charliesixpack:
The input stage Q2 is obviously a differential input with the common mode voltage at GND. Without the DC feedback you will still maintain symmetry between the differential outputs of Q2 (assuming, of course, you have 0 volts differential input).
The feedback input of the differential stage is still connected to ground via R20.
But it does not get any feedback from the drivers (R42) when the power transistors have been removed.
So the voltage on R42 might be anywhere between +55 and -55.
The 2x220ohm resistors restore the broken link, so the driver emitters are back to ~+.5 and -.5 volt again.
The bias voltage (over C12) will indeed always be ~2volt.
Leo..
Lot's of good input here. When you say to put in a 220 ohm Resistors where the output transistors are...Do you mean from Base to emitter?
What wattage?
Can I use this trick/values on future AB amps?
Yes I do have a scope BTW. Also...I made sure the voltage at the center line/output jack is 0v when there is no signal being injected. Have had that problem before and toasted a speaker :).
jarrod0987:
Yes I understand about the whole resistor thing but there is not one in the right place for that trick apparently.
Well then, you really need to make a place for them.
jarrod0987:
What about replacing the .1 ohm emitter resistors in the final stage for some 100 ohm ones? I don't know if that would screw up setting the bias at all?
jarrod0987:
When you say to put in a 220 ohm Resistors where the output transistors are...Do you mean from Base to emitter?
What wattage?
Can I use this trick/values on future AB amps?
Yes.
The resistor replaces the B/E diode of the power transistor, so there will be ~0.6volt over that resistor.
That's uhhhh, 0.0016watt. A 1/4watt resistor will do.
Maybe. Some darlington configurations already HAVE that resistor.
Leo..
Good point. I think maybe the problem is it stops adjusting up once it hits .6 or .7 etc. Might be nice if you had a resistor so that if you were set to high you would know? Of course then again, Diode's don't behave like resistors either. I will be happy to see the response.
Diodes there could be dangerous for the drivers.
Their emiter resistors are now two diodes with 0.4ohm in series.
If you turn up the bias, the drivers have to lift that heavy load.
Leo..
So I can drop in the 220 Ohm resistors and then safely probe,scope, and adjust the bias pot without too much worry? Keeping an eye on the temp of the driver transistors because there will be no heat sink at this point?
What is a safe way to see if the pre amp is putting out some kind of garbage without hooking it up to the power amp? it has a tiny connector and I do know what the pins are. All I can think if is try to set up all the pins and probe the outputs with and without a signal etc?
1/ Yes. Resistors, and bias to MINIMUM. Leave it there untill the amp is fixed.
Use a variac if you have one.
2/ Cross that bridge when you get there. Fix the main amp first.
Run the main amp without load and without input signal.
Carefully measure all the DC voltages.
Post back if you're not sure about something.
I don't expect problems with the preamp at this stage.
Leo..
I should probably mention at the recommendation of GK I changed about half the op amps in the pre amp due to the gain channel was ok until I turned it up then weird noises happened. Further more before I blew the amp last time I was scoping the output and there was a wierd spike in the second half of the positive alternation. If the Diff amp Q2 (tested good) was working....Must have been on the incoming signal from pre amp?
Also I am trying to really get a strong mental grasp on what is happening with this biasing issue. I get that we are trying to set up the bias of both output transistors so that that they amplify just a tiny bit of the signal that was on the wrong side of the alternation to avoid a dead zone in the middle of the waveform. These close to 0v center parts of the alternations would turn on both transistors at the same time but if biased properly the they cancel partly and leave just the right signal left. Am I right so far?
Just curios, I know .7 is a very popular number for diodes, including the ones in the transistor B - E junction. In your experience, is that the number you will always see? Or are there Audio transistors hanging around that that .6 etc?
I am trying to understand the purpose of D7/11 and D18/19. It seems like D18/19 would never turn on and D7 and 11 I don't get what they do at all.
Also Is Q4 just for phase inversion?
Q1/16 and Q8/19 are just earlier stages of amplification before the drivers and finals yes?
Thanks so much for all the help. I realize these are elementary questions. Most of the video's I find only discuss class A and almost none seem to discuss in depth audio amps of the AB design.
HF oscillations can indeed do damage. Evidence of that can usually be found at the amp's output.
R49/R50/R51 usually release their precious smoke.
I think you're starting to understand the bias circuit.
Give the four B/E diodes (drivers and power transistors) a certain pre-voltage, so they already start to work a bit.
Then a small (music) signal does not have to overcome that B/E diode threshold first.
The voltage that a diode just starts conducting is ~0.5volt, but higher with more current. And temperature dependant.
0.65volt is average, and should be used for calculations.
D18/19 is easy to understand. They have the same function as the kickback diode over a relay.
The speaker/crosover connected to the amp is an inductive load. Spikes generated by the speaker are dumped onto the power rails by these diodes.
D7/11 seem to be "soft clip" diodes. e.g. the base of Q17 can't be driven any higher than the volt-drop over R12.
Q4 is a level shifter. Takes things from +57volt to -57volt.
Q1/16 and Q8/19 are the voltage amplifiers. The power stage is the current amplifier.
Leo..
So the final driver and output stages have no voltage gain? Only current gain? I thought that for a while but eventually convinced myself they had some voltage gain as well. Also there for current gain of course. Is this wrong?
Also for "Soft clipping" is this just for circuit protection when adjusting the bias in case a person went to high?
Power amps usually have a voltage stage, and a current stage.
But there are also "compound" designs (rare).
The output (current) stage in this amp has indeed no voltage gain.
The bias block/driver bases are pushed/pulled to almost the supply voltages by the voltage stage.
The drivers/power transistors just follow this with more "balls".
Soft clipping has nothing to do with bias.
It is not possible to have a higher voltage out of the amp than the rail voltage (+60v/-60v).
If you try, it "clips" the tops from your signal.
When you turn up the voume more, your sine wave will turn into a square wave.
Clipping has sharp edges.
That distortion adds lots of high frequencies that are not pleasant to hear and kill tweeters.
Soft clip is a way to stop the amp from reaching 100%.
Sort of braking before you hit the brick wall.
Leo..
Soft clipping has nothing to do with bias.
It is not possible to have a higher voltage out of the amp than the rail voltage (+60v/-60v).
If you try, it "clips" the tops from your signal.
When you turn up the volume more, your sine wave will turn into a square wave.
Clipping has sharp edges.
That distortion adds lots of high frequencies that are not pleasant to hear and kill tweeters.
Soft clip is a way to stop the amp from reaching 100%.
Sort of braking before you hit the brick wall.
Leo..
I guess I can't understand how you would get a situation where you had a higher voltage then the rails because there is no place where it is available to get it form? Is it so that the voltage from the pre amp signal doesn't add on top of the rail voltage and give you rail + signal voltage?
Also...Does this 220 ohm resistor trick work on power mosfets (Gate to source) as well? I have a finicky switch mode that keeps blowing it's outputs while I'm trying to adjust the feedback pots etc.
The output from a preamp is normally a few volts top/top.
The voltage stage of the main amp has a fixed gain (R17:R19/20), so if you feed more than a few volts into the main amp, the voltage stage will clip (at about +55v/-55v).
Mosfets and SMPSs is a different kettle of fish.
Amplifier tricks don't apply there.
Post a picture/schematic, and I will see if I can help.
Leo..
Ipod Camera is shit. I have a different thread for that issue. I get back to that problem. This one is more important. Thank you for all your explanations and tricks. If there is something I can do for you please let me know.
Replaced every single transistor on the board with the exact parts right from GK except for Q2 which was a sub they shipped me. I have them installed right. I am getting exactly the same readings as before so I don't believe it was the cause of the problem anyways.
So here is the testing scenario:
No tone, No load, no Final output Transistors (Q21,Q22). I have 220 ohm resistors installed B - E in there place.
My rail voltages for + and - 60vdc are actualy +58 and -58.1 on my multimeter which is not NIST calibrated. + and -15vdc is reading +13.8vdc and -14vdc.
Is this going to be a problem? I have a nice digital Tek O-Scope that is probably way more accurate but for the life of me I don;t dare hook it to anywhere but cold ground which I can't find a good spot on this board. All I see is the earth ground going straight to the chassis. I see no place to clamp onto the neutral with this board. Thought about putting something on somewhere. Last time i tried to use Hot ground and that was a mistake Thought it would work like a volt meter but I forgot the probe is earth grounded. Should I just lift the ground on the scope with a wall adapter and use hot ground? Isolation transformers are expensive and I can't afford it right now.
Any ways...Moving on...
With R21 adjusted all the way down I am supposed to be able to set up +1.0vdc on Q17 and -1.2vdc on Q20.
However....Q17 is already at +1.030vdc and that is already to high (But probably ok?)
Q20 is -1.103vdc.
If I adjust R21 to make Q20 -1.2ish then Q17 becomes +1.1ish etc.
What should I do here?
Thought I would see if the fuse R42 is blown causing the emitters to float. Haven't found it yet. Measuring between the 2 emitters reads 472 ohms. That is not right but not open either.
Update: Yep, found it. It's wide open. Going to replace and hopefully Bias will be right.
Hi again.
Seems you might have found the problem. R42.
Tolerance is not that important there, but fuseable is.
If you can't find an affordable fuseable resistor, use a standard cheap 1/4watt metalfilm resistor.
Mount it a few milimeters off the board, so when it burns it can't do any harm.
Keep that bias DOWN untill you're sure everything is ok.
Leo..
OK so I had an Idea that I could buy 1 fusible one and then get some cheaper metal film ones just for setting it up. Put the real one in at the end?
Can this Fuse being open cause my bias range issue? I have 100 ohm resistors but I don't know what kind. They just the plain tan ones. Also tolerance is probably gold.
The voltages from the supply being ever so slightly low is not a problem either right?
BTW it hides under a heat sing so I can't give it too much height off the board.
How about this one? It's 5% instead of 1% but Fusible 100 Ohm 1/4 Watt.
Thought it would work like a volt meter but I forgot the probe is earth grounded. Should I just lift the ground on the scope with a wall adapter and use hot ground? Isolation transformers are expensive and I can't afford it right now.