Monday, April 6, 2020

"MWH" - 1920s UX-171A tube preamp

I was offered the challenge of building a DHT triode preamp and offered a schematic. But there was way too much sand in the design with an active load supplied by a FET. There was also a CCS reference in the cathode cct. Any delicate suppleties in the DHT tube were to be lost and buried in all that sand. Not for me. I was shaken and a little stirred and felt a Bond to the UX-171A tube which in the past I had built a power amp around ("Intermezzo"). I have a number of 71A pairs and now the opportunity to use a pair.

Fortunately an American DIY buddy, Bruce of oddwatt fame, came up with a simple single stage designe. Using close to century old tubes I felt a Hammond retro wood cheek Al chassis was the way to go. Add to that a Hammertone blue poxy enamel paint job, a big gold retro knob and ceramic sockets and the look is there. The tubes are RCA and they use nickel plates. 

Keeping old school the output cap is a Russian military PIO 1uf metal jaket. Never done it before but this time earthed the metal (which is actually copper) jacket. Not sure any improvement but like to think it can't hurt. Due to hum issues I moved the 24 step stepper pot to after the output cap. 

There are two selectable inputs and the preamp uses my standard external PS which works with any of my 6SN7 preamps of which I am currently using two. The sound is extremely transparent and captivating. Pleased with the build, both the way it looks and sounds.

Wednesday, February 5, 2020

"Image" - Low Profile 100W Class D amp

"Image" started off as a low profile 150W Class D amp but though it sounded very good I could not remove a very low level buzz. I changed the amp to a 25W chip amp but that played no music regardless of all voltages being correct and no cct. errors. 

Wanting to make my own Devailet right from the start I found a very inexpensive, extremely compact 100W Class D module using some newer Class D chips - TDA7498. The amp will produce 80W into 8ohms and works from the original PS I built in this chassis. A 36V single rail well filtered (19,000uf) PS supplies dead quiet current to this power amp. As with most of my builds the amp includes a RFI/EMI power filter and a ferrite choke on the power cord. Part of the supply filtering is a 10uf poly and two 0.1uf polies. These are to provide a better mid and high frequency signal path and reduce bridge (35A) and filter cap noise.

The amp looks super smart in its ash black stippled 1U 19" rack mount enclosure with handles. It sits on rubber feet and has quality gold plated RCAs and solid brass three way binding post. A retro-thermionic amber "ON" LED and power switch are upfront for easy access. The amp in operation is dead quiet has excellent bass extension and sounds balanced over the rest of the frequency range. It sounds much bigger than its 100W rating and appears to drive all my speakers very well. Even after hours of use it is cold to the touch above the heatsink. Very pleased with he compact build and overall sound. 

Thursday, December 26, 2019

"fiih"- Holton inspired 150W amplifier

This is my fifth Holton (assieamps) build. Riding on the success of two 150W and lately two 300W (500W into 4omhs) I have built another 150W amp. The last two builds (300W) are with Holton's latest amp modules which use dual die lateral FETs. These were specifically designed with hifi in mind. Each FET will handle 250W or 220V or 16A.  In the 300W versions there are eight power devices in this build only two per channel. 

Once more a beefy PS (the absolute heart of any amp) with 300VAC tranni and 80,000uf of filter/storage caps. Snubbers this time are X2s but still with a 47uf first ploy cap. Diode bridge is soldered directly to the first cap to ensure high current delivery. All soldering is with WBT silver solder (4%). Extra heavy speaker binding posts and quality RCAs used. An over-size heatsink and two 80mm silent fans (running low voltage) provide cooling. Enclosure is steel and AL 3U and with the PS mounted on a sub-chassis the whole amp is very stiff but not too heavy. Power switch is rated 16A and the enclosure is earthed and fused.

The overall sound is extremely smooth and natural from top to bottom. Bass again is the signature of a Holton amp. Deep, powerful and articulated bass with a linear delivery of mids and treble make for a very "listenable" amp. At this point, having compared the 300W "Precision" and this amp Precision has the edge when it comes to absolute transparency. Of course it always comes down to total synergy and personal taste. Either amp is a winner. 

Sunday, December 1, 2019

"Precision" - Holton 300W Class AB power amp

Over ten years ago I made two stereo power amps for a friend based on the Holton Precision 200W amp modules. I liked the amp so much I bought two of the 300W (500W into 4ohm) amp modules - nxv500. The modules lay dormant until recently where I built them into an amp. The amp was exceptionally good sounding but the PS I had made (as in the image above) was too big to fit into a 3U rack mount enclosure with two heatsinks etc.  

To finish the first build I built a narrower HS with 40,000uf of caps. I only enjoyed this excellent sounding amp for a week then it was sold. Happy customer and a repeat. Now I had this high power PS with 625VAC tranni and 80,000uf of 100V capacitors, 10A bridge and fully snubbed. What to do? Build another amp of course and use this sub-chassis mounted PS.

Anthony Holton's 300W modules have come a long, long way over the last ten years now in release R5. They now support very high performance dual die lateral FETs which can with stand 200V @ 16A EACH! There is eight per channel. The modules are; HPA-NXL500 R5. The power FETs are specifically designed for high-end audio not just power amplications. 

The amp is now complete with heavy duty 3-way speaker binding post and gold plated RCAs. The amp has a built in power filter, a choked power lead, fused and earthed. The 10A rec. bridge is soldered directly to the first cap bank to ensure he 100A pulses are not supressed before reaching that first cap. This technique I have used many times before with only positive benefits. 

Due to the fact I could not get the HS I wanted this amp runs hot without the lid off and a silent fan underneath. To rectify this this amp will receive a new lower voltage PS (but still 80,000d of snubbed caps) and be converted to a Holton 150W amp with the latest HPA-NXL200 R4.1 modules. These also support the high performance power FETs.

The 300W boards and high power PS will go into 300W monoblocks with appropriate HS to stop any other heat issues. In the meantime I am enjoying this high-tech power amp emencely; Bass is deep and very powerful with NO bloat or boom. Precisely articulated. My 12" ploy cone woofers in a 60ltr sealed box are 100% controlled. Even my 180W Class D amp do not give me the bass depth and control this amp does. Mids and treble are dead linear, smooth and extended from my Altec 511B horns with FailtalPro poly compression drivers. For me its good by Class D hello AB!

Thursday, September 5, 2019

Lagoon Take 3

Finally finished this JJ12AU7 preamp - SRPP. Finish is blue hammer tone on a Hammond wood cheek chassis. The cct. is constructed on a turret brd. with metal film resistors and Russian PIO caps. Pot is Alps blue velvet with 4oz solid brass gold plated knob. External PS supplies 260V HT and DC filament current. Two selectable inputs. Sound is very fast and snappy with good bass.

Sunday, August 25, 2019

"Gothic" - all FET headphone amp

Oatley Electronics Australia had been selling headphone (HP) kits for some years. These were based around the JAN6418 subminiature pentode and PT2308 HP driver chip. I've built and sold quite a few. Branching out from the normal this all FET HP amp has a two jFET input stage then two small power FETs arranged in push-pull to drive the HPs. A unique design but after  a few prototype brds. were produced the amp was dropped.

I got two of the brds, an "A" and "B". The "B" had better layout and LEDs were added. From the brd. I made "Gothic" so named because it is all black and quiet weighty. The compact cast Al enclosure was prepped, etch primed then sprayed Epoxy Enamel black gloss. RCAs on the rear with 6.5mm phone input. Front has volume, on LED phone out and ON/off switch. Quite simple.

There are two external power supplies. A compact SMPS of 24V @1A and a larger linear PS of 20V @ 2A. I woulkd have thought the HP amp to sound better with the linear PS but the SMPS may be the better sounding. Either PS is dead quiet and the amp works extremely well. Though in Class A the amp runs very cool. The small power FETs which do the driving are mounted on small heatsink sections of the cct. brd.

The overall sound is rock solid and weighty. You get the feel the music goes way back into the black background. With my Sennheiser 700 there may be too much detail but with the Technica AIRs there is a better balance. Overall an excellent HP amp.

Tuesday, July 30, 2019

"Overture" - LM4780 chip power amp

Originally this 1U rack mount enclosure was intended for another chip amp which really didn't work well.  Now with a LM4780 cct. brd. and repurposed this low profile amp really sings. Currently being used around the TV system and controlled by a single stage tube preamp. Lots of power (60W) into high efficiency single driver Mark Audio speakers and backed with a 175W sub-woofer. Pleased wih the outcome and the repurposing of the chassis and robust PS. Under test in the second image.