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Title: USB Oscilloscope for Audio Work
Post by: Steve-White on April 18, 2021, 12:19:12 am
I've seen this discussed a few times, probably here on PSW, but can't find what I'm looking for using the search engine.

I'm considering adding a USB scope module to the bench.  Seems like a 4ch setup would be real nice for amp testing.  Run both channels on a dummy load and be able to compare input to output.

As well testing DSC's to ensure they are doing what they are setup to do.

Anybody have any experience with these?  Which brands, etc?

http://www.hantek.com/products/detail/10164 (http://www.hantek.com/products/detail/10164)

A stand alone could be simpler overall:  https://siglentna.com/product/sds1104x-e-100-mhz/

Thx
Title: Re: USB Oscilloscope for Audio Work
Post by: Chris Grimshaw on April 18, 2021, 08:17:40 am
I don't know how useful it'll be, but there's a scope built into REW (NB - I'm using the Pro version, can't remember if it's on the free version) which seems to work fine. You'd be limited by the input of your audio interface in terms of sample rate etc. While my (old, analogue) scope runs at 20MHz, my audio interfaces run at 192kHz. Quite the difference.

Curious to see what you end up with.

Chris
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on April 18, 2021, 08:44:06 am
I'll give it a look, seems like I have REW on one of the laptops.
Title: Re: USB Oscilloscope for Audio Work
Post by: Riley Casey on April 18, 2021, 10:17:11 am
The Metric Halo Spectra Foo software has an Oscope function.
Title: Re: USB Oscilloscope for Audio Work
Post by: Frank Koenig on April 18, 2021, 11:11:13 am
In the absence of any specific recommendation (sorry) the first question I think is whether the input characteristics of an audio interface are sufficient or if you need conventional oscilloscope inputs. (I know you know this but I'll spew a few thoughts for my own benefit, if no one else's.) Scope inputs have bandwidth sufficient to show spurious oscillations, RF interference, etc. They typically have 10 M Ohm input resistance with a small shunt capacitance, have built-in ability for AC coupling , low-pass filters, a wide signal amplitude range and compatibility with probes that provide accurate transient representation. They are far more tolerant of out-of-range signals that might toast an audio interface.

An audio interface, on the other hand, will have finer amplitude quantization, which allows distortion and noise measurements, and is perfect for balanced mic and line signals.

For amplifier testing you'll likely need two inputs per amp channel since amp outputs cannot be relied upon to have one side grounded. The app should have the ability to subtract the signals from one another to create a differential input. (There are other ways around this, such as using a transformer, but that introduces its own aberrations.)

I'd take a hard look at the app. The finest piece of hardware won't matter if the app is unusable. Also cross-shop with stand-alone scopes. For a little more money you can get a "real" scope that will never want to update its OS right when you actually need to do something  >:(

Good luck and tell us what you end up with.

--Frank
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on April 18, 2021, 02:52:55 pm
Moving away from a PC based solution to a stand-alone scope.  Looks like something suitable with 4ch capability can be picked up in the $500 range.
Title: Re: USB Oscilloscope for Audio Work
Post by: Frank Koenig on April 18, 2021, 05:50:06 pm
Moving away from a PC based solution to a stand-alone scope.  Looks like something suitable with 4ch capability can be picked up in the $500 range.

Look at Rigol. No direct experience but a trusted friend with much industry experience recommended them to me a few years ago. --Frank
Title: Re: USB Oscilloscope for Audio Work
Post by: Ike Zimbel on April 19, 2021, 12:05:06 pm
Look at Rigol. No direct experience but a trusted friend with much industry experience recommended them to me a few years ago. --Frank
Hmmm. I have Rigol DS1102E on my bench and while it works, I do not love it. I know that's an entry level product, but that might be at least an indication to look above entry level. Another thing to keep in mind is that cheap scopes come with cheap probes, which can be a real liability.
Title: Re: USB Oscilloscope for Audio Work
Post by: Frank Koenig on April 19, 2021, 01:13:52 pm
Another thing to keep in mind is that cheap scopes come with cheap probes, which can be a real liability.

True. To put things in perspective, one probe for my Tektronix scope costs almost twice as much as much as an entire Rigol DS1102E. And those are not by any means the most expensive probes. We used to joke about the terror of running over a $1000 scope probe with your office chair. It would likely make a little crunching noise. I've come close. --Frank
Title: Re: USB Oscilloscope for Audio Work
Post by: Mark Wilkinson on April 19, 2021, 02:02:28 pm

Two $500 or less digital scopes that appear to be consensus favorites on EEVblog and other techy sites, are the Rigol DS1054Z at $350, and a bit more capable  Siglent SDS1104X-E at $500.
I have the Rigol and it is far more capable than I am...  If i were choosing again, i'd get whichever one had the easiest menu navigation & settings methods.
For amp testing I really like an old Fluke 123 battery powered handheld......it's isolated and i can't blow it up haha....and it's a handy little bugger.
Title: Re: USB Oscilloscope for Audio Work
Post by: Frank Koenig on April 19, 2021, 02:13:03 pm
For amp testing I really like an old Fluke 123 battery powered handheld......it's isolated and i can't blow it up haha....and it's a handy little bugger.

Good point about having an isolated instrument. If you get a battery-powered scope you can float the scope and not worry about a differential input. Just be sure that it's really floating and be aware that a big amp can put out enough voltage to give you a shock. -F
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on April 19, 2021, 04:35:51 pm
Thanks much guys for the comments.  The part about some amps floating both outputs could cause real problems with a 4 channel scope.  Maybe I better pick up two 2 channel scopes.  I've been thinking about battery operated for isolation.  On the input side I have a ART DI box that should work ok for just basic level checking and maybe some burst tests for output power.  Driving both channels and monitoring them independently would be fine.

Testing amps today is very different from what I did the past with a Tektronics 555, wire wound ceramic 250W resistors and a Phase Linear 700.  Back then we tested for RMS continuous and assumed 2+ db of headroom.  Today I have 5000W continuous rated amps with a 20A 120V power plug on it.  Yeah it will run on 240V and the pigtail can be changed, but I don't feel it necessary for any continuous power testing at those levels.

At power, planning on burst testing to confirm specs are met and low power level for gain and a look at waveform.

Won't just be testing the amps, DSC's as well to ensure unity is unity and limiter setting levels and such.

I just took a DVM and checked amp outputs, they are all single ended - so a 4ch scope will work ok.  Differential inputs won't be a big deal - probably just do a pick off from the signal generator signal or use isolation balancing transformers.

The Siglent looks to be plenty of scope for this basic testing:  https://www.siglent.eu/product/1139249/siglent-sds1104x-e-100mhz-four-channel-oscilloscope

Now to start looking at Function Generators as well.  :)
Title: Re: USB Oscilloscope for Audio Work
Post by: Ike Zimbel on April 19, 2021, 05:10:25 pm
Thanks much guys for the comments.  The part about some amps floating both outputs could cause real problems with a 4 channel scope.  Maybe I better pick up two 2 channel scopes.  I've been thinking about battery operated for isolation.  On the input side I have a ART DI box that should work ok for just basic level checking and maybe some burst tests for output power.  Driving both channels and monitoring them independently would be fine.

Testing amps today is very different from what I did the past with a Tektronics 555, wire wound ceramic 250W resistors and a Phase Linear 700.  Back then we tested for RMS continuous and assumed 2+ db of headroom.  Today I have 5000W continuous rated amps with a 20A 120V power plug on it.  Yeah it will run on 240V and the pigtail can be changed, but I don't feel it necessary for any continuous power testing at those levels.

At power, planning on burst testing to confirm specs are met and low power level for gain and a look at waveform.

Won't just be testing the amps, DSC's as well to ensure unity is unity and limiter setting levels and such.

I just took a DVM and checked amp outputs, they are all single ended - so a 4ch scope will work ok.  Differential inputs won't be a big deal - probably just do a pick off from the signal generator signal or use isolation balancing transformers.

The Siglent looks to be plenty of scope for this basic testing:  https://www.siglent.eu/product/1139249/siglent-sds1104x-e-100mhz-four-channel-oscilloscope

Now to start looking at Function Generators as well.  :)
That certainly looks like a step up from my Rigol. It does make me wonder how they can get so many features in there for the price. I'd be very interested to hear your impressions if you go for it.
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on April 19, 2021, 07:44:48 pm
That certainly looks like a step up from my Rigol. It does make me wonder how they can get so many features in there for the price. I'd be very interested to hear your impressions if you go for it.

I'll let you know Ike.  I picked up some 1000W load resistors, 2 x 16 ohm and 2 x 8 ohm.  Full power burst testing or close, then maybe 1/2 power or lower continuous runs to ensure the cooling system is working as it should.

I picked up about 1/2 the amps used, some B stock, open box, what have you.  Always been one to bench test anyway.  Thinking the Siglent may be worth a try.

Now on to a function generator.  :)
Title: Re: USB Oscilloscope for Audio Work
Post by: Mark Wilkinson on April 21, 2021, 09:48:59 am
I'll let you know Ike.  I picked up some 1000W load resistors, 2 x 16 ohm and 2 x 8 ohm.  Full power burst testing or close, then maybe 1/2 power or lower continuous runs to ensure the cooling system is working as it should.

I picked up about 1/2 the amps used, some B stock, open box, what have you.  Always been one to bench test anyway.  Thinking the Siglent may be worth a try.

Now on to a function generator.  :)

Hey Steve, don't mean to deter you from getting a scope, but since you started out thinking about a USB scope, this might be of interest.

I've been playing with wavelets as a better way to find sub timing, per Pat Brown's neat article https://www.prosoundtraining.com/2019/07/26/signal-aligning-using-wavelets/
At the end of the article, he shows how Arta (a $100 dual-channel FFT) can generate the wavelets and capture them with a little bit of built in digital scope recorder capacity.
It works so well, and is crazy repeatable.  (First screenshot below are wavelets from Arta of a sub (tan) and main (green) time aligned at 100Hz.)

Ok, this thread and the mention of REW having a scope made me take a look to see if REW could also do the wavelet thingy. Well... does it even better.
Second screenshot is a microphone capture of a 100Hz wavelet generated by REW and captured by its Scope.  It's a neat little scope, cursors and all!

Anyway, I'm realizing I'd rather use REW or Arta with their built in signal generators designed specifically for audio, than my Rigol Arb Function generator.
And with a scope as decent as REW's, as easy to use, and for basically no more than checking out limiting/clipping/distortion in the audio spectrum....well, i'm wondering how much i really need a hardware scope.
I guess the only thing i've measured that would still need a real scope, is how fast amp limiters kick in, using a scope's Roll Mode.....
But even there i'm beginning to wonder...2cents of fwiw
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on April 21, 2021, 02:59:44 pm
^^^ Mark, I appreciate your input.  My simple response is ALL OF THE ABOVE.

I need it all, as I'll be doing everything mentioned.  Bench testing amps on dummy loads, a stand alone 4ch scope will lend itself well to that.  Run 'em up, make sure cooling fans work and such.  Run some sine wave at lower power continuous levels, check cooling fans function and some burst testing.  Not looking for an amp dyno.  :)  Just to be able to week out problems.

Function generator or Laptop for the source.  Laptop may be the best solution, not sure there yet.  But yeah, some of the testing will be utilizing a microphone and there the laptop applications are how it's best done.

Will likely pick up a function generator and have both available there as well.  Have a couple of Behringer USB IO's  and a Scarlet as well.

Rather than overthink things, which by the way I'm good at - pick up mid line stuff so I can have the variety of options and see what my preferences end up being.
Title: Re: USB Oscilloscope for Audio Work
Post by: Ike Zimbel on April 22, 2021, 04:50:58 pm
^^^ Mark, I appreciate your input.  My simple response is ALL OF THE ABOVE.

I need it all, as I'll be doing everything mentioned.  Bench testing amps on dummy loads, a stand alone 4ch scope will lend itself well to that.  Run 'em up, make sure cooling fans work and such.  Run some sine wave at lower power continuous levels, check cooling fans function and some burst testing.  Not looking for an amp dyno.  :)  Just to be able to week out problems.

Function generator or Laptop for the source.  Laptop may be the best solution, not sure there yet.  But yeah, some of the testing will be utilizing a microphone and there the laptop applications are how it's best done.

Will likely pick up a function generator and have both available there as well.  Have a couple of Behringer USB IO's  and a Scarlet as well.

Rather than overthink things, which by the way I'm good at - pick up mid line stuff so I can have the variety of options and see what my preferences end up being.
This post on parameters to look at when choosing a scope popped up in my inbox today http://www.saelig.com/news/newsletters/saelig-042221-newsletter.pdf (http://www.saelig.com/news/newsletters/saelig-042221-newsletter.pdf) I do not have any affiliation with this company, but I have purchased from them in the past and was satisfied with the transactions. Just FYI.
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on April 26, 2021, 01:06:31 pm
Having deviated from the original concept of a USB interface with software.

I ordered:

* Siglent SDS1104X-E  4 ch LCD Oscilloscope

* Siglent Technologies SDG1032X Arbitrary Function Generator

Have two 1000W 8 ohm resistors and two 1000W 16 Ohm resistors.  Ordered two more 8 ohm, will build up a dual 4 ohm 2000W/ch dummy load.

* 1/4" Aluminum plate, fans, heat sink ordered.  Not planning on any torture testing, but want the load to remain on the cooler side to maintain consistency and just not fry anything as amps will be run up to test cooling.
Title: Re: USB Oscilloscope for Audio Work
Post by: Mark Wilkinson on April 27, 2021, 12:32:29 pm
Cool Steve, nice gear, enjoy.

You have better testing resistors than my cheap butt would spring for......I just have water heater elements in a bucket of water
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on April 27, 2021, 02:26:33 pm
Thank's Mark.  Definitely didn't start with this type of gear.

Here's a picture of the first audio load I used, built it in the early 80's - 2 of the new 1000W 8 ohm resistors (other two are ordered) sit atop the old 240W Heathkit.  It sported 4 x 60W wire wound resistors and would get quite warm running up a Phase Linear 700.  Today's amps would incinerate it in seconds.

I talked to a technical rep at Crown this morning for general testing regimen do's and don'ts - good conversation.  Learned a few things that I've never done in the past when testing amps.  When and how to use sine wave continuous testing, and 1/8 power and 1/4 power 12db crest pink noise testing, burst testing.  He suggested looking at the power and thermal tables for monitoring power draw on the amp.  To really determine overall health and confirm that an amp is working and healthy would be to monitor the current draw of the amp during testing, in addition to output, signal quality, heat, etc.

But, not on a 240W Heathkit dummy load - time to retire that one.

I'll post some pictures of the new dummy load when it's completed - not sure if that's anything of use as most guys are running powered stuff these days.
Title: Re: USB Oscilloscope for Audio Work
Post by: Ike Zimbel on April 27, 2021, 03:28:27 pm
Thank's Mark.  Definitely didn't start with this type of gear.

Here's the first audio load I have, built it in the early 80's - new 1000W 8 ohm resistors sit atop the old 240W Heathkit.  It sported 4 x 60W wire wound resistors and would get quite warm running up a Phase Linear 700.  Today's amps would incinerate it in seconds.
I'm jealous! My furnace has been out of action for a week...could have used some heaters like that! New furnace coming tomorrow. That'll keep me off the o'scope market for a while  :'(
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on May 07, 2021, 01:38:03 pm
I'm jealous! My furnace has been out of action for a week...could have used some heaters like that! New furnace coming tomorrow. That'll keep me off the o'scope market for a while  :'(

:)  That's something I need to do at my place in the next year or so - replace furnace/air handler simply because it's 25 yrs old now.

Yeah, these things should pump out plenty of heat.  Not sure where the resistors are used, but appear to be some kind of industrial application as they are referred to a "Braking Resistors".

Scope and Function Generator arrived.  Unpacked and put on the bench, fired 'em up last night to have a look at the screen.  Display doesn't have the contrast or sharpness I'm used to looking at on ancient Tektronics scopes of the past.  Not a problem, just an observation - footprint of the gear is fantastic as is the energy efficiency.  I do like the the full color display over the monochromatic stuff of the past.

Looks like some gear I'll be happy with for the duration.  Need to figure out how to setup the Function Generator for various modes, continuous sine wave output is simple to get going.  I ordered some 1/8" & 1/4" aluminum plate and a couple of 120V 120mm box fans.  As well a couple of large heat sinks.  I'm probably going to mount the load resistors up into a cheap blow molded SKB 4U road case for versatility in use and just overall packaging as well as functional airflow aspects.

First thing the Crown rep asked me was what power source I would be using.  He also said the amps could handle full RMS rated power output at 4 ohms for 2-3 minutes duration just fine.  Not sure at this point if I ever would want to do that - maybe for a short duration blast.  In which case it's going to take 240V supply.  Will probably end up doing amp testing in the main shop, use the welder plug and a generator extension cable for power when running the large racks up.  L14-30R twist lock for the welder plug in the shop, which is what I am going to use for the large racks anyway.
Title: Re: USB Oscilloscope for Audio Work
Post by: Ike Zimbel on May 07, 2021, 03:06:09 pm
:)  That's something I need to do at my place in the next year or so - replace furnace/air handler simply because it's 25 yrs old now.

Yeah, these things should pump out plenty of heat.  Not sure where the resistors are used, but appear to be some kind of industrial application as they are referred to a "Braking Resistors".



Diesel-electric locomotives are often equipped with "dynamic braking". The actual propulsion in these units is via electric motors in the trucks (bogies, running gear) and when these are not providing traction (ie when the train is coasting, or decelerating) they effectively turn into generators. The current they generate can be routed to resistor grids, which provide a load, which provides dynamic braking to the wheels. And, yes, they have big fans to cool the resistor grids.
Title: Re: USB Oscilloscope for Audio Work
Post by: Russell Ault on May 07, 2021, 03:37:54 pm
Diesel-electric locomotives are often equipped with "dynamic braking". The actual propulsion in these units is via electric motors in the trucks (bogies, running gear) and when these are not providing traction (ie when the train is coasting, or decelerating) they effectively turn into generators. The current they generate can be routed to resistor grids, which provide a load, which provides dynamic braking to the wheels. And, yes, they have big fans to cool the resistor grids.

It's not just locomotives; almost any electric motor can be effectively (and cheaply) braked using an electrical load (although, depending on how much momentum is involved, actually stopping using only dynamic braking can be a challenge). One close-to-home example is the auto-stop function when you release the handle on an electric lawnmower (which is often accomplished using nothing more complicated than a long piece of wire).

I would guess your resistors are designed for an application that falls in between these two extremes (probably, as you guessed, in an industrial automation setting).

(As a total aside, if your train is electrically powered you can use voltage converters instead of resistors to produce "regenerative braking", which effectively uses other trains as your train's braking load. Needless to say this represents a significant efficiency gain, especially on rapid transit systems where stopping is frequent, or on mountainous systems where a descending train can produce a decent portion of the electricity been consumed by an ascending train.)

-Russ
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on May 07, 2021, 05:17:12 pm
^^^ Yeah it seems I've seen some of that as it applies to hybrid automobiles.  The coast down and braking recirculated to battery charging.

These resistors are built, very solid and large, but the wire size is right at what would be rated power in terms of core area or gauge size.  For the usage in my case is fine, just an observation.  I did the math, 250W RMS is 25A @ 100V RMS so each 8 ohm resistor will be carrying 12.5A RMS at rated power.  That's just for awareness, so I don't get any surprises.

I plan on milling the bottoms of the resistor cases flat and using thermal paste at the interface with 1/4" aluminum plate - with large heat sink on the other side.  May not be needed, I don't know yet.  But, when using the old Heathkit it got hot - I only held the amps full tilt for about 30 seconds and that will likely be a protocol that's followed today.  Probably  a bit longer duty cycle for burst testing.  Overbuild, and you only build once.  :)

I'm only interested in giving the amps a clean bill of health for field operation.  A basic checkup, gain, low power signal, high power signal, output RMS & burst and make sure the cooling system is operating as it should.

The resistors are intended for use in a similar environment to amp testing.  No huge voltage/current spikes as judged by the wire size on them.
Title: Re: USB Oscilloscope for Audio Work
Post by: Steve-White on May 07, 2021, 05:18:48 pm
Diesel-electric locomotives are often equipped with "dynamic braking". The actual propulsion in these units is via electric motors in the trucks (bogies, running gear) and when these are not providing traction (ie when the train is coasting, or decelerating) they effectively turn into generators. The current they generate can be routed to resistor grids, which provide a load, which provides dynamic braking to the wheels. And, yes, they have big fans to cool the resistor grids.

I was thinking train braking when I looked at them.