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me pasa exactamente lo mismo, me podrias ayudar con la modificicacion.
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Just saw /g/BITX20/topic/22188783#65341, with Re= 2x 2e2. Now Q72, 73 = 2N3904/6 pair, Ic max = 200mA (0.2A), so 2e2 Re is about right. Mike's 5.02V at both emitters means both transistors turned essentially off. No wonder there's distortion! uA trickle Iq, just enough to give the readings. Vb-b around 0.6V each (and the diodes) confirms. Can't find Ic/Ib transfer curve (plenty of switching stuff),? so a guess at 10mA Iq? means E=IR=2x2.2x0.01=0.044V needed there between emitters. Easy to monitor adjustment of pot, and check of resistor replacement.
Further to the tutorial: High-power amps use a NTC (Negative Temperature Coefficient) thermistor in there (can even be transistor-assisted!) strapped to either a final transistor or heatsink so over-temp on the final(s) tends to bias them off.
73 all!
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Addendum: Emitter resistors are always necessary: High-power amps can go as low as 0.1e. I'd venture the originals might go down to 1e? or is 10e necessary for shorted load? Farhan?
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Please forgive me: I'm late to the conversation, but I take it this concerns the all-transistor AF output stage. This stage has been built 'to-cost', pared down to minimum using stock common components, to a well-known circuit (used in some form by EVERY transistor power amplifier).
If more power required, consider one transistor turned fully-on, and with the output capacitor shorted. What current? Now lower the emitter resistor/s (and load R?) until maximum spec of transistor. Does that deliver enough peak power to the load? If not repeat with beefier transistors. Once satisfied, now measure voltage across emitter resistor/s with no signal (THIS is the point where we consider distortion). Consult the transistor's spec sheet for its 'transfer curve' and find the point at which Collector current (Ic) just begins to increase linearly with increasing Base current (Ib). THIS is the desired quiescent current. Now increase the Ib by increasing Vb? (the diodes do this, but not enough) until Ic is at that point by measured voltage across emitter resistor(s). VOILA!
Some have suggested an extra diode in series with the originals will do the job - has anybody found this reliably repeatable/replicable?
Other than that, the small preset pot (500e? 100e?) for setting and measure for nearest standard value of resistor.
Tutorial ends.
73 de ZL2DEX
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Vic,
I suggest a preset for better result. For fixed start with 33 ohms and go up till you get satisfactory sound quality.
For me 39 was good enough. The output transistors should not get hot in idling condition.
Raj
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At 14/02/2019, you wrote: Raj, my uBitX v.4, just assembled after being purchased in June of last year, also has the slightly distorted audio into an 8 ohm speaker and I was looking at that biasing mod. Are you suggesting a 47ohm fixed resistor is enough to address this between D14-D15?
=Vic=
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Sounds a lot better than my Icon 718. I listened to this with earphones speaker might be a problem. (my Icom sounded about like this on headphones.
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Raj, my uBitX v.4, just assembled after being purchased in June of last year, also has the slightly distorted audio into an 8 ohm speaker and I was looking at that biasing mod. Are you suggesting a 47ohm fixed resistor is enough to address this between D14-D15?
=Vic=
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Hello Raj
BFO does no affect this "whistling" in my case. I did calibration to 11.996500 after firmware update. I need only? do basic calibration routine (10Hmz) within small home made crystal oscillator .. bud I don't thing that it helps.
@ I'm sorry I don't know that you mean with? "leave only the relevant bits"
Martin, OK5MCA
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OK you solved the "crispy" distortion as I suggested. 47 Ohms preset or 100 Ohms should do.
I suspect you have not done a BFO calibrate. Change the default to 11.996500 and see if that helps you.
Raj
P.S. If you are replying my mail, please edit the trailing mail and leave only the relevant bits. Just like below
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At 05-10-18, you wrote: Well ...¡°crispy¡± problem was solved by adding 470 preset between D14 and D15 .. this is now not the issue .. but .. If I take my backup receiver JUMA RX1 I build years ago as reference (powered on external battery without antenna), it has very nice noise (buzz) without any others interference noise hearing in background (sorry but English is not my mother language so hopefully it will be understandable what I mean). If I power on my uBitx (V4) it is in background something like high frequency noise .. like light whistling ¡ silent, but still audible whistling. Can be that this ? whistling¡± is coming form any arduino operations ? or ¡ ?
Martin, OK5MCA
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?Ron... please tell me it is solvable ...
Martin, OK5MCA
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Martin, I noticed the same background noise and very likely it is a result of signals coming from the nearby digital circuitry. The frequency is around 200-300 Hz.
--Ron? ?N7FTZ?
Well ...¡°crispy¡± problem ?was solved by adding 470 preset between D14 and D15 .. this is now not the issue .. but .. If I take my backup receiver JUMA RX1 I build years ago ?as reference (powered on external battery without antenna), it has very nice noise (buzz) without any others interference noise hearing in background ?(sorry but English is not my mother language so hopefully it will be understandable what I mean). If I power on my uBitx (V4) it is in background ?something like high frequency noise .. like? light whistling ¡ silent, but still audible whistling. ?Can be that this ? whistling¡± is coming form any arduino ?operations ? or ¡ ?
Martin, OK5MCA
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Well ...¡°crispy¡± problem ?was solved by adding 470 preset between D14 and D15 .. this is now not the issue .. but .. If I take my backup receiver JUMA RX1 I build years ago ?as reference (powered on external battery without antenna), it has very nice noise (buzz) without any others interference noise hearing in background ?(sorry but English is not my mother language so hopefully it will be understandable what I mean). If I power on my uBitx (V4) it is in background ?something like high frequency noise .. like? light whistling ¡ silent, but still audible whistling. ?Can be that this ? whistling¡± is coming form any arduino ?operations ? or ¡ ?
Martin, OK5MCA
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You may be attacking the wrong problem. If I interpret the description of the audio correctly, "crispy" may be audio that has significantly suppressed lower frequencies. I encountered this problem as soon as I fired my V4 uBitx. I found out that the problem was related to the output of the 1st mixer (45 MHz) falling too close to the edge of the IF filter. In my case it was about 2500 Hz. If you are using the CEC software you can verify the symptom by adjusting the IF SHIFT frequency. After figuring out the real problem and adjusting the oscillator frequency, I am getting very good audio and much increased receiver sensitivity.
--Ron? ? ?N7FTZ?
The distortion is caused by improper biassing the final transistor pair. There will be
no distortion by taking audio from volume pot.
Raj
At 30-09-18, you wrote:
>Would taking audio from the wiper of the vol pot on the v4 not just amplify the distortion? Or will this be bypassing the audio??
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The distortion is caused by improper biassing the final transistor pair. There will be
no distortion by taking audio from volume pot.
Raj
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At 30-09-18, you wrote: Would taking audio from the wiper of the vol pot on the v4 not just amplify the distortion? Or will this be bypassing the audio?
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Would taking audio from the wiper of the vol pot on the v4 not just amplify the distortion? Or will this be bypassing the audio??
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Then of course there's using the existing circuit with a couple more diodes in the base circuit to drive power transistors Darlington-fashion with emitter resistors of 0.05 ohms - and a duplicate circuit driven anti-phase? to the 'earthy' side of the speaker. Real advanced, but totally basic - and all from the junk-pile! Go burst your eardrums with Hi-Fi at your leisure!
<sarcasm off>
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and basic audio-amp practice: measure that standing current by the voltage across either or both emitter resistors. Report your finding of the V or I for cleanest audio here and a consensus will quickly be shown that can quickly be set in any rig - maybe with that preset measured could indicate a default value that can be pre-installed (factory mod?). It may vary per speaker impedance / ouput transistor type / emitter resistors value, giving more audio to taste. Honestly, has no-one any audio experience? That basic circuit is infinitely extensible!
73 de ZL2DEX
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If this is about uBitx V4 then you need to do the audio mod. Some boards may be overly distorted!
Cut the track between D15 and D16 and solder the center and one end lead of a 100 Ohm preset.
I use a bourne10 turn.
Set the trimmer to 0 ohm and switch on. Now slowly increase preset till distortion vanishes and the
audio volume will increase. This should happen about mid point. Do not advance it too much as you
will have hot output transistors.
Raj
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At 30-09-18, you wrote: Hi
THX. Because i will use stock volume control delivered with uBitx, I will disconnect VOL_MID from volume control and connect it to IN from external amp.
Martin, OK5MCA
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Hi
THX. Because i will use stock volume control delivered with uBitx, I will disconnect VOL_MID from volume control? and connect it to IN from external amp.
Martin, OK5MCA
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V¨ªctor rodrigo
Raj vu2zap
Vic WA4THR