It’s been far too long since I last posted on this blog. With the limited spare time I’ve got these days, I concentrated in setting up the new workshop and system since we moved back to our place. I’m nearly there, so now it’s time to get back to work
I made some updates to the “gyrator” PCB. I’ll stop referencing it gyrator from now on, since the name is misleading. However, it got popular that way. Nevertheless, it’s a hybrid mu-follower circuit but if only if you take the output from the anode, it behaves like a “gyrator” from a frequency response perspective. If you’re interested in this circuit in more detail, please read the lecture I gave last year at ETF.18. You can download it from here.
Back to the board, here are the few changes made:
- Moved the trimpot P1 for easier fixing of the board with the M3 standoffs.
- Added a gate stopper resistor (R9) to avoid oscillation at low anode current (<10mA) when using high-gm MOSFETs in the lower position J4. This was evident with devices like BSH111BNK
- Added an LED (D4) indicator and a series resistor (R8) at the drain of M3. This enables indication of:
- Normal operation subject to value of R8
- Source current into load (e.g. like in A2 operation) subject to value of R8.
- Short output to ground. Depending on duration and current limitation of power supply, this may prevent damaging M3 MOSFET. Not guaranteed, but in some scenarios will work.
Here is the circuit diagram for reference:
Of course you can continue using jFETs in the board. However, I found that the BSH111BK/BSN20BK and other high-gm perform really well in this circuit. They are still plenty and available out there, not like the BF862. Unfortunately this one is EOL (see lecture for replacement options).
Here is a sample test of the 01a preamp stage with this new board:
In this case with the use of an IXTP3N100D2 and BSH111BK operating at 3-4mA the frequency response is flat to nearly 380kHz. Same combination of FETs in a Rev07 board will oscillate at LF due to the lack of gate stopper.
With a Rev07 IXTP08N100D and BF862 board, the response only gets up to 150-180kHz depending on the valve. This is not a limitation whatsoever in audio band, let’s make this point clear.
A snapshot of a completed board:
in the following picture you will see the three new components added so you can compare to the previous Rev07 version. Also note that this is a prototype version so it doesn’t have an ENIG Gold board finish:
The new boards were tested on my 01a preamp. They sound amazing, as good as the previous ones. Interestingly enough, I made a mistake during breadboarding and the LED turn on really brightly with 100mA shunted through the output. The FETs survived this time thanks to the limiting resistor and the current protection I have on my bench power supply.
I would like a couple of those please Ale
If you make these available, I would like a pair or two.
Thank you
Hi John. I can sell some of the ones I have available. need to find the time to solder the BSH111BK FETs. Ping me an email when you can please? Thanks
If you make these boards available, I would definitely want a few pairs.
Thank you!
Hi all, as I have a few requests I will be able to give away 10 PCBs pre-soldered with BSH111BK FETs on a first-come-first-serve basis. I’ll keep just a few for myself.
I will offer them at lower price than usual as these aren’t ENIG finished but they work perfectly fine – and sound really well in my experience so far with the 01a preamp.
Please drop me an email as usual if you’re interested.
I am interested in getting 2 for testing
Hi Ale, I bought v8 and the source follower bords. But did not receive docs yet. Could you send them please?
Happy birthday!
Sorry. Forgot to say. I bought them from someone else.
Hi Ale,
it looks like there is a mistake in the wiring on the PCB around the added R9 resistor.
D1 is on the PCB still connect to G of transistor not to R9 and C1 junction.
Isn’t it?
Hi Radim, thanks for spotting that out! Yes, the diagram is incorrect and will amend it (when I get the time)
Is it possible the new boards may be direct gold over copper without the nickel layer? Nickel can create a slight hardness to the sound. Very much appreciate the exciting development on the Hybrid Mu follower!
Hi Ale, I would like to use this board for C3g to drive 300B. Is that feasible? Which mosfet should be used for this configuration?
Hi ALe, one more question, do i need to insert a source follower between C3g driver and 300B? as i saw another post you have this configuration?
Hi Kwok,
I used the C3g with this board at about 180-190V and 23-26mA of anode current when triode strapped. Cathode bias with LED (or alternatively SiC). Can swing 200Vpp at 0.25% max THD. Great driver
You can use either Rev07 or Rev08 boards. Lower FET in this case has to be BSH111BK or equivalent due to anode current level.
No need to use a source follower at the output given the anode current level.
Hi Ale,
Would like to use this with EML20A driving GM70. Since I will be using separate power supply for driver anyway, is it possible to stack and dc couple mu output to GM70? I can see the following advantages if it is possible to implement:
1. Omitting coupling cap, of course
2. Is driving ability better without coupling cap? I’m not going to drive the GM70 into deep A2
3. Bias supply is not necessary
Please advise me if I miss anything. Thanks in advance!
Hi Louis. Yes that’s what I did many years ago in my 814SE amp. Have a search on the blog as there is extensive information around it. The design was originally made by Michael Koster. There is a good thread in DIYAudio that you may want to have a look at which is on the 4-65a SE Amp. Same concept can be applied to the design you have in mind.
I used a 46 driver (with input SUT) in filament bias! Yes, crazy idea to burn that power on the filament resistors. It sounded amazing though.
Thank you for your valuable advice. I just found out 826 driving GM70 is another interesting idea, and would like know what components should be changed if 826 will be working at 1000V@60mA?
Not very familiar with gyrator and will be greatly appreciated if you could point out anything I missed.
Hi Louis. Sorry but I can’t do you a design from scratch. The 826 is a transmitting valve intended for A2 operation. It’s unclear how you’re looking to use this valve. The PCB is designed for 500V maximum due to the LND150 devices used for the CCS reference. You can build yourself a hybrid mu-follower using HV parts for 1000V if you wish.
Thanks
Ale
Thanks for your advice Ale. I just saw Greenvalve making 826-GM70 amp from Youtube and I thought his 826 is working at A1. I should return to previous idea, 20A couples to GM70 directly with stacked power supplies.
By the way, recommended working point for 20A is 380V, plus 250Vpp/2 voltage swing and 50V headroom, supply voltage will reach 550V. Could your gyrator handle that extra 50V?
Thanks
Louis
Hello Ale,
Do you have a recommended part number for the +B, GND, OUT, and ANODE connectors? Those look really nice and would be a definite improvement over soldering wires directly to the PCB.
Thank you very much!
Hi Paul
These are the ones used:
https://www.rapidonline.com/truconnect-2mm-pcb-mounting-socket-17-2878
Challenge is that you need to carefully hammer them as the holes are smaller. Patience, pliers and a hammer is all is needed. New boards will have wider holes
Cheers
Ale
Hello Ale,
Thank you for the link to the part number!
Best regards,
Paul
Hi sir,
Can you discuss more about the value of R8
How can I know the value of R8?
THANKS ALOT
Hi,
R8 has either two purposes:
1) Protect the top MOSFET(M3) in case of short circuit. You need to set the value depending the normal operating current and the SOA of the MOSFET. You don’t want to get more than 100mA however due to the power dissipation limitation of the resistor type it will act as a fuse and burn out.
2) work in conjunction with D4 to act as an indicator of either normal operation or source current (peak detector when driving the grid of the following stage in DC couple mode to indicate grid current). You will have to play with the forward voltage drop (Vf) of the LED. If a RED LED is used for example, you need to divide Vf/Ia to set the value of R8. Whereas Ia is the anode current in which you want the LED to start operating. Once the voltage across R8 reaches Vf (eg. 2.2V) then the LED is biased and the additional current goes through the LED (as is shunting R8). The LED will then turn on. Mind that also if there is a short at the output or anode, it’s likely that the LED will burn out. Either way, it’s much cheaper to replace the LED or R8 instead of the MOSFET!
Hope this helps
Ale
Hi, Ale,
Your meaning is the R8 if using on 01A preamp
We set 3mA
If R8 = 2.1v (LED) / 3mA
R8 = 700 ohm ?
Is it right?
If you omit R8 the LED will be an indicator of normal operation. You will likely need using a low current LED as few mA will just provide a dim light.
I think I used an 820R resistor if I’m not wrong. You just want the LED to show significant source current through the MOSFET cascode pair over the normal quiescent 3-4mA you have through the 01a (or whichever valve is).
Hi Alex,
I’m wondering why do you leave a place for TO-92 if SOT-23 Jfets are better for J1-J4?
Is there any TO-92 Jfets that match the performance SOT-23 (say BSH-111) ?
Thanks
No, just to provide flexibility as people may want to use low noise LSK170 or higher current J310 if they can’t or don’t want to solder SMD
So, how much worse SK117 compared to BSH111?
Hi Ale, have you found any sound differences between IXTP3N100D2 and IXTP08N100D? Thanks.
Hi Jan
Not really. Both provide higher VGS(th) to the lower device which turns out to operate in better conditions and deliver better HF bandwidth, etc.
The lower device is more audible in my experience.
Cheers
Ale
At first had no bass, due to lack of break-in Russian FT3 teflon capacitors, also used PCB separators of metal possibly causing a ground loop, once solved using plastic PCB separators & screws and the capacitors with the break-in got a very good sound quality, better than RC coupled or LC coupled, only have advantages, it’s so good that inmediately found easily the difference using in the CLC PSU different quality chokes, the bad Stancor 10.5H 110mA 225 ohms and the good Bartolucci 10H 100mA 10 ohms. A big thanks to make it available for the DIY community. Gracias Ale.
Ale,
I wonder if I could use the Supertex DN2535N3 (TO-02) depletion MOSFET in place of J1-J4. I have a quite a number of them and I don’t want to spare my precious 2SK170 there.
Regards,
Evangelos
If you have the DN2535N3-g in TO-92 you can use it, watch out the pinout as you may need to mount it reversed. Gate is middle pin and generally drain and source can be interchangeable, however try to mount is respecting the DGS orientation as per data sheet.
Yes, I meant TO-92, of course. Do you think that there is an advantage (sonically speaking) of using your favorite combination of IXTP3N100D2 and BSH111BK over the DN2540N5/DN2535N3 that I currently have in hand?