Hi Bate Vanko,
Go back to post #39... I have already read it, a while ago... Mine say 120 high-performance audio electronics projects! ;-)
Actually I did have a look at the picture 6.21 to see how it was mounted physically. But I bought this book a while ago when I was just curious and didn't have any interest to build my own at that time. My mistake as been to trust the other book blindly.
As for your suggestion to use an already PCB, I am more the kind of guy who like doing thing myself, to the most I can. As mentionned before, I am a senior drafter and designing my own PCB is easy and fun for me.
I received my new scope, made the Blode test on every channel configuration with full success. I upgraded the FIrmaware to 00.01.01 and I am doing the self calibration at the moment. I am happy!
I will now proceed with the modifications of the VAS above and came back with results...
@nattawa,
I finished the modifications. I had to use the Dremel to cut traces. I tested the board with the input shorted to ground and +/-5 VDC on the rails. Everything looks fine there, no hot transistor. I measured around 195mV across R305 and R405 (22 Ohms), those between emitter and base of Q302 and Q402. That is 8.864 mA in the branch.
But when I raised the rails to +/-10 VDC, the group Q301, Q302, Q303 become hot progressively. I can let my finger on them but the other group Q401, Q402, Q403 stay cold compared to the former. I measured 1.5 VDC across R305! Tha's 68.182 mA. So I stopped there.
Yesterday I measured all the 2SB649 and 2SD660 that I have, the old and the new, and I noticed a 100 hfe difference between the NPN and the PNP types. Could it be this the problem? (The first row are the old from the first replacement. The second row are the old from the second replacement. Two are bad from the first replacement; 18 and 53 hfe
Here are a few photographs after the cannibal had passed by...
I finished the modifications. I had to use the Dremel to cut traces. I tested the board with the input shorted to ground and +/-5 VDC on the rails. Everything looks fine there, no hot transistor. I measured around 195mV across R305 and R405 (22 Ohms), those between emitter and base of Q302 and Q402. That is 8.864 mA in the branch.
But when I raised the rails to +/-10 VDC, the group Q301, Q302, Q303 become hot progressively. I can let my finger on them but the other group Q401, Q402, Q403 stay cold compared to the former. I measured 1.5 VDC across R305! Tha's 68.182 mA. So I stopped there.
Yesterday I measured all the 2SB649 and 2SD660 that I have, the old and the new, and I noticed a 100 hfe difference between the NPN and the PNP types. Could it be this the problem? (The first row are the old from the first replacement. The second row are the old from the second replacement. Two are bad from the first replacement; 18 and 53 hfe
Here are a few photographs after the cannibal had passed by...
Attachments
The 0.22ohm resistors are doing the drain current sampling for protection purpose, not for current sharing. The FETs are lateral type and have negative tempco, so I guess they can get away without having current sharing measures in place.
That's definitely not right. By design the Q302/301/303 control loop forbids the voltage across R305 to get greater than about 0.7V. Now that you have much greater voltage than that, either Q302 is faulty or it's got hooked up a wrong way.
Well, I didn't make any mistake in the new wiring but, after having cut the two trace of the two emitters of Q302 and Q402, I totally forgot to wire them on their respective rails voltages. So after having test them with the diode test, they were still good and I wired them.
Now that it's done, I can raise the rails to +/-15 VDC without any transistor hot. I measured 0.278 and -0.278 across R305 and R405. In fact it is measured across the rails and the base of transistors respectively. The resistors are solder under the PCB. But again, if I go to +/-20 VDC, the Q301, Q302 and Q303 start to came hot to the finger...
+/-15 VDC : 0.278 VDC and -0.278 VDC, cold
+/-16 VDC : 0.280 VDC and -0.280 VDC, cold
+/-17 VDC : 0.288 VDC and -0.290 VDC, cold
+/-18 VDC : 0.293 VDC and -0.298 VDC, cold
+/-19 VDC : 0.305 VDC and -0.305 VDC, cold
The finger on the group, I raised the voltage to +/-20 VDC and I can immediately sense the heast coming...
I should add though that since your last modification, it is the first time that the RV1 BIAS trimpot does a difference in the voltage at TP5 BIAS when I move it. But I am not sure what value I should expect at TP5. now I have 109mV while it is fully clockwise. I noticed that it is fully clokwise that I have an equilibrum in the current sourced at the two power supply.
@Sevy , That was some positive development. If your scope is up and ready, would you see any oscillation? What is the voltage across D301? On a less related matter what is the driver transistors standing current (by measuring voltage over R503/603)? What is the DC voltage at the output node?
RV1 sets the output transistor standing current. With the output transistors out of circuit, however, you can temporarily set RV1 so that the voltage across R503/603 is at 1V - 1.5V. When the output transistors are in circuit, set RV1 while monitoring the voltage across one of the 0.22ohm resistor so that the output stage standing current is 100mA or so per transistor.
I can't think of any use of TP5.
D301 4.151 VDC
D401 4.134 VDC
R503 0.005 VDC
R603 0.005 VDC
OUTPUT -0.004mVDC
If I voluntarily turn the RV1 BIAS counter clockwise I can make an oscillation at the BIAS test point TP5. 94.78 Mhz at 13.9mV peak-peak:
When I turn back RV1 BIAS to the full clockwise position, the oscillation get off:
I tried to play with a 1 kHz input signal and raise the input voltage before I get saturation at rails +/-15 VDC. I can reach 8.6695 Vrms with a 462.02mVrms at the input, a 25.467 dB gain factor.
I push it upward to see the saturation, it is very symmetrical:
Then I lowered the input to about half the maximum output, at 4.1389 Vrms and checked the maximum frequency I could reach. The distortion start around 42 kHz. 42 kHz is only 0.063 dB above 1 kHz...
D401 4.134 VDC
R503 0.005 VDC
R603 0.005 VDC
OUTPUT -0.004mVDC
If I voluntarily turn the RV1 BIAS counter clockwise I can make an oscillation at the BIAS test point TP5. 94.78 Mhz at 13.9mV peak-peak:
When I turn back RV1 BIAS to the full clockwise position, the oscillation get off:
I tried to play with a 1 kHz input signal and raise the input voltage before I get saturation at rails +/-15 VDC. I can reach 8.6695 Vrms with a 462.02mVrms at the input, a 25.467 dB gain factor.
I push it upward to see the saturation, it is very symmetrical:
Then I lowered the input to about half the maximum output, at 4.1389 Vrms and checked the maximum frequency I could reach. The distortion start around 42 kHz. 42 kHz is only 0.063 dB above 1 kHz...
That oscillation can be the next problem you may have to deal with. It's a sign of instability. It shouldn't be there, especially with the output transistors out of circuit. I suspect it has to do with the VAS transistors getting hot.
Remove C3 and see if it helps making the oscillation go away, if success, then trim VR1 to raise voltage over R503 to 1V - 1.5V
Remove C3 and see if it helps making the oscillation go away, if success, then trim VR1 to raise voltage over R503 to 1V - 1.5V
Good news...
I removed C3. When I calibrate RV1 BIAS to try to get 1.5 VDC across R503, the oscillation came back, but at +/-15VDC on the rails, the transistors stay cold on my finger.
I measured all the values at INPUT (Yellow trace), TP5 BIAS(Magenta trace) and OUTPUT (Blue trace). Here are the screenshots with the 1/DeltaX and DeltaY for each trace respectively on the right side of the screen. The difference in frequency is due to my imprecision trying to move the cursor on the trace crossing the X reference. The OUTPUT is at 406.2mV peak-peak. The INPUT DeltaY isn't the right one captured. The cursor aren't on the peak of the sinwave. Also, the oscillation seem to be sensible to the DVM probe across R503. The oscillation frequency is around 102 MHz.
But the good new is I let my DVM across R503 and while constantly readjusting the voltage to always stay below 1.5 VDC and by observing the oscillation above on the scope, I was able to raise progressively the rails voltages until I reached +/-35 VDC without any difference on the scope and without any heat on my finger onto the Q301, Q302 and Q303 transistors. Neither for Q401, Q402 and Q403.
I removed C3. When I calibrate RV1 BIAS to try to get 1.5 VDC across R503, the oscillation came back, but at +/-15VDC on the rails, the transistors stay cold on my finger.
I measured all the values at INPUT (Yellow trace), TP5 BIAS(Magenta trace) and OUTPUT (Blue trace). Here are the screenshots with the 1/DeltaX and DeltaY for each trace respectively on the right side of the screen. The difference in frequency is due to my imprecision trying to move the cursor on the trace crossing the X reference. The OUTPUT is at 406.2mV peak-peak. The INPUT DeltaY isn't the right one captured. The cursor aren't on the peak of the sinwave. Also, the oscillation seem to be sensible to the DVM probe across R503. The oscillation frequency is around 102 MHz.
But the good new is I let my DVM across R503 and while constantly readjusting the voltage to always stay below 1.5 VDC and by observing the oscillation above on the scope, I was able to raise progressively the rails voltages until I reached +/-35 VDC without any difference on the scope and without any heat on my finger onto the Q301, Q302 and Q303 transistors. Neither for Q401, Q402 and Q403.
@Sevy That was great. Now lets put in a couple missing parts to the input of the amp. This 1k-ohm/470pF would rule out or confirm RF ingression being the source of the 100MHz we are seeing. These two parts should remain in circuit permanently.
If the 100MHz persists, try temporarily removing both R301/R401 and see if that makes a difference.
I'm heading off to my day job and most likely will be silent all day. Will be back later to catch you up.
If the 100MHz persists, try temporarily removing both R301/R401 and see if that makes a difference.
I'm heading off to my day job and most likely will be silent all day. Will be back later to catch you up.
I removed R301/R401 with the same result. Then I tried to turn off my two computers. Still there. But when I wanted to push the "Quick" button on the scope to capture the screen, I noticed that when my hand came close to the scope the sin wave was amplified. It was worse when my finger get close to the BNC "T" connector permamently conneted to the Output generator of the scope in it's back. I removed all these but stil get the oscillation. And also, I had to revert back to +/-15 VDC on the rails since the resistors are out because I could sniff a bad odor before I could touch the transistors. I turned everything off and let them cold down starting back from 5, 10 and finally 15 VDC on the rails.
Now that's something maybe revealing... After a test, I turned off the Power Supply rails AND the Power Supply themself and the oscillation is still present. The circuit isn't alimented in DC and my scope can still see the oscillation at the input. I even try the second channel with it's new probe and I have the same result. This oscillation seem to come from nowhere!