4P1L Line stage

EWS!Warning! Caution! Servicing this preamplifier involves work with high voltages that can kill you. Do not attempt to adjust or modify any circuits or equipment described below without the required training, experience and safety skills when working with dangerously high voltages. It is your responsibility ( I assume none) to ensure that work on this equipment, or DIY build based on information on this web site, is safely done and will not cause life or property threat or damage!



                                     ***Simple Pleasure  4P1L Preamplifier*** 2022 Edition

This is a new, upgraded version of this line stage/preamplifier.


I’ve started to experiment with the Loctal DHT 4P1L pentode many years ago. 4P1L is greatly praised for its awesome tonality and great linearity in triode mode.

This is single amplification stage, direct coupled input, filament bias and line output transformer coupled preamplifier. There are no capacitors in the signal path ( besides the last caps in the HV power supply-DC link, low ESR ones), which makes the sound of this device super transparent, fast and clear/clean. If you are looking for a laid back, tube sound preamplifier, this is not that one.

This design is the result as many hours of breadboarding, testing and listening. I’ve concluded that the best sound, lowest microphonic effect and highest driving capability, was offered by a 4P1L in triode mode, filament biased and line output transformer loaded.

This single stage 4P1L in triode mode is using a Lundahl LL1671/30mA line output transformer.

The preamplifier is built with 2 main PCBs of 2mm 2oz Cu, one has the power supply and the other one the 4P1L preamplifier stage. The boards are mounted on 16  neoprene damping mounts to reduce vibrations.

There are 2 delay relays. The first one switches the high voltage about 30 seconds after the power is turned on. The second relay is shunting the outputs for 1 minute, allowing the preamp to be fully operational before the signal will get to the amplifier. This way there will be no pops/transients at startup. At turn off however, the relay has a mechanical delay and there is a short time when transients can be heard on speakers. That’s why turn off the amplifier first.

This prototype built comes with 2 inputs RCA, and 1 output RCA and one output XLR, Gold Point 47 steps attenuator volume control and Seiden input switch.

The output impedance is about 140 ohms with high current capability. You can use this preamp as a line stage and can drive hard loads.

This preamplifier will enhance your sound experience whether you are using solid state amps or tube amps.

The preamplifier has one stage only, and all the functions of the tube are solid state assisted: Coleman filament regulators, constant current sources on each high voltage supply to maximize ripple rejection. Great care was taken to have a pure voltage supply to the filament and anode in order to deliver a hum free device.

“Shiny Eyes” 4P1L DHT line stage will be a very good sounding addition to one’s sound system.


  • Mains: 115/230Vac ( hard wired to either 115Vac or 230Vac)
  • Power consumed: about 55-60W
  • Frequency response: 17Hz-40kHz @ +0.2dB
  • THD: better than 0.1 % at 2.2Vrms output
  • Gain: x2.25 at 1kHz or 7dB.
  • Input impedance – direct coupled: value depending on volume pot used
  • Output impedance – transformer coupled: 140 ohms
  • Weight: about 30 lbs.
  • Dimensions: 475mmx400mmx130mm (about  7″x15.7″x5.1″)
  • Start-up time delay: First at 30 seconds and second at 1 minute

How to order:

This preamp has to be custom ordered. The barebone/ base line cost is $3000. This doesn’t include any of the RCA,XLR, IEC, Volume control and input switch devices. There is so much variety and cost varies so much then I will price the additional cost based on the specific preferences. Lead time depends on the time to procure parts and materials and can lately vary between 2 and 4 months.

For questions please email to radutarta@simplepleasuretubeamps.com

      ***Shiny Eyes 4P1L Preamplifier Older versions***

frontsidebackfront wiring

Here is a mini -review from the happy owner of the pre with Khozmo:

Hi Radu: 
I wanted to give you an update/mini review on my new “Shiny Eyes” line stage but I have been too busy listening to music.  Blush 
After further listening/break-in, I still find it to be super transparent and detailed but with a touch of tube warmth which is exactly what I wanted.  However, the biggest improvement over my previous LDR-based passive line stage is in dynamic impact!
I didn’t know what I was missing until I heard it.  It started with a well-known audiophile recording on CD that I never really understood what all the hype was about.  With the increase in dynamic range, not only did I “get it” for the first time, I really enjoyed this CD.
Obviously, one example wasn’t enough to establish a pattern but a couple of days later I played an SACD that I recently purchased.  It’s from one of my favorite groups but not one of my favorite albums.  The first time I played it, using my old passive line stage, I thought I had wasted my money of the SACD.  When played through the “Shiny Eyes”, I thought it was fantastic!
Again, it was that significant improvement in dynamic impact.  I know the following statement is a bit of a cliche, but the “Shiny Eyes” really is like my former LDR-based passive line stage but on steroids.  Not only is it a great component, it’s a great value as well!


Standard Version of the 4P1L Preamp

I’ve started to experiment with the Russian DHT 4P1L  years ago. This design is the result as many hours of breadboarding, testing and listening. I’ve concluded that the best sound, lowest microphonic effect and highest driving capability, was offered by a 4P1L in triode mode, filament biased and line output transformer loaded.

  • This is a single stage 4P1L in triode mode using Lundahl LL1671/30mA line output transformer.
  • The maximum gain is about  x2.2 or 6.8 dB.
  • Typical THD is less than 0.1% at 1kHz and 2.2Vrms output. Usually, earlier production ( 60s and early 70s) are better sounding tubes and lower THD.
  • The frequency response is better than +/- 0.5dB @ 20Hz-30kHz.

The preamplifier is built with 2 PCBs of 2mm, 2oz Cu, one has the power supply and the other one the 4P1L preamplifier stage.

In this case you will receive a finished product. It will come in a Pesante 3U 435x300x120 mm enclosure, American cherry sides, lighted power switch, 1 output and 3 inputs switchable and TKD stereo volume control. It is possible to be wired for 115-125V or 220-240V.


Dave posted here: http://hifiwigwam.com/forum/topic/121649-4p1l-dht-preamplifier-project/?page=5&_fromLogin=1, a review of this preamp built from scratch using my boards.

14 thoughts on “4P1L Line stage

  1. Dear Radu,
    Thank you for your so kind and accurate presentation and description.
    Your experience is precious for those who want to navigate in this ultimate-sound world.
    This is my case ans I’m currently working on a high level preamp, based on a DHT triode (maybe 4P1L), and Lundahl output transformers.
    Just to figure out the environmental system configuration, I have already built my SE 300B main amp and I’ve been pinned to the wall by the sonic performance elevation as brought by the SE configuration.
    My speakers are modified Altec VOT A5. Of course the global transparency of the whole system seems to be a pre-requisite and this is my assuption that only very agile drivers such as those having fly-like diaphragms can reproduce the additional micro-signal informations revealed by the SE technology.
    In the frame of myproject , I’m very interested in sourcing some Rod Coleman filament regulators.
    Could you be so kind to show me whom did you purchase yours from?
    Kind regards,

  2. I have ordered “Ready to use 4P1L preamplifier” a week ago, and I received it yesterday.
    I am still listening to the music from it.
    It is gorgeous. Very nice.
    I have used some tube preamplifiers – KK differential parafeed, 7 different Aikido, 01a DHT preamplifier and so on. And I also had built a preamplifier with 4P1L by myself.
    But, Radu’s is the best.
    Before order, I had asked some questions to Radu for its building or modification, and I got to know he had tried all the things I had done. This preamplifier contains his experience.
    For the sound, it’s the sound what I expected. Detailed, 3D, and less coloring.
    Furthermore, no noise and no microphonic, which is hard to solve when building a DHT preamplifier.
    Great build, Great sound. It’s just gorgeous.

  3. I have been using this preamp for a couple of weeks now. It was my first experience of building a valve preampflier. Radu was exceptionally helpful with my list of novice questions and always provided clear and concise guidance. This allowed me to finish the preamplifier build quite quickly. I never waited more than 20 mins for Radu to reply to my queries – great service.

    In my system I’ve had some nice preamps including a Shindo Aurieges and a Monbrison. This 4P1L pre is the nicest I have heard and it represents exceptional value for money.

    Hopefully news of this preamp kit will spread so that others can enjoy it. I’m looking forward to taking it along to local Hi-Fi meet ups.


    • Hi Len,
      The F5 has about 2.4Vrms input sensitivity. The 4P1l amplifies 2.2 times (7dB). The resulting input sensitivity for full 25W power will be about 1.1 Vrms.
      The 4P1L preamp has the power to drive almost everything ( low output impedance and high current drive-ability ). If the 1.1 Vrms satisfies you ( your audio sources will provide the needed output) then you’re all set.


  4. Hi Radu
    Great quality boards which l received a few days ago,now l look forward to completing the project.

  5. Hi there
    How would this preamp fit in with a Mark Levinson 27.5 power amp(solid state). Any modifications needed to be done to make it a good match?
    Input impedance: 50K ohms shunted by 1.5nF. Voltage gain: 26.9dB

    • Hello there,
      In general with DHT tube preamps the main issue is : Does the (amp + speaker) handle the mechanically induced microphonics of the DHT (4P1L)?
      In my experience, using both tube and solid state amps having an input sensitivity of 1Vrms/full power or lower, will be beneficial when using this 4P1L DHT preamp. So the answer is yes, you can use it with Mark Levinson 27.5 which has a 1.25Vrms input sensitivity.
      The preamp will comfortably drive loads down to 10kOhms and 5nF.

      • Thanks Radu… will think about upgrades I may add to order before completing it…Silly question but has to be asked…since my amp has balanced inputs and reportedly works better in XLR connection , is there a balanced version of this preamp and how much would it drive up the costs and is it really beneficial?
        Thanks and talk to you soon

      • I do not have a truly balanced version of the preamp. Yes it would be beneficial to use a truly balanced preamp if your amp is fully balanced/differential. Less noise and more dynamic.

  6. Radu, I have not written you lately because I spend all my spare time listening to the wonderful Shiny Eyes preamp and SE EL-34/6SL7 amp you provided me. These are truly the most enjoyable pieces on audio gear I have ever owned, and I have owned a lot of audio gear. Thank you.

    I am setting up another room that will require the use of monoblocks. Do you/could you/would you consider offering a version of your 4P1L preamp with filly balanced/differential XLR outputs? I would pre-order one now if you tell me it can be done.

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