Saturday, November 23, 2013

Mains DC blocker and RFI\EMI power filter

In Australia our mains supply has gone from clean power to not so clean power. For a number of years now I have built power filters into my power amps but not in media players or preamps.  DC on the mains can cause many problems with audio gear and particularly audio gear using a toroid transformer a DC mains blocker may improve sonic performance. This simple DC blocker also incorporates an RFI\EMI power filter. A choke on the input lead and 10A slow blow fuse round out a simple and neat solution to "dirty" mains power.

OPAMP phono preamp with high RIAA EQ accuracy

The creator of the oddawtt series of tube amps and kits has developed an OPAMP based, highly accurate, phono preamp. In my build you can switch between, MM and MC and in MC mode switch between 50 and 100ohms load. This MC loading suites my Ortofon Rondo Blue and I can switch between the two loadings on the fly to gauge the effect. I run my build on 4 X 9V batteries and on a 1.75" square single prototype cct. brd. I have a simple resistive network to provide +/- 18V. A couple of low ESR 220uf caps ensure smooth power to the chip an OPA2134. A battery test point is at the front.

The cap compliment is polies and MKT with a NP electro on the output. Better caps could have been used but a wanted a neat tight build. The preamp came out looking attractive and most of all sounds glorious. For an inexpensive build with great flexibility (two switchable TT inputs for my Rega (MC) and AT (MM) TTs)this would be hard to beat. Tested RIAA EQ tracking is better than 0.25db!

Monday, November 4, 2013

"Abstinence" - 6EM7 power amplifier

Yet another 6EM7 power amplifier. This time some expensive and different parts were use in construction and a different internal layout was employed. The power transformer and choke are isolated top and bottom by rubber grommets. This ensures no AC vibration reaches the tubes.The initial PS filtering is preceded with a on-board power filter, a 4uf\400V Russian PIO cap and 20H\100mA Hammond choke. The HT is supplied by a Hammond power transformer and a very expensive Sophia Electric mesh plate rectifier tube. The second stage of filtering is 47uf and 100uf electros. with a large limiting resistor ensure the amp is totally free from of hum and noise. All PS caps and cathode bypass caps are snubbed.

This amp sounds better than the last and I think it may be the Sylvania NOS 6EM7 tubes. Russian Teflon caps are used in the inter-stage coupling  and once more large NP caps for the bypass on the power tubes. For the first time I have used Chinese made H&K 15W OPTs, can't fault them. The look of the amp is almost under-stated "sexy" and the sound the same - sexy and natural.