It may just be that you cannot use the RP-HT60 headphones nor your 8 ohm speaker at max AF Gain with your KX1. A 4 ohm speaker or an amplified speaker may be the best solution, although you could reduce the gain of the LM386 to prevent the strong input signals from driving it to the zero voltage rail. 73, Don W3FPR Francisco Barros wrote: Hi to all, The scope shows a superimposed high frequency oscillation just on the negative peaks of the audio signal. It produces a distorted and annoying audio when the signal peaks, specially when listening a loud station on the external speaker. This happens when the volume is close to or at maximum volume. I fails just with Panasonic RP-HT60 20 Ohm headphones and external 8 ohm speaker (both work fine with my K2). It works OK with Panasonic earbud type headphone and with an external 8 Ohm load !!. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] KX1 speaker output with superimposed oscillation
Hi Don First of all, thanks for your answer. Well, I have checked it also with a signal generator: the output signal at R3 (AF Gain) R5 junction is always a clean 2,2 Vpp 600 Hz tone, with RF levels of 10 uV till 500 uV, except at close or maximum volumen. I this case, even with a 10 uV RF level, the strange oscillation is present, just on the negative peaks, as I told on my first post. At RF levels above 500 uV the signal is clearly distorted (almost square, even at minimum volume) due to overload, I think this is normal. At levels below 10 uV, the audio level is lower also, so no problem with this artifact. It is quite strange cause it works fine with a resisitive load of 8 Ohm, but not with a 8 Ohm speaker. ?? Again, many thanks. 73, Paco _ ¿Estás pensando en cambiar de coche? Todas los modelos de serie y extras en MSN Motor. http://motor.msn.es/researchcentre/ ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
[Elecraft] KX1 speaker output with superimposed oscillation
Try changing the value of R21 from 2.7K to 4.7K I had a similar problem with load sensitive parasitic oscillation of the LM386 with a Norcal 40 When this happens it can block the receive signal because of the agc action. The (high frequency rolloff) feedback may be causing enough phase shift to cause the amplifier to oscillate. Hope this helps GL 73, Bob Johansen WB2SRF ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
[Elecraft] KX1 speaker output with superimposed oscillation
Hi to all, The scope shows a superimposed high frequency oscillation just on the negative peaks of the audio signal. It produces a distorted and annoying audio when the signal peaks, specially when listening a loud station on the external speaker. This happens when the volume is close to or at maximum volume. I fails just with Panasonic RP-HT60 20 Ohm headphones and external 8 ohm speaker (both work fine with my K2). It works OK with Panasonic earbud type headphone and with an external 8 Ohm load !!. I have re-check every component all around LM386 audio amp: values are OK, soldering fine. It doesn't matter if the KX1 is working with external power (7-12 V) or batteries. At 12 V the RX current drain is about 35 mA (minimum volume). I raises to 71 mA at maximum volume with the external 8 Ohm resistor, and 75 mA with the external 8 Ohm speaker. Hope somebody can help me. Thanks in advance. 73, Paco Barros _ Acepta el reto MSN Premium: Protección para tus hijos en internet. Descárgalo y pruébalo 2 meses gratis. http://join.msn.com?XAPID=1697DI=1055HL=Footer_mailsenviados_proteccioninfantil ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
