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Sunday, 15 October 2017

New project - tone control

After building the last two amplifiers, I've had several months of not building anything, while I wait for the next firm customer. Lots of people expressing interest, but no-one ready to put any money down... yet.

So in the meanwhile I've been keeping an eye on the local auction site and pouncing whenever anything that looked suitable for a future project came up.

Latest score was a small power transformer with 213-0-213 secondaries (so good for 300V B+) and a handy 6.3v also.

Size suitable only for a preamp, this transformer cost the grand total of $8.00

With a couple of 12AX7s left over from my last project, thoughts turned to the possibility of making a headphone amplifier, nice idea except I didn't really need one.

Then I saw a site with some tone controls. My phono cartridge is a little down on treble so it seemed like a good idea to build a tone control based around a pair of 12AX7s. Idea being to put this between my RIAA stage and the integrated amp.

This is the schematic:


















Just to challenge myself, I plan to build this on a PCB rather than on a chassis with point-to-point. The reason for this decision is so that I can at some future point take this PCB and transplant it into something else, if needed. Or alternatively, make up another one quickly if needed.

The next challenge is to arrive at a PCB layout. Being a complete novice at PCB layout design, my preferred design method is the same as the constipated accountant: Work it out with a pen and paper.

Or in my case, because I am not a complete luddite - my Surface Pro computer with the drawing pen.

So. This is the concept drawing of the PCB - obviously I'll duplicate it for stereo - and there will also be a pair of relays to disconnect the inputs and outputs from the circuit and tie them together for a tone bypass.







































Next step will be to measure the components and design the PCB.

Work in progress.

Just for reference, my Phono preamp I am using is the 3-triode "Little Bear"


This machine uses 6N2 tubes, two for gain and one on the output as a cathode follower.

I've tested its RIAA response using the FFT function on my USB oscilloscope:

Blue is input 5mV RMS Sine sweep,  yellow is output. Scale is dB on the millivolt. Ignore the input below 70Hz, hum due to unshielded leads.

From this it's clear that the RIAA stage is behaving itself, so the blame for the slight treble loss from the turntable must be with the cartridge. An Audio-Technica ATS-11 with a band new Shibata stylus that cost considerably more than this tone control will.

(The treble loss is ascertained by listening tests by doing an A/B comparison with the same music played from vinyl and a digital source, synchronized at playback. Other than that, the vinyl sounds fantastic.)

Friday, 29 September 2017

Amplifier Horoscope eCommerce Service

Today I am pleased to announce my latest offering to the audiophile community.
  • Are you looking for a new way to give your system that "edge"?
  • Do you already have the best cables connecting your system?
  • Are your CD players already on isolating feet? 
  • Have you already traced the edges of your CDs with the special pen? 
  • Does your turntable weigh more than a small car already?

Have you experienced the crushing disappointment when you describe all the audiophile tweaks you've made, only to find someone else who has already done exactly the same?

Here at ATR Audio Designs, we feel you. The frustration is real. If only there was something more you could do, to give your system an extra advantage.

Well now, there is. At least for those of you with valve/vacuum-tube based amplifiers, that is.

For the first time ever, we have brought the benefits of the ancient art of astrology to the audiophile community. 

Using a patented process combining astrology with hysterio-magnetic resonance analysis and flux emission tomographic spectral emissivity coupling, we can now offer horoscopes for individual amplifier components with unprecedented accuracy.

In simple terms, here's how it works:
  1. Sign into our website and create an account for yourself (or use your Google or Facebook ID)
  2. With your amplifier cool, unplug the valves/tubes one-by-one, and make a note of the tube type, manufacturer, and the date stamp or code on the tube
  3. Using a compass, determine the magnetic heading your amplifier is pointing to
  4. Input this information into our site 





That's all it takes! Our patented algorithms will get right to work, calculating the optimum time to listen to your amplifier, based on the manufacture date of the tubes and the influence of the Earth's magnetic field at your given location, in the electron emissions inside the tube.

We will then produce a report which amounts to a horoscope for your amplifier. Complete with predictions for the next 3 months of which days you can expect the best results from your amplifier.

You will also have the option of subscribing to our site, this will add you to our mailing list and every three months we will automatically send you another horoscope as long as your subscription is active. 


Price list

Amplifier Type: Single Ended

Output tube type: 6550, KT66, KT88, 6L6GC, EL34 etc:
Initial Report: $US 1995.00
Monthly Subscription: $US295

Output Tube Type: 300B
Initial Report: $US4995.00
Monthly Subscription: $US495

Output Tube Type: T1610
Initial Report: $US19,995.00
Monthly Subscription: $US1995.00


Amplifier Type: Push-pull
Output tube type: EL84, 6550, KT66, KT88, 6L6GC, EL34 etc
Initial report: $US99.00
Monthly Subscription: Please make a donation to the Onion or Clickhole

Thursday, 28 September 2017

Philosophy

In the past, vacuum tube/valve amplifiers were not an expensive or exclusive option, because there was no alternative. Manufacturers typically mass-produced them, and like any manufactured item, margins were slim and they were manufactured to a price. Quality was variable and short-cuts were taken in the interests of cost-containment.

In the late '60s the first solid-state (=transistor-based) amplifiers became available. The increased reliability, smaller size, and greater power capabilities of the new gear quickly won favour and by the late 70s it looked like valve gear was relegated to history.

However, In the 1980s there began a resurgence in interest in valve gear, driven partly by nostalgia and partly because of the lingering dismay felt by audiophiles in particular at the sound of the early transistor amplifiers, which to be charitable, sounded bloody awful.

So the concept of valve amplification as a high-end audiophile alternative began to take root. And then it found a market and grew.... exponentially.

Now, we have hysterically over-engineered behemoths from Kronzilla and the like, using ridiculously overpriced tubes like the absolutely monstrous T1610. Try buying these, a snip at €3500 each.

Complementing this is the hi-fi cable industry. It's possible to spend €9000 on a 1.5 metre power cord. Interconnect cables and speaker wire runs to similar prices.

All of these excesses are targeted at one thing only: Relieving audiophiles of their money as quickly as possible. Nothing more.

The claimed benefits from the use of audiophile-grade cables and components are entirely quoted in pseudoscience terminology. There is no human alive who could discern the difference between a standard IEC power cord and something that costs more than most people earn in a month.

This point is worth exploring further because it's quite a frequent occurrence that someone will buy some high-quality gear, hook it up with ridiculously expensive cable, then credit the cable with the beautiful sound they experience, instead of the audio components. This is highly insulting to the engineers and designers who designed them. It is somewhat like the religious family with a child sick with a life-threatening condition which they take to hospital, and the medical staff work tirelessly to help, which is ultimately successful and the child recovers. Then the family publicly thank their god for the child's recovery instead of the medical staff.

I make a parallel with religion very deliberately; because belief in the purported benefits of hyper-expensive cables has much in common with it. 

Compounding this delusion is my personal view that an audiophile derives 80% of the pleasure from his equipment in bragging to others how much it cost him, and only about 20% actually enjoying the sound.

My use of the gender-specific pronoun is deliberate; I don't know any women who would be this misguided or frivoloous.

In fact, I suggest that Hi-Fi boutiques should not display prices on their equipment. Instead, as you walk in the door, the assistant asks "How much would Sir like to brag his system cost to his friends?" - you say $100,000 please - so the salesman puts together a system and charges you $100k.

The equation probably goes something like this:

Cost of manufacture of equipment: $3000
Cost of Sale: $2000
Rights to brag that your system cost 100K: $95000


Unsurprisingly, I utterly reject this form of thinking. I enjoy the sound quality from a well designed and constructed amplifier, and I enjoy the design and build process. But I dismiss all the pseudoscience.

With the amplifiers I build and provide to others, in the accompanying instruction leaflet, I include the following statement:


Monday, 17 July 2017

Fine tuning the big amplifier

So I've lived with the big KT88 amplifier for the last few months and have been mostly happy with it, though there were some niggles that I resolved to get around to. 

Specifically, the amp ran hot, and needed more ventilation holes drilled. 

Also, there wasn't enough Negative Feedback (NFB). I wasn't too worried about this until I built the EL84 amp featured elsewhere on this blog, which had more NFB. On hearing the difference, I resolved to correct the situation in the big amplifier, but this would need some equipment I didn't have.

So, first up was a shopping trip online to get some power metal film resistors for a dummy load, it's very important to have a non-inductive load for tuning NFB. These were duly mounted to a large heatsink.

Next I needed a pair of old-style variable capacitors, the kind you'll find in an old valve radio. eBay to the rescue, and these eventually came all the way from Bulgaria.

The method I intended to use for fine tuning the NFB was from Morgan Jones "Building Valve Amplifiers" p.290-291. 

Today, I managed to get the amp back onto the workbench. Pulled out all the valves and attacked it with the drill, to make some new ventilation holes. Problem 1 fixed, and it remains to leave the amp on for several hours to determine its effectiveness.

Problem 2 was also resolved, though this was a good deal more time-consuming. Utilising Morgan Jones' method, and armed with a healthy stock of film capacitors of various values, I started making the necessary modifications to the circuit, first with potentiometers and variable capacitors, before subbing in fixed components.

First order of business was to reduce the NFB resistor from the (fairly useless) 100K to something lower. After experimenting with the input sensitivity, I dropped this to 33K.

This got the amplifier's gain to where it needed to be, and eliminated the problem of the very touchy volume control.
It did however introduce another very serious problem: high-frequency ringing. The Williamson design is prone to this, and adding NFB in any quantity will exacerbate it. 


This was the result (red trace) at the speaker terminals of dropping a 10kHz square wave into the amp, after reducing the NFB resistor from 100K to 33K


Yeah :(

Don't know about you but I don't want to listen to that amplifier like that. Apart from anything else, it'll set all the dogs in the neighbourhood howling. And things will be getting mighty hot with all that high frequency energy to dissipate.

So clearly some compensation needed.

So watching the trace on the scope, using Morgan Jones' method, I arrived at these changes to the circuit:


Added compensating capacitor and resistor parallel to the anode resistor in the first gain stage



Dropped feedback resistor from 100K to 33K
Added compensating capacitor and resistor parallel to feedback resistor


This was the result:


Speaker trace in red


Yeah, I forgot to clip the CH1 probe back on to the input. No matter; it's the same signal at the same amplitude.

So far so good, this is all textbook from Morgan Jones. However in the course of my experiments I discovered something else that Morgan Jones apparently neglected to mention which I pass on here in the hopes that it may help someone.

Specifically. Jones' method calls for the feedback resistor to be bypassed by a variable capacitor and resistor, which I did, and I noted that the resultant waveform at the speakers looked pretty much like the above already. 

Thinking I wouldn't end up needing anything bypassing the anode resistor, I acted on Jones' recommendation and put a 220nF capacitor across the speaker terminals as a test, and watched the output go absolutely crazy. It looked much worse than the amp with no correction at all, and in fact was on the very edge of falling over into uncontrolled oscillation.

I then decided to bypass the anode resistor in the manner suggested, this resulted in some fine tuning of values as these are all inter-dependent. Eventually I got it to approximately the same level of cleanliness on the output as I'd seen with just the feedback resistor bypassed,

Then I tried the capacitor-across-the-output trick again.

Result: The amplifier barely even noticed the capacitor. A complete fix of the problem :)

Conclusion: the anode load resistor bypass doesn't do much to alter the oscillation into a resistive load, but it makes the amplifier much more stable into a capacitive load.

Morgan Jones did not mention this anywhere I could find.


So for reference this is the circuit diagram of the amplifier now (click to see full size)




The power supply implements the timer circuit which is not shown here for clarity, refer the circuit diagram of the EL84 amplifier on this blog for details on that.

This is the last modification or fine tuning I expect to make to this amplifier.

References

Morgan Jones "Building Valve Amplifiers", Newnes press, 4th. Edition. pp. 290-291

Wednesday, 17 May 2017

Last piece of the EL84 Amp design

So the circuit sketches and various measurements on assorted bits of paper have finally been consolidated into a coherent schematic... thanks in no small part to an incredibly patient and supportive wife who endured being an electronics widow for one final evening on this project!

The schematic is "as built" and there is one part I am less than happy with and if this amp is ever back on my workbench it'll get fixed: I am not happy with the resistance of R3 and the resulting voltage on the low side of it.

This leads to the voltage on the anodes of the RIAA stage for the second valve and the cathode follower. A little low, but notwithstanding, it passes the listening test with flying colours. 

But if it's ever back on my bench, R3 is getting swapped out for a 27K quick-smart.

Apart from that, you can see the implementation of the 555-based delayed HT switch-on circuit, the DC heaters for the first two valves of the RIAA stage, and the sneaky re-use of that voltage for the bias for the EL84s, and the 270K / 68K Voltage Divider for elevating the 12.6V heaters - the cathode follower needs it.

The Gain and phase splitter stages (V4 / V5) are based on the Fisher X100, but with higher quiescent current on the 12AU7.

This is the circuit. You'll need to click it to see full size and download / print / zoom / scroll, or whatever.




You'll probably need to download this to see it clearly

Monday, 15 May 2017

EL84 amp completed

So the EL84 amp is complete and has been delivered to its new owner, who has compared it favourably to his 60wpc Harmon Kardon Solid State amplifier.

The aesthetics of this one are much more favourable than the previous build. In testament to this, the new owner reports a high Spousal Acceptance Factor :) 

Looking good next to the turntable

In the end the chassis was about 5mm too narrow to fit the power transformer and the output transformers across the back in the usual configuration. So it had to be non-symmetrical


Making a virtue out of necessity: The non-symmetrical theme carried through from the transformers to the placement of the valves, and the controls on the front panel.



The front three small-signal valves form the RIAA Phono stage, the rear two are the line-level voltage gain and phase splitter.

The Bias test points sit between the EL84 output valves, with recessed trimmers to adjust, and test points to measure the voltage across the 10Ohm cathode shunt.
The Slovak-made JJ EL84 output valves on this one have quite a pleasing amount of light-leakage from the filaments and cathodes. Unlike the Russian Electro-Harmonix small-signal valves which are hard to see any filament glow from at all


Inside, the amp is crowded. Point to point wiring inside a tight working space.

Polyethylene Film capacitors are used for inter-stage coupling, and also in the RIAA stage, which as at top left in this photo. The power supply board sits under the output transformers, The power supply board has my usual 555 timer-based circuit to delay the B+ turn-on be 30sec giving the valves plenty of time to warm up first. By use of this circuit, combined with heater elevation for the small-signal valves, I can get away with avoiding the diode on the cathode follower in the RIAA stage, which is DC-coupled to the previous gain stage.

6 PCBs inside this case, including 5 home-made ones

Some specs and tech details

Main Amplifier

Topology
Line-level amplifier, grounded cathode gain stage, DC-coupled cathodyne phase inverter, push-pull EL84 output in class AB using fixed-bias ultralinear topology, global negative feedback. 370V Plate Voltage.
Valve complement
Gain stage: 1 X 12AX7 (ECC83)
Phase Inverter: 1 X 12AU7 (ECC82)
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
50 KOhms
Input Sensitivity
300mV rms for rated power
Frequency Response
6Hz – 55kHz ±3dB
Power consumption
230v 50Hz 190w nominal


RIAA Phono Preamp

Topology
Phono-level amplifier, cascaded grounded cathode gain stage, DC-coupled cathode follower, RIAA equalisation
Sensitivity
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Valve complement
Gain stage: 2 X 12AX7 (ECC83)
Cathode Follower: 1 X 12AU7 (ECC82)


Listening tests

The sound from this one is clean, detailed and very pleasing. The main amp stage is based on the well-regarded Fisher X100, with the 12AX7 gain and 12AU7 phase splitter, though this design runs that 12AU7 closer to its 5mA sweet spot for linearity from a 300V B+ than the Fisher does.

The output stage is EL84 in fixed bias ultralinear, biased to 8.4W quiescent dissipation (70% of rated maximum)

The RIAA stage works well. The Cathode follower is needed to drive the volume control which represents a 50K load across the input. The noise floor is low, hum is non-existent, and distortion does not occur even on the loudest passages. 


Lessons learned

During the construction of this amp several lessons were learned...

1) Hum was a constant problem
The amp was built backwards, with the output stages and transformers being wired up first, powered on to test, then the preceding stage, right back to the RIAA stage.

The 12AU7 Phase Splitter was putting a nasty hum into the output. After chasing that down and much testing with the oscilloscope etc, it was determined that the hum was on the anode but not the cathode. Many solutions to this common problem are available on the internet, in the end I opted for an additional level of decoupling in the power supply with a 10K resistor and 220µF capacitor, this fixed the problem. The lesson is that Phase Splitters have NO PSRR on the anode side. That power needs to have no trace of ripple on it


2) Grounding needs close attention in a RIAA stage
You can read as many books as you like but it's only when you build an RIAA stage that you truly get to appreciate how to ground the incoming signal... and how not to. This one had a nasty hum which was coming in through the Earth side, it would only manifest when there was a source plugged into the phono input. If the jacks were empty, the stage did not hum. But plug any source in, the hum appeared ... after bridging the input with a 1K2 resistor to simulate the cartridge, it was observed the hum was injecting into the live from the earth through the source. 
After moving all the signal Earth to a common point - which was the star earth off the first valve in the phono stage - suddenly it went dead quiet.


3) Take care with design and placement
There were a few too many near-misses with things fitting much tighter than planned, or almost overlapping other parts, 

There are also some minor changes I'll be making to the design of my bias boards and power supply boards for the next build, which is not currently planned.

Next post... I'll put up the as-implemented circuit schematic.

Monday, 24 April 2017

Some progress on the EL84 amp

The EL84 amp is well underway now. First of all, the name:

Iwa Orotuanaki Wairua

This is is the Maori language (explanation for people outside New Zealand: The Maori are the indigenous people of New Zealand).

It means (approximately) "Nine Echoes" or more accurately "Nine Sound Spirits"

Why this name? Nine because it has nine valves (tubes) and it's going to a friend who is involved in paranormal investigations

Progress

So the case has been laser-cut and etched, the front and rear panels have been filled, seats added to the top:





A slight error with the laser etching means we seem to have two sets of "Speakers A" ... oh well, it adds character

Circuit boards made up – the power supply shown in the previous post, plus the relay board for the speaker switcher, the feedback filter board for the Phono stage, and the bias adjusters for the output stage:


Feedback/RC Filter board for the Phono stage
Relay board for the speaker switch (Speakers A/B, 4Ohm and 8Ohm)



The Bias adjust boards, already mounted up. Yes it does say "Star Wars" inside the amp; this was a test of the laser etching before potentially messing up the top!


Construction


Bias boards mounted, tag strips, power supply and feedback board... starting to see how much of a squeeze this build is gonna be!


Wiring up the low voltage parts....



So that we can do this!

More later.