My jfet and mosfet amps

For the serious and not so serious in audio: DoItYourself and Tubes (SET, PP, Vintage).

Moderator: kabubi

My jfet and mosfet amps

Postby 6a3fan » Thu Dec 26, 2013 2:11 pm

I have built some solid state amps following some of the principles I like in my tube amps: single ended, few active devices and no global feedback. The preamp and amp chain shown here has only 5 active devices either jfets or mosfets, and 6 coupling capacitors in the signal chain from phono input to speaker output. In contrast, my tube preamp and amp combination with the shortest signal path has 5 tubes, 4 coupling capacitors and an output transformer in the signal path.

My preamp uses the popular Simplistic njfet Phono Preamp designed by Salas. The thread on this phono preamp in DIY Audio has over 10,000 posts. It uses two k170 jfets as amplifying devices with a passive RIAA compensation in between. I used the design with a source follower for a lower output impedance. The line stage is a MooseFET from the DIY Audio Projects Forum. It uses a single IRF510 FET per channel and has a gain of around 5. I am not a fan of high gain line stages as you then have to attenuate much of the gain in the volume control. I use a simple power supply for this preamp: 12 volt toroid transformer followed by a voltage doubler rectifying circuit, then the voltage regulated through LM 317 devices to get the 24 volts needed by the line amp and the 20 volts needed by the phono preamp.

The phono preamp board is on the upper right hand corner. The line amp is on the lower part near the volume control. The power supply is on the middle right hand part. I used a combination of components, some used, and it shows in the built.

Image

The power amp is the ZCA or zero component amp, again from DIY Audio Projects Forum. The amp uses a single power mosfet with a high powered resistor as load. Being Class A and with a resistor as load, this circuit is quite inefficient. This circuit is quite controversial: some people hate it while others love it. I used Hitachi K134 lateral mosfets for my amp. The power supply is a toroid, followed by a bridge rectifier then a capacitance multiplier. The original version used a CLC power supply. Behind the amp you can just see my tube preamp.

Image

I built the amp and the preamp on a wooden chassis. Inside the amp, you see the large heat sinks and the resistor load array. These resistors get quite hot in operation even if together they are rated at 50 watts. On the lower left side of the amp, you see the capacitance multiplier, and the large computer-based heat sink of the pass transistor. You might ask, where are the printed circuit boards of the amp? There are none since I built the amp right on the source and gate pins of the mosfet and an insulated tag connected to the drain. All point to point and short leads

Image

Following is the trace of a 10 kHz square wave on the amp. The trace was taken with the output just below clipping,

Image

I am quite happy with this combination of amp and preamp. Although the pundits are right: the ZCA amp is very low powered. At low listening levels, the sound is quite acceptable, very fluid and sweet. But the amp easily gets into clipping even with my high efficiency speakers and then the sound becomes quite fatiguing to listen to. I might try using a choke as the load, upping the voltage a bit, to get a bit more efficiency and power.
6a3fan
Citizen
Citizen
 
Posts: 191
Joined: Fri Jul 06, 2007 6:23 pm

Re: My jfet and mosfet amps

Postby AbadSantos 7 » Fri Dec 27, 2013 10:27 am

Congrats Sir... :rock: :rock: :rock:

Very inspiring....
User avatar
AbadSantos 7
Citizen
Citizen
 
Posts: 554
Joined: Wed Oct 26, 2005 7:00 pm
Location: Suzhou, China


Return to DIY and Tubes

Who is online

Users browsing this forum: No registered users and 2 guests