The idea was to have a small PCB which includes both protection and power-off function. The PCB itself is a one-sided board with black solder mask on both top and bottom layers. I’ve decided to include optional drills for other capacitor sizes. This will make it possible to “adjust” the board for other Thorens models of even other turntable makes like Linn or Ariston.
The following link is a great source of info about RIAA designs. A great collection of schematic topologies and measurements. A must read material. The site includes variety of industrial schematics. Some of them are pretty old but could be easily adapted and brought back to date using the modern component base.
Now this one is a real gem:
It’s a design by Linsley-Hood. This one could be easily optimized for even lower noise. What comes to mind is the Toshiba’s 2SK170 JFET. This will drop the input bias circuit and the need of input capacitor. Plus – more JFET’s can be paralleled to reduce the noise floor even further at no real penalty. The only problem is the hfe matching between the JFET’s.
Be sure to read the article here – >>> LINK <<<
This idea came to me when a friend from the forum asked for a simple schematic based on the ECC85 tube.
He had plenty of these around and wanted to try them out. So I sat and and made some calculations. Since we had a 300V B+ supply I had to make calculations for this supply voltage. The preamp itself is a pure classic. Two stages separated by the RIAA EQ network. The RIAA parts were calculated using this tool:
I have used it many times. It’s always spot on. Of course when calculating RIAA equalization some parts of the schematic must be taken into account. Since the output impedance of the common cathode stage is pretty high it can not be neglected. The output impedance is effectively in series with R1(from the link), altering it’s value. This of course leads to an incorrect RIAA equalization. The other part altering R1 value is the next stage input resistor. This must be taken into account too.
Of course before calculating the RIAA equalization network I needed to set the operating conditions of the two gain stages. Those stages are identical. Both sections are biased at 175V anode voltage, -2V grid, 6mA anode current. This setup gives about 28 of voltage gain using a 20K anode resistor. Here is the bias point:
The total gain of the preamplifier is about 37.9dB. This will give you around 400mV output from a cartridge giving around 5mV of output.
The complete schematic is available for download HERE.
Here is the predicted nonlinearity of the RIAA equalization for this project (+/-0.3dB):
Here are some pictures from a complete project. This one was built by a fellow mate Stoyan Tsonev.
This project started as a experiment after reading the Solidophono article at TNT-Audio website. I was interested in the SSM2017 opamp so I’ve decided to give it a try.
On my project I’ve decided to go for a bit different topology using a split RIAA equalization. All my previous projects were created using a traditional passive equalization so I was exited to see the results.
The whole preamp is made of two stages. The first one is built around the SSM2017 amplifier from Analog Devices.
One could use the modern substitutes – SSM2019 or INA217. The gain of the first stage was set at 60dB! The second stage uses the OPA627 opamp from Texas Instruments. One could experiment with a variety of opamps here. The gain was set to 2. I used a jumper to change the second stage gain to unity in case no further amplification was needed.
The complete schematic for this project can be downloaded >>> HERE <<< Some pictures of the finished PCB. The project is using two separate aluminum boxes. One for the PSU and one for the preamp PCB itself.
The finished PCB inside a nice aluminum box.
The four voltage regulators are placed on the PCB. I’m using a small 20VA toroidal transformer to power up the whole thing. Here is the PSU section:
As you can see, this is a serious power supply. Such configuration could be seen in some expensive power amplifiers. Such a PSU has a very high ripple suppression and since the current demands are no big issue in RIAA preamps, it’s affordable and easy to build as well.
So how does it sound?
Well, I’m not going to describe my own creations as my judgement could be a bit subjective. However I could say this preamp has some very good dynamics with good transient attack.
I encourage you to try this project and see for yourself.
WARNING: The project is provided to the DIY community for free. As such it can only be used for personal needs and NOT for commercial usage.
THANKS FOR READING THIS ARTICLE.