A marvelous low power 71A DHT amplifier

C3g-71A amplifier 2019

Iphone May 2019 299

Seeing an amp delivering such a low power, one might ask the question: ” How low can you go? “. Believe me, I can go even lower.

Here is a 750mW DHT amplifier using the amazingly good sounding 71A. Last year I’ve been asked by Kevin at Glow In The Dark Audio to build such an amp and here is the link to the description and his impressions about it: http://www.glowinthedarkaudio.com/radu-tarta-71a.html.

At that time I was not thinking to build one for myself, but after listening to the amp I’ve built for Kevin I’ve decided to make my own. It is almost the same design with the exception that I’ve used the C3g as driver. The reason I needed a bit more gain was to be able to use it without a preamp.

I’ve used same Sony power transformer and Tamura 5k/8/600 output transformers. I am impressed with the sound of these small output transformers: very detailed and balanced.

The enclosures are made with Cherry hardwood and the top plates are 12×12” sanded aluminum.

The power supply uses the 80 rectifier in a choke input LCLC filter, and separate raw DC supply for the filaments of the 71A and C3g. The umbilical cord is screened and carries only DC and it is terminated on a Mil spec Amphenol connector. I found two vintage Westinghouse panel meter that show the B+ voltage and the total current in mA.

The amplifier is using an active load for the C3g ( DN2540  CCS) and it is capacitor coupled to the 71As. Both stages of the amp are self biased and are using high quality components. The amplifier has a very smooth TKD CP2511 stereo attenuator.

The filaments of the 71A are powered by two Coleman regulators that bring the sound of this little power amp to a next level. There are two multi-turn potentiometers and a panel meter that allow the user to adjust the voltage to 5V for each 71A filaments.

The amplifier can also be used as a high gain ( 30dB) line stage due to a 600 secondary on these little Tamura transformers. This comes handy when one has a very low input sensitivity amplifier. It can also be used to drive a First Watt F4 ( which has unity gain) to its full power.

Due to its careful built and separate power supply this amp is perfectly quiet.

At this level of power output one needs ultra sensitive, easy to drive speakers of 106+ dB.