Amplificator simetric simplu(darlington).

[Vizitator]

Un amplificator BUN nu are voie sa oscileze in nicio situatie, cu sau fara sarcina rezistiva, capacitiva, reactiva sau cu scurt pe iesire (evident fara semnal de intrare).

 

Fiecare regleaza cum doreste pina la urma...

Cu 12mA, insa 50mV tensiune de offset mi se pare mult...

 Stii ce imi place, conteaza o virgula la THD, dar 12mA diferenta bias nu ar conta...

Uff...

 

Sa vezi amplificatoare care au si 100mV offset, evident nu full simetrice sau chiar full simetrice decalibrate (fara dc servo)...

 

Daca as fi facut cite o poza la fiecare jucarie reparata care avea initial peste 50mV offset, pina sa fac tot ca la carte, eram bogat acum...

[Marian]

Leco esti putin incorect pentru ca eu n-am zis ca 12mA nu ar conta, ci am zis ca 50mV e cam mult, si deci prin deductie logica, cu cat offset-ul este mai mic, cu atat eroarea aia este mai mica, iar eu asta am zis, si anume ca este de dorit offset mic, si nu este obligatoriu sa ai o schema simetrica pentru offset mic.

[Emil 1986]

conteaza daca este+50 sau-50mv?

[Vizitator]

Evident ca conteaza, in mod special la un amplificator full simetric care nu are iesirea ptin condensator electrolitic.

Sarcina este cuplata direct catre masa si iesire, si orice offset pozitiv sau negativ, va aparea pe aceasta, evident pe acolo va circula un curent electric ce va fi mai mare cu cit offsetul este mai mare.

Orice tensiune de offset existenta la iesire, va denatura reglajul de bias daca avem sarcina cuplata la iesire.

Tocmai de aia in manualele service de la scule care se respecta, cind citesti cum se ajusteaza scrie ca initial se face intii reglajul de offset cit mai aproape de 0mV (ideal dar aproape imposibil de obtinut), si apoi se regleaza biasul fara sarcina la iesire si cu intrarea data la masa (fara semnal pe intrare)..

 

Cine are chef sa citeasca ce am pus mai jos, omul vorbeste de scule care aveau si 700mV offset, dupa ani de utilizare...

 

Lecturare placuta.

 

I don't know why, but over the last few years there seems to be a race on among the manufacturers of audio in the low and medium class. They all seem hell- bent on finding the absolutely cheapest and worst trim potentiometers on the planet. When satisfied they got the worst possible, they happily solder them inside their products.

Far too many products I have seen over the last few years have had their trim pots degrade to unbelievably low levels of operation after a year or two of service. Being generally quite open, they are highly susceptible to common house dust, which slowly settles on them and degrades the initially not too good a contact, and your possibly being a smoker certainly doesn't help. The result is anything from deteriorated to damn poor performance from otherwise solid units.

To appreciate this, we have to look where such components are used. By and large, they are used in power amplifiers, mostly for two reasons. One is used to adjust DC offset in cases when the "maximum savings!!!" light was flashing and some wise guy decided a proper DC servo was far too expensive, adding all of $3 to the factory price. The other is routinely used to adjust the bias or quiescent current - the amount of current used by output stage transistors so as to keep them "open" or conducting even under no signal conditions.

Let's investigate this just a little further. When an amplifier delivers DC to a speaker, the speaker cannot do anything with that DC but turn it into pure and simple heat - how do you turn 0 Hz into sound? If this DC component of the signal is low, it will be effectively insignificant, of no practical consequence. However, if not so low, it will do two things. One is to produce more and more heat in the speakers, which is bad for the speakers as it heats them up and causes them to change their operating parameters, and the second is to effectively blur the sound you hear, since speaker cones are busy heating up instead of reproducing sound.

 

 

Sealed multiturn trim pot on the left, commercial junk pot on the right

So, to sum up. Cheap trim pots tend to have deteriorating contacts, offsetting amplifiers and degrading their original performance. They should be exchanged for better quality ones, definitely of the multiturn type, which are both sealed and allow for very fine adjustment. Just exchanging the standard poor quality pots for better ones will make your amp sound better - in reality, sound like it used to when it was new, after which it slowly and imperceptibly degraded due to dirt accumulation in the trim pots (so in fact, you'll be simply restoring it to its original state). They will allow you precise adjustment, which is especially important regarding DC offset and balancing the quiescent current on both channels.

Practice

 

Determine the values of the trim pots, they are bound to have them written on them somewhere. If not, if they are that bad, take your multimeter and measure them, extreme point to extreme point - no trim pot under this sun is smarter than you are. Then go out and buy the required pots - I suggest the type with the adjustment screw on top, for easy access. They come in 12, 25 and 40 turn minimum to maximum varieties, but I use only 40 turn types, nothing beats them for precision of adjustment. Also, they typically come in 10% and 5% tolerances, so you have a wide choice.

Set your amp on CD as source, all other functions on "off", turn the volume pot to minimum and set speakers A (if you have more than one set of speaker connections) to "on". Then, take some readings. Insert red multimeter cable into red speaker binding post, black into black, enable speakers and make a note of the DC value, which should be below 50 mV, but don't be surprised if you see something like 800 mV or even more. Bad trim pots do that. Repeat procedure for the other cannel, note the imbalance between them. Bad trim pots do that too.

Then, place one multimeter wire on one and the other on the other side of an emitter resistor and read the DC voltage drop across is referenced to service manual specfications. Make a note of it, initially and re-adjust if needed. Repeat the procedure for the second channel.

No load on output and volume pot set to minimum.

 

 

H/K 6550 after exchange - note small blue squares.

Again, carefully inspect the places against a strong background light, lightly brush with a natural hair brush (no plastics, plastics produces strong static!) and if necessary (and it will be!) gently scrape off stray solder. You don't want a short circuit, at this point it could be terminal for your amp. Then, close up the bottom, turn the amp the proper way, plug it into a wall outlet and switch it on, never for a second letting your finger leave the power switch. If no smoke appears, if no heat source becomes apparent and if you lightly touch the heat sinks and discover after about a minute that no point is beginning to heat up quickly, then and only then you can start breathing again.

Adjustment procedure

Usually, all measurements are made using CD as selected source on the source selector, volume pot at minimum, balance on center, all tape monitors and other functions on "off", no load, no input. Trim pots should ideally be adjusted using special plastic adjustments rods, available from usual electronics shops at very small prices. If not, make sure the screwdriver you are using is well insulated, but I strongly urge you to use the special rods, which you can buy when buying trim pots. They shouldn't cost more than say $1-2 per set of four or five.

Insert red multimeter wire to red speaker output of one channel, black into black. Adjust multimeter for DC, 2V range. Read it - if it shows some ridiculous value, switch down to smaller range, like say 200 mV. Change nothing, but make a note of the reading. Repeat for the other channel.

Then, put one wire on one side and the other on the other side of an emitter resistor, noting its value. Say it is 0.22 Ohms and say you have a voltmeter reading of 10 mV. The quiescent current is obtained by dividing the voltage with the resistance, i.e. 0.01:0.22=45.45 mA. Make a note of it, and then repeat the procedure for the other channel. Compare this with the initial values, just for your info. Do not be surprised at the differences, as the new trim pots are in an arguably "neutral" position and no attempt has been made to adjust anything yet.

The first thing you want to adjust is the quiescent current. In my experience, going below 10 mA per transistor is being too low, and above 15 mA very little, if anything, improves. This gives you a narrow window, but then again, it's very likely that your initial per transistor value is something like 10-20 mA for Japanese units, 40- 60 mA for European units, with an odd unit or two at around 100 mA.

For a start, adjust the quiescent current for 50 mA on both channels after set offset as much as close to 0 mV. Turn the amp off, connect a source, say CD player to it, connect the speakers, put in a CD with music, switch the amp on and let the CD play. Listen very carefully to it, it should be one of your favorites, one that you know well. Listen for timing, for tempo, for bass and above all, for ambience. Play the CD for no less than one half hour, preferably a whole hour.

Then, turn the amp off if you have only one set of speaker binding posts, or turn on Speakers B, and measure the DC offset. Whatever it may be, if it is not zero plus or minus 10 mV, you need to adjust it. Slowly turn the pot on the given channel one way or another and remember that it takes several seconds for the circuit to stabilize. Turn the required way until you have as near to zero as possible. Repeat the same procedure for the other channel.

 

I did all of the above on my two resident amps, both by Harman/Kardon, one an older model 6550, the other my newer 680. The third, Yamaha's AX-592 will also be coming along soon, I expect.

Initially, after 6 years of service, my 6550 showed a DC offset of 826/234 mV on L/R channels, both extremely high values, totally unacceptable. New trim pots cut this down to 0 +/-1 mV and the amp sounded much clearer than before - actually, it was restored to its "as new" condition. This is a single ended design, using a pair of Toshiba 2SC3281/2SA1302 power transistors per channel; its factory set quiescent current was originally 55 mA. As things stand, it is running on 136 mA, it runs hotter than before yet still only just warm to outer case touch, but it sounds better than it ever sounded before. SE stages have a reputation for best timing anyway, and at 2.5 times its original current, this is a lightning fast amp now, limited only in its absolute power. But it drives the not-easy-to-drive ARs with aplomb.

The 680 showed an initial DC offset of 136/171 mA on L/R channel; this is now also 0 +/- 1 mV. Its original quiescent current was set at 100 mA per transistor, for a total of 200 mA per channel.

[Blacksmith]

Dan, biasul se regleaza cu intrarea la masa nu cu muzica. Dupa ce ai ascultat 20 minute, nu in blana, asa acceptabil, sa se incalzeasca un pic radiatorul.

[Emil 1986]

reglezi prima data la rece!asculti muzica sa se incalzeasca rad si refaci reglajul si gata!

[zal]

Reglaţi la 25*C ca să nu aveţi surprize iarna.

....................

+/-50mV e absolut acceptabil ptr orice amplif. La acesta se putea reduce şi mai mult deriva prin sortarea trz de la intrare sau introducerea unui reglaj în diferenţiale. Despre reglajul deschiderii trz finali nu mă pronunţ, eu am alte păreri şi altă metodă.

Editat Februarie 27, 2016 de zal

[turcu_dan]

Merge si modulul 2. Am facut reglajul cu intrarea pusa la masa dupa ce am ascultat muzica cam 30 min, undeva la 1/4 din putere. Radiatoarele erau la 27 de grade, am reglat biasul la 13.5mV. Am lasat ampul sa mearga 1 ora la volum mic, si cand m-am intors am fost surprins cand am constatat ca biasul a scazut la 10mV la ambele module. Este normal? Am mai masurat tensiunea pe iesire, sau offsetul, tot cu intrarea pusa la masa, la un modul am 4.7mV iar la celalalt am 15mV.

Se pare ca functioneaza totul bine. Daca e sa fac o comparatie intre acest amplificator si celelalt pe care l detin, un amp cu Lm3886, pot zice ca acesta este mai puternic ........si pe langa putere mai e un avantaj, nu mai am nevoie de corector de ton....

Si mai e ceva, unul din module functioneaza mai rece ca celalalt, cred ca este acela la care am offset 15mV la iesire. Tensiunea retelei a mai scazut acum spre seara, am 38-39V la intrare in module.

Editat Februarie 27, 2016 de turcu_dan

[Blacksmith]

Bravo !

 

Nu cred ca se incalzea mai mult ala care avea offset 15mV. Poate bias daca avea mai mult.

Cu superdioda asta simpla e normal sa creasca sau sa scada nitel biasul in functie de temperatura radiatorului.

 

Pai si ce, la ala cu LM3886 aveai nevoie de corector ? Nu cred. La mine amandoua se aud la fel de bine. Ma rog, cine stie ce schema ai facut tu...

Editat Februarie 27, 2016 de Dudikoff

[turcu_dan]

Da, ma bucur ca a mers din prima, avand in vedere ca n-am mai lipit SMD-uri. Totusi de mare ajutor a fost si cablajul realizat de fabrica, altfel... Cum s-ar impaca amplificatorul cu o carcasa de lemn? Binenteles cu radiatoarele montate lateral pe laturile cutiei, aripioarele sa fie inafara carcasei. Felicitari si tie @Dudikoff, esti meserias.... ai facut o treaba buna....incepand de la schema, pana la proiectarea PCB-ului, incepatorii au invatat ceva din constructia acestui amp.

[Blacksmith]

In privinta cutiei de lemn eu nustiu ce sa zic. Am auzit ca mai fac unii... chiar si din plastic. Dar eu nu as face din altceva decat din metal, ca sa fie ecranata.

[maxente]

@turcu_dan bravo, sa te bucuri de el ! 

 Legat de partea cu corectorul ala de ton, unde simteai mai mult sa accentuezi, pe bas sau pe inalte ?

[turcu_dan]

  Simteam nevoia de mai mult bas si inalte la volum mic de 3-4W, insa la volum mai mare tebuia sa reduc inaltele si basul...... Suna bine si LM-ul, dar acesta are o rezerva de putere mai mare......

[Emil 1986]

asculta 1-2 saptamani si pe urma da-ti cu parerea!asa la prima auditie te poti pacali!

[zal]

Bravo! Încă unul realizat, cine urmează? Hai că nu e greu.