A really interesting reality check for audiophiles
Check out this series if you want to know how tube amplifiers work. Uncle Dougs videos are very clear and informative. He also has presentations of many vintage guitar amplifiers and building vids of guitar amps. Great stuff!
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. There is also a huge and active thread over on DIYAudio, and a discussion of powersupplys on audioKarma
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 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 tripling 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!
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):
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
So this will be my chronicle of tube or valve amplifiers and things associated, such as speakers and signal sources. Since I prefer to keep a computerized diary of things I learn, I might as well do it online so others might share my experiences.
Well – here goes…