A friend from Canada ordered a pair of boards and I used the opportunity to test the newly arrived hybrid mu-follower (aka as gyrator) Rev08 PCBs. He will be using these boards in the 300B design posted here.
Here is the driver circuit diagram:
As you can see, I increased the anode voltage to get about 14mA of anode current. This was due to the DUT I had at hand. The protection resistor R8 is set to 150Ω so you just get a slight dim light on the LED under normal operation. It will lit completely under an output short (which likely kill the LED) or under A2 operation (e.g. grid current)
The actual load resistor is 100KΩ which when connected to the audio test interface reflects a total input impedance of 50KΩ.
Here is a test of the frequency response:
And here is the harmonic profile under 200Vpp swing:
Nice results, very happy.
Hi Ale,
Try old type 5mm green LED instead of Rk.
I use my D3a (CCS loaded driver of my 300B SE) in a similar way about for a decade.
BTW for my taste 250V, 14mA is in the “hot” side, I prefer lower operating points (about 180V, 10mA, -2V).
Regards
Bela
Good reminder Bela. Will try the green LED as well as a pair of SiC diodes. I like the sound of them very much.
With the output resistance of the D3a and 10mA of CCS current, you’d get better performance in the driver as hybrid mu-follower for proper serial push-pull operation instead of the CCS anode load. Have you tried this in your amp?
Cheers
Ale
Ale
With the gyrator board and its mu output (which gives low output impedance) is the mosfet source follower necessary, as in your 300B design example?
Hi,
Low output impedance isn’t the only requirement. You can experience slew rate distortion (search the blog as there are few blog entries with a lot of useful information about this) when the driver hasn’t got enough current driving capability. If you get close to A2 operation and grid current occurs you will experience blocking distortion. Otherwise, the gyrator can be DC coupled into the driver.
You can also use stacked supplies to enable DC-coupling of the gyrator. It adds a lot of complexity, I’ve been there and done that. See the SE814 SE Class A2 design for more info on this.
Thanks
Ale