Hi my name is Guido Landry and I intend to build and sell Ultra Fi Class A amplifiers. I have been an electronics design engineer for 25 years and have built valve based guitar amplifiers before a 1W uTube amp - now being built by Weinbrock amplifiers and a 20W guitar amp, which uses an EF86 valve in it's preamp.
I have always loved the clean sound of the EF86 configured as a pentode and am aware of it's limitations in that mode - namely it's very high output impedance.
With this preamp, I decided to try it wired up in triode mode - as per the Mullard recommendations, with a 68K anode resistor and 250 volt HT power.
In triode mode, the EF86 has very low distortion and to ensure low output impedance, I am using a high voltage NPN transistor configured as a buffer. The calculated output impedance is now a mere 1500 Ohms.
The output sound through a Nelson Pass 5W Class A amplifier is absolutely superb!
Being a commercial venture - I will not publish schematics. But I will be free to give advice to DIY enthusiasts.
I have always loved the clean sound of the EF86 configured as a pentode and am aware of it's limitations in that mode - namely it's very high output impedance.
With this preamp, I decided to try it wired up in triode mode - as per the Mullard recommendations, with a 68K anode resistor and 250 volt HT power.
In triode mode, the EF86 has very low distortion and to ensure low output impedance, I am using a high voltage NPN transistor configured as a buffer. The calculated output impedance is now a mere 1500 Ohms.
The output sound through a Nelson Pass 5W Class A amplifier is absolutely superb!
Being a commercial venture - I will not publish schematics. But I will be free to give advice to DIY enthusiasts.
MOS FET makes a better source follower.
Zout ~= 1/gm
Chose a FET with a low reverse transfer capacitance and high gm like the SiHP5N50D-GE3 which has a gfs(gm) of 1.8S and Crss of 6pf.
Zout = 1/1.8 = 0.55 ohm.
In addition the input impedance of the FET source follower is orders of magnitude higher than a bipolar emitter follower. This results in less loading of the EF86, further lowering distortion.
Zout ~= 1/gm
Chose a FET with a low reverse transfer capacitance and high gm like the SiHP5N50D-GE3 which has a gfs(gm) of 1.8S and Crss of 6pf.
Zout = 1/1.8 = 0.55 ohm.
In addition the input impedance of the FET source follower is orders of magnitude higher than a bipolar emitter follower. This results in less loading of the EF86, further lowering distortion.
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But their input capacitance is far too high! Bipolar rules for buffer transistors - especially 300V NPN devices.
Zout = HFE x Zin
Zout = HFE x Zin
For the volume control, I followed a DIY thread and used a 1 pole 12 way switch.with discrete resistors. All my designs are optimised via Microcap 9 - even the preamp. The EF86 in triode mode is the most linear single stage device ever invented! Especially off 250 volt HT supplies
I would use an ECC82 as a cathode follower and not a semiconductor. Or ensure the input impedance of the amplifier exceeded 120k ensuring minimal load.
That was not the objective at all, I wanted the low impedance to drive the Nelson Pass Class amplifier - which I have modified. But that is a different thread altogether.
I believe in using valves for voltage gain and bipolar transistors for their low output impedance and low input capacitance @ 300 volts.
This preamp has only one gain circuit i.e the EF86 - I do not like the opamp sound at all! That, in my opinion is an easy route to meet high specifications to satisfy the marketing department of commercial businesses.
The slight compression of the triode makes the amp "breath" and sustain. I am a muso with a rare collection of guitar's and my first 20W guitar amp used an EF86 in the clean channel - as did all the original VOX amplifier's
This preamp has only one gain circuit i.e the EF86 - I do not like the opamp sound at all! That, in my opinion is an easy route to meet high specifications to satisfy the marketing department of commercial businesses.
The slight compression of the triode makes the amp "breath" and sustain. I am a muso with a rare collection of guitar's and my first 20W guitar amp used an EF86 in the clean channel - as did all the original VOX amplifier's
I would go with a two stepped attenuator like this one
BeeQuiet stepped attenuator upgrade for BeePre
It offer more granular control
Do
Thank you pinnocchio for the link, it certainly would be very useful for a stereo version of my preamp. The course control could be used as a balance control with the wiper connected to earth and the finer controls for left and right channels.
I will simulate the attenuation response with MicroCap 9. I love that CAD package and use it for all my designs since it is so accurate and saves you lots of $$$ in blown up silicon output devices.
I will simulate the attenuation response with MicroCap 9. I love that CAD package and use it for all my designs since it is so accurate and saves you lots of $$$ in blown up silicon output devices.
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