Tag: 4P1L DHT preamp

Gyrator Test Mule: 4P1L Preamp

4P1L gyrator test mule
4P1L gyrator test mule

It’s always great to come back and revisit a great design. The 4P1L preamp performs flawlessly so I tweaked the gyrator board to see how it worked with the BF862 FET. The result is great, it sounds as good as it measures:

4P1L Test gyrator board

The 4P1L is biased to 150V/25mA which is the maximum current that the BF862 can do (IDSS max). You can see that the frequency response is flat up to 1.5MHz. The LF response of my test mule is affected by the AC coupling of the measuring gear. However it should be around 5-10Hz.

The distortion of low-level signals is really good:

THD @ 4Vrms
THD @ 4Vrms

Predominantly H2, it’s very nice to see THD<0.015% for a 4Vrms output. The load is 100KΩ which is the typical input impedance of an amplifier (with exception of solid state gear)

This low distortion manifests across the entire audio band (ignore the THD below 20Hz which is a byproduct of my testing gear):

THD version frequency @ 4Vrms
THD version frequency @ 4Vrms

The nice thing to see also, it’s how well the 4P1L can drive larger voltage swings:

4P1L THD @ 10Vrms
4P1L THD @ 10Vrms

We can see H4 popping up, however odd harmonics are lower (H5 in fact is higher than H3). THD at 10Vrms is still below 0.03%!

 

 

4P1L DHT Preamp Siberian (Gen3) finished!

IMG_1568Introduction

Building a new version of the venerable 4P1L “Siberian” was very encouraging. This belated project finally came to life after some recent work on a new set of power supplies. So why 4P1L again? I always found the 4P1L sound to be unique. Great detail, overall tone and fantastic treble. What it makes it well suited for pre-amplifiers is not just its linearity (probably being the most linear valve out there) but the fact that it has a low anode resistance and current capability to ensure any challenging load can be handled effectively without any sound degradation. This can be heard particularly on the treble where the input capacitance of the amplifier is more evident and it is translated into treble loss. Other DHTs like 26, 01A, 30sp can only handle a few milliamperes of anode current and is not enough to charge and discharge the  parasitic capacitance at high frequencies. More importantly, the 4P1L has filaments which aren’t demanding. This is a unique feature amongst DHTs that is rare and very useful. Having low-current filaments that can be either configured at 325mA or 650mA, low grid voltages and high transconductance in a valve is very useful. This mean that filament bias can be easily implemented without burning unnecessary power by swinging many volts to perform the desired level of amplification.

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