Quad ESL-63 Resurrection

This post is being updated as I do different things to repair and refurb a pair of 42 year old Quad ESL-63 electrostatic speakers. Check back every couple days for updates that will be added to the bottom of this post.

I've been a fan of ESLs since I learned about them and started building them myself in Japan back in the 80s. In the early 90s I had acquired a pair of Quad ESL-63s that I repaired and used for a few years. I was always dissatisfied with their mechanical construction- much too complex and flimsy. I don't think Quad ever had a manufacturing engineer look at the design, because it could have been much simpler, more robust, and probably a lot less expensive to manufacture if they had. I designed and built new frames for the drivers from those speakers using t-slot aluminum, but never got around to rebuilding those speakers in the new frames. I still have all that stuff that I may put to use soon. Quad's follow-up to the ESL-63, The ESL989, was more solidly constructed, as are their current offerings.

I recently found a Craig's List deal on a pair of ESL-63s with their shipping boxes. The seller stated one works fine and the other makes noise. The speakers were listed for almost a month in Chicago, where you'd think there would be at least one audio nut willing to take on the challenge of repairs, or to strip them for parts to sell on ebay. Apparently not. After seeing them listed for a month, I decided to check them out.

Outwardly, they are in perfect physical condition, indistinguishable from new, except both were missing the original plastic feet and a couple screws from the bottom covers. Not bad for speakers made in 1983. You can learn all about Quad ESLs (the original ones, and the 63s) here. Here are the owner's and  service manuals for the ESL-63.

ESL-63 on the left, next to B&W 703 S3 on the right for size comparison.

I handed the man $300 and took them home. Thanks Tom! I assumed that I was going to have to work on them, so there wasn't much point in testing them before I bought them.

When I got home, I tested them. Sure enough, one works fine the other makes noise. Even though one is working, both will need some work to ensure reliable (is that possible with ESL-63s?) operation for the next several years. 

I started the usual way- replacing the electrolytic caps. There are exactly two in each speaker. A 1000 uF 16V cap on the power supply for the arc sensor, and a 220 uF nonpolar electrolytic input coupling cap. 

Accessing the caps is done through the bottom of the speaker. I turned it upside down and leaned it against my work table and removed the screws holding the bottom cover in place. On the inside of the bottom cover there are two strips of foam tape that have rotted and will need to be replaced. This foreshadows MUCH more to come...

Rotted foam tape inside the bottom cover. It only contacts the PCB in the chassis in two small areas, so I'm not sure why they used such long pieces of tape.

 

I ordered new caps from Parts Express- 220 uF nonpolar electrolytics, and a pair of Audyn 4.7 uF PP film caps to bypass them, and while I was waiting for delivery, I printed new TPU feet for the speakers. When the caps arrived, I went in to install them. 

That's where I ran into the first problems. In the first speaker (the one that worked), the 220 uF cap's solder connection to the input terminal had failed and it was making contact only by the strength of the spring force of the leads! I also found a black ground wire that was supposed to be soldered to a lug, just left floating in the air. Oops! 

The PCB that the caps both mount on is held in the enclosure by two + head screws. Those screws are at the bottom of the enclosure, making it almost impossible to remove them to release the PCB, even with a right angle drive wrench and screwdriver bit. Fortunately, there's enough room to get a soldering iron in without removing the board, so I was able to remove the old parts and install the new ones with minimal effort.

Bottom of one of the speakers with the cover removed. The nonpolar cap is the blue part located under the cement resistor, circled in green. There's plenty of room for the replacement caps I ordered. The only thing supporting the cap is the wire that wasn't properly soldered to the input connector and that thin yellow wire that's going to the PCB. Hmmmm. Not what I would expect in a very expensive speaker.

Input board screws. Now I'm starting to remember what my ESL-63s were like back in the 90s- a mechanical nightmare! The white wire is the antenna for the arc detector. I had to unsolder it to get to the solder pads for the input and 1000 uF caps.

Unsoldered ground wire. Not really a big deal- it's one of two wires that ground the aluminum bottom cover of the speaker.

After I removed the old input coupling cap and resistor you can see the 1000 uF cap on the input board. 

New input coupling caps installed. The new 1000 uF cap is under there, too. I was able to unsolder the old parts and solder in the new parts from the bottom side of the PCB without having to take the board out of the chassis.

I had forgotten what a PITA these speakers are to work on. My intention with this blog post was to get to the driver fix(s) quickly and get the thing working again, but as I got deeper into the speaker, I found a LOT of different foam and other tapes that had rotted away. I spent 3 days cleaning the old tape and adhesives off the different parts, replacing it with new stuff that I chose to hopefully last longer than the original stuff.

Keep in mind, that everything I am doing to replace rotted foam tape in the first speaker will also have to be done to the other speaker. The speakers won't work reliably without replacing all that tape- some of it is inside the dust covers. In all, I spent about $100 on all the tapes, solvents, and glue supplies needed to fix both speakers.

Accessing the drivers

The driver enclosure consists of two vertical aluminum rails, plastic top and bottom plates, and two vertical aluminum support rods, front and back, all bolted to the electronics chassis. 

The speakers are covered by cloth "socks" attached to the frame by hook type tape on the top of the driver enclosure. Behind the sock there are metal grids on the front and back side of the speaker that provide some mechanical protection for the drivers and keep curious fingers away from the high voltage inside. 

The step by step procedure to gain access to the drivers is to slide the top wood cover to one side and lift it off, then release the sock from the hook tape and slide it down on the speaker frame. Next, peel off the tape holding the metal grids at the top and bottom of the frame and pry them free of the crumbling foam tape that holds them to the top and bottom plates of the driver enclosure. Then pull the thin wires out from the vertical edges (apparently used to fill the space in the aluminum side rails), and remove the front and rear grids. Finally, snap the frames holding the dust covers off the speaker frame, slide them out sideways, and set them aside.

Sock attachment at the top of the speaker with the wood top cover removed. There's hook tape under the sock holding it in place.

Hook tape on top of the driver enclosure.

Gooey adhesive under the hook tape.

I found the best way to remove this stuff was to start by rolling it off instead of scraping it.

Adhesive booger removed from the top of the driver enclosure. This was the stickiest, gooiest, stuff I've ever seen.

The metal grids have what looks like gaffer's tape covering the vertical edges, and are held in place by more tape along the top and bottom edges, and by double sided foam tape also at top and bottom, and on two vertical supports near the center of the speaker, front and back. All that foam has rotted, and all the tape's adhesive has dried out, so all of it has to be removed and replaced before putting the speakers back together. It's a tedious and messy job.

Front upper left corner of the speaker with the sock pulled down. The metal grid with gaffer's tape on its vertical edge fits into slots in the vertical aluminum frame pieces. The grid is mounted using double sided foam tape at the top and bottom. The green wire fills up the space inside the vertical frame piece so that the grid can't rattle. It is removed by simply pulling it out.

Front upper left corner showing grid coming out of the frame after it has been released from the foam tape. You can see the two vertical supports to the right, just behind the grid.

Front grid removed from the speaker. Gaffer's tape still on the edges. Residue from the rotted foam tape visible along the top and bottom edges.

Brown foam tape residue on the back side of the front grid. This is easily cleaned off with a wire brush. The gaffer's tape at the vertical edges of the grid will also be removed and replaced.

Front bottom left corner of the speaker showing rotted foam tape and residue from the gaffer's tape that held the front metal grid in place.

Front top edge of the speaker showing foam and gaffer's tape residue. One of the two vertical supports (black) is visible at the center.

Poorly positioned gaffer's tape on the edge of the metal grid. That will be replaced, and positioned properly.

Rotted foam tape on one of the four vertical supports. You can see light reflecting off the wrinkled dust cover behind the support.

This is the scraper I use- it has a VERY sharp edge, so be careful!  After scraping, I wipe everything with Goo Gone, Goof Off, or IPA, to remove the adhesive residue. Goo Gone is reserved for parts I can completely remove from the speaker and wash with detergent in a sink after (you have to wash the orange oil off). 

Scraping the rotten foam tape off one of the two vertical supports.

Scraping the rotten double sided foam tape off the top edge of the speaker. The gaffer's tape residue has already been scraped off.

Front bottom of speaker showing the mess created when scraping all the tape residue off the speaker. I keep a vacuum cleaner handy and vacuum this stuff up. You can see the two vertical supports near the center of the speaker.

Wrinkles in dust cover. This is bad, but easily fixed. The wrinkles will cause rattling noises. This will have to be tightened, if I can get it off and back on again without breaking it, or it will have to be replaced. When it's back on the speaker, there should not be any wrinkles. The dust cover is double side taped to a plastic frame that snaps onto aluminum side pieces and plastic pieces at the top and bottom of the speaker frame.

Back side of the speaker with the grid off- requires the same foam tape cleanup as the front side. You can see the dust cover is badly wrinkled here.

 

Dust cover removed, you can see some foam tape hanging down on the driver grid. The black piece is where the dust cover snaps onto the top edge of the speaker. Note- that foam is inside the dust cover- as it rots, it crumbles, and can get inside the drivers and cause all sorts of noise problems.

Bottom corner showing the black piece where the dust cover snaps on, and foam tape. The wire spring like thig is there to ground the aluminum frame pieces and the steel protective grid (already removed). 

Removing the drivers

Each speaker has four drivers, two for "bass" located at the top and bottom of the speaker and two centrally located drivers that are divided into rings that are fed by a delay line to simulate a pulsing spherical radiator. The drivers can be removed without taking the top of the frame off. The procedure is to unsolder the wires from the drivers, then unscrew one of the middle drivers and take it out out of the frame sideways. Then take out the other middle driver the same way, and finally remove the top and bottom bass drivers by shifting them up or down and then remove them sideways through the frame.

You have to unsolder the wires, front and back, in order to remove the drivers. 

This is how you get the drivers out- remove the two in the middle first, then the top and bottom drivers can come out. Each driver is held in by 4 screws that go into aluminum L brackets that run the vertical height of the speaker. Once they are all out, set them aside- there's more foam tape to replace!

The top driver has to be dropped down a bit to get it out of the frame, so you have to remove the driver below it first. The black piece in the green rectangle is the part that the dust cover snaps onto. It comes out with the driver.

This is the top dust cover mount. That rotten foam tape has to be replaced before it falls into the drivers and causes problems. There is a similar piece at the bottom of the speaker that doesn't come out with the drivers.

All drivers removed, the rotten foam tape in the dust cover mount needs to be replaced.

It's a little hard to make out here, but those pieces of foam tape are mounted on an L bracket where the drivers mount. The brackets are on the right and left sides of the speaker. You can also see foam tape that runs the length of the bracket peeking out from behind it. All that stuff is inside the dust covers, crumbling, and has to be replaced.

Here are all the locations of the tapes that need to be replaced. A, C, and D are all inside the dust covers, so it's absolutely critical to clean the old stuff out and replace it with long lasting materials. Double sided tapes are used to mount the steel grids on the speakers and to mount the dust cover film. All others have adhesive on one side only. 

Adhesive tapes for 2 speakers
	location	total length	width	thickness	adhesive	type
A	driver mounting brackets	48"	3/8"	1/4"	single	foam
B	vertical frame supports	248"	3/8"	1/4"	single	foam
C	driver mounting brackets	120"	1/2-3/4"	3/4"	single	foam
D	top and bottom dust cover mounts	184"	3/8"	1/4"	single	foam
E	inside bottom panel	8"	3/8"	1/4"	single	foam
F	top and bottom of frame, holds grids	200"	5/16"	1/8"	double side	foam
G	top and bottom of frame, over grids	234"	1/2"		single	gaffer
H	vertical edges of grids	256"	1"		single	gaffer
J	dust cover frames	448"	3/4"	1/8"	double side	foam/other

Replace the tapes

The procedure with parts that can be removed from the frame is to first scrape as much of the old tape off as possible, then take them to a sink and spray with Goo Gone, an orange oil based solvent that dissolves the adhesive. Let it sit with the Goo Gone for a while then start wiping with a paper towel, folding frequently. Then spray the whole thing again, let it sit a while, then wipe with paper towels again. And again. And again. Finally, when it looks like all the sticky residue is gone, wash the parts with dish detergent and hot water to remove the orange oil.

For parts that aren't coming out of the frame (the bottom dust cover mount), wipe with paper towels soaked with Goof Off several times. Goof Off is very volatile, so use good ventilation. It will evaporate and not leave any residue of its own, but you may have to wipe several times with it to get rid of all the adhesive residue. 

I measured the old tape and looked for modern substitutes. I selected some EPDM rubber weather seal to replace most of the old foam tape, as it should last a lot longer than that stuff that was in the speakers. The thick foam tape that goes on the vertical driver mounting rails was replaced with some this weatherseal:

Thick foam tape for the driver mounting brackets, C in table above. I bought 1 roll of this stuff. It's 1" wide, so I cut it to the length I need (31") then cut it length-wise so I have two strips that are 1/2" wide. There will be some left over.

Small EPDM rubber foam tape for A, B, D, and E in table above. I bought three rolls of this for the two speakers and will have some left over.

I also bought some 3/4" wide Scotch brand "permanent" double sided tape to use on the hook tape that holds the socks on the speakers, and for the dust covers, and this 1/4" VHB tape to mount the steel grids. Finally, I also bought some 1/2" and 1 " wide gaffer's tape to mount the steel grids.

One of the two driver brackets (per speaker) with new foam tape installed.

Top dust cover mount with new foam tape installed.

I peeled the tape off the vertical edges of grids and found that the adhesive had completely dried out, much of it turning to dust as I peeled it off. The rest scraped off easily. The foam tape residue was a bit more tenacious. I used the scraper to get most of it off, then used a wire brush to get the rest. Finally, I vacuumed and wiped the grids with IPA.

At this point, I have put in at least 12 hours of labor cleaning up the old tape and its residue and shopping for and installing new tape. That's for one speaker, and I haven't even repaired the driver(s) or dust covers yet. This is why 40 year old ESLs can be purchased cheaply, and why it would be cheaper to buy new ones than to have a pair of old ones professionally refurbished!

The Drivers

The drivers have two plastic grids with two stator PCBs glued to them. The grids hold the two stators parallel and about 4.8 mm apart. There is a thin plastic diaphragm with a high resistivity coating glued to one of the grids, between the stators, putting it about 2.4 mm away from either stator. The speaker has a 5.25 kVDC bias supply that connects to the stators and the diaphragm. If the diaphragm breaks and contacts the stators, or the stators come unglued from the grids and contact the diaphragm, or some foreign material has found its way between the stator and diaphragm, the driver will make hissing, whining, and popping noises. My bad speaker makes such noises that you can hear, here.

There are three screws and some metal spring clips holding the two plastic grids together. Carefully remove the screws and then the metal clips to separate the two halves of the drivers. The diaphragm will be glued to one of the grids. Look carefully at both sides of each grid. Look for any loose stators by gently pushing on them with your finger tip, and look for holes or tears in the diaphragms and burn marks on the stators. A torn diaphragm, or one that has holes, has to be replaced. Loose stators can be reglued without replacing the diaphragm(s). 

The best case is that the stator(s) have come unglued, because they can be reglued without replacing the diaphragm. The worst case is that the diaphragm is broken and the stator(s) have become unglued. In that case, the diaphragm has to be replaced, and stators have to be reglued.

Upon inspection, I found that all four drivers had some areas where the stators had come loose from the grids. I will be regluing all of them, in both speakers.

One of the drivers opened up. The gray stuff is the high resistivity coating on the diaphragm- it's supposed to look like that. The metal tape at the edges of the top part makes contact with the coating on the diaphragm and allows charge to transfer to the diaphragm. You can see a thin, cloth covering the stator. That's there to dampen diaphragm resonance.

Regluing stators

There are couple ways to approach this- you can sacrifice the diaphragms and completely remove the stators from the grids, allowing you to clean off the old glue and reglue them to the grids with all new glue, but that will require replacing all the diaphragms- a tricky process, prone to failure.

Alternatively, you can remove the stators from the grids that don't have diaphragms, clean off the old glue, and reglue them with new glue, and then reglue the other stators with the diaphragms in place. A slightly tricky process, but not nearly as bad as replacing the diaphragms. 

I decided on the latter method, using clear polyurethane Gorilla Glue applied with a syringe. 

2/19/25

The last of the gluing and taping supplies I ordered arrived today. 

I took one of the stators out of one of the bass drivers today to assess the condition of it and the grid and to figure out how I want to proceed with regluing it. Taking it off the grid required desoldering two metal connection clips.

Once I had it out I could see that the copper side of the stator, the side that gets glued to the grid, is covered with some sort of clear plastic coating. There are a couple small spots along the edges where the coating seems to have failed and the copper looks corroded. 

I gently scraped the larger bits of glue off the stator, being very careful not to damage the clear coating. I also scraped the glue off the grid. Then I set the stator back into the grid to see how it sits. The three screw posts at the center of the grid protrude loosely through holes in the stator, and it looks sufficient to align the stator within the grid, but I may make some spacers to ensure alignment at the edges anyway.

The stator is pretty flexible and has a bit of a curve to it, so it will have to be weighted down to ensure that it glues onto the grid as flat as possible. It requires extremely light force to flatten the stator in the grid, so I cut three pieces of the 1"x1" foam weatherseal to the width of the stator and will stick it to a board. The weatherseal foam is very soft so the weight of the board I stick it to should be sufficient to guarantee that the foam will compress a bit and the stator will be glued flat in the grid.

Soft foam strips laid out on stator where they will be taped to a board to ensure the stators are glued flat in the grids.

Corrosion along the edge of the stator.

Check back here in a couple days to see how I reglued the stators...

New test equipment: Siglent DSO and AWG

I recently decided to step up my test equipment game by getting a digital storage oscilloscope (DSO) and an arbitrary waveform generator (AWG). These will come in handy for fixing stuff, or when making electronic devices. 

The AWG (left) and the DSO (right)

After some research, I chose instruments made by Siglent for four main reasons. First, they are both very capable instruments for their price. The nearest competitor is Rigol, but Siglent beats them in almost all specs and operations, for just a few $ more. 

Second, these devices are made in China. The very stable genius/convicted felon/insurrectionist in clown makeup says he's going to impose tariffs on everything from China. That means the prices of these instruments may rise dramatically in the coming months, so get them now or maybe never.

The DSO is the SDS804X which is a 12 bit (8.4 bit ENOB, SFDR 35 dBc), 4 channel, 2 Gsps, 70 MHz BW scope. Siglent also makes 100 MHz BW and 200 MHz BW versions of this scope with greater memory depth. In fact, all three scopes use the exact same hardware, the only differences are the model number labels on the front panel. The higher BW and memory depth models simply enable those features in firmware. 

This brings me to the third reason for buying these devices instead of others. Someone figured out (or leaked from the factory) a keygen that enables you to turn on the wider BW and memory depth so you can turn your 70 MHz scope in to the 200 MHz BW scope. Of course, making the modification voids the warranty, so it's a good idea to make sure everything works as it is supposed to before upgrading. If you had a warranty covered problem with the scope, maybe it could be downgraded back to 70 MHz BW before trying to get service, but who knows if there's a log file somewhere showing the mod and subsequent unmod.

The fourth reason for buying two Siglent products is that they can talk to each other via USB or network. That can be useful for certain types of measurements. More on that below.

The scope comes with 4x switchable 1x/10x, "70 MHz" probes. However, the 70 MHz probes are identical to the 200 MHz probes, minus some accessories that come with the 200 MHz probes. So even after the "upgrade" the probes will be fine.

I bought the DSO new, via amazon, and the AWG, used, via ebay. 

The DSO: SDS804x

When I started working as an engineer back in the early 80s, a scope with 1/4 this one's capabilities would have weighed about 80 lbs, and would come on a wheeled cart, and would have cost about 2 year's pay.

What sort of capabilities? The ability to do math on the captured waveforms is a big plus. That includes, among many other built in functions, the ability to do Bode plots (aka frequency response curves), and Fast Fourier Transform that converts time domain data captured by the scope to frequency domain - i.e. spectral - plots. Rise and fall time measurements, signal frequency, etc., with full statistics are just a couple button pushes away. Captured data can be stored on a thumb drive and then imported into Matlab or similar software for more heavy-duty number crunching.

Dual trace analog CRT based scopes could not monitor two points in a circuit simultaneously because the CRT could only provide one electron beam. A two channel scope typically alternated between the two channels, sweeping one trace then the other one the CRT. There was something called "chop" mode that would rapidly switch the sweep back and forth between the two channels to "sort of" monitor two points in a circuit at the same time, but you had to be very careful about interpreting what the scope was showing you in that mode.

OTOH, a single sweep on an analog scope showed the waveform in real time. There were also DSOs that used CRT displays, so they could capture a waveform and display multiple traces on the screen from memory. They suffered from the same limitations that today's DSO do, except these days the memory and A to D converters are much cheaper so today's DSOs use much more of both.

DSOs typically have multiple A to D converters and can sweep multiple channels simultaneously, so you can monitor multiple points in a circuit in "real-time" (within the limits of the sample rate, of course).

Like dual trace CRT based scopes, DSO's require constant vigilance to avoid being fooled by  what the scope is showing you. The Siglent scope always displays the sample rate and memory depth on the screen. That's not just for convenience. The actual BW of a measurement is 1/10th of the sample rate. Though the spec says it's a 200 MHz BW scope, that is the maximum BW it can achieve when it's using 1 channel operating a 2 GSPS and using the full 100 Mpts memory. The sample rate, and BW drop as a function of the memory depth, sweep setting, and the number of channels in use. For example, if I am using one channel and set the scope to 100 ms/div, it runs at 100 MSPS and uses the full 100 Mpts in memory. That's 10 MHz BW, not 200! If I turn on a second channel the memory is split between the two and the sample rate drops to 50 MSPS with only 5 MHz BW in each channel. If you're looking for a transient that lasts a few picoseconds, you're not going to see it.

There's a very good explanation of DSOs vs analog oscilloscopes here.

As the article linked above points out, DSOs can do a lot of math that the old analog scopes couldn't. Rise time, fall time, period, frequency, skew between channels, etc. can all be displayed, and since the numbers are calculated from the waveform in memory, you can set up multiple measurements and display them at the same time. It also presents statistics on the measurements including min/max, mean, standard deviation, etc.

The scope also does dozens of things geared toward factory production of electronic gear that I'll never make use of. The 243 page user manual (and several others) is here.

The AWG: SDG1032x

The AWG is a two channel, 14 bit, function/arbitrary waveform generator that works up to 30 MHz. Another keygen enables it to be bumped up to 60 MHz because it uses the exact same hardware as the SDG1064x AWG. I had some trouble getting the keygen to work on my AWG, but I'll keep trying. In the meantime 30MHz is fine.

The AWG has dozens of waveforms built in, and can use data generated by free software called EasyWave and entered via network to create almost any waveform you might want/need. It also has multiple modulations built in, and the two channels outputs can be combined or synced. The manuals are located here.

Sine wave specs for the AWG. Distortion at audio frequencies isn't spectacular, but adequate for many tests.

Siglent has a bunch of app notes that explain how to use their scopes and AWGs to make different measurements here.

So what can you do with this stuff?

I recently bought an Advance Paris A12 stereo amplifier. Among it's many features are two line level subwoofer outputs with switchable LPF frequencies of 75 and 150 Hz. I have inquired about the slope of the LPF and have been unable to get a response anywhere. So how about measuring it?

If I connect the AWG, the DSO, and the amp together like this:

The test setup. Channel 1 of both the AWG and the DSO are for the reference signal. Channel 2 is for the device under test (DUT-  the amplifier). There are BNC connectors on the AWG and DSO, and phono connectors on the amp.

I can set the DSO to do a Bode plot that will cause the AWG to sweep the signal frequency while it monitors the output from the amp and plots the gain and phase difference between the input and output signals. The AWG is set so that channel 1 and 2 are synced meaning their frequency and phase are locked together- as one changes, the other will change equally, under control of the DSO. The DSO will compare the signal coming through the amp to the signal coming directly from the AWG and plot the differences in amplitude and phase vs. the frequency. 

There's a good explanation of Bode plots here.- I'm going to have to do some studying- it's been a long time since I dealt with this sort of thing.

To verify my settings, I connected the AWG channels 1 and 2 directly to the DSO channels 1 and 2 and ran the sweep. The image below shows essentially zero difference in amplitude and phase as it should. The minor differences seen may be due to differences in the two cables I used- one was a BNC to BNC cable, the other a scope probe, just poked into the AWG output jack, and/or the limited resolution of the scope.

Bode plot of the AWG connected straight through to the DSO, frequency swept from 10 Hz to 5 kHz.

The LPF in the amp can be switched from 75 Hz to 150 Hz, so I ran two tests, one for each LPF setting, sweeping sine waves from the AWG over a frequency range from 10 Hz to 2000 Hz, measuring 20 points per decade. I set the AWG to 100 mVrms out so I wouldn't overload the amplifier, and disconnected the speakers and subwoofer before running any of the tests. I adjusted the volume control on the amp to get the test signal close to the 0 dB line on the Bode plot.

I connected the amplifier using two BNC to RCA (phono) cables with the LPF switch set to 75 Hz and swept again. This time I turned on two cursors (X1, X2) on the amplitude trace as well as 3 dB point, gain and phase margin measurements (P1, 2, and 3).

The 75 Hz LPF test. The DSO says the 3 dB point is 80 Hz (P1, lower left), and cursors say it's at 81 Hz. It probably varies a little depending on where you start. P2 shows gain margin. P3 is supposed to show phase margin- not sure what happened there.

I switched the LPF to 150 Hz and ran the test again.

The 150 Hz LPF filter test. I've turned on cursors and a couple measurements. The cursors show the 3 dB point of the filter is at 166 Hz and the scope's measure of the 3 dB point is 167 Hz. The scope and I probably chose slightly different starting points for the measurement.

The DSO can save the data from these plots as .CSV files like this one from the 75 Hz test:

Instrument Name,SDS824X HD

Serial Number,SDS08A0X806378

Software Version,3.8.12.1.1.3.8

Awg Type,USB

DUT Input Source,CH1

DUT Output Source1,CH2

DUT Output Source2,None

DUT Output Source3,None

DUT Channel Gain,Auto

Sweep Type,Simple

Awg Amplitude,0.1V

Awg Offset,0V

Awg Amplitude reference level,1V

Awg Load,HighZ

Awg Amplitude Unit,Vrms

Sweep Mode,Logarithmic

Start Frequency,10Hz

Stop Frequency,2000Hz

Sweep Line,50

Sweep Log(dec),20

Amplitude Mode,Vout/Vin

Amplitude Axis Type,Logarithmic

Amplitude Axis Range,-92dB,-28dB

Phase Unit,Degree

Phase Axis Range,-120Deg,80Deg

Bode Data

Number of Points,48

Frequency(Hz),CH2 Amplitude(dB),CH2 Phase(Deg)

10,-0.316206946,176.898533

11.2201845,-0.301349145,174.827017

12.5892541,-0.295593542,172.720797

14.1253754,-0.285307544,170.482712

15.8489319,-0.282871402,168.078595

17.7827941,-0.278952734,165.495564

19.9526231,-0.276636798,162.72877

22.3872114,-0.283139497,159.650485

25.1188643,-0.286777113,156.27685

28.1838293,-0.302849299,152.508918

31.6227766,-0.325629345,148.256579

35.4813389,-0.37922284,143.414019

39.8107171,-0.448551886,137.938013

44.6683592,-0.57699543,131.685234

50.1187234,-0.785198164,124.551014

56.2341325,-1.1070296,116.547249

63.0957344,-1.58892266,107.680373

70.7945784,-2.27801731,98.1979734

79.4328235,-3.19725386,88.408361

89.1250938,-4.36847629,78.6493903

100,-5.76128114,69.3966066

112.201845,-7.33886852,60.8239298

125.892541,-9.07129381,53.1396052

141.253754,-10.8753204,46.4291769

158.489319,-12.7822801,40.5519963

177.827941,-14.7305102,35.324053

199.526231,-16.6876153,30.6849254

223.872114,-18.6739893,26.8189145

251.188643,-20.6655185,23.2595056

281.838293,-22.6610596,20.0543472

316.227766,-24.6691554,17.3480928

354.813389,-26.6556163,14.7279327

398.107171,-28.6618495,12.4576434

446.683592,-30.6646106,10.2932372

501.187234,-32.6885167,8.5660686

562.341325,-34.6574611,7.25368276

630.957344,-36.5675565,5.67887063

707.945784,-38.7709119,2.96040434

794.328235,-40.6839971,0.931052533

891.250938,-43.4931255,-4.32564563

1000,-44.6084714,-1.41852283

1122.01845,-46.6796759,-4.34549142

1258.92541,-48.7276961,-5.72869398

1412.53754,-50.9672037,-6.92954931

1584.89319,-52.9628139,-10.5570804

1778.27941,-54.8710069,-11.4217083

1995.26231,-57.0067772,-13.761997

2000,-57.1240924,-13.1864203

From the plots and the data, we can see that the 75 Hz filter hits the -3 dB point at about 80 Hz, and the slope after that is about 12 dB per octave (or 20 dB per decade), making this a 2nd order LPF. The 150 Hz plot and data show the 3dB point to be about 160 Hz, with the same 12 dB per octave slope.

Why does knowing this matter? If you were going to drive a subwoofer that didn't have it's own crossover you'd want to know how much of the higher frequency energy is going to be presented to the sub's amplifier/driver. It would also be very useful if you were building an active crossover to drive a bi- or tri- amped set of speakers. Or if you were restoring old audio gear and wanted to see if the tone controls were behaving right, or wanted to see the limits of their effect. It has many other uses for tuning higher frequency circuits, too.

FFT

Another one of the interesting things the DSO can do is called Fast Fourier Transform (FFT). That is a process whereby time domain data is converted to frequency domain data and plotted. It essentially turns the scope into a crude spectrum analyzer.

I hooked the scope to my CD player and played a steady 1 kHz tone from a test CD, then ran FFT and plotted the result.

Spectrum of a 1 kHz test tone played from a test CD on my VRDS-20 CD player. The first 8 harmonics are listed. Note the sidebands on either side of the 1 kHz spike. Those are 120 Hz offset from the 1 kHz tone, indicating some ripple from the power supply is getting into the output of the CD player.

This is no substitute for a real audio analyzer (or spectrum analyzer) as the DSO noise level is high and ENOB is only 8.4, but you can see power supply ripple in the signal and the relative levels of some of the harmonics. The results also depend on the selected "window" used to make the measurement. Some window types are more accurate for amplitude and others more accurate for frequency.

I will be exploring other measurements and options in the future and my test cable collection expands, and I study and practice a bit more. A DSO can become a hobby all by itself!