My understanding is the LND150 is a mosfet. (N-Channel Depletion-Mode MOSFET)
Look up a data sheet on it. Please keep in mind that I have not built that schematic into a preamp. Perhaps someone more knowledgeable than I can make some
comment about it?
I personally would probably opt for three 12A_7 tubes so I could have a tube drive active FX loop and use a mosfet CF (IRF820) as a cathode follower to the active FX loop.
So the goal is to match impedances and not to add flavour of the MOSFET? That means that you’d also have to match output voltage levels. A tube will put out much more voltage than the power amp would require. That would at least something i’d Investigate too.
Secondly i’d Place the mosfet after the volume control so it doesn’t ever get overdriven.
I don’t know the output impedance of an lnd150, but the I’d think it’s far lower than any 12ax7 would do. Also keep an eye on frequency response of the follower due to capacitances of the MOSFET involved.
I tried my prototype the 3rd time. I used pots that are commonly used in the effects pedals. Is it okay or the rating of the pots for the tube preamps are higher? I found that the gain pot is always scratchy after a few days even if it is brand new?
My other question is, would i need a direct box when i hook this preamp to a Mixer or a simple class D power amp. Please help me. I'm no engineer at all. I need som light to my project. Thank you so much.
I have used small pots in amp builds when space was a concern with NO problems at all. Not saying this will work for you, but it's been fine for me. Not sure why your pots would be scratchy?
I have built a tube preamp with a 1/4" output jack and ran it into a direct box into a PA system and also used it that way into a EMu "recording" preamp that I use for recording into my computer.
I found the problem of the scratch. It is not the pot but a cold joints.
I'm sorry for not posting the schematics sir. My bad.
Thank you for the clarity and practicality of your answer 10thTx. You're very kind and a very bog help. I'll post an update soon sir. Thank you.
Must admit now: never experimented with DC coupled Source followers to triodes. Did AC coupled though. MOSFETS don't suffer from the compression a cathode follower would. Hence I'd not use them in say a Marshall to replace a cathode follower, but as a driver/last stage pf a pre amp they seem to do just fine.
The following is only relevant if you're still after a rather transparent MOSFET driver for your SS power amp. IF not, it's just ramblings.
Now I still don't really get why you'd want the MOSFET before a volume control if it is used as a driver. I see that on multiple schematics here. Could or could not be a problem, depending on cable length to the power amp (capacitance) and it's input impedance. What would the output impedance be with a 1 meg pot? 250k (500k//500k) max? I'd still place the MOSFET after any volume control to keep the output impedance low. That way it is a buffer as you want. Use a small MOSFET like a lnd150 or ZVN0545A, to keep impedance low and bandwidth high. Dump signal before the MOSFET: when looking at the normal non-HRM preamp, replace the master pot with a 100k one and place a 1 meg series resistor in front of it. With the right MOSFET the loss in highs should be minimal and you'd have a rather low output impedance (which relies on multiple factors though).
Like the article suggests I'd always go for small MOSFETS. Better for the high end bandwidth. Even better would be an OPAMP like a TL071 with an own supply (maybe single supply from the relays). But that would be quite another story altogether.
Mark, there are numerous examples of preamps with cathode followers at the output. From the top of my head, but correct me if I'm wrong: Bogner Fish, CAE 3+ (AC coupled), Soldano X88r (DC coupled). I can't see why you can't replace the cathode followers with source followers. On a technical level it would work. It can have different sonic properties, but that is something you can experiment with.
MOSFETS like the lnd150 aren't around that long. Since 2001 I believe. Many preamps were already on the market then. Take a look at the Engl e530 to see an approach with opamps. I find the solution for the line out in that preamp quite interesting and it should be great for driving a SS power amp.
rootz wrote: ↑Tue Oct 30, 2018 12:25 am
Must admit now: never experimented with DC coupled Source followers to triodes. Did AC coupled though. MOSFETS don't suffer from the compression a cathode follower would. Hence I'd not use them in say a Marshall to replace a cathode follower, but as a driver/last stage pf a pre amp they seem to do just fine.
The following is only relevant if you're still after a rather transparent MOSFET driver for your SS power amp. IF not, it's just ramblings.
Now I still don't really get why you'd want the MOSFET before a volume control if it is used as a driver. I see that on multiple schematics here. Could or could not be a problem, depending on cable length to the power amp (capacitance) and it's input impedance. What would the output impedance be with a 1 meg pot? 250k (500k//500k) max? I'd still place the MOSFET after any volume control to keep the output impedance low. That way it is a buffer as you want. Use a small MOSFET like a lnd150 or ZVN0545A, to keep impedance low and bandwidth high. Dump signal before the MOSFET: when looking at the normal non-HRM preamp, replace the master pot with a 100k one and place a 1 meg series resistor in front of it. With the right MOSFET the loss in highs should be minimal and you'd have a rather low output impedance (which relies on multiple factors though).
Like the article suggests I'd always go for small MOSFETS. Better for the high end bandwidth. Even better would be an OPAMP like a TL071 with an own supply (maybe single supply from the relays). But that would be quite another story altogether.
Is this how it should be? I think it is what Rootz pertains which i get the concept of making the mosfet after the master volume. making the mosfet follower as a somehow preamp/driver of a low impedance input of a poweramp.
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That arrangement of the MOSFET in your latest schematic should give you an interesting high end roll off. The 4n7 cap at the gate after the 220k//470p series network limits the high end severely.
I take it you tried to emulate the recovery side of a dumbleator with a MOSFET? If so you should take a close look of the Miller capacitances involved and try to emulate that.
I enclose two schematics. The top one is an idea I'm tampering with now, although my preamp is more a mix between classic low plate clean stage and HRM OD. The last two triodes are the ones of interest here though. First triode is half Dumbleator, the second triode a simple AC coupled CF. Output impedance should be well below 1k.
The second schematic is what I meant as solution to match impedances, preserve high end and get the levels down to something more friendly for SS amps (assuming line level is needed). With the level control full up, it will still push almost 10 V peak at the output. So it's usable for at least the first 50% of its rotation. Please notice I replaced the 1 meg level control with a 100k log one, preceded by a fairly large series resistor to create a voltage divider and dump a load of signal. With a 33k lower resistor in the SF the source should be 30V above ground. You can use higher value resistors, dropping some more voltage, which in turn isn't wasted as heat in the MOSFET.
I see your design doesn't use a protective diode at the gate. I routinely do that. Never blown a MOSFET since I started that.
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