All about electronic valves and hi-fi
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.
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.
I made the following changes to the RIAA stage to improve its accuracy:
All resistors were adjusted with a pair of 0.1% SMD resistors to avoid series connection to reduce noise level.
As shown below, the response is really good matching at less than 1% for frequencies above 50Hz. the error at low frequency is due to the rumble low pass filter, the hum and the soundcard LF response which introduce a slight distortion at very low frequency in the MLS measurement:
| Frequency (Hz) | Test (dB) | RIAA (dB) | Error |
| 20 | 18.80 | 19.27 | 2.44% |
| 100 | 13.00 | 13.09 | 0.69% |
| 1,000 | 0.00 | 0.00 | 0.00% |
| 10,000 | -13.70 | -13.73 | 0.22% |
Last week I started with my JFET RIAA stage. This stage is designed for my current MM cartridge (Ortofon 3e) which has a relatively high output at about 4mV. The stage is a variation of the Werner Ogiers’ circuit, inspired by the John Curl Vendetta. Rod Coleman helped me out here to get this design optimised, so thanks Rod again for the contribution.
A two stage RIAA stage with only about 42dB gain is not ideal. I’m looking at replacing my cartridge soon and hopefully buying an DL-103 MC cartridge. I will then easily tweak the gain and RIAA network to use the Denon one instead.
I built a breadboard of one channel and tested it:
Here is a first test of the RIAA response: Continue reading “JFET RIAA preamp build”
Now is the time to upgrade my current RIAA stage. A bit unorthodox approach for a valve enthusiastic but you will see me now with my hands on sand. A very simple JFET input stage with a shunt cascode pair to drive the passive RIAA network. The final gain stage is an OPA604 with remaining part of the RIAA network. The Op Amp is forced into class A by a nice pair of BC847C forming a CCS.
Given the shunt cascode bipolar is an PNP Darlington formed by 2 BC860C low noise and the CCS has also SMD devices, I started by soldering these into 4 PCB adapters for SOT-23 devices
More to follow…(just a start)