Ken Betts' audio innovations

Feature

Ken Betts
Ken Betts' audio innovations
Wednesday, October 24, 2012

Ken Betts is a retired electronics engineer who has a deeper knowledge of audio equipment than most. He built his entire system including turntable, arm and cartridge, CD player, preamp and power amp. He not only made his own speaker cabinets and finished them to a high standard but went right back to basics with the Lowther based drive units. Ken is not the sort to buy something when he can make it himself, an approach that includes rolling his own capacitors among other rare pursuits. He started building hi-fi components when he worked as an engineer for semiconductor companies including Plessey.

Ken Betts: “In the late sixties they made integrated circuits and came up with a chip called the SL403 and SL402 – the 402 was a lower power selected 403 – the SL403s were sold to Amstrad and the SL402s were bought by [Clive] Sinclair.

“My hi-fi story started with the SL403 which I used to build a stereo amp in a wooden cabinet with little tiny controls, it was a high quality dual mono amplifier that only produced 3 watts but it made music. I dabbled with that and started playing around trying to make better speakers than I could buy second-hand. As we had four kids and my salary was just about enough to pay the bills this was the only option. I got hold of a Goldring Lenco record player that was all metal with a type of rim drive that went on the bottom of the platter with a free running idler, it used a cone to drive the idler so that you could have infinitely variable speed. It was OK in those days, I didn’t know much better.

“The other way is to use feedback, even Sugden’s class A amps used a lot of feedback. I built a class A amp a few years ago and didn’t put the feedback in and it was dreadful, the frequency response was all over the place. Didn’t rate it at all, you needed so much feedback. The trouble with feedback is that you’re trying to cure an error after the event, which only compounds the error. It’s not actually sorting it out, it’s like trying to correct the audio problems by doing it down the other end of the room from the speakers, too late, the audio is already with you.

“I use transistor amplifiers but I use devices which I calculate very carefully so they always stay in that finite little bit, but they’re only low level. When I get the signal out of the DAC I want it to be big enough – 1.5 to 2volt – so that I don’t need an active preamp. I use good quality components and always avoid feedback.”

 

Unipivot arm made with thin wall brass tubing inside a carbon fibre sleeve, the cartridge is Ken's rebuilt Sumiko BPS

 

“At the same time I was making VHF transmitters and receivers for my amateur radio hobby, I had to teach myself electronics by getting books out of the library and learning from a colleague. Later on I realised that I could apply what I had learned to the process of building a ‘decent’ audio system.

“I always used solid state because I couldn’t hurt myself, I could grab something and it wouldn’t blow me up. I’d be looking at valves and read 350, 450, 750 volts and think I ain’t going there! Making transistor stuff sound good is more difficult than doing the same with valves, but that’s not to say that making valve stuff sound good is easy. There are many pitfalls with valves. A transistor is just a device that will turn on or off, in the middle there is a small portion where there is a variable bit of on and off which if you get it right can be linear. So if you get it wrong and bias a transistor at the wrong rate it will turn on fully, but there is a finite part where you can get a linear response.

 

Left from top to bottom: DAC, CD transport, preamp and phono stage, power amp. Right: Ken's variation on a Thorens TD 125

 

Ken’s system which he describes as “not such a bad system, it’s work in progress” consists of a turntable that started life as a Thorens 125 with the original bearing and Mazak platter, belt drive “of course” rather than idler drive and a plinth made from a chopping block. He tried an idler with a second belt to avoid pulling the bearing toward the motor a la Funk Vector but found that it didn’t sound quite right, a single motor belt providing the better result. It has a separate power supply cannibalized from the 125 for speed control.

A DIY unipivot arm is fitted that has interchangeable wands so that Ken can use his stereo and mono cartridges for the appropriate slabs of vinyl. Both cartridges are based on the Sumiko BPS body rewound with very low impedance silver coils and silver connectors out to the terminals, the mono having two coils only in the horizontal plane, one has even been rebuilt with a ruby cantilever. The main beam on the unipivot wand is carbon fibre loaded with thin wall brass tube, the metal being bonded to the inside of the carbon tube with araldite, so very well damped, but slightly higher in mass to suit the cartridges. A magnetic stabiliser system keeps the stylus upright in the groove and a potted toroidal transformer, drilled out in a mounting bush, is employed as a record clamp/weight.

His CD player has a Pioneer drawer front but this is only to cover the large slot in the cabinet, it’s not fitted with the stable platter mechanism that originally accompanied it. Instead it has a Philips swing arm loader and laser mechanism because Ken wanted to solidly fix the laser unit to the frame. This is usually suspended with the U-frame unit attached to the chassis but he has fixed the whole mechanism to the main PCB and that sits on eight anti-vibration mounts that provide isolation. The Pioneer platter is inverted, placed on the CD and clamped by a modified Philips spring plate, this stabilizes the disc clamp and is driven by a Philips low current motor that’s modified to be free of cogging. All of this is was done to minimize jitter.

 

Left: totally rebuilt Lowther PM6A in home made cabinet, right: squash ball feet under the heavy chopping block turntable

 

The DAC is made up of two TDA1541AS1 (single crown) multibit chips fed by I2S data from the laser control chip. They are run in dual differential mode to minimize noise. He chose the DAC for its high current output, reasoning that the more milliamps you have to describe your signal the higher the resolution, or to put it another way the higher the current available to share amongst the samples the bigger the differential between them. I’ve not heard that reason being used before but there are plenty of well regarded designers out there who have a lot of respect for this classic chip. The casework is MDF in order to avoid EMF and minimise resonance.

The passive preamp has galvanic isolation between input and output using 1:1 transformers with a high 100kOhm value TKD passive pot to control volume. This is because Ken says that “with almost any capacitor except massive ones (relatively speaking) a 10kOhm pot makes a lovely high pass filter”. 10kOhmn is the norm for such devices of course but he reckons that “the input capacitor on the next stage is usually about 0.47uf. To get anything above 60Hz through that you need 33kOhm to ground, anything below that and you lose bass.”

If that weren’t controversial enough the power amp has an output of 300 milliwatts at best because the output devices are 6021 miniature valves with fixed bias (battery source). This is the type used by Audio Note and Musical Fidelity among others in preamps and the first stage of power amps. Originally made for ICBMs these triodes have been wrongly described as Nuvistors by some and are only 25mm long and 6mm in diameter (a Nuvistor is a metal can device that’s half this height). The amp has four triode sections in parallel for each channel, this in order to keep total internal resistance down and make transformer output possible without a massive turns ratio to achieve the impedance match to eight Ohms. Ken uses a JAN 5751 driver valve, interstage coupling transformers in parafeed mode, and his own paper in oil capacitors in the parafeed chain. He rebuilds these from old ‘grey tin’ PIO caps from ITT or Dubilier, using the aluminium foil and Kraft paper he rewinds and fits them in recycled electrolytic cap cans which are filled with some of the oil and then sealed with epoxy. He also makes his own output transformers and converted a line transformer into a choke for the power supply, he does however bow to convention in some areas and uses Blackgate caps in the choke regulated power supply, describing them as “the best you can lay hands on”.

 

The guts of Ken's 300 milliwatt power amp with homemade output transformers

 

The loudspeakers are from a Lowther design for a folded transmission line and have heavily modified Lowther PM6A Alnico drive units which were purchased with damaged cones and no surrounds. As you might imagine this meant a re-coning but rather than take the conventional route of sending them to Lowther, Ken bought some parchment and fashioned himself new cones and whizzers. Even looking closely you’d never guess that this was the case because the job is so professional, Lowther connoisseurs might however notice that the whizzer cone is a little bigger than usual and that the surround is from a Wharfedale driver as is the unseen spider. What you can’t see are the line transformers that Ken has added, they have a pot to tune out this speaker’s rather exposed midband by lifting the frequency extremes. This is done by altering the impedance match between the speech coil and the output transformer. Since my visit he has also replaced the wooden phase plugs with shorter duck egg size and shape ones that apparently improve off axis response, also the 6021 valves have been changed for CV3986 as these are far better matched, improving the overall sound and frequency response.

As you might guess Ken is not a headbanger, he listens at relatively low levels and favours classical material from satellite radio and on stereo and mono records, he doesn’t appear to need more power than that delivered by the micro valves in his power amp. There can be few enthusiasts in this business with the breadth of knowledge that he has and word has got as far as one far eastern manufacturer who he has been assisting, but I can’t divulge any more than that. Suffice to say that Ken Betts knows his audio onions and if some of his ideas seem unconventional they have a lot of experience behind them and I shouldn’t be surprised if they prove influential.

Jason Kennedy