Cap Multiplier PCB: build guide and BOM

Finally after many years, I managed to document this great PCB. If you already have this PCB and want the build guide and BOM please let me know.

Here is an extract of the build guide for the ones who are interested in this circuit:

Introduction

 The capacitor multiplier circuit often gets overlooked in the valve circuit designs. It’s a fantastic circuit to achieve an extremely quiet power supply. It’s not a regulator as such, as it’s doesn’t have a feedback loop/mechanism to regulate the output voltage and therefore it doesn’t affect the sound of the amp in the same way a regulated power supply does. Therefore, it’s an attractive circuit to be used in hi-fi designs in my opinion as many get put off by the “sound” of regulators. In particular, when you are looking for a very quiet power supply (e.g. phono stage or preamp).

With a capacitor multiplier circuit, you can achieve high level of PSR (Power Supply Rejection) compared to passive filtering networks. Easily rejection of over 50dB can be achieved and you should expect it to be in order of 80 to 100dB. In practice you will get noise coupled/induced to the circuit through cable layout, etc so at least 50dB is easily achievable, which is a fantastic result. 

However, there is no free lunch am afraid. There is a price we pay on this circuit, and it’s not the cost of the components. MOSFETS and even good quality film capacitors (e.g. WIMA DC-Link) are relatively inexpensive in the grand scheme of things. The big issue that we may end up with is the heat dissipation depending on the load we are planning to use this circuit. You may end up wasting too much heat and requiring a big heatsink for the pass MOSFET, hence this circuit isn’t practical for an amplifier. I don’t recommend using it for an amp, you have been warned! 

The circuit

I assume you have a good electronic engineering background and you know what you are doing here. If you’re not able to design your capacitor multiplier circuit, I’d recommend you replicate an existing one or not use this circuit at all. 

Having said that, it’s a very simple circuit and easy to use. If you want to learn about this topic, do a bit of a research on classic text books as it is well covered or the following one: “Designing Power Supplies for Valve Amplifiers”, “Designing a MOSFET capacitor multiplier”, page 138, Merlin Blencowe, 2010.  

The basic capacitor multiplier circuit is shown on the figure below. The output voltage is set by the VREF voltage minus the VGS (sat) of the PASS MOSFET. To dial this voltage, you need a resistor divider which is formed by several resistors on my PCB. R6 and R7 are split to allow you using different resistor and not bother about the voltage rating too much. The total resistor divider is formed by R5+P2 as upper part and R6+R7 as the lower part. The combination of R5 and P2 gives you fine tuning as well as ensuring the power rating of the trimmer resistor. The charging time of C2 will dictate the gentle ramp up of the voltage. You can do the maths or an LTSpice simulation will help you if you are a bit lazy. 

You may be asking what is the role of C1 and can you omit it? The answer is no, you cannot. C1 is key to provide a low impedance to the drain of the MOSFET. The drain is high impedance and you certainly don’t want a high-impedance node at the input as higher ripple and/or oscillation may occur. It will also provide additional filtering. Anywhere between 10 and 47uF will work fine. 

C3 is not that critical but I find it to be useful to have a film capacitor there as they sound much better than electrolytics in my experience, the WIMA DC-LINK in particular. The PCB has flexibility to accommodate multiple version and sizes, so have a look at the BOM to see what works best for you. 

The higher the overall resistance across R5+P2 and R6+R7 the better. You will get a slower turn-on constant which is beneficial to your valves during the warm up period as well as lower dissipation requirements on the resistors due to lower current consumed.

Let’s talk about the protection diodes in this board. D2 allows a discharge path for C3 (and beyond) when the MAINS power is disconnected. This will protect M5 for a reverse voltage. 

Second protection is provided by D3 and D4 zeners. These are required unless the device used in M5 has protection diodes embedded. Otherwise, you may damage the gate by exceeding the maximum VGS allowed. Clamping it to 15V is safe enough. C2 gets to discharge through the resistor divider when the power is off.

Now, if you want to go a step further in power supply noise rejection, you can implement the below circuit which is the ultimate cap multiplier contraption:

The above diagram shows the addition of the CCS in replacement for the upper leg of the resistor divider network. In this way, we provide further noise rejection and a small constant charging current to C2. To ensure you deliver a voltage below IN level and desired to your application, there is R4 and P1 to set the output voltage like in the resistor divider circuit explained earlier. In this case they will set a reference voltage from the constant current set by the CCS. It’s important that you keep 25V at least across the CCS and 30V between output and input to operate properly. The addition of the CCS will increase the power dissipation requirements of the MOSFET due to this constraint. 

The way the CCS is implemented on the board is shown below:

The cascoded pair of FETs formed by M1/M2 and M3/M4 will provide you the CCS required. You can either use the simple pair of LND150s or replace the top LND150 for a beefer MOSFET if you are operating closer to 500V at the input. In that case you can install an IXTP08N100D for example in TO-220 case instead of M2. If you want much better stable temperature, you can also implement a J112 jFET on M4 instead of the LND150 on M3 and set it to about 1mA of operation with a 2K2 resistor in R3. If you use a jFET then you will need to install D1 15V Zener. You can play with different jFETs you may have around to create the reference current. Make sure you set R3 correctly for the right current and jFET TEMPCO. There is an outstanding reference book to look into if you want to get further in-depth view of the CCSs: “Current Sources and Voltage References” from Linden T. Harrison, Newnes Elsevier, 2005.

You want to protect the MOSFET from a short circuit scenario or for overcurrent in some cases. There is a big resistor R10 to be installed in the board for some basic current limiter in series with For a more advanced current limiter circuit, you can implement the addition of the T1 transistor. The T1 transistor will cutoff the MOSFET once the voltage across R10 reaches the saturation voltage of the transistor. This makes it simple to dial to a degree the current threshold for the limiter circuit. Simply divide 670mV by the maximum current before the limiter kicks in. Be sure that this limit is beyond the maximum peak current required by your load. I’d give good headroom here to avoid the cap multiplier imprinting some sonic signature in your preamp/stage. For example if normal operation is 20mA, I’d set the protection to 200mA to ensure that it only operates in an abnormal situation. 

Optionally, you have a NEON bulb to install in case you want some indication of the output voltage and cap multiplier operation. Useful to have, but if the board is inside a case, you will need to solder wires to extend the connection to the NEON bulb. R11 needs to be sized according to the NEON bulb used and the output voltage of the cap multiplier. 

Below is a simple workbench test example for my phono circuit. A noisy raw power supply was used during test and a 20mA load used. You can see the 50Hz peak reduced over 50dB. Likewise, all upper harmonics over 100Hz get squashed down to -100dB floor, quite impressive for a simple circuit. This test shows how 300mV ripple is reduce to less than 1mV. In fact, the circuit was tested on a bench and without any case/shielding so there is Mains noise creeping everywhere on the leads. 

With a passive cap multiplier (i.e. without a CCS) you should ensure at least 10V between input and output. That will ensure the correct operation of the PASS MOSFET whilst keeping as low as possible the dissipation on the MOSFET. Similarly, when a CCS-type circuit is used, you should allow 30V between input and output. The higher the input voltage, the more you will push the PASS MOSFET to dissipate. Sometimes is even easier to have a high-power dropping resistor (e.g. clad wirewound type) to dissipate and lose a few volts before the input of the cap multiplier. It’s easier to dissipate Watts on the clad resistor compared to a MOSFET which will need a bigger heatsink instead.

Circuit example: Phono stage

Below is an implementation I used for a phono stage. I used both electrolytic caps for C1 and C2, whereas the final capacitor C3 is film. R10 is set to provide 200mA current limit. Output voltage is dialed by P1 which is 500K trimpot. 

Today, I would use a better MOSFET like the STF3LN80K5. This is a plastic package TO-220FP which is more convenient as it’s isolated.