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Upside-down Single-ended MOSFET AMP (originally posted in Headwize)

post #1 of 3
Thread Starter 
<Received no response in Headwize DIY formum - trying here ... Apheared, CaptBubba, Daniel Pumphrey, Possum, rickcr42, Tomo, others who have built Szekeres and even ppl - any coments???>

Well actually it is a buffer not an amp (no gain).
This one is similar to the Szekeres, but building it upside-down (ie the positive rail is ground) eliminates the input cap and draws a constant current from the power supply.

Note that there are two distinct current loops, both through the bottom MOSFET (Q3):
1. the input loop from the input, through R7, through the MOSFET's gate and back via the drain (which is ground); and
2. the output loop from the MOSFET source, through C4, through the headphones and back via the drain

As I envisage it, the input loop is the throttle for the output loop.

The top MOSFET just supplies constant current continuously. Note with the normal Szekeres:
1. the power supply is in the output loop
2. the current drawn from the power supply fluctuates according to what is going through the phones
With this design, both of these problems are eliminated, placing less demands on the quality of the power supply.

The input is tied to the drain through a resistor which typically results in a midpoint around 4 Volts across Q3, depending on the Vgs (shown as 3.75V in the diagram - simulation output). Alternatively you can include an input capacitor then bias the gate to whatever you like.

The current can be set by varying R11 - this resistor will have 0.6 to 0.7 volts across it by virtue of Q4.

Note the negative rail really is meant to be top (and the MOSFETs are upside down)!!!

I have build versions with various components and it sounds great - dynamic with good bass. Need a really good output capacitor (C4) - I am thinking of 100 x 2uF polycarbs in parallel (rather than the 1000uF electrolytic)!!!

Any comments are most welcome ...

post #2 of 3

By the time you read this, you probably would have read my message at HeadWize. Your design seems different from the original but in essence, it is the same as Szkeres with Constant Current Supplies. Whether you put CCS before MOSFET or after MOSFET means very little since it will conduct a constant and fixed current to the MOSFET. (If you are wondering if it will work, it WILL work.)

However, note that your CCS may not be temperature compensated and changes its current output as the temperature changes. This is not really good. I recommend you to devise some ways to take temperature differential into account.

In addition, paralleling 100 capacitors is simply is not the best idea. In theory, it will work. However, note that capacitors are NOT simply two parallel plates ... they are not ideal. So you will end up with leakage and contamination due to the interference between capacitors. Before I found a bulk of 24uF 250V Solen, I was paralleling 4.7uF's. This set up was not worth it for me. I instantly abandoned this approach. (Currently 4.7uF's are sitting in my PSU.) I found 220uF paralleled with excellent 1.5uF to be the second to 24uF SOLEN paralleled with 2.5uF.


P.S. Of course DC coupled will be theoretically favorable, however, I leave that to others to contemplate. ...
post #3 of 3
Thread Starter 
To all readers:

Discussion on this topic is happenning at Headwize:


Suggest you respond there.

Tomo - I am most interested what you had to say on paralelled caps.

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