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4P1L preamp DIY

Posted by Johno on March 17, 2017 at 06:07:19:

This preamp is designed by Radu Tarta he sells the two raw PCB's for $100 plus shipping you source the components, populate, test & house.

Radu has been great to deal with a true gentleman, he calls this preamp the***Shiny Eyes 4P1L Preamplifier***
He also sells this unit completed.
https://simplepleasuretubeamps.wordpress.com/products/

It's a very well thought out design based on Ale Moglia - Siberian Gen 3 (4P1L pre-amplifier)

My initial concern were reports of excessive noise using this valve but this has so far proven unfounded. Radu recommends both PSU & Audio board be mounted in the same enclosure but this assumes you have approx 450mm of available real estate regarding width, otherwise use a 2 box solution.

Upon completion I soak tested the unit for 3hrs then connected the pre to my main audio system, there were a few pings & mild bursts of noise as the valves bedded in but this disappeared after 10 hrs use. I suspect if you had a stash of 4P1L valves you could weed this out but the noise after warmup - 5 min disappears. Still shifting selector switch due to no physical decoupling with this chassis creates pings but who listens to selector switch switching ? Designer Ale Moglia has mentioned the 4P1L valve is a 'dancing lady' ultimately this unit will be installed into a wooden enclosure.

My N&D meter is not operational but output noise on CRO is non measureable. I hear a very small amount of noise & hum if I put my ear against the spkrs but who does that when listening ? !

The voltage gain is approx 2 giving a power gain of around 6-7db.

This design uses the Lundahl LL1671/30ma strapped in Alt Q (4-1 stepdown)

I used a TKD 2CP-2511 motorized volume control 50K ohm pot from 'hificollective' combined with there 'Glasshouse remote control module & remote'. These components have worked flawless they appear transparent to the pre-amp coupled via the superb Seiden's 2 pole 6 way switch. The pre-amp in question is worth spending money on front end parts.

I have been using a 15yr old British 'World Audio Design' - 6DJ8 trannie coupled pre-amp kit for sound comparison. There are similarities between the two where the gain of the 6DJ8 pre-amp in question was double that of the 4P1L which for my needs was excessive. But the difference in sound quality indicates to me the 4P1L pre-amp is superior. Cymbals, guitars, piano keys & voices are razor sharp more focused more immediate sounding. The 4P1L is a DHP but triode connected in this design and it has a toughness to its sound which blends nicely with DHT's downstream. The 4P1L has time / space decoding ability in spades. Thankfully there are no Blackgate or paper & oil caps in this design so no tonal shifts during break-in. The majority of the PCB components are affordable & of industrial quality.

I would heartily recommend this pre-amp for a front end feeding a 2 stage DHT amp. It has low output impedance approximately 150 ohm and runs thermally cool.

The only upgrade I would consider would be to substitute the 2 x 20 ohm Ohmite 10W kathode resistors for some MRA-12 non-inductive wirewound resistors. The cathode resistors are un-bypassed so quality is essential. Use 3 x 30 ohm Mills in // to get the 10 ohm rating. The design dissipates around 3.5W across these kathode resistors but they do get finger burning hot even though they are well de-rated. Have not fiddled with the Coleman regulators currently they are set at 6V. The anode current is set to 26ma by the cascaded DN2540 regulators. The B+ noise across Anodes is not measureable with a CRO & periodic monitoring has shown HT & LT regulators to be very stable.
Finally this preamp is a keeper, cheers Johno.

PS - Thanks to Radu Tarta, Rod Coleman, HiFi Collective & K&K Audio for supplying great designs & parts.

Regards Johno