I used a aluminium bar structure to hold both LL7903 transformers and the Russian PIO output capacitors. The LL7903 will be wired in 4:1 and the capacitor will prevent the DC current from the DAC to shunt to ground. Also used some silicon rubber to protect the mu-metal can of the LL7903:
Here are the DAC boards mounted on a PTFE board with a set of copper strips that will be used to solder 10 x 100μF SMD low ESR capacitors to improve decoupling of DC rails:
After testing the accuracy of the R and 2R network, thanks to Jon managed to complete my discrete DAC boards for the Starlight CD player. Not very keen to solder SMD ICs but couldn’t get away from the tedious task of soldering 10 ICs:
Now need to continue with the DAC (or the transport). Lots of work to do still…
It was time to take the 814 SE Class A2 amplifier measurements. The challenge though, is that the amp is so heavy that I will never take it up to the workshop. Therefore, I decided to take my workshop PC down from the loft this time to see how the 814 really responded.
First test was to do a THD analysis as a function of the total output power. As you don’t want to do this with your speakers, and also the classic wire wound resistors (Alu-clad) are inductive, you want to use non-inductive resistors like this test jig:
I bolted on to a large heat-sink an array of resistors to form 8Ω in value by using a pair of 10Ω in parallel and three 1Ω in series. I added a set of binding posts and connectors for the speaker cables. this way you can easily wire your speakers and connect your audio test set to take the measurements.
After enjoying the RIAA phono preamp for several weeks with the new battery pack, I decided to build the battery charger just not to be surprised by the lead-acid battery pack running out unexpectedly.
The lead-acid battery pack is formed by a 12V and a 6V 1.3Ah batteries. Between both batteries there is a total of 6 +3 cells = 9 cells. Each cell requires 2.27-2.30V of charge at 15-24C. Therefore the charging voltage should be 20.43-20.7V. Exceeding this voltage will reduce the battery life. The charging circuit has to be constant voltage, not current.
Rod recommended me a simple but very effective design based on the TL431a. The regulator output voltage is set by R1 and R3. 0.1% precision resistors are needed, otherwise a trimpot should be used as R1. R3 should provide at least 2mA for both Q1 and TL431a operation:
A simple PCB was built in less than 1 hour. You need to place the BD439 in a heatsink as it will get hot when charging the battery:
In a question of 2 hours the battery was charged back fully. I used it for around 20 days so far I think. The initial current is as high as 370mA and drops as the battery charges and the voltage goes up. The residual charged voltage was about 19.4V. This drops quickly after some current starts to flow through the preamp.
Not music related, but it has to do with my other passion: brewing
Straight from Czech Republic, brought on a flight from Prague today, a nice pint of Pilsner lager!
Building a new version of the venerable 4P1L “Siberian” was very encouraging. This belated project finally came to life after some recent work on a new set of power supplies. So why 4P1L again? I always found the 4P1L sound to be unique. Great detail, overall tone and fantastic treble. What it makes it well suited for pre-amplifiers is not just its linearity (probably being the most linear valve out there) but the fact that it has a low anode resistance and current capability to ensure any challenging load can be handled effectively without any sound degradation. This can be heard particularly on the treble where the input capacitance of the amplifier is more evident and it is translated into treble loss. Other DHTs like 26, 01A, 30sp can only handle a few milliamperes of anode current and is not enough to charge and discharge the parasitic capacitance at high frequencies. More importantly, the 4P1L has filaments which aren’t demanding. This is a unique feature amongst DHTs that is rare and very useful. Having low-current filaments that can be either configured at 325mA or 650mA, low grid voltages and high transconductance in a valve is very useful. This mean that filament bias can be easily implemented without burning unnecessary power by swinging many volts to perform the desired level of amplification.
Recently I finished the filament supply for the latest incarnation of my 4P1L pre-amplifier. Here is the next instalment of this project. The HT power supply was refined after builiding more than 7 stacked HT supplies for the 814 SE Amplifier.
It’s been some time since I built my JFET shunt-cascode RIAA MC stage. I have listened to several well know records for some time with this phono stage in my system and others. I took it to the London Circle Audio meeting recently and it was very well received by most of the circle members. I even had some requests to build units!
VT-154 SE’s warm sound
Now I can say that it’s completed. Albeit I haven’t built the charger for the lead-acid battery pack, it sounds fantastic and has great autonomy. Long gone days were I was recharging my NiMH pack! Now I can relax and enjoy music without worrying about charging the batteries now and then. I haven’t measured the voltage across the battery cells so far.
More lasting than bronze – John Coltrane
Usually I play my John Coltrane records. Not just because I personally find Coltrane to be one of the best musicians ever existed on earth, I have some specific songs I use for testing. When I was younger, I used to listen for hours the same Coltrane records when learning saxophone. I was keen to listen to every single detail on the phrases, solos and arrangements of the Quartet, Quintet or the Sextet formations. As a sax player, you learn many solo parts, tunes and you know by memory every expression made when blowing the horn. If you don’t have any Trane’s record or even you haven’t heard any of his compositions, I urge you to seek his wide and bast repertoire. From “Love Supreme” to “Giant Steps” and many other gems. Several years ago when I started to rebuild my vinyl collection here in London, I found this excellent compilation of Coltrane: “More lasting than bronze“. It’s a great sample of Coltrane’s music. I use it to test my system. The horn bass notes, the pulsed bass, drums and pianos have a great level of dynamics and detail I constantly listen to. It grabs my mind and takes me to another planet. That is music, more lasting than bronze…
I built a new pack of 12V+6V lead-acid batteries to provide +/- 18V for my JFET RIAA MC phono stage. Despite the bad reputation of these being noisy, Geoff tried them with great success. The Haze brand are the recommended and the low capacity ones (i.e. 1.3Ah) are very quiet.
I decided to build and test it. As the proof is in the pudding!
differential chokes and OSCON caps
Battery pack
Battery pack completed
Measuring noise
Here is a simple test on my workbench of the 18V battery set with a 20mA load to simulate the RIAA stage consumption. The LC is formed by a 33mH choke in differential mode + 100uF/20V OSCON capacitor.
No twisted cable pair, just banana alligator clips. The noise level is really low and is obvious that 3.3μV 50Hz hum is picked from the workbench. I also listened to it for a while and can hear the difference. I monitor the FFT with no average and lower FFT size and also couldn’t see any spikes due to chemical reactions. They may happen in future though 🙂
The performance of this battery pack is outstanding, so far so good!
Here is the 4P1L Siberian DHT preamp (Gen3) socket board. I’m using a pair of custom made teflon sockets mounted on a PCB sandwich with a 4mm silicon rubber sheet. The lower board is mounted over 4 silent blocks:
drilling the socket boards
Loctal sockets fit and silentblocks
finished socket board
4P1Ls
4P1Ls ready!
This should be a great improvement to reduce any further microphonic noise in the preamp!
I think all fits in the box I’m planning to use 🙂
