Hybrid Mu-follower Output Impedance

Where to start?

Well, I often get the question “what is the output impedance of the gyrator circuit?”. My response has always been “it gets close to 1/gfs as a good approximation”. Recently, I was asked the question again, but this time I decided to crack on the formulae, which is a daunting task for someone who has ditched calculus after obtaining Ms in Engineering degree more than 20 years ago.

For simplification let’s start with a simple hybrid mu-follower stage (a.k.a. Gyrator load) like the following:

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Flexible CCS board prototype

I’ve been prototyping a flexible CCS PCB. The intent is to provide a cascoded FET pair with some interesting features:

The lower FET can be multiple devices depending on the choice of reverse capacitance and transconductance. These include jFETs and depletion MOSFETs like the 2SK170, J310, BF862 and of course DN2540. For this purpose several pads are provided for SMD devices as well as TO-92 ones, just like the gyrator PCB. A protection Zener diode between drain and source can be soldered when using low VDSS devices.
There is either a string of trimpot plus a resistor to set the CCS current manually during test given the variance in the FET parameters. There is also an option to put a fixed resistor.
There is a mu-output connection provided.

The board is very flexible and can be used for multiple purposes:

shunt regulators (including VR valves)
Anode load for phono preamps, drivers, LTPs, etc.
LTP tail CCSs

I’ve been running some tests with excellent results.

If there is interest, I will run a batch of PCB to offer to the DIY community.

Cheers

Ale

JFET Buffer

Fall 2016

End of summer is here, and for some the beginning of the building season. Well, not for me am afraid. My parental duties and work are keeping me very busy these days. I don’t have the free time I used to have before (I guess I’m not the only one on this so won’t rant on it). Today, building DIY audio gear is a matter of a well planned and negotiated free-time that worths more than gold to me. Well, that’s the way it goes. Anyhow, I picked up my daughter from nursery yesterday and on the way back I was faced with this beautiful landscape. I guess nature give us some gifts from time to time, you just happen to be on the right place at the right time:

Standing on the middle of the street with the pram was a bit dangerous so had to park my daughter on the side whilst I managed to take this picture. Time ago, I’d have taken probably a long time to take this snapshot, but now it was as quick as a bank robbery. Just take the phone out and shoot – you can’t take your time when you have a crying toddler on the pram!

A tail of buffers

I think I have spent far too much time designing, building and testing preamplifier, perhaps more than amplifiers lately. I don’t know why. I guess I fell in love with the preamps and their contribution to sound overall. Who knows, who cares.

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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:

BF862 configured for 4P1L preamp
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:

Tony’s 01a Preamp

I went to see my friend Tony today and helped him to fix his 01a preamp implementation. Time ago Tony used a prototype version of my gyrator PCB to build the Gen2 preamp with the addition of an output follower to address the slew rate limitations he had on his system due to the larger capacitive load.

Luckily we found the fault easily and it was a bad solder in one of the smoothing HT chokes. Once fault was rectified, we proceeded to take some measurements of this preamp.

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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:

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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)

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