Tuesday, May 20, 2014

Celebrating 100.000 visits by moving to WordPress

Thank you to everybody who visits my site, I sincerely hope some of my posts have been useful. The 12.may 2014 we reached 100.000 page loads!

Totally unrelated to the above:
The site will now be gradually be moving, to wordpress.
All new posts will be published at taooftubes.wordpress.com, and I hope any comments are posted on the relevant blogpost over on wordpress.
This site will still be here for the time being, for reference

The choice is due to my dislike of googles data gathering policies and pressure to join Google+. (don't ask,  its a personal choice;-)

Thursday, April 17, 2014

Phono preamp for moving magnet turntable pickups

It has been 2 years since my last post which i due to a foray into RC planes, multicopters and helicopters as well as Arduino building, and life happening.
But now, its time to tackle a box of parts I have been saving for a while. RJM audios VSPS or Very Simple Phono Stage, for short, at http://phonoclone.com/diy-pho5.html  It´s a simple dual op-amp design with a modified Allen Wright RIAA stage and only 27 parts.
You can read a detailed description of the design on RJMs site, above.

http://beavishifi.com/articles/phono-preamps/ has published this simple diagram of the circuit. But note that RJM also suggests adding adjustable gain by switching between three values of R2.

 My plan is to build this phono pre-amp with a battery supply and install it in a case that is suitable for display in the living room system. I am using strip board to build on. And I´m planning to power it with batteries.

Starting with the socket for a dual op-amp NE5532 in the top middle, I established the power strips next. The chip is powered by +12V  and -12V with reference level, ground, in the middle. I decided to use the bottom strip as ground and the nr 8 pins strip near the top as +12 V.   -12V is not yet assigned a strip, I just soldered on a piece of black wire as a reminder, but later changed this to orange to remind me it is not ground.
Next I connected the right and left  + INputs from pins 3 and 5 to ground via two 47k resistors to establish input level relative to ground.


Next I decided to solder in three R2 resistors. Considering that the inputs may be more sensitive that the outputs, I soldered these resistors from the -IN strips  As suggested by RJM I will use 2.2k for 30dB, 680R for 40dB, and 220R for 50dB. I started with a nice 1% Vishay metal film closest to the middle for 2.2K, but only had ordinary 10% for the 680R and 220R. For the 680R, in the middle, I only had 670s but the nice thing about 10% resistors is that you can find some deviants, and after sorting through 10 of them I had two 675R, which is as close as I will get for now. I also matched the 220Rs and found two that were spot on. All three were soldered to their own strip in the other end where I will later have a 3 way switch to ground to choose gain. Then the strips were Dremmeled away in the middle to create separate channels, like I did, previously, under the op amp.


Next is the RIAA stage between -IN and OUT which is pins 1 and 2 on the left side and pins 6 and 7 on the right side. Due to the amount of hardware going into these strips I need to move one strip down a bit. Intuitively I would move OUT, since -IN might be more sensitive to noise.
I have substituted some values for what I have and this fits RJM´s recommendations for higher precision, which is a plus: 2.2K , 105K and 732K all 1% Xicon Metal Film resistors. For capacitors I am using 1000pF 100V 2.5% WIMA polypropylene and trippeling them for the 3nF value. As shown here:


For the final stage in the build its time to add the OUT components. First the bottom strip of the RIAA stage is connected to OUT on pins 1 and 7 with green wire. Then the output coupling components go into place; a pair of 47R 5% Kamaya Carbon composition resistors followed by a pair of 2uF 250V 10% Sprague polypropylene capacitors. unfortunately I did not have 2.2uF, hope it works OK anyway. I mount all these along the OUT pin strips so I have to cut the strip under the components. Finally a 33k resistor to ground to drain of the capacitor. For this resistor I am using a generic 1/4 watt 10% again, I have a 32k and the value is not critical, according to RJM, so I´ll go with it.


Oh, and the Vcc- with the orange wire - I stuck it an an unused strip so I could power the board from the right edge.  Let the the debugging begin!




Wednesday, May 23, 2012

Jecklin Float model 2 restoration

Time to get that dorky nerdy cool look going here at TAO of Tubes. After looking and dreaming for a year or so I recently bought a Float model two on Ebay, which in this case earned its name as FleaBay. They were smudgy and dirty, the foam was crumbling big time and all glue bonds on the headphones had let go.
A good thing about them, though, is that they come with a original box in reasonable condition.

Today I have finished restoration of the left side, and soon i'll post a detailed, illustrated, refurbishment description for you. Just give me a day ot two to finish them first.




Friday, May 4, 2012

Finalizing the 6S45P Part four

The simplest amplifier obsession Part 0
Breadboard amplifier Part 1
Chassis building Part two
Assembling the 6S45P amplifier Part three
Finalizing the 6S45P Part four - this one

After having played the 6S45P for a few days I decided to try to improve hum by rewiring the heater and signal wires and reworking the star ground system. I ran the heater wires along the wall, and stretched two coaxial wires directly to the sockets, across the inside of the amp, Taking care to cross other wires at close to 90 degrees. This reduced hum somewhat, but it was still present.

The next step was to rebuild the power supply on a tag board. This would eliminate the ground plane on the PCB and create a cleaner single ground point to chassis.

This is a developing story - sorry for not posing it completed

Wednesday, February 1, 2012

Assembling the 6S45P amplifier, part three

The Christmas holidays are coming and the darkest part of the winter is here, only 4 hours of light each day. I have time, again, to work on my 6S45P amplifier continued from part two.
I´ve finished it in this part so here are all the links to this story:
The simplest amplifier obsession Part 0
Breadboard amplifier Part 1
Chassis building Part two
Assembling the 6S45P amplifier Part three - this one.
Finalizing the 6S45P Part four

Her goes the story:
Today I began soldering the output transformer to the banana sockets in the back of the amp. The wire was bent and zip-tied in place to give a neat appearance. I am hoping to finish this amp really neatly inside since it is so simple. I also mounted the signal input wires from the RCA plugs. I used CAT 5 twisted pairs encapsulated in thick aluminum foil to avoid hum. The wire package was tacked to the inside frame to be as far from the power supply as possible.

Now I had to decide on an operating point. The datasheet recommends 30mA at 150V, but several builders have tested the tube to be robust far beyond that point. After experimenting on the breadboard earlier this year I decided on operating the 6S45P at 175V and 40mA which will put the operating point just below the recommended power dissipation limit shown as the purple curve. In the diagram below the red lines point at the operating point, and the green line shows the voltage swing with an input of 2 Volts, typical max from a modern CD player. The slope of the green line is calculated by finding how much current is lost over 100Volts when loaded with the 5000ohm transformer. I=U/R so 100/50000=0.02A. So the slope is 20mA per 100Volts.

 Next I built the power supply, which is the most components in the design. This amp is almost only a power supply and 5 other components. I simulated the power supply, roughly, in Duncan Amps PSUD2. Since I was using the Russian 5Ц4С  I found nearest equivalent 5Z4P (unfortunately not G) data on Duncan amps pages and added to PSUD2´s rectifier list. The transformer is 240 to 190-0-190v and I used rectifier load of 5K to simulate my 5K output transformer. Usually it is recommended to use current draw for the operating point, but it did not seem to match my breadboard results. A confounding factor was not having the precise manufacturers data on the transformer or 5U4S in PSUD2.

Closest approximation in PSUD2 with Resistive load 5k.
I built the power supply´s smoothing section on a piece of experimenters board bolted at 90 degrees to the chassis. I could have isolated the board creating a better star ground point, but was impatient to get going. The power transformer has several optional taps that I cut short, isolated with heat shrink, and zip tied. At this point the dream of clean wiring started to crumble - as it usually does!
First capacitor 3uF to the left, then 100uF, 100R, 100uF, 100R, red wire i B+.

Angle brackets keep the board in place. Green/Yellow wire is heater for the rectifier.
Important: the bleeder resistor is connected between the two 100uF caps to drain them at power of.
Overview of the power supply. The switch is not mounted to the brown wire yet. I prefer switching through a ground fault protection switch.

Next I wired up the amplifier tubes. The 6S45P has several connecting points for cathode and grid. you only need to use one, so I chose the ones that reduced crossed wires to a minimum.
for novices pins are counted clockwise as seen from under the socket. starting from the gap on nine pin mini or from the key on octal sockets. 
Socket wiring:  Yellow pin 4 and 5 is heater, Orange pin 6 is cathode, Blue pin 7 is plate, Green pin 8 is grid and black in the middle is a local start ground for this channel.
The mA meter is connected to ground and then the cathode bias is created by series connecting a 22ohm resistor to a 50ohm trim potentiometer. A 470uF 35V capacitor is connected in parallel to the resistors. the light in the mA meter is fed 6.3v ac in parallel to the heaters.
Here is the first attempt at wiring. Not so neat after all. As usual. The yellow ac heater wires should not be routed out into the circuit, but kept along the side.
First power up. Oh why do I never get round to finishing that variac and light bulb load? I connect three multimeters to B+, filament 6.3v and 5 volt, the onboard mA meters show me the current. Connect the amp through a ground fault switch,  and hope for the best .... NO Smoke! The rectifier takes about 10 seconds to conduct properly. All voltages pretty good. I need to reduce minimum cathode resistance to get to 40mA. I can´t adjust it with the 33ohm limiting resistor I have now. Test with sound. One channel has a bad contact. After powering down and waiting for capacitors to drain I locate it by poking at contacts with a screwdriver. Its a resistor in the cathode circuit.  Now all I have to do is mount the switch and build the bottom.

I use some pieces of wood to support the transformer

Three voltmeters and the dummy load connected for first power up. Also I have a automatic ground fault switch connected to the mains cord, and switch mains of remotely.

The final schematic with measured voltages in red. A perfect beginners project!
UPDATE: I am not completely happy with Musical Power supplies for this application due to to the magnetic field inducing hum in trafo cover. I did have some hum, and when reworking the layout hum in the transformer also was reduced. you might consider equivalent Edcor or  even Sowther power transformers.
There a often discussions about what is the best beginners tube amplifier on forums like DIY Audio etc. I think this amplifier is the best place you can start because voltages are relatively low. (still deadly tho). You can follow up the amplifier build with a nice 95dB/w full range speaker and spend some quality time wondering why people spend tens of thousands on high end equipment.

If you want to build this amplifier really simply, take a look at how I did it on the breadboard version. Drop the ampere meter and the trimmer pot on the cathode.  Drop the tube rectifier and go for a cheaper lower power output transformer. With a cheap 220v to 120v transformer in a 240v country you will get close to 175v B+ using a normal bridge rectifier, 1 amp 600v should do, and capacitor resistor capacitor (CRC) smoothing, but make sure the transformer is a little over size to handle the extra voltage. Maybe 500mA or so. For heater use a 3 Amp 240 to 12 volt transformer and wire the tubes in series. And never leave out the grid stopper resistor soldered right up to the grid tag on the socket. The 6S45P can become a high frequency ADHD squirel if you do :-)

Here are all the links to this story:
The simplest amplifier obsession Part 0
Breadboard amplifier Part 1
Chassis building Part two
Finishing the 6S45P amplifier Part three - top of this one.

Monday, January 30, 2012

Music Angel XD-800MKIII

Since my Audioromy 383 FU13 died almost a year ago I have been researching numerous amplifiers capable of driving the MagnepanSMGa ´s 90db/W. I don´t want to repair the Audioromy and continue using it because its B+ voltage of 870V is to high for comfort. After some research and posting on DIY audio I decided I needed at least 20W. Ideally an amplifier with B+ of approximately 300v would be nice. My first though was the 6C33C by  Dmitry Nizhegorodov, but in single ended it needs some huge and expensive transformers and does not really reach 20 Watts without parallel single ended or push pull, I considered an OTL, output transformer-less version like Tim Mellows which is popular on DIY audio now. But the Magnepans low 4ohm resistance is not really suited for OTL tube amps. So my attention turned to Push Pull amplifiers, classic Mullard 5-20 circuits, with output tubes like 6L6, 807, EL34, KT88. The problem was that B+ needed to be higher than 300V to get enough power. I was pretty interested in the EL84 for a while as it is cheap and has a nice operating point. But I would probably need Paralell pushpull for power reserves which means cost and complexity rises.
My studies resulted in slowly loosing interest in building from scratch. I looked at the Engineers Amp/DCPP by Pete Millet which was very well suited, and I looked into building a Dynaco S70 based on PCBs from Triode electronics, another great amplifier. Both would drive the Magnepans well and have a lot of support for sorting trouble or upgrading. But at this point I had moved into territory I wasn't really feeling engaged about. I need a sense of exploration and wonder in my projects, not just walking beaten paths. So if I was going to work on a typical push-pull I might as well buy a finished amp. I decided to buy a Music Angel XD800 MkIII. This is a Chinese amplifier based on the classic Williamson circuit of 1947. it has a B+ of 450V, a 100V higher than I wanted but still half of the Audioromy. This particular Music Angel has a strong following in Norway, where I live, and is a cheap basis for experimenting. DO NOT confuse it with the YC808 - which is all about trouble. This amplifier I consider sort of a kit. There are several versions and a lot of experience to be found on "improving" or changing the amp for personal preferences. I looked long and hard at this amplifer when I bought the Audioromy a few years ago, and now I feel I should have gone this rute from the beginning.

Anyway, a Music Angel XD800 mkIII has been bought, used, and is in the mail to me now. It is the version built until July2008, the last version based on 12AT7 as preamp tube.
This may be the amplifier for transplanting into the Empire Chassis
Here is an older circuit diagram:
 
Variations to the build. (this is just a sketch  for now - more details will follow):
 
Preamp tubes. Both versions use 12AU7 (ECC82) as the driver tube. For several years the MA KT88 came with 12AT7 (ECC81) preamp tube - this is the original version. In autumn 2008 there was a change - the amplifier was modified to use the ECC85 in the input - typically it is now the Chinese 6N1 sitting there (because it is cheap)
Grid stoppers. Early versions had grid stoppers, then Lampizator interfered and made them produce without gridstoppers introducing instability. Later they reemerged.

Chokes. Although all MA XD800´s have the pots from chokes, not all models actually have chokes.

Straight/slanted cabinet. From 2010 to 2011 the cabinets were built with slanting fronts, although some were made using left overs of the opposite type.

I got my Music Angel in the mail yesterday. It looks and works well, but today I decided to open it up to take a look around and adjust bias, if necessary. I found that the cathode resistors to the KT88s were wired unusually. both were connected to the same cathode resistor. I reconnected them, properly, and readjusted bias, discovering that one tube was not conducting current properly. it cut out after about two minutes. So now a new set of power tubes need to be bought. See picture:
Both cathode wires go to V7 instead of one to V7 and one to V8

Same with V5 and V6
I also found there was no safety earth, So I soldered one to the mains socket and connected it to the choke-pot bolt with a crimped ring and lock washer. There are no chokes in this amplifier, just their holes and cover.




Follow this thread for updates

Wednesday, December 28, 2011

Fooling arround with acoustics inspired by Geneva Labs

A colleague at work bought a Geneva Labs model xl this insipred me to take a closer look at the grandest of all iPad docks. I had always assumed that it had a huge bass speaker in the middle and a couple of angled tweeters firing from the outer corners to create a degree of stereo effect. At least this is what the semantics of the design said. But in our time semantics are for telling stories, not for Describing functionality. Inside the Geneva xl has a stereo set of three way speakers placed as close together as possible. What was going on here?
A quick search found a thead on DIYAudio which researched the Geneva. There is a professional speaker using the same technology the EMES Owl. The technology behind both speakers acoustics comes from a company called Embracing Sound Experience AB located in Sweden. Embracing Sound have published a paper describing the principles involved, called (PDF download) "Single Bipolar Loudspeaker System for Stereo Reproduction"

So... I had to find out what was going on here. I had a couple of Fostex FE103´s lying around, and glued together two closed speaker boxes side by side. In between the speaker elements I cut the MDF so I could slot in a piece of aluminum or plastic. Listening tests showed that this divider contributed somewhat to dividing the output of the two speakers. When the listener was positioned in front of the speakers.
The 30mm dividing strip is not in place in this picture. But you can see the slot it goes into.

By reversing polarity on one of them I got the typical effect of floating the sound out on each side of the listeners ears. I used Rouge Amoeba´s Audio Highjack Pro to hijack system output from my MacBook Pro. I set up a high pass and a low pass filter where the lowpass was bass boosted a bit and sent straight to output, creating a in-phase bass line, while the high pass was divided in left and right channel and phase shifted using the Flux SST stereo tool. This combination gave a nice mixture of center image and wide open sound. My Good Wife was impressed. Though not by the looks. "That´s not coming into the living room, right?" I don't think this is the trick Embracing Sound Experience is using, but it sounded quite OK. Only problem was the SST tools were pulling 100% of the computers system resources. Oh, and the FE103´s don't deserve the name full range - they are mid-high tweeters.

Screen dump of Audio Highjack Pro and the Flux SST tools.