Category: electronics

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,

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.

 

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.

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

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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