Oscilloscope question...

[ibuski]

While Spending time in idleness in Christmas/newyear holidays, I started thinking it's nice to have an oscilloscope just for fun mainly.  Probably I'd buy some recent low cost digital model (typically Rigol 1054z, etc.).  I know you many veterans are satisfied with old Tek high-end models, but it doesn't sound reasonable to buy those old models on ebay in 2022 as a first Oscilloscope for a beginner.   TBH I don't know how oscilloscopes are useful in building audio amplifiers, and which specifications are required for that purpose.

Do you recommend 4 ch for HC newbies or 2ch is enough?

What analog frequency bandwidth or sampling rate is necessary in the adjustment/troubleshooting of amplifier DIYer?

Does the record length matter in our purpose of audio frequency?   There are old models sold for cheaper price, and their biggest disadvantage to the recent model seems very short record length (less than 10k points), to my eyes.

Is there special note for use in e-stat amplifier particularly, considering its extreme voltage specification (up to 900V?)?

Any suggestion would be appreciated.

[jamesmking]

6 hours ago, ibuski said:

While Spending time in idleness in Christmas/newyear holidays, I started thinking it's nice to have an oscilloscope just for fun mainly.  Probably I'd buy some recent low cost digital model (typically Rigol 1054z, etc.).  I know you many veterans are satisfied with old Tek high-end models, but it doesn't sound reasonable to buy those old models on ebay in 2022 as a first Oscilloscope for a beginner.   TBH I don't know how oscilloscopes are useful in building audio amplifiers, and which specifications are required for that purpose.

Do you recommend 4 ch for HC newbies or 2ch is enough?

What analog frequency bandwidth or sampling rate is necessary in the adjustment/troubleshooting of amplifier DIYer?

Does the record length matter in our purpose of audio frequency?   There are old models sold for cheaper price, and their biggest disadvantage to the recent model seems very short record length (less than 10k points), to my eyes.

Is there special note for use in e-stat amplifier particularly, considering its extreme voltage specification (up to 900V?)?

Any suggestion would be appreciated.

quick answer:

Scopes are useful for two things 1. verifying that your device is working as expected, normally this also involves a signal generator and inputting a set of known waveforms and seeing what comes out and 2. troubleshooting. The issue with multimeters is they can tell you that Ac voltage but NOT what the waveform looks like, for example is it noisy, has the top and or bottom been squared off due to clipping distortion etc etc. with the scope you can work backwards from the output and work out where things are starting to go wrong in the circuit...

for example looking at a square wave output you can check for instability (ringing on the top and bottom edges of the square wave), bandwidth, (a rounding of the corners of the square wave) etc. using a triangle wave you can easily see clipping of the output (loss of the sharp top and bottom of the triangle wave) etc. In other words you need a signal generator to provide test signals for the scope to be able to see what is going on.... You can look at the outputs of DC power supplies and look for noise. ripple, start-up overshoot, undershoot etc. A scope allows you to dynamically see changes in a way a multimeter simply cant convey.

The probes decide the voltages you can measure (most scopes internally can only handle low voltages typically a few 10s of volts) . For example a 1x probe passes the voltage unmodified. A 10x probe divides by 10, a 100x probe divides by 100. probes themselves will have a voltage rating - which will depend on if they are 1x, 10x etc. Some probes are switchable between multiple divisors e.g. x1, 10x switchable.

The main issue with most scopes is that the ground lead is earth referenced (this is not the case for almost all multimeters) so if you clip the ground lead to something in your device which is not ground then you have just shorted that thing to ground and this can kill the device and the scope as well as being dangerous to the user. You can buy differential scope probes which don't suffer from this danger but these are far more expensive than normal probes and far more noisy. A battery power scope would normally not be earth referenced and so be fine without differential probes...

4 channel scope is a luxury, it enables you to look for example at both the + and - outputs of both channels of a stereo a differential pair, or the output of one channel and the input at the same time. A 2 channel scope is ok but to check output against input on both channels you will have to unclip the probes and move to the other channel.

I have the rigol ds1054z... its an oldish design but it quite good value for money for its features... especially if you can get a deal with the extra software options enabled for free e.g. more memory, serial bus decoding etc.

If you dont want to spend 1054z even an old second hand analog scope will be useful - just most dont make automatic measurements and require a bit more skill to use and read the screen to measure manually

do you have a signal generator? either software you can run on your computer and output from the sound card or an actual standalone signal  gen? without one a scope is of much more limited use... Some scopes have the option of a built in signal gen but with some you pay more than a standalone gen, lose some versatility etc. So dont assume built in is cheaper.. check.

For audio you probably will not need a bandwidth higher than a few 100Khz both for signal gen and scope. Almost all scopes go into the 10s of mega hz or higher so that's not a problem. Cheap scopes will not be able to measure distortion or signal to noise ratio etc or give you a good FFT analysis of the harmonics in a signal but this is only necessary if you are benchmarking, developing, debugging or comparing etc.

If you are more serious about audio quality measurements there is a keithley 2015thd multimeter which has a built in signal generator and can do distortion, fft and noise measurements but its quite old, many of the advanced features require talking to it via a program and they are grossly overpriced on ebay. I managed to get one before they become crazily expensive and by modern standards they are behind the curve for example the signal gen is only 10hz to 20khz, the fft has a maximum of only 1000 bins and can't analyse bellow 20hz or above 50khz. it cant calculate distortion + noise for a fundamental frequency bellow 60.8hz etc etc. The user interface is not great for the advanced features and programming it is a pain (I have written some control software for it but its in beta and desperately requires more testing and refinement). I wish someone would make a modern version of the 2015 but i guess the potential market is just too small....

Don't connect the output of a stax amp to the input of a line level sound card , audio precision analyser etc.to try to do some measurements these devices are only designed to take a few volts input and a stax amp can fry them really easily.

If you need a good multimeter I highly recommend the brymen bm869s it can handle very high voltages in any mode. For example, I by accident had it in resistance mode and connected it across a 350V DC power supply. Multi meter survived no problem.. its very robust... and the dc power supply fortunately was robust and had current limiting and also survived. the brymen will handle multiple KV across any input: (Joe smith does some comprehensive multimeter testing)

 

 

 

 

Edited January 3, 2022 by jamesmking

[Rinat]

@jamesmking, thanks for another awesome post. 

I remember that you made resistors/capacitors selection to reduce the tolerance. Did you use a LCR meter ? And which one ? 

[jamesmking]

1 hour ago, Rinat said:

@jamesmking, thanks for another awesome post. 

I remember that you made resistors/capacitors selection to reduce the tolerance. Did you use a LCR meter ? And which one ? 

good quality repeatable lcr meters are not cheap and in general an lcr meter is a bit of a luxury unless you absolutely need to match caps for example in a RIAA phono stage or want to check esr of caps. I was not prepared to pay £1000+ for a bench LCR meter + test fixtures + software unlocks so I hunted around for a handheld. I ended up purchasing the Mastech MS5308 LCR Meter. It was a bit less than £200. Its unnecessarily big, ugly, the instruction manual is garbage and it unnecessarily takes 8 AA batteries. It can take dc power BUT due to a design fault don't put in more than DC12V or very fresh non-rechargeable batteries  because you will burn out one of the DC regulators inside which is only designed for 12V input absolute maximum.... I had to replace mine. It does not come with a dc power supply anymore because of the 12V burnout issue.... (nimh rechargeable are fine because they are nominally 1.3V and 1.3*8 is less than 12V) I replaced the regulator with one that had about 19V max input tolerance and its been fine ever since. I also replaced all the tantalum bypass caps with ceramic caps as in the link bellow:

https://anteh.ru/notes/repare/repr_ms5308.html

on the plus side it has a large screen, sub pF measurements, comes with kelvin clips and smd tweezers and can do 100hz, 120hz (for large psu caps), 1K, 10K and 100Khz test frequencies. I have some low drift resistors measure in a uk cal lab and found the mastech to be within 0.1% worst case 100ohm to 10Mohm and often better than that, around 10K-100K mine is almost 0.01%.

I don't really measure inductors and don't have any lab tested caps but for +- channel matching absolute values are often not needed - most designs here are built with 1% resistors any 5%+ caps. I do have some 1% caps and although not lab measured the mastech measures them within the factory specified 1% tolerance so that's at least a sanity check. Its possible I got lucky with mine, the headline specs are nothing special but getting something that has guaranteed specs of 0/1% or better is prohibitively expensive for my use cases.

The chipset and screen used in it is used by several manufacturers for example

https://www.ebay.co.uk/itm/224253889293?epid=20032261934&hash=item343692cb0d:g:9iQAAOSwUspgbT5C

has the same/very similar screen and almost certainly the same chipset and i have seen multiple rebrands of this model.

 

 

 

warning I tried a APPLENT lcr meter (https://www.ebay.co.uk/itm/333757039315?hash=item4db57842d3:g:624AAOSw22Zfip8G), similar specs to the Mastech but smaller, cuter colour screen. BUT I found it to be very poor at low capacitance values and high resistances - almost 10% out on 10Mohm. After some initial investigation I found that the kelvin probes where fake... the meter was only 4 wire up to the connection slots and no further. The entire probe system could only run in two wire mode. I queried this with the seller and eventually they admitted it was not 4 wire measurement all the way to the device under test and I sent it back for a refund... At least the Mastech is true 4 wire measurement.

 

note using an LCR meter takes a bit of getting used to, you need to get a feel for which test frequencies to test what values at. For example putting 100khz into a 4700uF psu cap is going to give you a measurement of a very small capacitance and fairly high esr. These types of caps are best measured at 100 or 120hz. On the other hand putting 120hz into a 5pF cap is going to give you crazy esr... that's what your 100K test frequency is for. resistors seem less effected by frequency so I standardized my testing at 1Khz.

 

 

 

Edited January 4, 2022 by jamesmking

[Rinat]

Thanks again, I am going to buy this DE-5000. I've heard many times that it's accuracy in a lucky cases can be better than in specs. It gives a hope 🙂

[ibuski]

On 1/3/2022 at 5:24 PM, jamesmking said:

do you have a signal generator? either software you can run on your computer and output from the sound card or an actual standalone signal  gen? without one a scope is of much more limited use... Some scopes have the option of a built in signal gen but with some you pay more than a standalone gen, lose some versatility etc. So dont assume built in is cheaper.. check.

Thank you Jamesmking for your detailed explanation and lot of information with many links to the valuable videos.  That's exactly one great post and should be pinned to the top of thread for others, too.   I have to take time to read/view them.

One point about the signal generator.  No, I don't have signal generator.   I think I can start using PC software audio signal generator.  Response to square wave should provide enough (or some?) information about high frequency behavior of the DIY amplifier, right?  I also confirmed that cheap AWG (...6600, typically) are good enough and costs less than 100 EUR.  So I won't consider paying extra 200 EUR for the additional AWG feature of Oscilloscope.

Regarding DMM, it sounds that Brymen is specialized for E-stat amplifier builders, hearing its high voltage robustness up to kVs....   I had bought DMM last year, and don't know how robust it is for kV in resistance mode.

[jamesmking]

1 hour ago, ibuski said:

Thank you Jamesmking for your detailed explanation and lot of information with many links to the valuable videos.  That's exactly one great post and should be pinned to the top of thread for others, too.   I have to take time to read/view them.

One point about the signal generator.  No, I don't have signal generator.   I think I can start using PC software audio signal generator.  Response to square wave should provide enough (or some?) information about high frequency behavior of the DIY amplifier, right?  I also confirmed that cheap AWG (...6600, typically) are good enough and costs less than 100 EUR.  So I won't consider paying extra 200 EUR for the additional AWG feature of Oscilloscope.

Regarding DMM, it sounds that Brymen is specialized for E-stat amplifier builders, hearing its high voltage robustness up to kVs....   I had bought DMM last year, and don't know how robust it is for kV in resistance mode.

I love the bm869s "it just works" (tm) is well built and just inspires confidence, has good accuracy and good resolution - especially on dc volts, has great input protection and a micro amp range. Its a great set of features for not crazy money. Many compare it to Flukes - which are double to triple the price. Ok, nit picking is in my DNA: it does not come with a case and the included probes are only okish and maybe the back light could stay on longer but otherwise its really nice.

 

pc sound card into stax amp input works fine. I do this myself - its actually lower distortion and noise than most signal generators (many low cost arbitrary digital signal gens are more interested in high frequency in the 10s of Mhz rather than low noise and distortion in the audio range and are often only 12 bits of resolution - bellow compact disk resolution). The only things to bare in mind is the limited bandwidth of pc sound. Many soundcards have a capacitor in the output so the output will be down by multiple DBs at low frequencies ( 5 to 10hz) whereas many of Kevins Stax amps have no coupling caps and have a very extended low frequency response. Also at the upper end even a 192Khz sample rate sound card will probably have little output above 50khz-70khz. But for basic functionality testing it should be fine. A second hand old school analog signal gen might also be an option if you are on a budget and don't need the features of all digital gen.

Windows can and often will re-sample the sound adding more noise and distortion so ideally you need asio drivers which can bypass most of the windows sound system including the windows sound mixer and deliver a far more pure output.

if you pc has a digital output you can do digital output to external dac which can result in even lower noise and better distortion and sometimes better bandwidth (depending on if the dac has coupling caps and how it implement high frequency filtering). I also do this.

personally I find arbitrary wave forms a gimmick - you cant analyse crazy wiggly waveforms and the software for making them is often primitive and basically you "draw" the waveform with a mouse and end up with something not really meaningful for audio. About the only time I used awg in audio was I created a symmetric wave so I could check absolute polarity and because of the asymmetry it effectively had busts of dc and so you could see the effect of the coupling caps reacting to the sudden changes in dc conditions.

The ability to do a sine wave sweep is useful - for quickly getting a visual feel for frequency response on a scope and detecting peaks and trough's. Same with square wave sweep for quick look at instability vs frequency and triangle sweep for determining clipping vs frequency. 

(glad to help - my way of contributing to the community. In return I hope the community does not flame me if I express something they don't agree with 🙂 ) 

personally if you have no test equipment I would say in order of priority

1. good reliable robust multimeter with a good selection of probes, hooks etc and preferably a temperature probe input and K type thermal couple. For checking temps are sensible, searching for shorted and therefore overheated components, checking components are running within thermal spec, checking transistors are properly bolted down to heatsinks etc etc. 

2. audio signal generator and scope (one is not much use without the other in my opinion)

 

if you plan to do quite a few builds/start to get serious:

3. variac for slowly starting up new builds in a controlled way

4. semi conductor analyser - useful for checking for fakes, checking for failed transistors, matching small signal transistors. e.g. peak dca75

5. LCR meter

6. high voltage transistor breakdown tester - for testing fake transistors, diode breakdown and high voltage zeners one possibility is the dy294 although in stock form the clips are crap. I modified mine to have flying leads with good quality clips on the ends. The manual is also pretty bad. (common theme with cheap Chinese stuff but it does work and I have found it useful e.g. measuring and matching the output of high voltage (75V+) zeners.

7. capacitor leakage tester 

 

if you take electrical safety importantly and have separate power supplies from the amps

8. high voltage insulation tester for testing the wiring chord and connector between psu and amp

9. pat tester for testing primary winding of psu transformer, chassis safety earth  etc

 

if you plan to do lots of surface mount sense peak do a set of probes which are self standing and have very fine spring loaded tips. great of looking at individual pins on opamps etc.

https://telonic.co.uk/product/4018-sensepeek-pcbite-kit-with-2x-100mhz-and-4x-sp10-probes/

and can be connected to mutimeters, scopes etc.

 

real luxuries because you are taking things too seriously:

ability to measure distortion and noise e.g. keithley 2015

x10 x100 differential scope probe

more multimeters

dim blulb tester (arguably as important or more important as a variac) if you are dealing with a known bad or unknown condition amp

 

NOTE this list does not cover tools for building/repairing amps... only for testing/troubleshooting.

 

I'm sure other headcase members can add to this list and include their own favourites. (I will not recommend anything I have not owned and used multiple times myself.)

Edited January 4, 2022 by jamesmking