Gyrator hack: Enhancement MOSFET option

Happy Easter to all! (whatever you celebrate, doesn’t matter, it’s always good to have some days off)

I have my preferred gyrator setup which includes a top (depletion) MOSFET IXTP08N100D, which has a unique high VGS(th) which helps improving the performance of the bottom FET, in my case the BSH111BK. The combination of both is superb and they do measure (and sound) superb. The frequency response is flat until 3.4Mhz (-3dB). Yes, a high bandwidth amplifier, so you need to be mindful of this when using high gm/gain valves. I read somewhere people complaining that gyrator “oscillate”. Well they don’t, however they create a high bandwidth amplifier which is therefore prone to oscillate if you don’t take the right measures. If you don’t know what you’re doing, it will oscillate for sure, you have been warned.

Ok, if you can’t get hold of (any) depletion MOSFET as the top device, there is an option, a la Gary Pimms.

The circuit can be tweaked slightly, as can be hacked the PCB (I can show you how if you’re intending to use this circuit)

Here is the design:

The main difference is that D4 provides a stable reference voltage (18V) which ones you subtract the VGS(th) of the top MOSFET (typically 2-5V) then will give you enough headroom to allow the bottom FET to operate under low output capacitance due to higher VDS. This is the common limitation of the cascoded pair of depletion devices. You can’t get more than 2-3V. As the top device forms a “cascode” with the bottom, it also limits the maximum voltage possible to the drain of the bottom device. The protection zener of the bottom device can be removed to ensure maximum swing. This stage can do 20Vpp easily. C5 provides some filtering to the zener noise, which is very low. I can’t see an issue at the driving levels in place.

The protection zener (D2) for the top device is needed unless the MOSFET comes with a pair of back to back as some do.

There are multiple options for the top MOSFET. I like the (nearly EOL) STP3NK60ZFP which is a FP TO-220 device, very handy for heatsinks and high voltage and comes with the bonus of the protection zeners. The best option is the AOT1N60 and also the easier to get hold off FQPF2N60C.

So, the performance is great. You can get flat response up to 2.1 Mhz. Here is a snapshot with my buffer which limits to 1.5Mhz:

However, my prefered stage can do 3.4Mhz under same conditions!

cx-112a DHT preamp

Introduction

This was one of my first DHT preamps. I found a quartet of NOS CX-112a Cunningham (globe) back in Buenos Aires many years ago and built one of my first DHT preamps. I loved it. I played with it before I moved into the 26 and then started the long exploratory journey with DHTs.

The CX-112a can be easily fit in an existing 01a preamp. Take a look at what Thomas Mayer recently blogged about this valve, worth reading it.

Well, you can get more current drive than 01a (nearly double) but no thoriated tungsten filaments. Anyhow, the gain is slightly higher but is very easy to adapt to my gyrator-based circuit, that I couldn’t resist to take the quartet out of my valve stash and make them sing again after so many years.

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VT-25 DHT Preamplifer

VT-25 in action

Now I’m back from our long trip, I found some time to play with the “Mule“. I wanted to revisit my old VT-25 preamplifier. Many years ago I had my first VT-25/10 preamplifier which was based on a gyrator load. Then it morphed to a transformer coupled (LL1660/40mA) version to drive my TVC before I settled into the 4P1L for some long time.

The circuit design

The VT-25 has always been on my list of favourite DHTs. It’s gone ridiculously expensive these days and is hard to get. I have a couple of pairs in very good shape luckily.

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“Schade” SE Amp Example

Introduction

On my last post I covered how the gyrator PCB can be used in a pentode driver. The pentode driver is the best candidate in a “plate to plate” / shunt feedback or “Schade” feedback amplifier which is the name typically used in the DIYAudio world. The triode doesn’t work well here as you need high gain and low distortion with a load which can get quite low (due to the feedback effect of the feedback resistor). I’m not going to cover the subject as it has been covered (and discussed) extensively before by many people, so I suggest you do a bit of research yourself if you are interested in the subject and want to learn more.

A Study example

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C-299/CX-299 DHT Preamplifier

The start of a different DHT experience with the Mule

I built the “Mule” to provide enough flexibility to test other DHTs as pre-amplifier / line stage. Using the gyrator board, the flexibility is fantastic. Can share same HT and dial the right anode voltage. The LT supply can also be shared amongst many DHTs and Rod Coleman provided me with a set of different resistors to test the list of 9 or 10 DHTs I have in mind which haven’t listed carefully on this design.

The C-299

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Gyrator FET options (More!)

Someone had to invest and sacrifice some gyrator boards to test various lower FETs (either depletion or enhancement devices as well as TO-92 or SMD options). That was me.

Why? Because I want to push this circuit further and find the best options as well as provide to the builders out there some other device alternatives when they can’t solder SMD components.

So let me present you the abused test mule and the various boards under the mercy of my tests:

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Gyrator PCB board updated (Rev06)

After some further testing and prototyping, I’ve updated the gyrator board PCB to provide additional protection to the lower FET device with:

Protection Zener (D3) between drain and source (through-hole)
Back to back protection Zeners (D1 and D2) between gate and source to ensure positive gate bias for higher currents on jFETs and use of enhancement MOSFET

Layout was carefully adapted to ensure track separation due to HV in place. Result is that the new gyrator board provides all protection needed on the lower device and simplifies the build process

Here is an example of a completed board tested:

DHT preamp “The Mule”

The birth of the Mule

The name I guess says it all. This is yet another DHT preamp with the gyrator PCB. So what’s different? Simply, a breadboard DHT preamp module ready to be abused. I’m planning to mod this to death and try a long list of other DHTs with the gyrator load.

I will only need to change the valve sockets (or build an adaptor) as well as the filament resistors and Rod Coleman filament regulators. Simple changes which can be done fast, will open the door to quick tests on my system.

In order to make this simple and a rapid build, I opted to use an IKEA chopping board. These are made of a laminated hardwood and are dirt cheap. A couple of hours are required to drill all the board like this:

Job done. You only need to do this once. Here is another look at the half-build Mule:

The initial sockets are NOS short pin UX-4/UV-4. I will play around with the 01a before I move to other DHTs. I still need to add the tag strips for filament resistors, output capacitors and the filament regulators.

Wiring will take a couple of hours and we should have another DHT amp to play with 🙂

4P1L: pump up the current!

Background

I’m a firm believer than sharing knowledge and experience is the best way forward to continue learning yourself. It always pay pack at some point. This time Paul Prinz, a fellow implementer of the 3B7 DHT Preamp using the gyrator PCB, came back with a great suggestion. He found a MOSFET which could do high drain currents, it has high transconductance and most importantly the parasitic capacitances were low even close to the BF862. Hooray, I thought. We may have a great solution here to use the gyrator load for currents above 25mA and with similar performance to the great BF862. There are some other depletion MOSFETs that can do high currents, however they all have relatively high capacitances and low transconductances when VDS is low, like in the cascoded gyrator circuit.

The BSH111BK is an enhancement MOSFET, so doesn’t have a “depletion” behaviour like the jFETs. This isn’t a problem as the bias voltage can be set by the reference CCS.

For comparison, here is a brief summary of the key characteristics of these three devices:

	BF862	BSH111BK	MMBFJ310L
Ptot (W)	0.3	0.3	0.225
VDSmax (V)	20	55	25
VGS off (V)	-1.2		-4
IDSS (mA)	25	210	60
Gfs (mS)	45	640	18
Ciss (pF)	10	19.1	5
Crss (pF)	1.9	1.5	2.5
Coss (pF)		2.7

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01a Gen2 Preamp Build from Barry French

Barry French has recently build his version of the 01a Preamp Gen2. Here are a couple of pictures:

And Barry’s impressions:

“The 01a Amplifier is a stunner, personally I feel it leaves the 26 out in the cold, better top & bottom by a Country Mile, this was built using the Russian FT-3 Caps on the Output, Russian PIO Caps on the Boards with Jupiter 0.1 μf Wax/Oil Caps from B+ to Ground, the Power Supplies for both Filaments & B+ are from my original 26.”