Oscilloscope

Anything to do with the things that make the music we listen to.
User avatar
Greg
Shed dweller
Posts: 2670
Joined: Wed May 23, 2007 11:14 am
Location: Bristol, UK

Oscilloscope

Post by Greg » Thu May 22, 2014 11:50 pm

I believe a while back, someone posted a link to an online Oscilloscope user tutorial. I can't now find it. Can anyone point me in the right direction?
Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?
Douglas Adams (HHGTTG)

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6974
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Fri May 23, 2014 5:41 am

I don't know what link you're referring to but this is a basic Tektronix XYZ's of Oscilloscopes. which isn't too stuffy to read.

If you want a low level lesson in what different shaped waves mean to us in audio I have a basic book which may now be out of copyright so I could put pictures of the relevant sections on this thread.

I'll find it and see when it was printed. I have referred to it for myself a few times, but it isn't comprehensive enough. But there isn't a great deal of information out there on reading waveforms.

On the whole our need for an oscilloscope is vertually just to make sure a SW looks square doesn't slope either way or ring. Non of these conditions will be fully met at both extremes, but at 1khz at least it had better be square.

Secondly we can check our frequency response by seeing where the singnal amplitude reduces significantly. With two channels it helps because most signal generators amplitude also varies with frequency so you monitor that on one channel and compensate for it with amplitude control either at the sig gen or in the scope sensetivity.

This way outside the audio range of 20hz to 20khz (p.s. it is rare for a analogue scope of the type we can afford to be much use down to 1hz) we can find resonances. Like for instance in your output transformer and just make sure they aren't too close to the audio frequency.

What Alex is doing with HF AC heating for instance. It would be interesting to scope the output of an amp heated with DC up to 100khz then scope the HF AC heated amplifier and see where any peaks between 20khz and 100khz alter or interact.

I'll see if I can find that book.

final p.s. I much prefer a quality analogue scope to the moderately priced digital one I have now. But the quality analogue scopes are expensive. the cheap ones are not much use for anything more than "oh yes there is a wave form".
I can do it right or I can do it now, but I can't do it right now.

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6974
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Fri May 23, 2014 8:24 am

Picking stuff off from my old threads when I was checking a design with the square wave and referring to the book. (I looked for the book but no luck, sorry).

Here I was really just playing with a toy oscilloscope.

But the shape of the square wave sloaping slightly down hill reveals a slight but not damaging bass response.

The slight rounding of the nose shows a slightly rolled off HF response.

Not shown here but uggly symetric wobbly shapes on the horisontal section shows complexed distortion patters. Wobbly shapes like a spring uncoiling deminishing to the right represent ringing. The comments below are of some value.
Paul Barker wrote:Just playing with my new £15 toy oscilloscope.

Using the SW 1v 1khz calibration output from the sig gen.

This is the output measured across an 8 ohm resistor. 10v per division.
Image

This is the primary of the output transformer
Image

This is the input of the 6080 (output of the 12sj7 as it is DC coupled)
Image

Now we drop the sensitivity of the scope to 0.5v per division to show you the input signal.
Image

The traces of stages in the amp with no transformer show fair to good low frequency response. But this is corrected to excellent low frequency response on the output of the OPT. The OPT is wound for stupid amounts of inductance.

So let's talk LF response firstly.

The only mechanism of LF reduction is the screen bypass cap. There is no other component to affect it. When I build the amp it will have VR on the screen so this issue is gone.

Next we have to look at the phase. According to the shape of the waves the phase of the bass is not a problem.

Now the output waveform. On the output we have excellent bass response, but we now have ringing. Whether or not that could be snubbed or whether one would bother I don't know.

On the whole I am pleased. It certainly sounds good.

The drop in amplification across the Cf was a shocker.

I may change the amp by cap coupling into the grid of the bottom triode, take output from cathode of top triode. This would win back the lost gain. Doesn't mean it will sound better. Time will tell.

It's good enough to build into a box as it stands. I don't want to break it trying to fix something that doesn't matter.

For power output and sensitivity, a rough approximation of a SW is the peak voltage is the same as the RMS. So this input signal is 0.5v RMS which produces 1.56 Watts. So sensitivity is pretty acceptable, but this is with the VA cathode resistor unbypassed where VA is X 33 we loose nearly half that at the Cf. If we fully bypass the gain is 120 which will bring out decent power for my living room. Lets put it this way, I am not allowed to use full volume without bypass. Diana is complaining there must be something wrong with my hearing.
I can do it right or I can do it now, but I can't do it right now.

User avatar
Nick
Site Admin
Posts: 11983
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Post by Nick » Fri May 23, 2014 8:31 am

On the whole our need for an oscilloscope is vertually just to make sure a SW looks square doesn't slope either way or ring.
Cound me out from that "our" there Paul, I use my scope all the time, its got a hell of a lot more uses than just that one.

Its like any tool, if you don't have one, you cope, once you have one you can't imagine how you could have got on without it.
Resistance isn't futile it's V / I.

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6974
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Fri May 23, 2014 8:42 am

This one is quite interesting, because it shows you that the distortion in this push pull amplifier occurs because of driving something in the SE portion of the amp positive grid, up to that point the sign wave shape is good. In a push pull amp the clipping would be asymetrical if it were in the push pull output stage where the clipping occured.

But the power output I had up to that clipping (as seen in the sign wave), which occured at 10 watts creating 3% distortion, was sufficient.
Paul Barker wrote:It tested up much better.

Top trace is output of amp bottom trace is input signal.

Image

THD is 0.2 at 7.2 Watts
0.5 at 8.67 watts
1% at 9.15 watts
3% at 10 watts (at this point half the sign wave flattens so we are beyond this point driving the grids, hence we don't want to go further)
Image
But for measurement purposes
5% 10.8 watts
10% 11.88 watts

No doubt I could bias it more towards Class B for more power and to keep it away from Class A2, but the bias point is for Class A1. If it is pushed to far and goes nasty that is because it is not designed to go beyond 9 watts.

Edit: the flattened peak is probably over drive of the 6EM7 section I, as the 6b4g has considerably less gain than the 1619 it requires greater voltage amplification. If I altered the bias on the 6EM7 sec I to fixed (or bypass the cathode resistor) the sensitivity would double and we could probably take this amp up to 12 watts in class A1 with <5% THD. But life isn't all about geting the most power out of everything. I don't want that capacitor in my amp. I could add a portion of fixed bias but I must have some control (because of the dc leveling issue) and I wish to have some degenerative feedback, so it is not a quick change to the circuit, and therefore shall not be done at this time.

Frequency response is

-3.6 dB 10hz
-1.45 dB 20hz
-0.95 dB 30hz
-0.5 dB 40hz

You may note this is not 3dB per octave (it;s about half that rate), which it would be if transformer primary inductance were the reason. There must therefore be other contributory factors and the primary inductance of the transformer is not wholly responsible. It may be that there is an element of tanking going on a bass frequencies such that there is a bloomb right down at these frequencies which is mitigating the LF performance of the output transformer. That is my most likely explanation. In this instance as there is a net decline of linear scale it is assisting us.

Fritz's quality point where phase is 15 degree altered is 40hz. I can live with that on this occasion. If I wind some transformers for it I would put the 15 degree phase angle at 20hz, and lift the HF performance with more layering and the use of Nickel (which in any case, as a material, makes the very small level sounds much more distinct and natural).

HF is
-1dB at 16 khz
-3dB at 23 khz

With two transformers in the circuit you could expect little else at HF. There are no nasty resonant peaks. I stopped measuring at 60khz but all the way to 100khz the amplitude just tailed out without peaking.

The results are much better than the 1619 results, which clearly needs a transformer with plenty of inductance and probably a 10k load. So unlikely to be of much use as a Hifi output valve, because by the time you get where we have arrived you would be better off with the VT62.
I can do it right or I can do it now, but I can't do it right now.

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6974
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Fri May 23, 2014 8:53 am

As a guide also

The first square wave image in the above quote represents the output of an amplifier with a frequency response as follows

-3.6 dB 10hz
-1.45 dB 20hz
-0.95 dB 30hz
-0.5 dB 40hz

-1dB at 16 khz
-3dB at 23 khz

SO that is useful to understand the interpretation of square waves. The bass response is represented by the sloaping down of the square wave horizontal sections and the HF response is represented by the round nose front edge. the wobbly horisontal probably represents slight overall distortion, but it doesn't have the look of classic ringing, despight the use of two transformer stages it is a fair amplifier. It could be bettered with quality transformers rather than the cheapest of the cheap which is what was in it.

The slight peak at the front of the horizontal section is supposed to portray an HF peak, but when I ran a sign wave sweep up to 100khz I found none. So interpretation of a square wave needs accompanying with other tests as demonstrated here. Therefore what was that slight up turn? Next most likely explanation is ringing. So inspight of what I though the IT's are ringing slightly. The SW shown is therefore complex and showing a group of happenings in a combined form. You have to then do tests in each area to see which is the problem. I don't see any great problems here, and it sounded good.

I took it apart to reuse the bits in the next project as tends to happen. But revisiting it for you makes me wish I had kept it.

I probably shall return to IT phase split PP at some point. I bought John Caswell's Sowters but rather than using them for 300b's I'd probably use 2a3's (or equivalent).

I was so close to hitting buy it now over Neal's 211 PP transformers. But I don't have sufficient matched NOS 211's for a PP project and wouldn't use Chinese 211's if you paid me to.
I can do it right or I can do it now, but I can't do it right now.

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6974
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Fri May 23, 2014 9:06 am

Nick wrote:
On the whole our need for an oscilloscope is vertually just to make sure a SW looks square doesn't slope either way or ring.
Cound me out from that "our" there Paul, I use my scope all the time, its got a hell of a lot more uses than just that one.

Its like any tool, if you don't have one, you cope, once you have one you can't imagine how you could have got on without it.
I think when you are making stuff for market and also fixing stuff, you would use it a lot more.

can you give any examples without giving away any secrets?
I can do it right or I can do it now, but I can't do it right now.

User avatar
ed
retired
Posts: 3945
Joined: Thu Jun 21, 2007 4:01 pm
Location: yorkshire
Contact:

Post by ed » Fri May 23, 2014 10:08 am

Hi Greg I can't believe that you've missed this, surely the answer will be in here somewhere?

http://www.world-designs.co.uk/forum/sh ... php?t=7045

there is also this if it's any use:

http://electronicsclub.info/cro.htm
Last edited by ed on Fri May 23, 2014 10:42 am, edited 2 times in total.
There's nowhere you can be that isn't where you're meant to be

User avatar
ed
retired
Posts: 3945
Joined: Thu Jun 21, 2007 4:01 pm
Location: yorkshire
Contact:

Post by ed » Fri May 23, 2014 10:12 am

Nick wrote:
On the whole our need for an oscilloscope is vertually just to make sure a SW looks square doesn't slope either way or ring.
Cound me out from that "our" there Paul, I use my scope all the time, its got a hell of a lot more uses than just that one.

Its like any tool, if you don't have one, you cope, once you have one you can't imagine how you could have got on without it.
+1

I also use mine for frequency checking, data logger, volt meter and a few other things such as phase checking.....it's probably the 2nd most used gadget I've got.......I didn't use my Farnell half as much as I use this digital storage scope.

The only thing this digi scope can't do is error checking...with the old farnell it was possible to lay the out trace on the in trace and sum the difference, thereby showing the error wave in isolation...now that was something.
There's nowhere you can be that isn't where you're meant to be

Neal
Needs to get out more
Posts: 1863
Joined: Fri Aug 03, 2007 10:57 am
Location: From the land of the Bodgers

Post by Neal » Fri May 23, 2014 12:37 pm

Greg, this is quite a good get-you-started page:

https://learn.sparkfun.com/tutorials/ho ... cilloscope

There's a link to the Tecktronics XYZs guide but there are a number of versions out on the web none of which seem current. I downloaded what is the latest version before Tecktronix pulled it for some reason...here's a link to my drop box: https://db.tt/BR8NRLcb

User avatar
Paul Barker
No idea why I do this anymore
Posts: 6974
Joined: Mon May 21, 2007 9:42 pm

Post by Paul Barker » Fri May 23, 2014 12:49 pm

your xyz is better than my one so I have downloaded yours. thank you.
I can do it right or I can do it now, but I can't do it right now.

Michael L
Old Hand
Posts: 507
Joined: Tue Feb 19, 2013 5:10 pm
Location: Shrewsbury

Analog/ Digital Oscilloscope

Post by Michael L » Fri Jan 02, 2015 10:27 pm

Can't seem to find the thread now but I was reading on here recommendations to buy a used tektronix oscilloscope 2445 for around
£200. I see there are some impressive digital scopes available for
around £300 or less new. Would these be good for the audio DIY jobs?
How would they lose out to the Tektronix?

User avatar
Nick
Site Admin
Posts: 11983
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Post by Nick » Sat Jan 03, 2015 10:27 am

Not sure it would. I used to use analogue scopes, but the EHT never got on with the damp air in my workshop, so I got a cheap digital one. The only thing the Tek was better at was you could see finer details on the display, as it didnt have the pixelation of the digital scope. Howver given that digital scopes are storage scopes as well, you can just save the trace and zoom in to see details.

Other than that, much as I love old test kit, I can't fault the cheap Chinese scope. This is the one I got (well, in fact it was the 25Mhz version I got, but for audio use that bandwidth has been fine)

http://www.rapidonline.com/test-measure ... ope-518761
Resistance isn't futile it's V / I.

simon
Eternally single
Posts: 4112
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

Post by simon » Sat Jan 03, 2015 11:24 am

They're SO much smaller on the bench too. I keep telling myself I just can't justify one...

Michael L
Old Hand
Posts: 507
Joined: Tue Feb 19, 2013 5:10 pm
Location: Shrewsbury

Post by Michael L » Sat Jan 03, 2015 11:33 am

This one looks good to me

http://www.amazon.co.uk/SDS7102V-100MHz ... ds=sds7102

I think most of the cheaper ones like my example are 8bit so that may be a drawback
for audio work.

Post Reply