JFET RIAA Phono: Battery Charger

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.

Fresh pilsner

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!

4P1L DHT Preamp Siberian (Gen3) finished!

Introduction

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.

Continue reading “4P1L DHT Preamp Siberian (Gen3) finished!”

4P1L Siberian DHT Preamp (Gen3)

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.

The supply design is very simple. Perhaps the selection of components and the refinement of some aspects of it is what makes the difference to me: Continue reading “4P1L Siberian DHT Preamp (Gen3)”

Listening to the new RIAA phono stage

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!

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…

JFET RIAA Preamp – battery supply

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!

4P1L Siberian Gen3: Loctal socket board

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!

Siberian DHT Preamp Gen3: filament supply

Time to upgrade my pre-amplifier (again) and is perfect timing to go back to 4P1L. The Siberian preamp had a fantastic bass response and detail.

Here is the new filament raw supply. It has split bobbin transformers, schottky rectifier bridge and input choke LC filter stage. It also has a CM choke and EPCOS electrolytic capacitors:

Power supply ready

Input chokes

Chokes and capacitor bank

Schottky Rectifier Bridge plus snubber

Building the power supply

Dead quiet at 16V output and 550mA which is the load used by 4P1L starved filaments in parallel with filament bias.

Soon to build the preamp!

Building your own interconnect cables

I’ve been struggling for some time to find an affordable interconnect cable choice without falling into the trap of the ultra-expensive hi-fi commercial cables. In the end, an interconnect cable has to provide low capacitance, low resistance and proper shielding to protect our desired signal from external noise. We don’t need exotic cables or connectors to achieve this. Ideal dielectric is air, but is not practical build cable using air as dielectric 🙂

Continue reading “Building your own interconnect cables”

RIAA phono stage completed

FET RIAA Phono Stage

Finally completed today. It seems impossible from me to get a project finally boxed properly. I’ve done it this time with the RIAA phono stage built recently thanks to the help and guidance from Rod Coleman. This MC stage has a gain of about 70dB for MC cartridges and currently running my DL103a with a 200Ω input load. The beauty of the folded cascode (or shunt cascode as Rod refers it to) is that Miller capacitance is not a problem thanks to the fixed voltage at the drain of the FET input stage. This helps us to have a low input capacitance stage. The second stage is op amp OPA637.

I need to take final measurements of this Phono stage but this is how it looked when I was initially testing it on the bench:

Nice RIAA compliance to +/- 0.1dB. LF noise and gain levels made it difficult to capture the LF response below 100Hz.

The distortion is also really nice with less than 0.025% @ 1kHz and nice harmonic distribution thanks to the CCS at the output which is forcing the operational amplifier to operate in class A:

This stage was well received at the recent London Audio Circle Meeting. It has a nice clean and detail sound. I tried it with multiple MC and even low MM cartridges and has a nice response across the whole bandwidth. Great bass and delicate treble.

I think it is a very quiet phono stage given its high gain. I really like the overall sound and response and I guess that the negative side of it is the 18V battery pack made up of PP3 rechargeable batteries. It needs charge every two weeks and may be a pain in the back. I will try a DC supply, but will have to be really quiet to substitute my battery pack.