Testing the Wolfson Audio Board and Raspberry Pi

The trouble with birds is they get up earlier than I do, so a time-delay start recorder lets me scout locations without loads of early starts.Autonomous recorders are sometimes known as frogloggers in the nature recording community. Commercial variants and great, reliable, but dear. I want something I’m prepared to take the risk of losing to some inquisitive dog-walker, and preferably something I can make enough of to scout several locations.

A Raspberry Pi via Wifi is also a good remote startable recorder over WiFi . A bit like the Tascam DR-44WL but without the nice display. the trouble is the Raspberry Pi has no record facilty. If it had, you can start recording by logging in via SSH and issuing the arecord function. The audio can even be transferred off the Pi remotely via SFTP over WiFi. Enter the Wolfson Audio board – a piggyback audio card for the Pi, which takes over all the IO so you aren’t going to be running any other custom hardware on that Pi.

Installing the Wolfson

Physical installation is easy enough, the Wolfson board uses the bizarre approach of connecting to the GPIO using a standard header and the P5 header using a set of pogo pins.[ref]I did have trouble with these once – what happens is you issue the record for x seconds command and in simply sits there and never times out. Then you press the GPIO down again and it comes good… P5 carries the i2c bus  SCL0 and SDA0 pins which control the Wolfson, lose contact on one of those and you aren’t talking to it any more.[/ref] I’d have been easy with soldering an extra set of pins or a header myself and this is probably a reliability hazard, but I’ll run with it for now. Just as well I’m not going to use the badly aligned yellow and white SPDIF sockets, eh?

I started with a Model A running a stock Raspbian image, 2014-06-20-wheezy-raspbian.zip and ignored Wolfson’s recommendation to avoid a USB hub, because I needed that to see what I was doing to set up WiFi.

bizarre use of pogo pins, it's a wonder these make enough contact for the board to work at all!
bizarre use of pogo pins, it’s a wonder these make enough contact for the board to work at all!

No standard Raspberry Pi Drivers for the Wolfson

Unlike other bits of hardware, to run up the Wolfson card you can’t use the stock Raspbian and do an apt-get install some-sort-of-wolfson-driver. You’re in for a world of hurt here.

You either download the SD card image which is recommended by Wolfson. It’s 8Gb and it means 8Gb, and wouldn’t fit on my 8Gb card, because it requires a card with no dead sectors presumably.

Maybe time to compile the drivers myself following this? Nope – life is too short and I do not have the skillz to firefight what goes wrong. And what with the takeover of Wolfson by Cirrus it looks like the drivers are delayed still more. I like the Torygraph’s opener

Wolfson Microelectronics, the struggling Scottish microchip company, has been acquired by its American rival Cirrus Logic for £278m.[…]
Wolfson has become increasingly reliant on a few big customers including BlackBerry, which as seen sales of its smartphones collapse.
The company reported flat revenues of $179m and mounting losses of $13m last year.
Wolfson admitted it had been blindsided by the rate at which consumers were adopting 4G smartphones, which gave Qualcomm an advantage because it had developed an all-in-one 4G microchip that included an audio processor.

Damn. Those drivers aren’t going to happen any time soon, or maybe ever… I then used Ragnar Jensen’s zip described in this post, and the usage instructions here to install it. Which worked for me. I have no real idea how.

Don’t get the Wolfson card if you want playback until there are normal drivers available

My only interest in the card is to record – everyone else seems to want to take advantage of the whizzy playback options. To be honest there are alternatives if you want playback only, and it looks like the product is at risk of getting orphaned, since it is Pi Model A or B Rev 2 only, not B+, and is still driverless. You run the risk of getting stuck on an old version of Raspbian. That doesn’t bother me, as I will only use this for recording and not run anything else exotic on the Pi. If you want to run a media centre then you could start to hate being stuck on older versions of Raspbian.

How does it record, then?

I made the mistake of firing up ALSAmixer after installing, it certainly showed a lot of options and stuff going on which gave me a good feeling that the Wolfson card was present. But it is easier to adopt their installed ‘use cases’ that are installed in /home/pi

./Record_from_lineIn.sh

then issue

arecord -c 2 -f S16_LE -d 10 -r 44100 record_from_line_in.wav

to record a 10 second stereo track from Line in (that’s the -d 10 seconds, -d3600 would do you an hour, etc)

I experienced random buffer errors every 30s or so. Mindful of Wolfson’s warning about USB hubs I removed the keyboard (which has a hub) though I still used a hub to power the device, and because this was a Model A I had the wifi on the hub, and still took overruns

arecord -c 2 -f S16_LE -d 130 -r 44100 record_from_line_in1.wav
Recording WAVE 'record_from_line_in1.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo
overrun!!! (at least 90.703 ms long)
overrun!!! (at least 50.601 ms long)
overrun!!! (at least 15.111 ms long)

Although it looks ghastly here is an MP3 of the original file that I played into the Pi –

and here is the file recorded with the overruns above, converted to MP3

Which doesn’t sound so terrible to me at all. I should still not be such a cheapskate and buy SD cards from ebay, if a Class 6 card is what’s needed for audio 😉

I still got overruns with FLAC but they were shorter, which points towards disk IO as being the problem, since FLAC ups CPU load a lot but reduces disk writing, because that’s its job

arecord -c 2 -f S16_LE -d 130 -r 44100 | flac -o test1.flac - --channels=2 --sample-rate=44100 -f

flac 1.2.1, Copyright (C) 2000,2001,2002,2003,2004,2005,2006,2007  Josh Coalson
flac comes with ABSOLUTELY NO WARRANTY.  This is free software, and you are
welcome to redistribute it under certain conditions.  Type `flac' for details.

Recording WAVE 'stdin' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo
-: 23% complete, ratio=0.468overrun!!! (at least 20.023 ms long)
-: 69% complete, ratio=0.513overrun!!! (at least 0.553 ms long)
-: wrote 11620131 bytes, ratio=0.507

Using FLAC (free lossless audio compression)

you must apt-get install flac

to get the codec first

Pipe the output of the record chain into FLAC to reduce file sizes by about 40% on field recordings

 arecord -c 2 -f S16_LE -d 130 -r 44100 | flac -o test.flac - --channels=2 --sample-rate=44100 -f

flac 1.2.1, Copyright (C) 2000,2001,2002,2003,2004,2005,2006,2007  Josh Coalson
flac comes with ABSOLUTELY NO WARRANTY.  This is free software, and you are
welcome to redistribute it under certain conditions.  Type `flac' for details.

Recording WAVE 'stdin' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo
-: wrote 11692886 bytes, ratio=0.510

with the flac command the single – means  process stdin and -f means overwrite existing file

FLAC used to be a resource hog so I thought I’d look at the CPU usage, which seems to run about 12-15% on a stock Raspbian (no overclocking etc)

top - 18:07:28 up  1:54,  2 users,  load average: 0.05, 0.04, 0.05
Tasks:  74 total,   1 running,  73 sleeping,   0 stopped,   0 zombie
%Cpu(s): 12.4 us,  2.3 sy,  0.0 ni, 84.9 id,  0.0 wa,  0.0 hi,  0.3 si,  0.0 st
KiB Mem:    187592 total,   145604 used,    41988 free,    14664 buffers
KiB Swap:   102396 total,        0 used,   102396 free,    98588 cached

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
 2713 pi        20   0  7488 1380  940 S  12.1  0.7   0:03.71 flac
 2710 pi        20   0  4672 1372 1036 R   1.0  0.7   0:00.50 top
 2676 root      20   0     0    0    0 S   0.7  0.0   0:03.14 kworker/u2:3
   13 root      20   0     0    0    0 S   0.3  0.0   0:00.96 kworker/0:1
 2683 root      20   0     0    0    0 S   0.3  0.0   0:01.61 kworker/u2:0
 2695 pi        20   0  9260 1584 1000 S   0.3  0.8   0:00.12 sshd
 2711 root      20   0     0    0    0 S   0.3  0.0   0:00.18 kworker/u2:1
 2712 pi        20   0  4944 1336 1128 S   0.3  0.7   0:00.19 arecord
    1 root      20   0  2148  720  616 S   0.0  0.4   0:04.29 init
    2 root      20   0     0    0    0 S   0.0  0.0   0:00.00 kthreadd
    3 root      20   0     0    0    0 S   0.0  0.0   0:00.46 ksoftirqd/0
    5 root       0 -20     0    0    0 S   0.0  0.0   0:00.00 kworker/0:0H
    7 root      20   0     0    0    0 S   0.0  0.0   0:00.75 rcu_preempt
    8 root      20   0     0    0    0 S   0.0  0.0   0:00.00 rcu_bh
    9 root      20   0     0    0    0 S   0.0  0.0   0:00.00 rcu_sched
   10 root       0 -20     0    0    0 S   0.0  0.0   0:00.00 khelper
   11 root      20   0     0    0    0 S   0.0  0.0   0:00.00 kdevtmpfs

Wolfson Card Analogue Line-up

So it’s time to line the card up and find out what analogue levels it takes. I’m going to need outboard audio processing anyway to bring mic level up to line level, and to be honest that’s probably better done off the board anyway away from all the digital power-supply sizzle. I can control levels in the analogue domain, so no need ot run alsamixer unless I want to do remote live recording.

I injected 1kHz tone from a Farnell Wien bridge oscillator ad found that the default gain setting is exactly right for a 1V rms input

1V rms exactly...
1V rms exactly…
on a scope, not the readout is 10x too low because I used a 10x probe
on a scope, not the readout is 10x too low because I used a 10x probe, so that’s 2.8V p-p or 1.4V peak

When I ftp the file to my PC and look at it with a WAV player I see it is as close to 0dBFS as you can get

You aren't going to get closer to 0dBFS than this without clipping
You aren’t going to get closer to 0dBFS than this without clipping

The audio doesn’t start recording instantly, there is an elegant fade in combined with an inelegant DC shift

Audio fades in softly over a short period
Audio fades in softly over a short period

It isn’t a big deal, but you probably want to start it .1s before the desired sound. That’s neither here nor there with a manual start but if auto-started from a sensor trigger that would be a bear.

Audio performance

I terminated both inputs with 150 ohms and used Audition to gather the stats on silence, starting 3s in, a reasonable way past the initial DC bump.

internal noise stats. I'm not going to complain about this
internal noise stats. I’m not going to complain about this

I then scanned the spectrum of the quiet recording to look for any frequency spurs etc, on a fairly narrow IF bandwidth (wide scanning window). I’m not going to argue with the results –

Can't really argue with that
Can’t really argue with that

For reference here is the 1kHz tone (if you analyse it all the distortion comes from my 1970s era Wien Bridge oscillator)

and here is the quiet recording

I didn’t run rightmark on it since I don’t have anything good enough to generate the test signal and don’t know how to play and record at the same time on the Pi.

Since I want this for recording I didn’t bother to test playback – here’s a description of replay.

Time delay recording

The way to use this as a time delay recorder is to set cron to start on boot:

shutdown -h -t 3700

and

arecord -c 2 -f S16_LE -d 3600 -r 44100 record_from_line_in.wav

Then power on the Pi and the microphone preamplifier about half an hour before dawn and pull the power after about an hour and a quarter – the Pi should have halted by then. I will use a PIC microcontroller for that job, because it draws a very low power in the rest state, but an Arduino would work too, though it’s typically 7mA drain is higher than it has to be.

Conclusion

The Wolfson audio card records well, with low noise. You need to use a fast SD card otherwise you will get overruns. But it is poorly supported and the devil’s own job to get going. However, it seems the only game in town for high-quality recording.

Since that’s what I want to go I have to put up with the poor support and idiosyncrasies, it works well enough.

 

Raspberry Pi Soundcard From Logitech Headset

The Raspberry Pi has no audio input, and I had a pair of Logitech USB headphones that had gone faulty – the earth on the headset was a dis. The USB part seemed to work, but headphones live on borrowed time, connectors and wiring tend to go, and everything is sealed, So I wondered if I could reuse the circuit board as a Pi soundcard. Stereo output but mono on record, of course, but still useful. It’s all part of experimenting to make a low-cost audio field recorder that can start by itself. Nothing is designed to be repairable here, so the plastic USB module case was pinged apart with a flat-bladed screwdriver. Initially I wondered if the black stuff was a blown capacitor, but it was on both sides of the board so I figured it was probably glue from the connector, which was confirmed when I unsoldered the connectors, it came off like a gluey film.

Logitech A-0365A USB headphones control board
Logitech A-0365A USB headphones control board

Headphone cable uses fine strands of individually enamel insulated wires wrapped around a synthetic fibre core. The good thing about this is your headphone cable breaks later than if you used normal multi-strand audio cable, given all the flexing it has to take. The bad thing about it is it’s the devil’s own job to solder, because as you heat the strand the synthetic coating melts, robbing you of heat and re-insulating the enamel you’ve burned off. So I unsoldered the existing cable and threw it out, and soldered a stereo jack plug for the headphones and a phono line socket for the microphone. I then extracted the original microphone capsule from the headset and soldered it to a phono socket just to test this out in the original circuit conditions. Continue reading “Raspberry Pi Soundcard From Logitech Headset”

Flat Screen UIs make Fumbling Fools of us all

Did the music for a wedding using my PA today. The bride and groom were lovely, friends of ours and at was a great day. A wedding is an odd PA/DJ sort of job, It’s not the same as a party, or even running a mixtape for a party. Most of it can be scheduled up front – to the extent that there are firms that hire out a no-DJ wedding system

The job was made easier for me as the bride had made her selection of classical music, with a section for the getting seated bit and a long stretch for the after the event part. So far so good. The venue was the lovely Christchurch Mansion in the main park in Ipswich, and the weather smiled on the happy couple for the event and afterwards

So all I have to do is run the playlist. Because this is classical music the sequences are long, and you don’t have to do the DJ crossfading thing between tracks. Easy-Peasy, right?

Wrong

Some parts of a wedding are a stage show. Stuff needs to happen in time. Usually there’s the entrance of the Bride, and the signing of the register. Both of these are a performance, and timing is crucial. Whatever you do as the music operator you do not want to screw this up the entrance of of bride, in short you need to know that your music is gonna start on time, every time, and you are going to fade it out at the right point. The bride selected Pachelbel for this

The registrar also has to say various things at various points, and they don’t want the music to be running while they’re talking. With classical, pulling the music is easy as a fade down – it’s a bit more of a drag if you have to fade a pop song just as they are getting into the main part, you probably have to rehearse timings, but this wasn’t a problem I had.

Making the right thing happen, on time, on cue is not easy with an i-anythingwithatouchscreen.. It might be easier with a Nano or something with an old-skool gonzo keypad and LCD, but a touchscreen is a vile UI for this job. Not only is there a little bit too much latency between the press on the screen and something happening, but it’s far too easy to screw up and then you have to pad your way through a series of menu choices to say no that isn’t what you want, you want this to happen instead. Continue reading “Flat Screen UIs make Fumbling Fools of us all”

How to replace the volume control on an Audio Research SP8 preamplifier

I bought this preamplifier secondhand from Subjective Audio in 1984. My amplifer is a Mark 1, by the way, other models may be different.It has provided me with thirty years of listening pleasure, a good investment by my younger self. However, it is getting elderly and the volume control got scratchy. I shut it down for a few months while I mulled over my options. Remember this is a product that can ship out a signal of 60V, you don’t want a scratchy pot getting worse and sending 10V peaks into a power amp designed for 0.775Vrms full scale. You just don’t wanna go there…

This one can bite

Before you even think of reading further, note that opening the lid of this amplifer could easily kill you, not only is it mains powered but the rectified HT line operates at 630VDC. A high voltage DC shock is a totally different experience to a mains shock. If you have survived a mains shock you won’t necessarily survive a DC shock 😉 you have been warned. Here is ARC’s take on the matter:

Technical servicing of Audio Research vacuum-tube models should only be done by a trained audio electronic technician. Operating voltages inside these products can be lethal, and owners are advised against any tampering with internal components. Unauthorized modifications or circuit changes to Audio Research products immediately voids any protection under the terms of the 3-Year Limited Warranty.

I’m not just any old owner of an SP8. I learned electronics as a teenager in less litigious and more enterprising times picking junked tubed TVs out of skips and scavenging parts. Later on I worked as a broadcast engineer years ago and worked with high voltage TV cameras and monitors, before moving on into electronics design. If you have only worked on low voltage electronics then beware. Old stagers tended to work with one hand in their pocket because an electric shock across the chest is particularly bad for your health.

You have been warned. If you have any doubts then don’t do this. It is a lot easier to buy a new preamplifier than to buy a new life.

Now one option would be to return this to ARC in Minnesota, however, a glance at the T’s and C’s of their servicing indicates this isn’t likely to be a cheap option. The secondhand cost of an SP8 is about £650 these days, which sets the limiting case of what it’s worth to spend on one.

How to change the Audio Research SP8  volume control

The nice thing about this is that you can do this job from the top. Remove the screws holding the top lid, and save them carefully. They are some curious coarse American Imperial thread, fortunately the same as the sort that come on many PCs if you lose some.

First familiarise yourself with the patient. The circuit diagram is here,

Audio Research SP8 preamplifier circuit diagram (schematic)

You’ll observe a thin circuit board at the front, right on top. Your mission is to replace the volume control which is on the left hand side. First unplug the line stage valves, remember which are which. Indeed, get a digital camera and photograph where everything is before you dismantle it. Then unsolder the three wires that go to the line stage from the top control board.

unsolder these three wires to be able to lift the control board to replace the volume control of the Audio research SP8
unsolder these three wires to be able to lift the control board to replace the volume control of the Audio research SP8

 

These have stiff wire wound round the end, poked through a hole in the PCB. You can heat up the pad and lift from the top. I only realised life gets easier if you remove these after I had unsoldered the pot. It’s easier to unsolder these first, then remove the pot.

Remove the four knobs with an Allen key. remember being American, the ARC is gloriously unreconstructed imperial feet and inches, none of that cheese-eating surrender monkey metric hoo-hah here, so use an imperial Allen key set. Set the knobs aside, marvelling in their honest-to-God solid metal construction. Observe that there seem to be two diameters of shafts here, the pot shafts on volumegain and balance are slightly thinner than the switch shafts on source select and that odd mono/stereo/left/right thing which I’ve only ever seen on ARC gear.

Next get yourself a ½inch AF socket, or if you are careful, a ½” AF ring spanner, and unscrew the nuts holding the pots and two switches, plus the associated star washers. Set to one side. All the threads seemed to be the same despite the different shaft diameters.

Now very carefully move the board back and lift the volume control end. Remember that your amplifier, like mine, is likely to be very elderly, and being vacuum tube gear will have been thermally cycled zillions of times. That means the wires and plastic will be stiff and perhaps brittle. The last SP8 was produced in 1985. That means at least thirty years will have passed. Go easy on the old girl, right 😉

A solder sucker and a 25W or more temperature controlled soldering iron will be your friend. This is US gear, they don’t bother with all that namby-pamby Euro RoHS rubbish. They’re Real Men in Minnesota. You’re on fully leaded 60/40 tin/lead solder, like in the good old days, which makes life easier. Have at it, but note that more heat is required, this is not miniaturised SMD circuitry. A spot of WD-40 in your solder sucker to give it a good strong recoil and good suction will do the trick.

Audio Research SP8 Volume control
The offending part – the Audio Research SP8 Volume control (right) next to its replacement (left)

You’ll see I managed to snap off the tag from one section of the volume control, though I could have patched it if necessary. It is a 100k log-law (A-law or audio taper) part. ARC run the whole control section at a very high impedance, cascading the 100k linear balance control (presenting a 25k impedance to the volume pot assuming a low source) into the 100k log volume pot, which is then direct-coupled into the grid of V4.

Now you could replace this with a cheesy Chinese 100k log law pot, but bearing in mind you’ve probably got the same problem of being wrapped down at the low end of the travel, you will hate yourself all the time when one channel is louder than the other. So don’t do it. Spring for an ALPS high-end pot. This is your main control interface, you’ll use it every day, and a cheap pot will annoy you every day. I got mine from Tisbury Audio on ebay, look for ALPS Blue Velvet.

I got a 50k version. What’s that all about, I am replacing a 100k pot? Well, this is my chance to lose about 6dB of gain – by putting a 47k film resistor in series with the hot end of the pot I make it as if I have a 100k pot but never get to use the top half, thereby extending the bottom part of the travel which is where everything happens. I make the gain distribution of the SP8 a bit better suited to the modern world (I’ve already got the low-gain mod ARC recommend in Note 2 of the schematic), but without upsetting the circuit conditions. The balance control still feels as if it has a 100k pot after it, and to V4 it feels like there is a timid listener who never gasses the amp up past the halfway mark  on the original volume control (OK probably about 10 o’clock due to the log taper).

 

ARC SP8 preamplifier volume control post-mortem

ARC SP8 preamplifier volume control
ARC SP8 preamplifier volume control carbon track after 30 years

It was clearly old age that’s done for this. It was well made, with multiple tracks and multiple fingers covering the track. It might have been possible to clean this one up with switch cleaner, but in the end it’s got a lot of miles on it, and the carbon track just wears. A cheesy touch that honestly I wouldn’t have thought ARC would have done was the volume control has a stepped detent built in. There is no reason for this, indeed I was usually only on the first four detents and sometimes used the halfway positions. It isn’t a stepped attenuator and there’s no need for ARC to make out it is. However, they had to lubricate the detent mechanism; and this may have hastened the pot’s demise, as it was the first wafer that went scratchy. You can see a gunkiness on the slip-ring.

ARCs cheesy faux-stepped attenuator mech on the SP8 volume control
ARCs cheesy faux-stepped attenuator mech. And all that grease, oh my…

Over thirty years, that grease wants to move. It migrated to the shaft of the control, that was sticky and greasy when I removed the knob, and I suspected some of it eventually got onto the track. Even geniuses like ARC have bad days, and introducing a part than needed greasing and stopping me using every part of the volume control was a Bad Idea in my view.

The other trouble is the replacement is smaller, it has a different pin pitch, and the pins are in a different plane to the old one. This isn’t about to sit on the PCB as a drop-in replacement 😉 That’s a blessing in disguise, since there’s enough space to solder in some extensions to the pins and space to lose the 47k series resistors without modding the PCB in any way. Which would feel kinda rude.

First there’s some mechanical argy-bargy to sort. The ALPS pot has a anti-rotation pin. There’s no place for that to go on the SP8 chassis. Simple decision, do I drill the hole in the chassis of the SP8 or do I saw off the pin? I went for the saw off the pin option, funnily enough.

ARC SP8 replacement volume control
See that pin on the Blue Velvet? It’s gotta a go, because I’m not drilling my SP8. You can also see that the plane of the tags front to back and side to side is different between the old and new

The eagle-eyed will see there’s a great big slot in the Blue Velvet to avoid the tags shorting out on the front metal plate. That is where all the swarf is going to go from sawing the tag, because swarf is like that. So tape over the gap before you saw the pin off, okay. Otherwise you get to replace a scratchy old pot with a scratchy new one.

ALPS Replacement ARC SP8 volume control
Sellotape stops the swarf falling into the ALPS Blue Velvet giving me an instant scratchy new volume control

Next up was matching up the 47k series resistors. I want them the same, I don’t care about the exact value. They were nominally 2% but what the hell, it’s easier to check this before soldering them in. They both read 46.8k on my AVO M2007. Good enough for me.

ARC SP8 volume control replacement
matching the series resistors. The both read this, I won’t bore you with the second picture showing the same reading…

Observing the difference between old and new, I figured some extension wires would take out the slack, since the new control is smaller it would stand a little off the board. Obviously I no longer have the control to locate the left-hand side of the PCB, but I figure three other controls are good enough. So I wire some pins to the volume control and sleeved them. I sacrificed a couple of resistors for their leads which seemed about right for the holes in the Audio Research circuit board.

ALPS replacement SP8 volume control prepared with flying leads
ALPS replacement SP8 volume control prepared with flying leads
ALPS replacement SP8 volume control prepared with flying leads

Then it’s a case of carefully matching up pins and holes, pre-bending the wires to get the plane of the replacement to roughly match the old one. Don’t solder anything yet, just bend the outer leads over to stop it falling out into the guts of the amp.

ARC SP8 replacement control in board
ARC SP8 replacement volume control loose in board
ARC SP8 replacement volume control loose in board

Now it’s time to lower the board back into the amp and marry up the switches and pots with the holes. The new volume control has some play because of the unsoldered wires, to take up its rightful position in the front panel. Tighten up all the nuts, using the ½” AF socket for the three old controls and socket for a M9 nut for the ALPS pot. I used a 3/16ths W ring spanner, because it fitted.

Now everything is seated, solder the flying leads of the pot to the board, and wire back the ARC flying leads that go the the line stage input. Be very careful to make sure no bits fall into the amp, or if they do that they are extracted. Remember there is 630V B+ in there, looking to cause some trouble for someone… It was hard to get the camera in behind the installed pot to show it in service but this gives you the general idea

Installed replacement ARC SP8 volume control
Installed replacement ARC SP8 volume control
Installed replacement ARC SP8 volume control

Now double check everything, then power up the amp. I fed mine to a oscilloscope in parallel with some computer speakers. You don’t want to be down a preamplifier and the rest of your hi-fi if it all went horribly wrong. I fed a CD player into the back, and with some judicious manipulation of the unmarked switch shafts established that I had a working amplifier. On powering down the CD, I rotated the volume control and observed the absence of scratchiness. On the scope I was looking for any bursts of oscillation or general spuriae and bad attitude but there was none at any position of the volume control. I tested all the inputs and switch positions, and they all worked apart from the tape monitor input, where the switch had a low level on one side. I don’t use that so I can live with it, though I stuck a label on the monitor socket to remind myself should I ever want to use it. It would involve demounting the PCB on the switch panel and changing the switch, which I can’t match aesthetically. I’d have to change all the switches for diddy little ones.

It was now down to breaking out the IPA – that’s Isopropyl alchohol, not India Pale Ale, and cleaning some of the accumulated grime of the last 30 years from the recesses. Not the innards – in the end thirty years of thermal cycling means if it ain’t broke or in obvious distress don’t touch it, but it was time to clean my knobs.

a dirty knob or two
A dirty knob or two. Particularly the one on the left, where the grease from the detent of the pot worked its way out along the shaft, through the grub screw on the underside to gunk up the anodised finish
A dirty knob or two. Particularly the one on the left, where the grease from the detent of the pot worked its way out along the shaft, through the grub screw on the underside to gunk up the anodised finish

The IPA worked well, but couldn’t get more than about half of it off, and didn’t get into that groove. However the remarkable power of a cheap ultrasonic cleaning bath with some tepid water and washing-up liquid worked wonders.

Happiness is... an ultrasonically cleaned knob
Happiness is… an ultrasonically cleaned knob

Time to refit them, starting with the two switches on the right-hand side (which need the larger diameter inside for the slightly thicker shafts).

I didn’t want to change this pot, and indeed I first changed the tubes, in the hope I had a knackered tube. I didn’t, but they were due a change anyway, being over 10 years since I last did this. I got mine from Watford Valves. Overall the total job cost me about £110 for the new valves and pot, and about three hours of time.

The alternative would have been to get something like a Naim Audio NAC 172 XS. Eventually I will have to move away from the Transporter because I have just discovered that Logitech is end-of lifeing the whole Logitech Media server and players, and going to cloud. I am not having my hi-fi depend on anybody’s cloud 😉 Will I still be listening through this when I am 80? Probably not, but hell, hopefully it has a few more years in it yet.

The SP8 – a legacy from days before CD

Thirty years ago there were no CDs, good audio was to be had from vinyl LPs or not at all. It might have been different if Compact cassette hadn’t killed of reel-to-reel, but vinyl was where it was at. The SP8 is a legacy from vinyl days – records chucked out loads of out-of band signals and crud, you needed decent headroom to pass clicks and pops through the RIAA de-emphasis[ref]seems funny to write about the RIAA without cursing them, their lawyers, and the horse they rode in on, but they did a good job in the early days, standardising the frequency equalisation curves for LPs[/ref] without stretching them into dirty great thumps. The downside of the SP8 was it was noisy with low-output MC cartridges, but I used an AT1000T moving-coil transformer ahead of it which sorted that.

The SP8 served me well in my vinyl playing days, and it worked well with digital sources too, with one proviso.  The trouble with moving to digital is that line level took a big hike from the old 0.775Vrms[ref]Wikipedia claims domestic gear was lined up at -10dBV which is half that at 0.316 Vrms in which case it’s gone up by 12dB rather than 6. I don’t know if this is US-specific [/ref] to typically 2V p-p with the move to CD. The Naim CD-5XS has a line level of 2.1Vrms and the Transporter runs 2Vrms. And the trouble is the SP8 line stage is high gain – about 26dB, so I was always wound way down on the volume control, it would rarely get to 9 o’clock whereas in vinyl days I’d be happy at 11. This probably contributed to making it scratchy, as the first part of the track on the volume pot gets all the action.

I have tried patching the Transporter straight to the Naim power amp and using the digital volume control but it wasn’t that special. And up-down buttons as the main volume control user interface sucks. Whaddya do when you want to listen out for if it’s your phone or doorbell ringing?

The SP8 doesn’t owe me anything – thirty years of listening pleasure is a decent ROI on its capital cost. However, my hifi system is stable and well matched to itself and to me. Any change risks running into the Diderot effect so it was worth taking a flyer on fixing this.

The surprising repairability of old gear

Vintage gear is surprisingly repairable, because you get to replace component parts. It’s surprising that in three decades of progress people still haven’t come up with a better volume contro lthan the humble potentimeter, and these are still widely available. Whereas I’m not going to be fault-finding to component level on an iPod in thirty years time. or even still using it daily. However, an awful lot of portable audio like that is still repairable, because you most commonly get trouble with jacks, power supplies/batteries and cases. Most of these are to be found on ebay or are standard parts. I’ve serviced mobile phones and the like – case replacements in particular are an easy win, but I’ve replaced keys and joysticks before. However, portable gadgets aren’t going to be serviceable in the far future like this was, and nor are they liekely to be usable in the long term future.

For completeness’ sake, here is the rest fo the SP8 manual and parts list for the Mk1.

Continue reading “How to replace the volume control on an Audio Research SP8 preamplifier”

Minidisc hacking

I used a Hi-MD to trawl for the Robin’s first song of the year. Set it off at 9pm, recording from the mic. Trouble is that it split the file into one group with a bazillion tracks – about 1500. Must have tripped a new track each time the audio dipped below the meter indication and came back. This isn’t useful. There must be a way to can that other than editing all the marks out. I’d really like to not do that again.

Set me thinking to how to record the Wolves Wood sounds later in the year. What’s needed is a remote timed start. Could set all the controls to manually record and pause, then hit the go button on the remote using a timer/alarm clock module etc. Let’s see what’s inside the remote.

MD remote innards
MD remote innards

The worst part is the plug, which is not only Sony proprietary and nonstandard. The pitch of the pins isn’t on a 0.1” grid.

MD remote plug - nothing is standard here
MD remote plug – nothing is standard here

That’s just what you don’t need. Having said that, a stick remote sucks without display, and LCD remotes are on Ebay for about £23. So I could stump up for a remote upgrade and get this one to donate it’s plug to the cause.

Other than that this is a pretty straightforward resistive ladder job – the play/pause button is a 330 ohm resistor across two of the pins. The resistor values used in Sony Minidisc remotesare documented hereWhat I need now is an alarm clock module but Maplin doesn’t seem to do them anymore. Which could be loads of pain using a PIC. Or maybe a 4017 clocked in with a 4060 set to output one hour intervals. On second thoughts the James-Bond-esqueness of the PIC and LED display might be too much to resist 🙂