Hi
I recently recommissioned a mic preamp that a friend built for me about 20 years ago. It was 2 channels with 1 ecc83 in AF followed by 1/2 ecc83 in CF per channel. I modified absolutely everything but I kept the input transformers (OPEX187B) and found a complementary pair on ebay in order to mount a Quad pre as well as the power transformers (230/230 20VA • 230/12 16VA • 230/2*18 20VA). The pre is actually active with an AF circuit (12AY7 > 12AU7) and an Jfet ouput for lowering impedance. It offers 10dB (OEP) + 42dB (tubes) and it works great ! There is no gain level but an audio out level (after the 12AU7 > pot. 100KΩ log > Jfet ouput).
But I quickly encountered a problem: the HT transformer (20VA) was not keeping up, not being able to power 8 anodes without dropping in voltage. Not wanting to buy a transformer (because I wanted to reuse as many components as possible) I thought about setting up a "hybrid" 2 tube channels + 2 transistor channels. The advantage of such a preamp is that it offers 2 types of amplification depending on the sources to be recorded.
Currently, the transistor circuit is made of BJT BC550 & BC560 with an OPA at the input. It seems that this circuit is inspired (?) by an Accuphase circuit...
I would like to replace it with a jfet-based circuit, more "tube-sound-like" because the sound of the BJT circuit is not of much interest.
I know, however, that jfets are "capricious and delicate" to implement and I would like to get your advice and if possible a viable schematic.
I post below a schematic that I found on the net as well as the link leading to it. And also a link to the excellent article on Jfets by Rod Elliott.
Thanks in advance !
N1HFX • Mike Martell : https://studylib.net/doc/18599697/design-guidelines-for-jfet-audio-preamplifier-circuits-by...
Desingning With Jfets by Rod Elliott : https://sound-au.com/articles/jfet-design.htm
I recently recommissioned a mic preamp that a friend built for me about 20 years ago. It was 2 channels with 1 ecc83 in AF followed by 1/2 ecc83 in CF per channel. I modified absolutely everything but I kept the input transformers (OPEX187B) and found a complementary pair on ebay in order to mount a Quad pre as well as the power transformers (230/230 20VA • 230/12 16VA • 230/2*18 20VA). The pre is actually active with an AF circuit (12AY7 > 12AU7) and an Jfet ouput for lowering impedance. It offers 10dB (OEP) + 42dB (tubes) and it works great ! There is no gain level but an audio out level (after the 12AU7 > pot. 100KΩ log > Jfet ouput).
But I quickly encountered a problem: the HT transformer (20VA) was not keeping up, not being able to power 8 anodes without dropping in voltage. Not wanting to buy a transformer (because I wanted to reuse as many components as possible) I thought about setting up a "hybrid" 2 tube channels + 2 transistor channels. The advantage of such a preamp is that it offers 2 types of amplification depending on the sources to be recorded.
Currently, the transistor circuit is made of BJT BC550 & BC560 with an OPA at the input. It seems that this circuit is inspired (?) by an Accuphase circuit...
I would like to replace it with a jfet-based circuit, more "tube-sound-like" because the sound of the BJT circuit is not of much interest.
I know, however, that jfets are "capricious and delicate" to implement and I would like to get your advice and if possible a viable schematic.
I post below a schematic that I found on the net as well as the link leading to it. And also a link to the excellent article on Jfets by Rod Elliott.
Thanks in advance !
N1HFX • Mike Martell : https://studylib.net/doc/18599697/design-guidelines-for-jfet-audio-preamplifier-circuits-by...
Desingning With Jfets by Rod Elliott : https://sound-au.com/articles/jfet-design.htm