SiC MOSFET Follower Driver

How many more times

Led Zeppelin wrote a fantastic song on their first album: how many more times. You may not be a rock fan, but hey: what a great song. How many more times do I want to get back to this “slew rate” theme? I don’t know, as much as I have to. Plenty of comments out there of bad designs with wimpy drivers attempting to take the 300B/2A3 or even 45 valves to full tilt with disappointing results. Either way, they always blame the valves.

I came back to revisit the driving of capacitive loads effectively as I’m working on a new 4P1L PSE amplifier. Slowly, but getting there. Previously I looked at adding a buffer to the 01a preamp as a result of slew rate limitations found in Tony’s implementation of this preamp.

buffer

 

 

The circuit design

Continue reading “SiC MOSFET Follower Driver”

Building gyrator boards

I’ve been on some business travel so haven’t had much time to work on stuff, however I did get a set of gyrator boards for a friend and a customer:

  1. BF862 configured for 4P1L preamp
  2. 2SK170 configured for 01a preamp

4P1L preamp with BF862 gyrator

Many have asked me about this preamp with gyrator load. Here is the latest implementation which I preferred most in terms of sound. The mu resistor is 470Ω which is a nice compromise between BF862 transconductance and distortion. I adjusted it on test. I use a 100nF for C1 so R6 is 10MΩ. R4 can be either 300KΩ, 330KΩ or even 390KΩ. Difference would be only on the voltage range for the CCS. I found running it at 25mA to be perfectly fine, some BF862 can even do J310. I prefer this SMD compared to the J310. It performs much better even at high frequency:

4p1l-preamp-gen1b-gyrator-pcb-detailed

 

4P1L gyrator boards
4P1L gyrator boards
Continue reading “Building gyrator boards”

27/56 Preamp from Jose Martins

Jose Martins sent me an email with his recent built on the 27/56 preamplifier using the gyrator load and these PCBs. I recently posted an idea using the lovely 27 IHT valve here.

Here is a picture of the finished preamplifier:

DSC01108 Continue reading “27/56 Preamp from Jose Martins”

Abusing the Gyrator Load

On my previous post, I covered my initial build work on the gyrator test mule using the gyrator PCB. I did all the lovely soldering work (which I do enjoy not like milling or drilling) and proceeded to do several tests.

Some interesting observations based on my abuse of the gyrator which yielded on several MOSFETs and JFET damaged as a result:

  • CCS reference: I used an external multi turn 5KΩ potentiometer via lead cables. I wired it incorrectly and that contributed to one of the initial faults. Be sure you look carefully on this if you use an external pot. if you use the on-board trimpot, this is not an issue.
  • JFET: this is the interesting one. If you want to run the lower JFET at very low biasing current for a larger jFET (e.g. J310) you will find that the JFET needs to operate close to cut-off voltage (somewhere between -2 and -6V). This VGS required will definitely forward bias the Zener protection diode D1 and prevent from reaching lower bias current (I found it about 10mA for J310). To resolve this you just need to add a back to back zener as shown below. This isn’t a problem for an BF862 or a 2SK170 as their cut-off voltages are quite small.
  • Failure: if you abuse the FETs, they will die. And if they die you will get a nice short across them and you will measure nearly HT at the mu output. Just replace the MOSFET and JFET (probably both are damaged)

Continue reading “Abusing the Gyrator Load”

Gyrator load – test mule

I’ve done several tests using a simple gyrator PCB test mule. It was time to build a proper and flexible test mule for extreme abuse:

  1. 2 Boards for current flexibility
  2. Board 1: BF862
  3. Board 2: J310
  4. Top FET is IXTP08N100D2 for 1000V operation
  5. ZIF socket pins for CCS reference resistor and RMu. This will give the necessary flexibility to try any combination in the gyrator depending on the triode and or the power supply
  6. External pots for ease regulation of anode voltage

test muleThe top MOSFETs are bolted on the aluminium case which will act as heatsink. For tests this should be sufficient.

The top plate of the case looks like this:

test mule1

There are 4mm posts are for HT supply, GND, mu-output, Anode. There are also a pair of 2mm posts for current sensing per board.

Some further soldering to do and job done!

45 SE Amplifier

IMG_8261Introduction

More than 5 years ago, I built a fantastic single-ended amp with the unique 45. The 45 has a distinguished tone and personality despite its mere 2W of output power. If you have high efficiency speakers, then it’s a great amplifier to build. With 2W you can enjoy music in a mid-sized room. You don’t need more, this amplifier performs at its best at low output levels and in particular when playing jazz or classical music.

The 45 Amp design

There are plenty of design circuits out there. I settled for a simple triode driver using a gyrator load. The choice was down to the 6J5 and 7193 (a military version of the 2C22). Both triodes are medium mu and sound really nice. Depending your needs, you may opt for a different driver (even pentode). However, they need to be able to drive the large voltage swing required by the 45. I’d go for a 6J52P, 6e5P, 6e6P, D3a or C3g these days. It all depends on your needs and available valves. The driver is biased at 7mA to provide enough grid current to avoid slew rate issues. An improved version would be to add a MOSFET follower to provide better performance under grid current. An example of a follower implementation can be found here.

45 SE AmpThe 45 is biased hot at 34mA/300V. The anode can handle 10W and this operating point provided best sound in my view. The OT is crucial, so invest as much money as you can afford. Rod Coleman regulators are needed to implement this amp without hum and the unwanted  inter-modulation effects.

IMG_2401I carried out several tests on the driver to find the sweet spot for minimum distortion and full swing. The driver is a hybrid mu-follower composed by the gyrator and the 6J5/7193 triode. The valve is biased by a set of 5 red LEDs to about 8V. I think I had a combination of a white LED and LED to provide 8V in my implementation. The dynamic resistance is minimum and won’t impact the performance of the stage.

I used the Sylvania metal-base 6J5 but then settled for the 7193 valves. They sounded better and I was quite pleased with the overall performance of the amplifier.

The amplifier design is very simple. With the gyrator PCB you can simplify the driver build and also use different valves to experiment with them. I originally didn’t have a PCB so I built my gyrator in a prototype board.
IMG_2403

45SE Amplifier upgraded with the 7193 drivers
45SE Amplifier upgraded with the 7193 drivers

I’d highly recommend you building this amplifier. If you want to experience the single-ended sound, then this is one of the amps to build. Of course you can go for higher power levels with a 2A3 or 300B, however, the sound of the 45 is unique. Worth trying

 

 

 

 

2P29L – Preamp and driver for 4P1L PSE Amp

A very interesting Russian directly-heated pentode related to 4P1L is the 2P29L. It has a similar mu (μ=9), much higher anode resistance 2.8-3KΩ and transconductance of 3mA/V when triode-strapped. The filament requirements are much lower at 120mA. I picked one valve from my collection to submit it to the mercy of the curve tracer:

2P29L test point (pentode)
2P29L test point (pentode)

The triode curves are really nice:

2P29L triode curves and model
2P29L triode curves and model

This valve is as linear as the 4P1L (hooray). As a preamp it can be easily implemented like the 4P1L Gen2 preamp using a gyrator PCB which simplifies the building process:

2P29L preamp
2P29L preamp

Running it at 15mA and slightly above the recommended 160V achieve its lowest distortion.

We could also use this valve as a driver for a 4P1L preamp, which comes very handy for filament bias:

2P29L-4P1L PSE Amp

Filament Bias: a practical example with 3A5 DHT

Introduction

cartoon-july4_0006

Recently I was asked about whether I could write on my blog about how to design a filament bias stage. My immediate answer was:

  1. I don’t have much time these days am afraid to write extensive articles (and sometimes to even write-up at all)
  2. Thomas Mayer has written about it (see here). Of course, I completely forgot that Thomas never completed his intended series of posts around filament bias, so I decided to attempt explaining the practical aspects of its design in this blog.

Before you continue reading this post, I suggest you read first Thomas’ article above and get yourself acquainted with DHTs and triode amplification. I’m not going to cover any of that theory which I will give it for granted that the reader is experienced with valve circuits and in particular with the hybrid mu-follower amplification stage with gyrator load.

3A5 DHT example

Continue reading “Filament Bias: a practical example with 3A5 DHT”

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%!