Jfet pre

[Emmathom]

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

 

[Jarno]

That's a general purpose preamp, not a dedicated mic preamp. I would say try to find a schematic for a dedicated mic preamp.
Either something Neve 1073 based, or API.

 

[rock soderstrom]

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.

Check this one, you will find a lot of info for the Hamptone JFET mic pre on groupdiy.

 

[Emmathom]

That's a general purpose preamp, not a dedicated mic preamp. I would say try to find a schematic for a dedicated mic preamp.
Either something Neve 1073 based, or API.

That's a Line In (Ω) pre designed for hi-fi I guess.
Neve 1073 does not use Jfet but BJT and most of "the sound" comes from Tr In & Out > I don't intend to use Tr Out but a Jfet output follower...

 

[Emmathom]

OEP X187B

 

[Emmathom]

Check this one, you will find a lot of info for the Hamptone JFET mic pre on groupdiy.

Very intersting ! Thanx Rock Soderstrom ! Will study this with attention as it seems "simple" to implement...

 

[Emmathom]

Very intersting ! Thanx Rock Soderstrom ! Will study this with attention as it seems "simple" to implement...

I've got in stock a 2*12V 30VA R-core so I think that I've got much enough current wired in serie... ? 30/24 = 1,25A
No that's not enough voltage : I can use a 2*18v • 12VA (330mA)

 

[Emmathom]

@rock soderstrom : since the 2N5457 seems to be out-of-stock at Mouser's could you please give a equivalent ref. Jfet ? (I've got MPS102 & J113 in stock)

 

[rock soderstrom]

@rock soderstrom : since the 2N5457 seems to be out-of-stock at Mouser's could you please give a equivalent ref. Jfet ? (I've got MPS102 & J113 in stock)

I think you can use other similar JFETs but I would stick to the 2N5457. Buy more than you need, because you have to select them by hand.

This is a good source:

https://www.musikding.de/2N5457

This one is cheaper, but I never bought from them:

https://www.amazon.de/Chanzon-2N5457-N-CHANNEL-Feldeffekt-Transistor/dp/B08M3YJR9Z

 

[rock soderstrom]

I've got in stock a 2*12V 30VA R-core so I think that I've got much enough current wired in serie... ? 30/24 = 1,25A
No that's not enough voltage : I can use a 2*18v • 12VA (330mA)

2x12VAC is ok, why not? If you rectify it you have enough margin for a good regulation (LM317, LM7824 or similar)

You need roughly 40mA per gain block, so a 2 channel mic pre with 4 gain blocks needs at least 160mA.

If you need 48V phantom power, you can use a voltage doubler or tripler.

 

[rock soderstrom]

another good source for 2N5457

http://www.banzaimusic.com/2N5457.html

 

[Emmathom]

2x12VAC is ok, why not? If you rectify it you have enough margin for a good regulation (LM317, LM7824 or similar)

You need roughly 40mA per gain block, so a 2 channel mic pre with 4 gain blocks needs at least 160mA.

If you need 48V phantom power, you can use a voltage doubler or tripler.

48v is already implemented (the tube & the BJT channels are already operational).
Yes (2*12v!)*1,414 = more-or-less 38v DC under 1A

I suppose I need to buy at least 20-24 Jfet to get 2 pairs with same IDSS... ?
The problem of living in France is the shipping cost > so I like to buy at the same place to combine shipping.
For ex. Mouser's shipping is 20€ to France...

Banzai has 2N5487 and also MPSA14 but no ZTX653 in stock
Maybe TIP122 will do ?

 

[Emmathom]

@rock soderstrom I just read your intervention in https://groupdiy.com/threads/looking-for-recommendations-preamp-a-perfboard-edit-fetboy.70359/page-2
May I ask you how you get "tons of colour" ? by different R source capacitors values ?
You said : My advice is the Hamptone JFET Preamp aka Fetboy. Easy to build, tons of colour as micpre and DI for instruments.

 

[Emmathom]

I redraw the Hampton schematic... but I have a few questions :
- is the 100nF as signal input blocking cap is ok ?
- is the output at a sufficient low impedance to avoid any adaptative circuit ? it's gonna "feed" a Line In sound card (10KΩ)
* I think 100KΩ at output after R100 is better than 10KΩ (?)

 

[jokeramik]

I redraw the Hampton schematic... but I have a few questions :
- is the 100nF as signal input blocking cap is ok ?
- is the output at a sufficient low impedance to avoid any adaptative circuit ? it's gonna "feed" a Line In sound card (10KΩ)
* I think 100KΩ at output after R100 is better than 10KΩ (?)

Why do you use a 10µF bipolar elektrolytic at the output. At the junction with the two transistor are +12Volt i think. What are measuring?
Best regards!

 

[Emmathom]

Why do you use a 10µF bipolar elektrolytic at the output. At the junction with the two transistor are +12Volt i think. What are measuring?
Best regards!

Hampton insists on this value but a 10uF MKP is big, almost bigger than the whole board
Yes there are 12v at the 2 "circuits" junction so I must bypass DC :
- at Pot 10KΩ (-3dB at 3Hz) = 5,30uF (4,7uF is ok I guess)
- at 100KΩ (output to ground) 530nF (-3dB at 3Hz) > so 680nF or 1uF is enough
... I must put 100KΩ to ground at output instead of 10KΩ
> so with these values I can easily put MKP capacitors (Wima at least)
What do you think ?
BR

 

[Emmathom]

BP Muse Nichicon are very good quality for coupling caps in audio and often used

 

[k brown]

I think John Broskie had some ideas about using J-FETS or MOSFETs in some of his circuits originally disigned for tubes. Glass Audio or Tube CAD Journal articles; don't know specific issues.

 

[Emmathom]

I think John Broskie had some ideas about using MOSFETs in some of his circuits originally disigned for tubes. Glass Audio articles; don't know specific issues.

The information about Scott Hampton given by @rock soderstrom seems very interesting since it's a simple and proven schematic... I would stay on this circuit unless someone advise against it

 

[k brown]

The information about Scott Hampton given by @rock soderstrom seems very interesting since it's a simple and proven schematic... I would stay on this circuit unless someone advise against it

Yep. I built the Hamptone a few years ago - sounded quite nice for such a simple circuit; worth exploring.