All,
I am planning to build Eli Duttman's Phono Preamp, which is, as I understand it, an update and modification of the classic RCA phono preamp. I already have many of the parts from when I purchased parts to build RJM's 6dj8 phono (does everyone else always buy a few extras?), but I have a few questions before putting in a final order. I would be grateful for any advice and help in clarifying:
Best,
Dan
I am planning to build Eli Duttman's Phono Preamp, which is, as I understand it, an update and modification of the classic RCA phono preamp. I already have many of the parts from when I purchased parts to build RJM's 6dj8 phono (does everyone else always buy a few extras?), but I have a few questions before putting in a final order. I would be grateful for any advice and help in clarifying:
- Are there any suitable substitutes for the ZVN0545A Mosfet? I'm not seeing any in stock at Mouser, Digikey, etc. I could search for alternate devices, but I just don't know what characteristics are necessary (polarity, Vds, Id, Vgs, etc.?)
- It looks to me like there are 2 coupling capacitors on the output -- one at 0.47uf and one at 5uf. Is that correct? I don't think I've seen that before. Could anyone help me understand why it's done this way in this amp?
- How critical are the values of 30uf of the capacitors connected to the B+? Would 32uf be ok? I see Parts Express has a cheap electrolytic can capacitor w/ two 32uf caps inside. Would that work? Is using a electrolytic here a bad idea? Also, would it be OK to use one of these capacitors on the cathodes of the first triode stages?
- Generally, I'm not sure where and when to use the different types of capacitors (electrolytic, film, polypropylene, paper in oil, etc.) and where it makes sense to spend a bit more money. I would be grateful if anyone could point me in the direction of some useful readings on the matter. My initial searches didn't find much.
Best,
Dan
Re: Sub for ZVN0545 --
There's no exact sub that I know of, but what you want is a high voltage MOSFET with a very low Crss. The 12AX7 runs with very low plate current (about 500uA to 1mA) so it cannot sink much current into any capacitance loading its plate (output). I used a humble FQPF2N60 and it seemed to work well enough (I made sure the MOSFET had at least 5mA Ids going through it to keep its Crss in a lower region).
Paralleling coupling capacitors is done to reduce equivalent series resistance at high frequencies... I think. Actually, I'm not 100% sure it's a good idea. Try the paralleled 0.47uF and 4.7uF caps, or just use a 4.7uF by itself. See if you can hear a difference. Stick with the config that sounds best to you.
Subbing 32uF for 30uF is fine. Or use 33uF. That's fine too. Or 39uF, or even 47uF. The exact value is not critical.
For reading on capacitor types, and a whole lot more useful information, check out Morgan Jones' "Valve Amplifiers - 3rd Edition" or "Valve Amplifiers - 4th Edition", or Merlin Blencowe's "Building High Fidelity Tube Preamps". Both have good chapters explaining how passive parts are made and the advantages/disadvantages of the various types.
There's no exact sub that I know of, but what you want is a high voltage MOSFET with a very low Crss. The 12AX7 runs with very low plate current (about 500uA to 1mA) so it cannot sink much current into any capacitance loading its plate (output). I used a humble FQPF2N60 and it seemed to work well enough (I made sure the MOSFET had at least 5mA Ids going through it to keep its Crss in a lower region).
Paralleling coupling capacitors is done to reduce equivalent series resistance at high frequencies... I think. Actually, I'm not 100% sure it's a good idea. Try the paralleled 0.47uF and 4.7uF caps, or just use a 4.7uF by itself. See if you can hear a difference. Stick with the config that sounds best to you.
Subbing 32uF for 30uF is fine. Or use 33uF. That's fine too. Or 39uF, or even 47uF. The exact value is not critical.
For reading on capacitor types, and a whole lot more useful information, check out Morgan Jones' "Valve Amplifiers - 3rd Edition" or "Valve Amplifiers - 4th Edition", or Merlin Blencowe's "Building High Fidelity Tube Preamps". Both have good chapters explaining how passive parts are made and the advantages/disadvantages of the various types.