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A Better Elevator?

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    Posted: 15 Dec 2020 at 11:24am
Crystal Palace Transmitter, London

"Come, let us build a tower that reaches to the heavens, so that we may make a name for ourselves"

So, what happens when you want to play your records with a low-output moving coil cartridge?

You either use a SUT that gobbles-up the bass, or a universal phono stage which seldom satisfies, or perhaps an Elevator EXP?

The problem with that last option, is that if you're a reviewer who lives in a flat within a mile of the pictured beast, you complain to Graham about interference.

Therefore, owd Graham must now embark on another sharing story, as to how he will tackle another MC head-amp that might stand a chance - a kind of "David and Crystal Palace" tale.

Where to begin? As usual, I haven't a clue, but that's as good a place as any to start!

(Warning: this topic could go on, and on, and on....)
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Post Options Post Options   Thanks (1) Thanks(1)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 15 Dec 2020 at 11:35am
My first thoughts are, as has been discussed before, what about the interconnects and more importantly the tonearm wiring and MC itself? How do you overcome the RF picked up by these often poorly shielded components?

I imagine that an effective RF filter at the phono stage input would have an unsatisfactory impact on sound quality to say the least.

Ian
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 15 Dec 2020 at 9:22pm
You like a challenge Graham and most of us will gladly read and learn...

As Ian says the RFI we are bathed in is a challenge for the little box of coiled cables we call a cartridge and I fear many cables are not up to the standards of a Cusat50. Silly suggestion, could you use the first stage of an Accession MC as a starting point?

Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Dec 2020 at 10:24am
On the few occasions I stayed up past 3 am, head-worn phones, volume set high from Solo, nothing playing, 1.7 miles west of Hallam's Barnsley repeater AM and FM transmitters; line-of-sight across the river Dearne valley...

I admit I could only make out an occasional extremely quiet burst of the German language. Absolutely nothing at all, from the all-night Hallam and Magic AM broadcasts the transmitters were relaying.

I am now 2.2 miles east from the same transmitters and nearly 17 years older. I don't stay up all night if I can help it. However, a few years ago, I had a visit from somebody with an iPhone 6. Most of the rack picked up its interference. The phone might have been faulty as we all have mobiles of our own, including iPhones, without that interference.

Going back those 17 years again, I remember making up a shielded DC cord, thinking it was acting as an antenna, but it didn't change a thing.

Over recent years, our South Korean importer had to comply with the strict EMC legislation over there. They had to send all our products for EMC testing, including the Elevator EXP, and every item passed the tests easily.

However, I estimate from enquiries received over its years of manufacture that there are about one in a hundred Elevator EXP users who experience interference problems - one being a London based long-standing hi-fi magazine reviewer.

What's the difference between set-ups? I have never been sent a diagram showing positions of possible interference sources - it would have helped! In my office (which doubles as a listening room) and the "lab" next door, there are a total of five Windows machines, all needed for one job or another. There is a "landline" digital cordless phone (base station in the house); my mobile, an iPhone, is often on my desk within 1.5m of the rack.

I have electronic ballast fluorescent lighting, data cables, impromptu mains distribution, and I'm untidy. I have neighbours with solar energy switched-mode grid inverters, mobile phones, and all their technology, plus varying degrees of wi-fi and Bluetooth signals everywhere.

There's a giant "fashion house" distribution centre just over the hill, which draws its power from the same "ex-mine works" sub-station, and there are two 30kV three-phase overhead transmission lines at the end of the garden. There are also three giant wind turbines and a control centre with a radio link to the national grid.

There is an automatic washing machine in the house, which causes the giant toroidal variac "lab" transformer to grumble due to the lop-sided "DC" on the mains.

We received an EMC training visit from Keith Armstrong of Cherry Clough Consultants and were told the 100uH DC input inductor we'd been using on most products should be replaced by a more effective RF choke. That was done within the last five years.

The very last thing I looked at was the stereo interference loop, which quite a few commercial amplifiers unwittingly cause. How that can extend to a head amp is beyond me, but on the reviewer's Elevator EXP, I ensured the "loop" couldn't travel inboard of the output RCA jacks - all to no avail.

Knowing that the grounding and shielding of the Elevator EXP enclosure can only admit RF of wavelength the widest distance of its front and rear panel screws (3.2GHz), and to permeate a gap of much less than a thou - RFI can only enter via the LED - I am satisfied we did everything possible to prevent interference.

Having looked it over from these directions, perhaps the fault lies within the circuit?
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Dec 2020 at 3:28pm
When I began the Elevator EXP design, I decided to use op-amps with the conventional series pass monolithic voltage regulator (78 series) used successfully on just about every channel card in any studio or broadcast mixer. It was the done thing because it always worked.

The subtle difference was the lack of a third (or negative) rail, and so the single supply's ground became the negative rail, with a resistive split generating a pseudo ground. The current through the resistors was orders of magnitude greater than each op-amp's supply current, and these were also bypassed by large electrolytic capacitors, which in turn, were bypassed for high frequencies.

That was the second version as the unreleased first version used an op-amp to generate the pseudo ground. I didn't like the sound even though it measured perfectly well.

Initially, it was offered as two versions with the cheaper using OP37 op-amps, but when auditioned, the EXP version was given the thumbs-up, and the OP37 version a thumbs down. The OP37 version was scrapped.

To offer the highest slew-rate along with the lowest noise, I had chosen the OP37 and the expensive AD829, both requiring special care to ensure stability. Even using gains greater than their specified stable-gains, although nothing showed up in measurement, my hearing detected a sound signature associated with high-frequency oscillation.

Musical transients and the rising treble of audio-vile moving coils present the input with signals far removed from that of an audio analyser, and so all kinds of ad-hoc tests were tried. One included a rusty old mains Black and Decker drill to induce spikes into the input wiring. Another had modulated tone onto an RF carrier inputted it to the circuit while sweeping the short wave bands of a radio receiver - whose aerial wire was wrapped around the circuit - to hear its transmission. Absolutely nothing.

The AD829 isn't easily understood because its stability components also determine its slew-rate. At the same time, bandwidth can differ under capacitive loading due to its internal RC output stage network. It is similar to a Zobel network but used in a bootstrap fashion. The available output current and load capacitance determine the possible slew-rate, and the AD829 has plenty of current on-tap.

Standard stabilisation capacitance was chosen from its datasheet, and that was that. The vast majority of users were happy, and it received some "rave" reviews.

Occasionally, however, I'd get a radio frequency interference complaint - not audio detection  - but buzz, crackle, or data transmission noise. I'd ask about arm cable and output cable shielding, but every customer seemed adamant that their wiring was correct.

It hadn't been long before that I'd met with somebody selling boutique cables, who'd explained that bell-wire sounded much better than a properly shielded coax cable, and went onto explain silver ribbon cable, in which the flat sections of the signal and return were at 90 degrees to each other, and thus canceled any interference.

Knowing it was absolute bullsh*t, I nodded politely and hoped not to meet the individual again. However, such practices had already entered the hi-fi "industry," and boutique manufacturers had sprung up seemingly overnight. Some are still around to this day, and with the help of individual reviewers and magazine owners, have substantial balance sheets.

Each time I asked a complainant if they were using any of those interconnects on the EXP's inputs or outputs, it was flatly denied. The customer is king and always right?

Also, there were lots of wealthy Charlies at that time, hoping to become the next new thing in hi-fi and launching ill-advised wholly DC-coupled phono stages. When coupled with wholly DC-coupled integrated amplifiers, the slightest suggestion of DC on the phono stage input made the woofers pop outwards and stay there.

Although my phono stages are all capacitor coupled, the EXP was DC coupled and featured an output offset to be proud of - provided its input was connected to a cartridge.

It had become something of a fetish to unplug the turntable input cable to "test" how noisy the product was. Obviously, some non-technical reviewer must have written something, or it had appeared on a forum somewhere. The result was woofer pop and the evident noise from an open circuit high gain stage.

It made me wonder if I wanted to continue doing anything related to low output moving coil, plus the fact that I couldn't stand the rising treble harshness others found so endearing.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Dec 2020 at 9:53pm
A designer must have originality to make a difference. It is no good doing the same thing over and over and expecting a different result.

The problem in audio electronics projects such as the Elevator EXP is under the hood. It doesn't look much different to hundreds of other circuits. That's due to the simple fact that there are only a few ways you can use an electronic component, especially an op-amp, which will always look like an op-amp.

Every component of a circuit serves a purpose, and if it doesn't, it shouldn't be there.

Every component has a rating, whether it be a value of capacitance, resistance, inductance, voltage, or current. Each value is either calculated or determined by testing.

The op-amp was configured for non-inverting gain inboard of the loading resistors, using a simple two resistor negative feedback network whose paralleled sum needed to match the approximate DC resistance of a typical moving coil cartridge. The reason being that the resistors would not generate more noise than that caused by the source resistance - the cartridge.

The op-amp had a noise figure of 1.7nV/sqrt Hz, which over a 20kHz bandwidth resulted in a 1.7 x 141.43 = 240nV of noise voltage compared with a 300uV output cartridge; the S/N was 62dB.

However, to achieve the required gain of just under 13, plus the approximate resistance matching, required a feedback resistor of 220 ohms into a ground resistor of 18 ohms. Like many op-amps, my choice was not too happy driving 220 ohms, and a minimum of 1k is stated in the data for its stability condition.

With 1k, the gain would be too high, which would mean increasing the 18-ohm resistor value considerably, which would compromise the noise. The solution was to make a total value of 1k, tapped off at 220 ohms.

With a cartridge connected to the input, the output DC offset was relatively small. It didn't cause a problem with a capacitor input phono stage, nor a DC input phono stage with AC coupled negative feedback ground. However, and probably due to the bias cancellation circuit in the often-used OP27 equipped phono stages, inputs had started to be DC coupled. That would be perfectly OK if not for their DC coupling of the ground resistor, which resulted in a DC gain of around 100, and that meant 100 times the Elevator EXP's output offset.

The above problem was probably due to the "all capacitors are crap" chorus emanating from numerous places around the web, added to the poor comprehension of what the "Charlies" were doing.

The problem was worsened where those who made valved phono stages dispensed with the DC blocking input capacitor without considering the possible use with an MC head amp. It upset the valve bias one way or another, depending on the polarity of the DC offset.

Although the op-amp had terrific sonic performance, it became evident that it didn't behave as advertised, and in attempting to capacitor couple the circuit, it burst into oscillation. This was behind closed doors during continuing R&D and was never released until the problem was solved.

The AD829 is featured in many AD data sheets as part of ancillary circuits, often as a buffer amplifier with a similar gain as the EXP. I noted that the AD829 compensation was totally ignored by its manufacturer in these applications, and slew-rate had obviously been sacrificed for greater stability.

By following suit, the EXP circuit could now be capacitor coupled, remaining stable. The whole circuit was rejigged and built using a heavily modified Revelation PCB.

After the iPhone 6 incident, I increased the compensation capacitors further. Piling up both iPhones and Androids performing data functions on top of an EXP, with no ill effect, decided it was of good merchantable quality, and we started manufacturing them that way.

As very little else had changed, the new version measured within less than a dB of the original specification. It was sonically identical to the original.

During the development of the Accession MC and the frustrations I had using low noise op-amps to up the input stage gain, I started to cast an eye over other designers' shoulders. One you will probably know about is Doug Self, who publishes books on audio electronics. Studying his transistor assisted op-amp moving coil step-up amplifier, I realised he was using the same negative feedback tap idea, but employing a NE5534 op-amp, it required a small gain transistor stage on its input. The EXP had not needed this because the op-amp has lower input noise than the NE5534.

However, my interest turned to the transistor input stage. It felt like we were all using a sledgehammer to crack a nut. We only require a stage gain of between 10 and 14, so do we really need an op-amp at all?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote patientot Quote  Post ReplyReply Direct Link To This Post Posted: 17 Dec 2020 at 12:22am
A very interesting read for me. I don't use MC carts but one of the things that led me to your phono stages was an RFI problem I had with another unit. 

Edited by patientot - 17 Dec 2020 at 12:38am
SL-1200 MK7 (modified) + Reflex M + PSU-1 used with AT150-40ML, AT VM95ML, Stanton 680mkII + Ogura, and Shure M35X cartridges.
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