Monday, December 17, 2012

Menatzchim- C3m metal valve preamp

These petite C3m metal valves are quite inexpensive. After making a 6N7 (also metal valves) preamp which sounded positively excellent,  I decided I would make a preamp with the C3m(s) in a very attractive 1UR 19" enclosure. I could find no preamp schematics incorporating them but a fellow DIYer in the States designed one for me.

This preamp uses the same power supply I have used in the last dozen preamp builds. An external 24VAC 1A wall wart is stepped up to 250VAC by using a step down transformer backwards. Double filtration and a DC filament supply ensure a dead quiet preamp. Wire wrap wire is used for signal hookup wire and heavy duty gold plated RCAs are employed. No shielded wire or PCB is used. An Alps blue velvet pot is on the output for attenuation. Jensen Copper, paper and oil caps with silver leads are used on the output. Three sets of input RCAs are selected by two small power switches.

The sound stage is extremely deep and voice and instruments have a uncanny natural sound and feel about them. With symphonic pieces you get the unnerving feeling you are in row twelve and right in the center. I would highly recommend the construction of this preamp.

Saturday, November 24, 2012

"Heavy Metal"- 6N7 metal valve preamp

In a box of very old valves, I was given recently, was a new looking pair of black metal can RCA 6N7s. These are dual triode with a common cathode. I could find no schematic for a preamp but a DIY buddy came up with a design. Construction is in a timber box. There are three switchable inputs and power is supplied by an external 18V 2.2A AC power pack I was also given. Internally 255V is developed with a linear PS and regulated 12V for the heaters in series.

An Alps blue velvet pot is fitted to the output for attenuation with a fine solid brass knob to compliment the gold right-angle RCAs, name plate and ventilation holes. Wire Warp wire is used in the signal path with PS and amp on separate brds. No PCBs or shielded wire is used. Bitumanised heavy Al foil lines section of the preamp for resonance control.

The preamp is dead silent at idle with stunning drama and dynamics. It's fast, crisp, clean and makes every track sparkle. Best preamp I have ever built, SS or valve based. Nothing comes close to this one. Nothing!

Monday, November 12, 2012

4S Universal valve preamp in a Tea box (4SUTea)

All housed in a Twinnings timber Tea Box: Bitumanised Al foil controls enclosure resonance and Russian paper in oil caps link the modulated HT to the real world. Silver solder is used throughout. Heavy duty high grade gold plated RCAs grace the back and all is powered by a 24V 1A plug pack. There are two switchable inputs, Alps blue velvet pot and black chromed solid brass knob.

Power supply and amp reside on a single piece of vero (strip) board. No PCB or shielded wire is used. Any 12A?? tube can be inserted with no components changes. Sound is big, bold and fast with a Mullard 12AX7 in place.

Tuesday, September 11, 2012

retro-34UL power amp

In an effort to use-up so many spare parts I have I put together another EL34/KT88 amp. Part of the inspiration to build the amp came about when I was given four 1996 NOS Svetlana EL34s. These bare the old Svatlana logo. Also I had dismantled a bread board amp which had a large power tranni and choke which were perfect for this build.

The base is Australian Spotted Gum and the power tranni and choke are Hammond. The OPTs are Edcor 15W open frame types, reside in the base and are shock mounted. The feet are filled with lead sinkers for resonance control. A CLCRC filter network ensures the amp performs dead quietly and has an extremely high head room. Frequency response is to 40Khz and power is 5W RMS into 8ohms. Russian paper in oil caps are used in the inter-stage and bipolar caps and low ESR caps are used in the cathode bypass. Silver plated wire-wrap wire hooks up the gold plated RCAs to the nearby 6N1P and connects  the secondary of the OPTs to the binding post.

The paint work is epoxy enamel on 2mm high grade Al plate. Silver solder is use in the driver stage. Component-2-componet wiring is used to ensure signal path length is kept to a minimum. There is no shielded wire or PCBs to degrade the sound. This is a fast amp with a huge sound stage and a rich lush EL34 sound.

This amp has been built to sell.

Wednesday, September 5, 2012

Fostex FE206En drivers in dual bass chamber box

I had the boxes made some years back with no idea what I was to do with them. When I went back to 2 Channel theater with a 3.5W 6EM7 integrated amp (Paris) I knew they would be perfect around my new 46" LED TV and Oppo BDP-93 blue ray player. The finish is satin black stipple and the grills are held in place with four rare earth button magnets. Easy to pop on and off. The stipple effect was produced with a roller going over the fresh paint a number of times as it dried. The drivers huge magnets have bitumanised Al foil dampening attached and the internal wiring is two stands of lightly twisted wire wrap wire to each binding post.

The top chamber is lined (but not stuffed) with Dacron but the bottom chamber is empty. There is no Xover in these full range speakers. Efficiency is 96db at 8ohms impedance so they work well with flea power tube amps. Sitting higher than the tweeter (wizzer cone) the sound is quite natural with just a touch of forwardness in the mid-range. The speakers not only look stunning they sound excellent for a $500 spend.

Monday, March 19, 2012

"Luciano" - 807 SE Power Amp

I managed to purchase a couple of 807/ATS25 tubes for $7 each and were giving two in excellent condition. Both pairs of tubes test 85% emissions. Matt, an American electronics engineer, volunteered to design an amp around the tubes. Though this project stagnated for years finally it has come to completion.

The bases are made by a local amp builder and are from Australian Spotted Gum. The top and bottom plates are 2mm Al and the colour "Red Delicious". Edcor 30W OPTs and power tranni grace this smart looking amp. The design is SE with no feed back apart from Ultra-linear. The driver tube is 6SN7 and the rectifier tube is a 5Y3. I have a few different 6SN7s and rectifier tubes to roll this amp and another pair of JAN 807s.

The sound appears natural with a little forwardness in the upper-mid and excellent head room. Loud and dynamic passages are handled with no compression or strain. Voice clear, stands out and is natural. Instruments are individually defined. A pleasing sound and a well performing amp.

Tuesday, February 14, 2012

"Paris Duo" - 6EM7 monoblock power amplifier

Building on the success of the Paris amplifier (dual mono construction) I was asked to build a pair of monoblocks. Paris Duo has a two stage power supply filter using a small choke and resistive network with large electrolytic capacitors. Tightly twisted wire-wrap wire feeds the signal to the first triode and the same wire connects to the speaker binding posts.

The power transformer is rubber mounted to reduce resonance on the cast Al chassis. The finish is epoxy enamel paint with an acid etch primer. The tube compliment is a 5Y3 rectifier and Sylvania (NOS USA) 6EM7. A small Hammond choke and Edcore 10W output transformer are inside the heavy chassis. There is also a EMI mains power filter in the chassis and a choke on the power lead. The bottom of the chassis is drilled for cooling with a layer of bitumanised heavy Al foil to control resonance. Soft silicon feet secure the amplifier and also help provide resonance and vibration control.

The sound is similar to the original Paris amp with an ultra clear mid-range, heavy in detail and good bass extension. In room testing proves a low 40hz tone with only a modicum of attenuation. Treble is on the sweet side rather then bright and is extended. An excellent sounding amp providing a touch over 2W of delivery.