Ok I didnt know that PSRR thing
Yes there Will be CF after the cascade. Do you mean FB from the output of CF to the gate of the jfet?
Valve mic preamp design incoherent rambling
- Thread starter GussyLoveridge
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Yes there Will be CF after the cascade. Do you mean FB from the output of CF to the gate of the jfet?Winston OBoogie
Well-known member
Ok I didnt know that PSRR thing
I didn't know or think about it either, but Merlin certainly knows his stuff so, try it.
Yes FB (if you need or want it) from the CF to the gate, but via the bottom of your IP transformer. An example would be the V72A although you could go lower values depending on your CF drive ability.
P.S. Do you have 2SK170's?
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Ok
But the cascade is double inverting and CF are non inverting dosnt that make positive FB or am I rambling?
Ok I got that under the transformer thing
But the cascade is double inverting and CF are non inverting dosnt that make positive FB or am I rambling?Winston OBoogie
Well-known member
The Cascode is just a single inverting stage
Edited my brain fart typo.
Edited my brain fart typo.
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Now I dont have any 2sk170 but banzaimusic got them....Ok
But in the v72 first stage is inverting second stage are not. I got 2 non inverting stage that messes with the feedback right?
Winston OBoogie
Well-known member
OK maybe I misunderstood your configuration. I thought the cascode just fed a cathode follower yes? Or is it feeding another stage too.
The top device of the cascode - ie. the 6SK17 in your case - is non-inverting if that's what you were referring to so: 2SK170 is inverting, 6SK17 is non-inverting, cathode follower is non-inverting = inverting stage.
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Winston OBoogie
Well-known member
Ah, OK sorry. I thought that was a separate stage or idea.
IOW, I thought that the cascode and CF were a separate item.
So you're just using the CF to avoid miller in the 6n6p common emitter stage then yes?
I probably need to see a complete schem, but if there's no global FB loop around the cascode and 6n6p then you could still take some signal from the CF as FB under the IPT if you needed to trim things.
Anyway, sorry if I got the wrong end of the stick
IOW, I thought that the cascode and CF were a separate item.
So you're just using the CF to avoid miller in the 6n6p common emitter stage then yes?
I probably need to see a complete schem, but if there's no global FB loop around the cascode and 6n6p then you could still take some signal from the CF as FB under the IPT if you needed to trim things.
Anyway, sorry if I got the wrong end of the stick
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I can draw a complete schematics tomorrow so get idea of the thingAh, OK sorry. I thought that was a separate stage or idea.
IOW, I thought that the cascode and CF were a separate item.
So you're just using the CF to avoid miller in the 6n6p common emitter stage then yes?
I probably need to see a complete schem, but if there's no FB loop around the cascode and 6n6p then you could still take some signal from the CF as FB under the IPT if you needed to trim things.
Anyway, sorry if I got the wrong end of the stick
Winston OBoogie
Well-known member
Just checked and the price for the BL grade K170 from Banzai is good @ €1,01 each. Cool Beans 
Think it over...
Winston OBoogie
Well-known member
Think it over...
Oops. Typo brain fart. Honestly. I meant to write inverting, otherwise I wouldn't have suggested that FB scheme. I'll edit it quickly. Thanks
Here, earlier I said this:
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I know, it's past bed time for us old geezers...
Winston OBoogie
Well-known member
I know, it's past bed time for us old geezers...
I know. I was gonna go but figured I'd only be up again in 2 hours to pee so, no point really!
Thanks for catching the error though before too many saw it!
I'm still following when I can and loving all of this folks. I really appreciate it. My wife just had a baby so pretty much everything in my life is on pause until the little dude makes it out of the ICU. All good though.
Is it reasonable to think that I can pull an old power transformer from an old guitar amp and repurpose it to build a decent linear power supply? I am thinking that as long as the transformer satisfies the voltage and current requirements I should be fine right. It's mostly the design of the supply that will ensure a quiet supply? I think that I might have better luck with a toroidal transformer but a used one should get me at least prototyping right?
I know there is a whole lot to the design consideration from the power supply as well. So I won't go there for now. I feel like I have a reasonable understanding. Would you recommend just picking up a low voltage filament supply transformer right off the bat? They should be inexpensive enough right?
Is it reasonable to think that I can pull an old power transformer from an old guitar amp and repurpose it to build a decent linear power supply? I am thinking that as long as the transformer satisfies the voltage and current requirements I should be fine right. It's mostly the design of the supply that will ensure a quiet supply? I think that I might have better luck with a toroidal transformer but a used one should get me at least prototyping right?
I know there is a whole lot to the design consideration from the power supply as well. So I won't go there for now. I feel like I have a reasonable understanding. Would you recommend just picking up a low voltage filament supply transformer right off the bat? They should be inexpensive enough right?
Winston OBoogie
Well-known member
Oh wow, congrats to you and your wife Gus, hope your little dude gets out of ICU soon.
That'd be fine I think. The only issue with something like that is that it'll probably be 6V3 AC which doesn't really give you enough headroom to implement DC. A pre amp is one place you really need very low hum and DC makes that so much easier.
Yep, pretty much it.
As an alternative, there are inexpensive R-Core transformers made in China you can find on Ebay that have a lot of the benefits of a torroid in terms of low noise radiation.
Failing that, just keep your E-I type guitar amp transformer away from sensitive areas of the circuit. Distance is your friend.
If you're going to go with a separate supply for heaters then sure, just get one that has a bit of extra voltage margin for DC is my personal recommendation.
Quite a few folks use an inexpensive switching supply for heaters so you could consider that maybe.
Regardless, none of this is that important when compared to your new baby so, just check in when you have time, funds, or questions,
take care
Congrats, man!
Here we go a new schematics with FB.I can draw a complete schematics tomorrow so get idea of the thing
