| chris ma |
This thread is intended for newbies like me not knowing what to do with a oscilloscope. What setting on the scope and where in the circuit to probe etc etc..This is only for the JLH High Power class A circuit. No need to be too detail. Just a rough idea as to.
1/ Checking for amp stablility. How to measure and understand what the scope is telling in a simple way.
Where to safely connect the probe, illustrated on the schematic diagram of various probe points.
Establish benchmark figures and interpretation at various point in the circuit with regard to output transistors. Driver transistor etc.
2/ Checking for noise. At the PSU. At the speaker output etc.
Simple task list with the scope to get a clean bill of health for the newly assembled JLH before daily use.
All pointers and helpful hints are welcome and appreciated.
Best Regards |
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| Mooly |
Hi,
Before you can undertake any measurements you have to know the 'scope is calibrated correctly. From the pics in the other thread it looked like you were getting the overshoot with a X1 lead. The 1 Khz squarewave output-- have I got that right, is that what your scope puts out at it's cal socket -- should be square on all volts/div ranges when connected with a X1 probe. If it's not - compare also with channel 2- then the attenuators inside the 'scope require adjusting. Note, unless these have been twiddled in the past they should never need touching.
When you use a X10 probe you adjust the trimmer in the probe itself to obtain a "flat" squarewave and this adjustment should be correct then for all volts/div settings on the 'scope.
The squarewave response has to be right as it will affect all other measurements.
Remember the scope shows peak to peak voltages not RMS. So in the U.K. our 240 volt mains would be 680 pk/pk on the 'scope.
DO NOT try and measure the mains by the way.
Is the volts/div accurate ? The 'scope measures DC as well as AC so get some batteries 1.5v 9 v etc, read them on a DVM then read them on the scope and check the readings agree.
Safety, The 'scope should be earthed (mains earth) at all times.
This however means the probe is also connected to mains earth. Be aware therefore that you cannot connect the ground lead of the probe to say a positive rail in an amp that is also earthed. Big bang if you try that :hot: .
Do you have an audio signal generator etc or a test CD with various tones on it ?
Regards Karl |
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| chris ma |
Hi Karl,
When I connect the probe to the (4. Select CH1 VERTICAL MODE and insert the tip of the Channel 1 probe into the PROBE ADJUST output jack.) Cal socket I left the other clip of the probe lead not connected to anything. I assume this must be the (ground) as to the main tip of the probe be +ve am I correct?
The volts/div seems ok. It is just me not getting used to reading the scale on the screen sometimes. That is the only thing I dare to poke the probe, batteries for now :)
Are you trying to set me up using a working system as my sand box for using the scope?
Sorry, I have no signal generator.
I have a test CD with different frequency on different tracks. I also have a small DVD player that can be used to play that CD.
Once the new probe delivered to me I can start check to see if it is calibrate correctly. I wanted to buy 2 probes but the guy selling on e-bay only has 1 left. I will get this one sometime next week.
So the sand box consists of all working:
1/ DVD or CD player
2/ Power amp
3/ Speaker
Let me try and setup the sand box first this weekend...
Best Regards |
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| Mooly |
Hi,
Having a measure with your test disc is as good a place to start. Just connect the 'scope to the line out sockets on the DVD, ground lead to the line out ( phono socket ? ) outer.
Try and calculate the frequencies you see on the 'scope. This will help confirm the timebase is accurate as well. Freq = 1/T So 1 millisecond for a complete cycle on the scope screen would be 1Khz. The test disc frequencies will be absolutely accurate to fractions of a percent.
I am still confused with the probe test thing on your 'scope.
Is this a 1Khz squarewave output. If you connect the channel 1 input here can you get a squarewave on the screen ? If you can, then with a X1 lead it should be square - no over/under shoot.
Regards Karl
And whats a sandbox ? Never heard that expression before :) |
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| chris ma |
Sandbox is the like playground for kids so to speak.
I have connected the dvd player to channel 2 with a interconnect (coax) with a RCA adaptor, see in picture in blue. The dvd player is set to play on repeat of a track that plays 1kHz square wave.
I will attach 2 pictures in the following posts.
Picture one showing the settings.
Picture two showing the result of those settings. |
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| chris ma |
Top Display
Channel 1 the black coax is connect to the scopes own CAL output at 1kHz 500mv P-P
Bottom Display
Channel 2 is the 1kHz square wave signal from the Sheffield test CD. But it does not look square no matter what settings are those dials are at.
Do they make any sense?
I tried to attach the second picture but for some reason I keep getting the picture size is too big although it is only 42kb |
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| chris ma |
| Bad news, while doing these pictures I fried the digital camera by not connect the USB to the camera last! When I plug the USB cable in my laptop with the cable already connected to the camera and I heard a crackle the camera became dead |
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| KP11520 |
Ouch chris ma,
Sounds like not only the butcher but the fryer too! Yikes!
But Fried Chicken Thighs are delicious, not sure about a Digital Camera though.
Do they warn about this in the instructions? Maybe there is a fuse. If not, hopefully it is still under warranty!
Good luck!
BTW, I am a newbie too and I am interested about this as well for when I get a scope too.
Regards//Keith |
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| Mooly |
Hi,
That's a pity about the camera. Isn't USB supposed to be plug and play, I think I would have a whinge about that !
Right --- I am still confused. The picture in post 7. Where has that squarewave come from. Is it the CD or the 'scope cal output.
Just confirm what we are looking at :)
When making any measurements you need all the "variable" controls on the 'scope ( Any pots in other words ) in the cal or calibrate position. If this isn't done the settings on the attenuator and the timebase are meaningless.
O.K. Amplitude of the sinewave on the screen. As an example if the 'scope is set to 1V/DIV and the sinewave is 4.25 squares from tip to tip this equals 4.25 Volts peak to peak. To find the RMS value ( only for sine waves ) divide the pk/pk by 2 to get the peak value. 2.125 volts peak in this case. Then divide this by the square root of 2. So the RMS value of 4.25 V pk/pk is 1.5 volts RMS. So working the other way a C.D. player with an output of 2Vrms will show on the scope as 2*1.414 *2 which is 5.65 volts pk/pk.
Regards Karl
Edit, just had another look at your pic, it's from the 'scope cal output isn't it. That looks fine, and when you get your X10 probe you adjust the trimmer in the probe so that the tops and bottoms of that squarewave are flat. When you try it it's obvious what you do. |
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| awasson |
Chris are you sure that disk has a square wave 1k tone?
I have a number of test disks and I think they are all sine waves.
Andrew |
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| chris ma |
Hello,
| quote: | | That's a pity about the camera. Isn't USB supposed to be plug and play, I think I would have a whinge about that ! |
Good news the camera is fine after replaced it with new batteries.
| quote: | Right --- I am still confused. The picture in post 7. Where has that squarewave come from. Is it the CD or the 'scope cal output.
Just confirm what we are looking at |
The squarewave is from the scopes PROBE ADJUST connector / scope cal output
| quote: | | When making any measurements you need all the "variable" controls on the 'scope ( Any pots in other words ) in the cal or calibrate position. If this isn't done the settings on the attenuator and the timebase are meaningless. |
Yes, this is what the manual suggested procedure “OBTAIN BASELINE” before use each time.
| quote: | | O.K. Amplitude of the sinewave on the screen. As an example if the 'scope is set to 1V/DIV and the sinewave is 4.25 squares from tip to tip this equals 4.25 Volts peak to peak. To find the RMS value ( only for sine waves ) divide the pk/pk by 2 to get the peak value. 2.125 volts peak in this case. Then divide this by the square root of 2. So the RMS value of 4.25 V pk/pk is 1.5 volts RMS. So working the other way a C.D. player with an output of 2Vrms will show on the scope as 2*1.414 *2 which is 5.65 volts pk/pk. |
When scope is set at 1V/DIV
RMS in Volts = Number of squares (Pk-Pk) / 2 / 1.4142
1.5 = 4.25 / 2 / 1.4142
Does it matter what frequency? Or is it a normal practice that 1 kHz test frequency used in this case?
Experiment: try to find out what the C.D. players output at in RMS compare with what the manual stated in the specification section of a particular C.D. player.
1/ Reset Base Line
2/ Feed the scope with 1 kHz signal from C.D. player by connecting the probe to the audio output RCA center pin with probe ground to RCA outer jack.
3/ If the scope is set at 1V/DIV then count the number of squares peak to peak displaying on screen.
4/ use formula RMS in Volts = Number of squares (Pk-Pk) / 2 / 1.4142
Question:
a/ What if the scope is set at 2V/DIV then?
Will it become RMS in Volts = Number of squares (Pk-Pk) / 4 / 1.4142
b/ What if the scope is set at 0.5V/DIV then?
Will it become RMS in Volts = Number of squares (Pk-Pk) / 1 / 1.4142
c/ With this scope I should be able to measure from 2mV to 50V DC like any Volt meter am I correct?
| quote: | Regards Karl
Edit, just had another look at your pic, it's from the 'scope cal output isn't it. That looks fine, and when you get your X10 probe you adjust the trimmer in the probe so that the tops and bottoms of that squarewave are flat. When you try it it's obvious what you do. |
It is from the cal output.
Thanks a lot Karl:) |
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| chris ma |
| quote: | Originally posted by awasson
Chris are you sure that disk has a square wave 1k tone?
I have a number of test disks and I think they are all sine waves.
Andrew |
Hi Andrew,
I am not sure, but it is the CD's cover stated that it is supposed to be same as the other track of 100 Hz square wave but at 1 kHz. I tried the 100 Hz track as well but it only shown as sine wave as well. I guess it may have been misprinted on the CD cover. |
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| Mooly |
Hi,
You always read the "number of squares" as accurately as you can. If your 'scope is set on 5Mv/div and you have a signal of 3.85 squares then that is 3.85 X 5Mv or 19.25 Mv. Same goes for the timebase, at say 20microseconds/div a signal that "repeats" every 3.65 squares along the horizontal axis is 3.65 X 20E-6 or 73 microseconds. The frequency is therefore F =1/T or 1/73E-6 or 13698 Hz.
A 'scope is not as accurate as meter and is only as accurate in any case to how well you can read it.
When measuring on the horizontal axis for determining frequency pick a point on the waveform (anywhere but usually easiest where it crosses the center line) and count the number of squares untill the same point is repeated on the waveform.
For amplitude you have to stick to sine waves for converting peak to rms values. For other repetitive waveforms there are various form factors that can be applied, but stick to sine waves for now.
Regards Karl |
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| Mooly |
I see your back :) , The setting of the attenuator and timebase don't of course affect the actual reading, a signal of 4 squares at 1V/Div is 2 squares at 2V/div and so on. In either case it's still 4 volts pk/pk. Same for the timebase. A signal that repeats every 20 milliseconds will occupy 1 horizontal square at 20ms/div and 4 squares at 5 ms/div.
You asked about a 1 khz square wave from your test CD, well it just so happens I have an actual screen shot taken yesterday for another thread so I have attached it. All CD players will reproduce this identically. |
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| chris ma |
What settings did you use when taking that 1KHz square wave screen shoot?
I could not make it look square no matter what settings I tried with my test CD. |
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| Mooly |
Hi,
It would be 1 volt/div if I remember, but it was via an attenuator not direct from the line out. Now the timebase ---- try and work it out ;) I will post the result in a mo. |
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| Mooly |
| How are you getting on, it's a 1000 hz signal so that is a time period of 1 millisecond. F=1/T so T=1/F. Now the signal is taking 5 horizontal squares so that is 1 / 5 or 0.2 milliseconds per div. (200microseconds/div) |
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| Mooly |
| Should say that the 'scope I use is a dual timebase type so the lower trace is part of the same waveform at a higher sweep speed to show detail--- sorry that would confuse :) |
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| chris ma |
Hi Karl,
I am at work and you've beat me to it:)
I wonder about that your screen shot was showing two time base. I will try again at home tonight. May be my CD mis-printed the sine wave instead of square wave. |
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| Mooly |
| :) Let us know how you get on. |
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| chris ma |
Hi Karl,
Thank you for you input. I have used your example of 1 kHz frequency input from CD to the scope.
Vertical ------>Set at 1 volt/div
Horizontal--->Turn the timebase dial until I get the 5 squares for 1 cycle similar to your top channel display screen shot.
However, mine is not square like yours although the insert of the CD clearly stated it is at 1 kHz and it is a square wave just like track 67.
My test CD is "My Disc" 10045-2-t The Shefffield/A2TB Test Disc and the track that I was using here is Track 68. |
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| chris ma |
The good thing about this test/ check is that I now found out that the marking on the dial of the SEC/DIV is off.
The two black lines are the original came like that with the scope and it is overlaying both the 50 and 20 [us] millisecond/1000....this is very confusing when I tried following the manual to obtain base line.
But according to your example it should be at 0.2 millisecond. Now I have used a marker and made a spot under the 0.2 point on the dial. May be you can see it in the picture. It is at 12 o'clock I made the black dot on the transparent disc of the dial. That is the 0.2 ms with 0.5 ms on its left and 0.1 ms on its right.
Sorry about the quality of the picture. My camera is not very good with micro setting. My photo skill is not good at all.
Still waiting for the probe.. |
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| Mooly |
Hi,
Does your test disc have a lot of different spot frequencies up to 20 khz. If so I would look at each one on the 'scope and make sure the timebase seems accurate. Do you have a multimeter with a frequency range on it. If you do check each track on the disc with that as well. Also if your meter measures small A.C. voltages accurately measure the output of your CD (choose a track recorded at as high signal as possible and make sure it is within the frequency range of the meter), and see if the RMS value shown by the meter agrees with the pk/pk value shown on the 'scope.
Is it possible that the switch or knob has been moved accidently on the 'scope. Might be worth seeing if it can be repositioned correctly, and now you know how to work the frequencies out you can be sure you get it in the right place. I see the cal knob does not line up with the attenuator one next to it--- may be nothing that !
I can't quite make out some of the text on the front panel but the top right knob, you want that on normal--no hold off. The left hand switch of the three at the bottom. This provides the timebase with a signal to trigger from. If you set it at ch1 it takes the signal from this channel only. Same for ch2. This is normally the way to use it, use channel one for measuring and trigger off ch1.
If you are using the two channels together, for example displaying the input and output of an amp then still use this mode. Provided the two signal you are looking at are "related" in terms of frequency and phase the display will be stable. Only use the "vert" mode triggering when the two inputs are unrelated. For example the output of your CD on channel 1 and perhaps say the secondary of a transformer on ch2. The vert mode will then give a stable display as it first shows ch1 triggered from ch 1 and then ch 2 triggered from ch2.
Again hard to make out but have you got it in "add" mode. The switch above ch2 attenuator. You want "alt" or alternate. If you turn the timebase speed right down ( and with both channels enabled )you will see alt sweeps ch1 and then ch2. Chop will appear to show both sweeping together, only use this at very low speeds where it make the display easier to view. Add numerically adds the signal of ch 1 and ch2 ( Quite useful for taking differential measurements ) If you have ch1 and 2 displaying the same signal at the same attenuator settings then add will show the sum of these- try it. Also if you then "invert" channel 2 the phase is inverted (180 degree shift). If you now add these together the result -- if the attenuators and calibration are correct--will be zero. Again try it !
That should keep you busy for half an hour or so ;) |
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| chris ma |
I do not have any multimeter with frequency measurement.
I managed to get the 1 kHz square wave from the Sheffield Test CD. This time I am using an older Sony CD player that I bought for $2.0 in a jumble sale years ago instead of the DVD player.
Volts/DIV is set at 0.2
SEC/DIV is set at 2ms
See picure below
I also managed to test another CD with a 20 kHz sine wave track.
No picture for this one though.
Volts/DIV is set at 0.1
SEC/DIV is set at 10us...means 10microsecond? or 0.01ms?
The sine wave full cycle takes 5 squares on screen
5 X 0.01ms=0.05ms (5 squares on screen)
1 / Time in second= Frequency
convert ms to second 0.05ms / 1000 = 0.00005s
1 / Time in second = Frequency
1 / 0.00005 = 20000 Hz
Am I correct? |
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| Mooly |
Thats it ! You will see your squarewave looks identical to the one I posted, slow risetimes and ringing from the filters in the player.
Now you have a 'scope I think a function or signal generator would be next on the shopping list. I have attached another image, this is a squarewave from a function generator. Notice how fast the rise time are compared to the CD. This is really the sort of signal you need for measuring amps etc, but having said that try your test disc squarewave through an amp. The function generator I use covers from 0.1 hz to 2 Mhz sine/square and triangle waveforms with a max output of around 7 volts RMS.
You mentioned at the start about checking your amp. The only point to watch is as I mentioned earlier, that if the 'scope is connected to mains earth then the probe earth becomes a "shorting link", to anything referenced to earth. That does not read well does it ! If your JLH psu ground is also connected to mains earth, then touching the probe earth to say the positive rail will apply a dead short across it. If in any doubt measure on DC then AC volts between the points you want to apply the ground wire first.
Try some measurements on your amp, perhaps the ripple on the rails.
Regards Karl |
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| chris ma |
Hi Karl,
I have just finished replacing all 4 outputs MJ21194 and emitter resistors of the faulty JLH. I think they are all blown as the emitter resistors were all blown apart to pieces
I replace them with 2n3055 (from dismantled 1969 JLH my first diy) of what I have in hand and the emitter resistors are 5% 0.10 ohm 2 W.
Use the variAC bring up the power slowly. The DC offset is about 110mV start up and slowly going down and the Iq is at 130mV across one of the emitter resistor. Too tired to carry on and wait until it warm up. Will continue...
But what to do to measure ripple on the rails? The rail voltage is at 24V. This is a CLC PSU not regulated.
If I want to put the 1kHz square wave through the amp do I just connect the output from the CD player to the amp just like I connect it directly to the scope. Do I then connect the 1X probe's tip both ground and tip to the speaker outputs?
Best Regards, |
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| Mooly |
Hi Chris,
Yes to measure the rail ripple connect the 'scope negative to the zero volt line. Is all this about being careful where you put the ground wire putting you off ? And with the 'scope on AC coupling (so you can see small AC signals sitting on top of a large DC component) for the Volt/Div attenuator (set around 5 volts/div to start) and a timebase speed such that a couple of cycles at 50/60 hz will fit on the screen connect the probe tip to the positive rail starting at the rectifier and working toward the amp.
You will be able to see how the ripple voltage increases as you alter the quiescent current of the amp. Try also measuring directly on the speaker outputs with the input to the amp shorted and see if there is any measurable ripple at 50/60 hz. If you feed the squarewave into the amp remember to use a pot, or feed it from your preamp so you can control the level.
Regards Karl |
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| chris ma |
Hello there,
Yes I am still a bit shy to poke the probe into the PSU. When you say " connect the 'scope negative to the zero volt line " do you mean the shield of the 1 X probe and the not the a connector on the front of the scope with a label GND?
Best Regards |
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| Mooly |
Hi Chris,
They are one and the same, but-- there's always a but, when measuring HF and we are talking R.F. really grounding of the probe becomes important, so important that even the earth lead on the scope probe will give incorrect readings.
But we don't need worry about any of that here.
I know mains arrangements vary the world over, but you are not going to blow yourself up :D , just remember if in doubt to check with your meter that no voltage exists between the points where you intend to clip the earth lead and the 'scope.
If the case of your 'scope is earthed ( and it should be ) then this means that the scope probe earth lead is also connected to mains earth. Measure on your meter to prove to yourself it is.
If your amp case is earthed and you have connected your PSU zero volt line to the amp case then this too is earthed.
So connecting the scope to the amp case or zero line is fine- but if you clipped the earth of the probe to say the positive rail can you see how that would short out the PSU. From the rail positive through the probe earth wire to earth in the mains, back through the amp earth in its mains lead to the zero volt line-- a dead short.
The input impedance of the scope is high, 1 Meg, or 10Meg with a 10/1 probe so no problems there. The probe tip wont cause any current to flow, so stick it on the rails and have a measure.
In fact if both items are earthed as I describe above you don't actually even need the scope probe earth lead ( the probe clip) connected, it's done anyway through the mains leads, but you will find the "noise" ( on the 'scope screen ) decreases if you ground properly when measuring.
Can we expect a "ripple reading" :D :D by the end of the day.
Edit, Chris, I saw your post on the JLH thread. I don't know what "variation" you are using but a single ended input stage draws its current supply from the output "midpoint" -- is this causing the current "inequality" you mention. |
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| chris ma |
Hi Karl,
The version of the high power is here:
http://www.diyaudio.com/forums/show...808#post1517808
By the time I try it tonight after work it will be two in the morning in your part of the world I think.:)
I was hoping the moderators would have merged that thread with this one the other day but I guess my english is not good enough to express that point clear enough to be understood precisely.
I will be checking that feedback resistor 2.7k (R8) might need a cap across it as the others were saying. But I only have a 100pf smallest at hand. Do you think that can help?
Best Regards, |
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| Mooly |
Hi Chris,
See your on line, is it the drop across R15/16 that is not equal.
Edit, Well you were ! ;) |
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| Mooly |
Had chance to read your other post again. What DC voltage have you at the emmiter of Q4, just interested on that one :)
I suspect the outputs are not sharing the current equally for a couple of possible reasons.
1. The characteristics of the transistors vary enough that the very low 0.1 ohm resistors are unable to allow the currents to equalise properly. Although the efficiency would be slightly worse perhaps try 0.33 ohms or 0.47 ohms and see if that helps.
2. Are the outputs thermally coupled i.e. on the same heatsink ? The characteristics change wildly with changes in temperature.
Adding a cap across R8 won't change the DC conditions unless the amp was unstable and oscillating to begin with. A small current flows through R8 to provide Q4 with its correct operating conditions and this will very slightly ( not what you are getting ) imbalance the current in the output halfs (Q1/Q1A and Q2/Q2A combined). |
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| chris ma |
Hi Karl,
The top and bottom half are not on the same heat sink but they are identical heat sinks though.
Emmiter voltage of Q4 will be measured across the emmiter and ground? Same as the base of Q3 and ground? I hope it is not too stupid a question.
Best Regards, |
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| Mooly |
Hi Chris,
Emmiter of Q4, goes to junction of R6/8 and collector of Q6.
Am I looking at the right circuit ? ;) |
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| chris ma |
Sorry Karl,
That's my mistake I thought emitter is the one that with the arrow :D
So it is to measure voltage across the the collector of Q6 junction of Q4 emitter and R6/8 and ground then? Thank you for your effort. Will post result later on tonight..
Cheers, |
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| Mooly |
Hi Chris,
Have a look at this, just been posted today-- if I knew (could get to work more like :) ) I would put a link in.
Look in the " everything else " forum, the thread you want is "Oscilloscope in audio tutorial" post # 57. Could be just what you want.
As I type this it's next one to this thread ;) |
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| chris ma |
Thank you Karl,
I will take a look tonight can't do that from work:whazzat: |
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| chris ma |
| quote: | Originally posted by Mooly
Had chance to read your other post again. What DC voltage have you at the emmiter of Q4, just interested on that one :)
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Hi Karl,
Measured across the emitter resistors (100m) after 20 minutes warm up.
Bottom half
For Q1 R14 ---> 49 mV
For Q1a R16 ---> 85 mV
Top half
For Q2 R13 ---> 37 mV
For Q2a R15 ---> 96 mV
The DC voltage of emitter of Q4 is 605 mV
DC offset stable around -6 mV at speaker outputs
The emitter of Q2a and speaker Vout is 100 mV
DC Supply rails +26.7 V and -26.7 V at C6 and C7
Best Regards, |
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| Mooly |
Hi Chris,
That's fine, what I was getting at was to make sure C4 was not reverse biased at all -- it's not - just !
Going back to the current inequality- in percentage terms it's a lot. Do try higher value emmiter resistors, that might well do the trick, and even at 2 amps per transistor the loss is small. A 0.47 ohm 3 watt resistor (non inductive) would be fine.
No ripple voltage yet ! ;) Get it measured. If you post the result here at least we know you lived to tell the tale. |
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| chris ma |
Hi Karl,
I removed the earth from the PSU last night. Then I took a stab at it with the probe connected between the rectifier and the first set of capacitor banks. It was difficult to make out the exact value. I just adjust the Volt/Div and Sec/Div until I saw a couple of /\/\/ not sine wave the upward curve is steep but the downward curve is not steep at all. (Will take picture tonight it was 12:45pm late and felt very tired). But when I move the probe to after the inductor coil and the second bank of capacitors it was even more difficult to get the display properly no matter how I adjust the dials. May be I need to use the 10X probe? I am still waiting for it in the post.
I may try different 2N3055 one at a time to see if it get any better first before trying different emitter resistors.
Best Regards, |
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| Mooly |
Hi Chris,
What do mean you "removed the earth" |
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| Mooly |
O.K. bit more time now. One of those days :)
Connect your 'scope ground lead on the probe to the PSU zero volt line, that's the negative end of C6 and the positive end C7 on your circuit or just to the negative speaker socket if you prefer. If you set your scope to say 5 volts/ div and AC coupled so that the trace stays central and is not offset by the rail voltage you should be able to read the ripple. Set the timebase to around 5 milliseconds per div so you can get a few complete cycles on screen and make sure the triggering is on internal and set for Channel one if that is what you are using. By adjusting the trig controls you should get a stable display. The ripple will be in the range of, what, 2 to 3 volts peak to peak, says he taking a wild guess. Depends how much current the amp is drawing. |
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| chris ma |
Hi Karl,
Remove earth, meant lifted the E not neutral and live would be the green wire in UK for the mains.
With the above settings and measurements
I thought the amp is drawing
(0.135Volt X 2)/ 0.1R = 2.7Amp
Regards, |
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| chris ma |
The AC measured before the smoothing caps and right after the rectifier.
Volts/Div is set at 0.1
Sec/Div is set at 1 ms
The cycle takes 8.2 division
8.2 X 1ms= 8.2ms
Convert ms to second 8.2ms / 1000 = 0.0082
1 / Time in second = Frequency
1 / 0.0082 = 122 Hz |
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| chris ma |
This is the best I can do with the probes at + C6 and 0V ground
Volts/Div is set at 2mV
Sec/Div is set at 10 ms
Can not stable the display no matter what:confused: |
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| Mooly |
Hi Chris,
That looks O.K. You have worked the frequency out to 122 Hz. I take it in your part of the world it's 60Hz mains. So what you are seeing is the effect of the bridge (full wave rectifier) topping up the caps each half cycle-- thats why you are seeing 122 (It will be 120Hz, but measuring that accurately on a 'scope is not easy).
Your first pic shows the voltage on the cap rising as the bridge starts to conduct, the voltage keeps rising untill the mains is at its "peak" and then the caps maintain the rail voltage as the mains falls away. The falling part of the waveform is the cap discharging as the amp draws current, when the next half cycle comes along the process repeats. The bridge only conducts when the voltage on the rising part of the incoming half cycle exceeds the DC voltage on the cap ( +0.7 volts diode drop ). If you vary the quiescent current of your amp you will see the ripple voltage vary. This ripple is sat "on top" of the D.C. voltage of the rail. So at 0.1 V/Div you only have 0.6 volts peak to peak-- that sounds very good to me.
Second shot, if you are on 2mv/div ( 2 MILLIVOLTS :D ), that really is a half of a quarter of nothing ;) .
Must be those coils :D , they look like something out of a particle acccelerator !
I am sure you are right but can you just confirm, pic two looks as though there is a low frequency component lurking there. Try and keep the timebase the same as in shot one and just alter the volts/div setting-- and make sure any variable gain pots are in "cal"
Try measuring directly across the speaker outputs for interest, with the inputs to the amp shorted. If the 'scope trace seems to "thicken" but you can't make anything out turn the timebase to much higher speeds, this will show if there is any high frequency oscillation of any sort. |
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| chris ma |
OK,
Volts/Div is set at 2mV
Sec/Div is set at 1ms
Input is short---the whole band(line/trace) is like the picture below very steady
Input NOT short--- the whole band wave a little bit and not steady
EDIT: But this is the same even after the power is turn off. The same band(trace) appeared anyway:xeye:
EDIT#2: I connected/shorted the probes together and get the same trace. |
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| Mooly |
Hi,
Nothing strange going on--- you are discovering the mysteries and limitations of doing simple but sensitive measurements with simple test gear and wiring arrangements.
The 'scope trace is ( looks like from here-- let's put it that way ) just white noise. You say with the inputs to the amp NOT shorted you get a:wave2: :wave2: on the trace. That's just the noise being modulated by the stray 60 Hz pickup.
Just to be absolutely sure, with it as in your picture turn the 'scope gain up as high as it goes, and expand the timebase to much higher speeds-- is it still just noise or is there any kind of "sinewave" lurking there. Almost certainly not, it's just a check to make sure it really is just noise.
Now your last comment-- with the scope set to 2mv/Div and timebase back at 2m/s Div and the scope leads connected together but NOT touching the amp is the trace still "thick" with noise. It should not be. If it is disconnect the probe from the front and see it it goes back to normal.
Did that info arrive by the way ? |
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| chris ma |
| quote: | Originally posted by Mooly
Did that info arrive by the way ? |
Yes it did thank you very much.
I will try what you suggest later this weekend. |
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| chris ma |
After fixing the fish tank water pump broke down problem the last few days back in business.
The 10X probe arrived and I used it set at 10X across the speaker output with input shorted. Can not measure anything wrong with all kinds of settings.
The only thing I noticed is that since it is still connected to the mains via VariAC
When the DC rail at the amp board is 25V
Q2a/R15 is at 56mV
Q2 /R13 is at 86mV
Q1a/R16 is at 88mV
Q1 /R14 is at 55mV
When the DC rail at the amp board is 29V
Q2a/R15 is at 14mV
Q2 /R13 is at 107mV
Q1a/R16 is at 81mV
Q1 /R14 is at 41mV
When I connect to the mains directly the DC rails are at 30.5V and the load is really tilt to one side the R13 and R14 is taking the 90 % of the load:(
Could that be because of the driver transistors causing this effect with higher DC rail supply? Or it is still due to the different charactericstics of the 2N3055?
Any other way to measure and see if the amp is stable? I am trying to find a pre-amp to connect to the CD player or can I just use a 25ohm / 3 watts wirewound potentiometer as volume control instead?
Regards, |
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| Mooly |
Hello Chris,
Easy one first-- 25 ohm pot is way to low to use as a volume control. 10K to 100K is fine, you need a dual gang pot preferably with a logarithmic track but a linear is OK as well. Try to get 10 or 22 k if you can. Do you know how to wire it up ?
These unbalanced currents. You have to understand the way a transistor operates to visualise whats going on. I will just take the lower pair as an example Q1 and Q1 A. Firstly the characteristics of each device can vary wildly and still be "in spec"
The current gain or hfe may be as low as 15 for one and as high as 50 for another. The base emmiter junction will not pass current until a voltage of around 0.6 to 0.7volt is across it at which point the device will begin to conduct and current will flow from collector to emmiter. How much current depends largely on the gain of the device. Temperature also plays a big part, even if you had matched them, if one were hotter than the other the "gain" and other characteristics vary.
The circuit designer should ensure that these variations do not upset the performance of the circuit and that the circuit will work with all devices, whether they are at the top or bottom of the specification.
So back to your circuit, the volt drop across R9 biases the transistors into conduction. If Q1 and Q1A were identical and R14 and 16 are equal the current MUST be shared equally. In practice this will never occur, so what can be done ? This is where R14 and 16 come in. Imagine Q1 begins to conduct a bit more-- it gets hotter-- this increase the hfe-- so it conducts more still. This higher current will increase the volt drop across R14, so the effective base and emmiter voltage is now slightly less. This reduction will (but in your case it's not enough) cause the current to reduce and hopefully share more equally.
This is why I suggested increasing the value of all the emmiter resistors to say 0.33 or 0.47 ohms. This will tend to force the outputs to equalise the currents more equally as the variations in current will cause more of a change in that all important base emmiter voltage. If you can also match the transistors then that helps as well.
Remember the bjt or junction transistor is NOT a "voltage" driven device but rather a cuurent driven one. This is why a tiny change in base emmiter voltage can cause such a large change in the collector current.
If the amp were unstable you would see it on the scope as an unwanted continuos signal probably in the 600khz to 5 Mhz range. |
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| chris ma |
Hello Karl,
Am I being silly to suggest that replace those 4 emitter resistors with trimmer resistors then the current sharing can be tuned ?
Thank you for the quick reply:)
Regards, |
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| Mooly |
Just a minute, fell off the chair, right O.K. erm what can I say.
Give me minute :D |
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| Mooly |
| Presets eh. Well, it's a new one on me. If you can get some 0.5ohms pots well why not. Actually though, that set me thinking, its not so strange an idea. Well a bit maybe. Problem is the current Chris, it's to much. Many years ago TV's had such things in them for convergence adjustment etc, but they were big arkward things. No try increasing them first 0.33 or 0.47 and then perhaps you could ADD a parallel resistor across them on a selected one off basis to trim them--- there you are, so it wasn't a strange idea after all. |
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| Mooly |
| Have you got any low eg 0.47 0.33 0.22 etc 3 or 5 watt resistors spare. The transistor with the lowest voltage (current flow) of each pair is the one to trim by adding a resistor across it's emmiter resistor. |
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| chris ma |
| quote: | Originally posted by Mooly
Have you got any low eg 0.47 0.33 0.22 etc 3 or 5 watt resistors spare. The transistor with the lowest voltage (current flow) of each pair is the one to trim by adding a resistor across it's emmiter resistor. |
No I do not have them at hand. I have to go and buy them. Please let me know what values trimmer I do need as well.
Shopping here in Canada is not as easy so I try to make the trip worth it. I will have to drive 40km each way and hope they are not out of stock :(
Regards, |
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| awasson |
Hi Chris,
I think Mooly is suggesting that you trim with the 0.47 0.33 0.22 etc 3 or 5 watt resistors instead of using a typical trimmer because of the amount of current they will have to sink.
I think that was the gist of it.
Andrew |
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| Mooly |
Spot on Awasson ;) .
I still think it best to increase all four to at least 0.33 ohms first. You will never get it 100%, certainly not if the outputs are not all thermally in contact with one another but I would guess that if you can get it within 25% or so imbalance, thats not so bad.
If you are going shopping especially for these I would suggest you get 4 each of 0.47 and 0.33 and 0.22. Don't forget the volume pot 10 or 22K log dual gang. |
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| chris ma |
Hello there,
The place have moved. Took me awhile to find it again.
Just got them now. But they are only 2W 5% for 4 each of 0.47
and 0.33 and 0.22
I did not want the white rectangle ones. They have 5W but not the exactly the same values as you have specified. I also have the 10k log dual gang now. How do one wire this pot up? It has 6 contacts. From CD audio out to pot and from pot to the RCA input of power amp. One channel at a time as my power amps are mono blocks.
Birds eye view of the 10k log pot
* * *
* * *
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| Mooly |
Hi Chris,
All pots are basically the same. Hope this makes sense. |
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| Mooly |
The wiper is always the middle leg. Ground one side as shown and apply the input between the other leg and ground. The signal is always taken from the wiper and ground. If you got the two end connections reversed the pot would work back to front !
This shows just one channel-- just copy it for the other one. |
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| chris ma |
Hey Karl,
That's the one I got shown in your pic. Thanks. |
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| chris ma |
The sharing of the current is fixed after I replaced the 2N3055 with the Mj21194G that came through the post from On-semi.
They are not perfect but they measusred at 80mV and 71mV for Q2a/Q2 and 83mV and 69mV for Q1a/Q1
So the PSU ripple seems fine. Next thing to try now is to wire up the pot and connect the CD player to the amp and play with the scope to see how well the JLH reproduce the sine wave and square wave. Is that the right approach? I have warmed up the amp for an hour or so after putting in the new output transistors and the DC offset and Iq is very stable. With the above voltages across the emitter resistors what Iq current I have set the amp at ?
80 + 71 = 151mV ; that would be 1.5A Iq is this right?
Regards, |
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| Mooly |
Hello Chris,
Thats good, sounds like they are much better matched gain wise.
Yes 80 mv across 0.1 ohm is 0.8 amp, and 71 mv is 0.71 amp. Just add them together as you have = 1.51 amps.
I am not sure what the original recommendation was now for the 1969 version. And yours is the "high power version" with multiple outputs. It was all of that though ! 1.6 Amps I seem to remember.
A lot depends on your PSU and the thermal management you have.
Wire the pot up next, just make sure you get the earth at the correct end as shown in the previous picture.
Regards Karl |
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| chris ma |
Well some scary moments:eek:
After I wired up the pot and played with the scope and amp for a little while, everything seems fine and then I unclipped the clip from the RCA input of the amp and I made some sparks after that the scope was showing the amp was acting strange with some strange looking sine waves comes and goes with no input from the CD player(disconnected); also the Iq current went up from 70mV to 200mV so I shut off the amp immediately wait for 5 minutes restart everything and it seems normal again.
Now evrything cool down a bit ,,, so am I:)
Took couple of pictures
Scope settings for both channels
Sec/Div set at 0.2ms
Volts/Div set at 0.5V
Top trace is from CD player thru the volume pot to the JLH and using 1X probe connect at the speaker outputs.
Bottom trace is direct from the CD player to the scope using RCA interconnect for compare purpose.
The CD is playing a 1Khz square wave.
Something is not right here. My camera seems to over expose but I can not use flash to take the pics.
Hope you can make it out of what the traces are telling
:) |
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| chris ma |
Here is the second pic with the lights turn off in the room. But it is still over exposed.
P.S.
Karl, thank you for your input.. Without your encouragement I would not have gone this far. Although I have no idea what the scope is telling me at the moment some how I feel that I have accomplished something and it is a very good feeling. Thanks again. Owe you a pint!:D |
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| Mooly |
Hello Chris,
"Sparks in the dark" scary stuff !
First question, did you have speakers connected while you are taking the reading.
This is where it gets really difficult without it all in front of you.
Try the measurement again, NO load attached , with just one channel of 'scope connected direct to the speaker output. We know the output from the CD is OK so lets keep it simple.
See if the "overshoot" on the waveform varies at different gain settings of the volume pot. Try from a few 100 mv output up to just below where the amp begins to clip which will be a value just a bit below the supply rail voltage. Make sure you connect your probe ground to the actual speaker negative point, not some point in the PSU. |
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| chris ma |
There was no load attached. The only thing connected to the speaker binding posts were the DC offset checking multimeter and the probe of the scope connected correctly.
The wave form as in the pics looked the same with different volume setting. But when I made a mistake of passing the amp with very high volume the trace moved right of the screen and the amp started to make faint mechanical hum/buzz noise and I dared not to overload it for more than 2 seconds. |
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| Mooly |
Hello Chris,
What you are seeing is called overshoot, the reason why -- well thats another thing altogether. Have you used your own wiring layout for constructing it all ? I can't remember now without re reading it all. Is it on PCB's ?
In all honesty this is where it gets a bit difficult now. There are so many posible reasons why. It could be the way the returns to the zero volt line are made. Every piece of wire has to be treated as a potential "resistance" that could cause an unwanted voltage (signal) to appear across it. The returns for the C4/C2 & R2 are the most important in the amp. They should all go to the star earth not to just any "convenient" earth. I am not saying you have done this -- it's just something to bear in mind.
C2 the 330 pf -- thats fitted is it.
I don't really know what else to advise Chris, at the end of a keyboard there are so many posible pitfalls.
If you are really keen you could try rewiring one channel at a time to a star grounding scheme. |
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| chris ma |
Ok,
then in grossly/ losely term when amp showing overshot behaviour what ill effect it would have to the sound quality of reproducing?
What do you mean C2 330pf ?
This amp is point to point hard wired, no pcb , the butcher style:D
Sorry C2 yes I have C2 intalled in amp |
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| chris ma |
Hey Karl,
Problem identified. It was due to the wiring of the volume pot I just move the aligator clip and the wiring a bit and it changes the ringing to no overshot.
It was due to this mess of wires:rolleyes: |
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| chris ma |
So I should have a better setup for the pot and wiring instead of loose wires soldered to the pot and to 4 aligator clips. And they are not shielded. Now the trace look exactly the same as the CD player connected direct to the scope except the vertical divisions is more with the amp output control by the pot.
This is the amp trace at 1kHz
PS. now I twisted the wires together and the overshot can be reduced now. It seems the amp is reproducing the squarewave without adding further extra freebies.:) |
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| Mooly |
That looks great !!. It's all in the detail isn't it. Earlier on you asked about the quiescent current and I mentioned 1.6 Amp for the original. The updated JLH (not yours exactly -- the one I sent you) mentions I think 2 amps.
What you may find interesting to do is to connect your 'scope across the speaker when it's all playing and see just what voltage you actually put out when it's turned up loud. Try and calculate then what the peak and RMS power is into 8 ohm. You may be suprised how little power you need for normal listening. For ex 6 volts peak (not peak to peak) would be 4.5 watt's peak into 8 ohms. If it were a sine wave swinging 6 volts above and below ground that would then be 12 volts peak to peak or 12/2 which is 6 divided by root 2 ( 12/2=6. 6/1.414=4.24 watts RMS)
So whats next ???:D |
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| chris ma |
I will check and re-bias the other 3 JLH amps before I build more.
I am using regulated PSU Mj15003 for mid, CLC PSU Mj21194 for high and Chipamp for subs doing active 3 ways per channel.
These JLH amps are so picky about the matching of output transistors may be I should build a matching tool with regulated power supply? Any suggestions?:) |
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| chris ma |
| quote: | Originally posted by Mooly
What you may find interesting to do is to connect your 'scope across the speaker when it's all playing and see just what voltage you actually put out when it's turned up loud. Try and calculate then what the peak and RMS power is into 8 ohm. You may be suprised how little power you need for normal listening. For ex 6 volts peak (not peak to peak) would be 4.5 watt's peak into 8 ohms. If it were a sine wave swinging 6 volts above and below ground that would then be 12 volts peak to peak or 12/2 which is 6 divided by root 2 ( 12/2=6. 6/1.414=4.24 watts RMS)
So whats next ???:D |
Do I set the scope at DC or AC ?
I guess it would have to be AC. |
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| Mooly |
| Think about it :) The output is at zero volts DC. You only need set it to AC coupling if the signal you want to look at is sat on top of a DC voltage. Yours isn't so use DC coupling. You will find if you switch back and forth there is no difference. |
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| chris ma |
| Thank you very much |
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