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Old 20th October 2008, 05:00 PM   #5601
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Location: Recife - Brasil Northeast
Default Nephew will come from London.... in January.


Will come from a nice weather to this hot spot here.

Try to imagine...the guy was living in London since he had 7 years old...now he has 30 years old...imagine he facing the weather here.

regards,

Carlos
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Old 20th October 2008, 07:50 PM   #5602
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Default Initial power-on

Hi,

Tonight I made the initial power-on of my amp, things seems positive in a general sense (no smoke...). I'm able to adjust the bias level to about 100mA using 10ohm sense resistors instead of the fuses, but there seems to be some oscillations related to the bias level.

If I put a scope on the speker output there is a sinewave (with the input shorted) at around 16MHz, the amplitude varies with the bias level but is around 50mV with a bias level of 100mA. If I increase the bias level to around 120mA the output amplitude goes up a lot and the current across the 10ohm sense resistors goes up to around 700mA and stays there until a turn off the power.

Touching the trimpot on the bias "board" with a finger makes the output amplitude drop, so I suspect that this arrangement is not completely stable. I assume this behaviour is not normal, is it? Should I adjust some component values?

Thanks
Daniel
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Old 20th October 2008, 08:07 PM   #5603
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Default From London to Brasil...

...I'm sure he'll be looking forward to it! We've had such a poor summer I bet he'd much rather be there than here.

Although I was in Kefalonia, Greece, last year when it got up to 43-44 deg C some days. I'd never felt heat like that before. They had huge problems with bush fires - we watched the fire planes picking up sea water to put them out (fun to watch, but a bit unnerving to see fire and smoke on the hills).

Back to the DX - a quick question about volume control - just a potentiometer in series to the signal input?

Thanks,
Darren
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Old 20th October 2008, 09:01 PM   #5604
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Default Yes Forever...put a potentiometer and be happy


Dnilsson..... 50 milivolts is a very small signal... seems you are picking some nearby oscilator.... 300 microwatts of power into 16 Megahertz i think will not bother your ears or speaker, or circuits.

Check your input condenser, your feedback condenser and the miller condenser.... increase the miller condenser you have into the VAS.... observe if you have connected the zobel to ground..this is made with an external wire.

Check your scope with your amplifier out from the scope probe points and switch the power amplifier supply off.... pick a good piece of wire and connect to your scope to observe if your signal remains.

Really.... this is the first time i listen that kind of problem....well... if you understand as problem..because 300 microwatts is not a big problem inside my mind...maybe you feel unsafe with this signal you found.

This frequency can come from everywere... a lot of appliances have oscilators transmiting clock signals around you... even some instruments produces some spurious signals.

If was amplifier oscilation you would have 50 volts peak to peak into this frequency... not 50 milivolts

regards,

Carlos
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Old 20th October 2008, 09:40 PM   #5605
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Default Just to remember you Forever.... there are two most common uses to


Install potentiometers.

The first one is more often used... it "loads" the power amplifier input as the power amplifier "will see" a variable resistance to ground (if amplifiers have eyes to see... means feel).

The power amplifier has an input condenser...so...this will not bother the input circuit biasing... base will remain with the needed voltage there.... electrolitic condenser into the input will have the negative pointed to the ground...so....no problems to "put the power input to ground.... means.... AC to ground"

This sketch (scratch) signed "A" will present a stable resistance to the audio source... as the audio source will "see...or feel" a stable resistance to ground.... and high resistance to ground...so...even if you have no output coupling condenser (blocking DC) into your CD DAC output, you will not face problems.

See the first sketch... the number 1

...numberone.... this sounds good...excelent name for my next amplifier....numberone!

Dx Amplifier does not bother using this connection... the unit is stable and this does not represent any problem to the amplifier input circuitry.

regards,

Carlos
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Old 20th October 2008, 09:47 PM   #5606
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Default This other way.... of course there are other ways...


But those are the most often used..... this other way is good to the power amplifier... some of them can operate without input condenser... the trimpot will be the base resistance to ground... a fixed resistance.....BUT...... the source equipment will face something that can be a problem.... the lowest volume put the output "live" wire connected to ground as a short...this sometimes is a problem when source equipment does not have output condenser..

Also the source will "see and feel" a variable resistance to its output stage... this may be bad sometimes... when you adjust the resistance will change from the "live" wire to ground.

Of course you can use another condenser into the input....but those folks...potentiomenter input condenser and the power amplifier input condenser will be in series one respect to the other... this will need you to increase both not to have losses into the low end.

Well... this is very basic things...do not be offended with big old fat uncle Charlie..... i am just trying to help... i have no "cristal ball" or "sonar"....no "radar" to discover how much you know... you can be better than me about those things and i am writting foolishes to you....but.... for sure there are young beginners that will appreciate those informs.

See sketch number 2

regards,

Carlos
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Old 21st October 2008, 07:25 AM   #5607
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Default I am a complete beginner...

When it comes to amplifiers, dealing with mains power, etc, I know next to nothing - I've read most of this huge thread and picked up a little knowledge here and there, but in all honesty I'm simply building the circuits out of a passion for hi-fi ... which will often mean that I'll go without understanding them!

So a big thank you, uncle Charlie, for the diagrams above - I wouldn't have known to set up the volume in this way. I know my questions will be rudimentary to many on this forum, however I hope you don't mind me asking them.

All the best,
Darren
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Old 21st October 2008, 07:34 AM   #5608
Mooly is offline Mooly  United Kingdom
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Carlos,
" A bull ship---- a big boat filled with cows"

Be kind to cows, you never know when you might want to eat one.
http://images.google.co.uk/imgres?im...%3D2%26hl%3Den
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Old 21st October 2008, 08:17 AM   #5609
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Default Ahahahah.... sorry Mooly


I forgot you were into the forum.

ahahahah!

Carlos
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Old 21st October 2008, 06:54 PM   #5610
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Default Oscillations...

Hi Carlos,

I made some more measurements, first of all I'm not at all worried that the 300uW of power at 16MHz would harm anything so I don't consider that a problem. What worries me though is that the amplitude of the oscillation depends on the bias level. At 0mA bias, there is 0mV of oscillation amplitude at the amplifier output. At 100mA bias there is about 50mV (peak) of oscillation amplitude. This might be fine, but the bias level seems very hard to adjust and it depends on the temperature. In fact, the bias current increases constantly as the amplifier warms up. Once the bias current goes above 110mA (either by adjustment or by warming up) the bias current suddenly jumps to about 700mA and can't be adjusted back down. I have to turn off the amp and turn it back on for the bias current to be adjustable again. Maybe this is not related to the oscillation seen at the output, I just though it might be...

Anyway, with the amplifier off there is no noise picked up by the scope. The frequency of the oscillation also varies with the bias current level so I'm pretty sure it is not coming from a nearby clock source. Touching any of the pins of the BD139 transistor mounted on the bias servo with a screwdriver also makes the oscillation go away.

I started wondering why the bias level is not drifting and remembered something... I bought the component kits from Nordic but my kit was lacking one pair of BD139 transistors. I bought another pair and mounted in position Q8 (one transistor for each channel). So Q8 is a different brand compared to Q6, Q7, Q9, Q10 and Q13. Maybe this was not a good choice? Looking at the circuit a little closer I suspect there are some assumption maybe regarding the gain of Q8 compared to the drivers and Q13?

I checked the input capacitor again, it is 2.2uF. Which one if the feedback and which one is the miller capacitor?

Trying to attach a picture of the current state as well...

Thanks
Daniel
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