Long time ago I developed a CCS board which provides full flexibility in terms of FETs/MOSFETs used. It was a 2-terminal CCS, a very well known circuit.
Continue reading “Mini CCS”Mini CCS
Tag: DN2540
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
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.
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:
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:
Slew Rate (Part IV) and the Tale of the three Source Followers
Some of you may be a bit fed up already with these slew rate posts, however I find this fascinating as is taking me through different routes of experimentation.
On my last tests, I abused the DN2540 to an extent which meant the dead of it. So I ended up adding the appropriate back to back protection zeners on the gate:
Continue reading “Slew Rate (Part IV) and the Tale of the three Source Followers”
CCS: not everything that glitters is gold (Part I)
Introduction
This is the first instalment of a series of blog posts around CCS for valve circuits. Hope you enjoy it as much as I did with the experiments conducted as a result of my interest in CCS-driven circuits.
The depletion cascoded CCS
It’s been long time since I’ve done some circuit analysis and algebra, hopefully I’ve got this right. Seems to get to the expected result, so hey: I’ve done it ok.
The analysis of this circuit starts by using the T-model of the MOSFET. I’ve omitted the parasitic capacitances to simplify the analysis. I leave you the challenge to add them in though. If we look at the typical self-biased depletion FET CCS we can find the output impedance by doing the following formulae crunching:
In summary, the output impedance looking from the source side is:
Continue reading “CCS: not everything that glitters is gold (Part I)”
Testing the line stage
Introduction
I couldn’t resist the temptation to try and build quickly the SLCF design proposed here. It was question of building a simple PCB for the tail CCS and the top MOSFET follower. Wiring it then point-to-point could be done in a matter of minutes and a “rat nest” was built fast enough to enjoy this learning experience.
The usual challenges we face when breadboarding circuits
One of the challenges we face when building a cathode follower with a high-gain / transconductance valve is that it can easily oscillate widely into VHF. So we then are a bit more precocious when building the test jig and “try” to have short connections (something which I didn’t do), add some ferrite beads to anode, grid and screen. Also some grid/screen stopper resistors (e.g. 300Ω) are always very useful. If you pay attention to this and check with an oscilloscope with sufficient bandwidth (e.g. 200MHz) you can spot out any nasty oscillation from the valve. I didn’t, thanks to the ferrite beads and stoppers.
The clear challenge of the SLCF is establishing the correct bias point on the top follower due to the high value of the resistor divider and the high-variance we typically get on the VGS(th) of the MOSFETs.
High-value resistors are available on 1% but the variance on the FET defeat the purpose of accurately building the resistor divider.
813 triode SE with 4P1L Pentode
A monster DHT amp
Lately I haven’t had any time for audio work unfortunately. Changing nappies to a 4 week old baby whilst working long hours is tough. I can get the odd 30 minute here and there and every time I try to get upstairs to the workshop something pops up. Never mind, hopefully things will get easier in the near future.
I’ve been asked about the 4P1L pentode driver. It’s been a long time since I did those tests and never got around to listen to the driver sound. Tests were promising but never managed to include this driver on my amp.
Driving transmitting valves is a challenging task. Especially if we want to take them to A2-land (unless they operate in A2 whilst in zero grid bias). Driving big transmitting valves like 211, 805, 845, 813 or GM-70 require a large swing of volts for the driver which should do this linearly. The load is quite demanding in particular when we approach the grid to 0V (or biased positively) and using a triode as driver also puts a daunting task to the previous stage due to the Miller effect. It’s not easy to find triodes that can swing 300Vpp with very low distortion.
4P1L Siberian Gen1 upgrade
Some of the DIYAudio fans have built this version of the 4P1L with great success. There are several upgrades that can be easily implemented to improve this. I haven’t tried this myself, but my recent experience with the Gen3 and the 01 preamp gen2, I think are worth trying:
- Replace the gyrator FET for a cascoded pair (M2 and M4 below) to improve PSR
- Replace voltage reference by a cascoded LND150 for better HF and PSR response
- Optimise the LF pole of the gyrator load by increasing R4 to 4.7 MΩ and reduce C1 to 220nF
- Bias 4P1L to about 30mA. This will reduce distortion
Hope this is useful
Ale
