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.

A common challenge most of the owners of SS amps is: input impedance is too low. Hey, do you remember I had to put out all of the dangerous HT equipment when my daughter was born? So I’m one of those who own an SS amp (despite I keep building a new 4P1L PSE and 300B amps at this moment). Mine sounds very nice and not like a traditional SS amp, though. The main disadvantage is its low input impedance (below 10K). There are many out there with SS amps with low input impedance so if you are one of them, you’d find this post interesting (if not, just keep reading it for fun!). Also, I have seen too many cases (or comments around the audio forums) about the poor performance of an AVC or preamp which in cases was attributed to the demands of a low input impedance or high capacitive loads. They are hard to drive, so well. What do you expect?

The circuit design

For once, I think I will surprise you. I’m not using any hollow device here. Pure sand instead.

What?? Yes, you read well: sand, sand, sand!!!

JFET devices perform really well (within certain designs and conditions) and they do sound nice. I love them. I have used them in phono stages as well as the lower FET of my gyrator boards.

My friend Tim kindly sugested reading this post (which I highly encourage to devour its site as it has plenty of fantastic info and experience) whilst I was experimenting with SiC MOSFET followers as driver of output stages. I played with a similar design when I was building a SLCF stage. Adding a servo is very useful to avoid cap coupling.  Despite you all purists, having a servo is ok. It sounds great, just chill out. The beauty of this circuit is the fact that there is no cap on the signal path. The cap on the servo is clearly on the feedback loop and has limited involvement as provides zero gain in AC to keep the bias point where is needed.  I still use a nice film part here anyhow.

The trick of this circuit is matching the FETs. You will have to play with LTSpice if you want to optimise this circuit depending on the FET IDSS you have. I carefully measured and traced several FETs to match pairs and get the right ones using Locky’s tracer. The FETs are NOS but still able to get hold of. Probably not for long.
The circuit is very simple. A buffer stage formed by a push pull pair of jFETs. In the post above, a nice combination of valve +pFET was used. In this case, we drop the valve, simplified the power supply and instead we use the complementary pair of 2SK170 + 2SJ74 instead. The supply can be as low as a pair of 9V batteries or a quiet supply ±12V.

The key thing on this circuit design is the matching of the jFETs (between channels)  as well as balancing the degeneration across the two jFETs (P and N types) to minimize distortion. Low degeneration will lead to higher distortion, whereas the opossite to higher impedance but lower distortion. Unfortunately, I had to manually select the FET with the tracer and then look at the THD response whilst replacing R2 and R3 with 100Ω trimmers. I manually balanced the circuit.

In my case I matched a pair of 2SK170 with IDSS of 8mA @12V as well as a pair of 2SJ74 with an IDSS of 7.6mA @12V.  Here is an example of the 2SK170:

 

You could argue that R1Ω could be increased to 100KΩ or more. However, you will impact the noise performance of the stage. If signal to noise ratio is high, then so be it. You may get better distortion performance overall if the previous stage distortion is very sensitive to the load (R1)

Now, let’s talk about the servo. Yay! Many will dread the excessive sand involved in this circuit. Well, I have to say that the servo circuit sounds really nice. I’m not the first one saying this (check Meno on this). Ok, have you accepted the sand on this design? Let’s move forward. The servo is a simple design. U1 is operating at open loop at DC so given its high gain it’s forcing both input pins to match their levels so to speak. In other words, the op amp will try to make the negative input as close as it can to the positive input. The positive input is grounded. Hence, the negative feedback loop via R6 will draw current down from J1 and change it bias level through R6 to keep the output at 0V (or at least as close as it can be). C1 forms a low pass filter with R7 to ensure that under AC conditions the gain of the op amp is unity so the op amp doesn’t interfere with the circuit performance. The positive input is at ground, remember. The pair of diodes limits the input levels in the servo to ensure a smoother response on transients (e.g. Power up).

The circuit (in my case given IDSS of FETs) bias at about 7-8mA. This ensure it can drive heavy loads without Slew Rate distortion issues. The measurements below are a proof of this.

Why would you bother with this circuit? Well, you may not need it and it that case forget it and don’t build it. Don’t add an extra stage to your system unless you really need it in view. Less is more.

However, if you’re looking to drive an AVC/TVC or SS amp with low input impedance or a valve amp with an input transformer, then you may want to consider this circuit.

Surprised? Well, I’m a valve fan, however I appreciate where the sand performs at it best. And we don’t have P-type valves anyhow! I’m very fond of jFETs and their sound in general. As I said before, I use them in RIAA stages as well as gyrator loads. Sand can bring an interesting twist in our hollow designs. In this case, I ended up gobsmacked with the results myself (see below)

In my view, the downsides of this circuit are:

The 2SK170 and 2SJ74 are discontinued devices.

FET’s IDSS parameters are all over the place. So be prepared to buy a handful of them and match them manually. Be patient

If you don’t match them, then the circuit will be unbalanced and higher distortion as well as an offset voltage will be at the output.

Good news is that Linear Technologies have revived these FETs (Thanks to Rick for reminding me of this!). You can use the LSK170 and LSJ74 instead. You can get them already matched by different sellers. Check the DIYaudio store for example.

Building the circuit

It took me a day to build this completely. For once, I took a day of from work and locked myself up in the workshop to get this done. A very pleasant experience as it all run smoothly as planned (at least for once!).

For simplicity and speed I used prototype PCB boards (one for each channel). Here are some pictures which may help you inspire in your version:

I firstly build a mule test circuit with a protoboard to ensure I could test the pair of jFETs and minimise the distortion. Then, I proceeded with each board build and test independently.

The entire circuit can be fit in a small aluminium box which you can drill easily and fast enough. The circuit also has an LED power indicator as well as a ground lift circuit comprised of a 10Ω 3W resistor in parallel with a 100nF ceramic cap and a reversed dual Schottky diodes for high current faults through ground. Given is a line stage, you may not need this and a 10Ω resistor will suffice. However, I had the set already soldered point-to-point so I used it as-is.

How well it measures?

The measures below speak for themselves. The circuit is dead quiet and has great PSRR given the nature of the follower topology. The distortion is minimal and in the case below, my new sound card has a higher H3 component and distortion level can get lower than 0.0048%-0.005%:

Well, the interesting thing is when you put the stage to drive a heave load: 4.7kΩ Most preamps will struggle. Here is the output at 1kHz and 1V, which shows that distortion has increased only 0.002% approx due to H2 raise by 8dB:

Of course at higher frequency and output levels, distortion will increase due to slew rate. A

What is also interesting to show here is the frequency response with same heavy load (4.7kΩ) and 2,200pF capacitance to drive. The output can do very well up until 2Mhz:

 

 

How does it sounds?

Very clean, as it should do. In fact, I can’t hear any change other than the improvement on response. This buffer is feeding my subwoofer amp as well as the SS main amp. I can notice a more solid and sharper bass as well as clearer treble. Subtle difference, but worth to my ears. I have to keep running with this setup for some time to get further impressions.

It took me some time to write up this blog post as I struggled to find the time after building this piece of equipment. Hope you enjoy it (at least reading it)