Our first test for the comfortINA starts – controlling the amplification of an instrumentation amplifier by a switched resistor. The DigiPot uses two low-resistance analog switches (ADG451) and an 8-bit shift register. The shift register is used to simplify external connections to a 2-wire serial bus (DATA & CLOCK). On board LM317L/LM337L reduces the external +/-18V voltage to +/-15V. For the tests we will use our platINA board and the control will be handled by an Raspberry Pi.
Are you waiting for the comfortINA? We’re still working on it. There are still some components that have to be tested. To do this in a real-world setting we started the DigiPot project. The name is a bit misleading. DigiPot can be used as a digital potentiometer, but also for many other use cases. You want to switch the input capacitance of your MM phono preamplifier remote? DigiPot is the solution! You want to create a variable gain amplifier? Use DigiPot in the feedback loop! With some tricks you can even use it to use it as a input switch in a line preamplifier.
Do you already own a platINA and want to upgrade it with some remote-controlled features? Guess what you can use to do this: DigiPot!
The ingredients: A shift register and low resistance analog switches. Initially I had planned to use a LDO voltage regulator from AD (you still see it in the design above), but I’m gonna test this device independently and stay with old-fashion LM317L/LM317L for this project. This makes it easy to solder the circuit even for beginners. PSSR is very high for the analog switches and power consumption is extremely low. Therefore there is no real need for regulators with better specs.
Does this sound interesting? Stay tuned for more information.
One challenge of creating a phono stage that works for both MM and MC cartridges is to implement a amplifier stage with a wide gain range. For our comfortINA project we’re looking for an input stage with a gain between 30db and 60db – that’s a voltage gain between 30 and 1000!
There are variable gain low noise amplifiers available. One example is the PGA2505. Unfortunately this chip can’t be used for a MM stage, because the input impedance is only 9.2kOhm, which is too low for a MM system. Another option would be a THAT1570/5170. However, this chipset is only available in a QFN packages, which is hard to solder for the hobbyist.
Therefore we go back to our previous design using an instrumentation amplifier. If you use an instrumentation amplifier like the INA163 or an operational amplifier, the gain can be controlled by an external resistor. The value of the external resistor has to be between about 200 Ohm and 6 Ohm. On the low end, a low tolerance is needed. There are no digital potentiometers available in this range. Therefore we will use low-Ron analog switches and external resistors. This design is also shown in the CN0146 application note from Analog devices.
Note, that the circuit diagram only shows the implementation of the variable resistor.
The minimize tolerances, we’re using parallel switches for the low value resistors. Let’s do some calculations. If the tolerance of an analog switch is 0.5 Ohm, the maximum gain difference between 2 channels would be 0.25db. In reality much lower tolerances are expected. With a 0.1 Ohm tolerance, the maximum gain difference between two channels would be only 0.05db – looks good!
For the comfortINA project we were looking for analog switches with a very low resistance. They should be able to use a symmetrical power supply with at least +/-5V.
There are more ICs available for unsymmetrical power supplies. These might be an interesting alternative if you can shift the level of the logic inputs.
Here are the interesting switches with a symmetrical power supply that we’ve found:
Analog Devices ADG1401/1402
These single switch circuits have a Ron of 1Ohm with supply +/-15V. Unfortunately the MSOP package with 0.65mm pitch requires serious soldering skills.
Analog Devices ADG1411/1412
This quad switch has a Ron of < 2 Ω at +/-15V supply. Unfortunately it is only available in TSSOP package, which makes is not so easy to use for DIY projects.
Analog Devices ADG451/ADG452/ADG453
Another (older) quad switch from AD. Unfortunately Ron is about 4 Ohm with +/-15V power supply. The good thing about it: it is available as SOIC an even DIP version.
The Ron of this chip is 2.5 Ω, It is available even in DIP packages which is great for DIY (I still would recommend to use SOIC package, which allows a more compact layout. Unfortunately it is relatively expensive (about 10 US$ in small quantities).
You can see, that the more modern ICs provide lower Ron values. Unfortunately this comes in packages that are harder to solder.
When connecting a MC or MM system with a phono stage, the most important things to consider are input capacitance (for MM systems) and input resistance (for MC systems). Usually this is done be DIP switches or external connected capacitors. That means you cannot do this from your listing position and it is not easy to hear the differences between different settings. This the following input stage, all switching is done by low-resistance analog switches. These switches can be controlled by a digital circuit (e.g. a microcontroller). That makes it easy to implement a switching of capacitance and resistance using a remote control.
The selected analog switch is the ADG1411 from ADI. It features 4 independent switches, can run at +/-15V 8 and has an Ron of only 1.5Ω.