Valve mic preamp design incoherent rambling

[solkatten]

OK got it. I completely misread your earlier post, thought your chokes were 20H.

Voltage across the J-Fet is fine. On this, you could try degenerating the J-Fet a little if gain's too much.

Your next stage, is cathode follower right?

A little feedback from there returned under the input transformer to trim gain?

Edit: Merlin Blencowe states on another site that PSRR will be better without the cap across the lower resistor of the voltage divider

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?

 

[Winston OBoogie]

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 there Will be CF after the cascade. Do you mean FB from the output of CF to the gate of the jfet?

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?

 

[solkatten]

Ok

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?

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]

The Cascode is just a single inverting stage

Edited my brain fart typo.

 

[solkatten]

The Cascode is just a single non inverting stage

Ok

 

[solkatten]

Ok

Now I dont have any 2sk170 but banzaimusic got them....

 

[solkatten]

The Cascode is just a single non inverting stage

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]

But in the v72 first stage is inverting second stage are not. I got 2 non inverting stage that messes with the feedback right?

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.

 

[solkatten]

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.

1/2 6n6p CF second half anode choke loaded gain stage
Thanx a lot for great

 

[Winston OBoogie]

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

 

[solkatten]

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 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

I can draw a complete schematics tomorrow so get idea of the thing

 

[Winston OBoogie]

Just checked and the price for the BL grade K170 from Banzai is good @ €1,01 each. Cool Beans

 

[abbey road d enfer]

The Cascode is just a single non inverting stage

Think it over...

 

[Winston OBoogie]

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:

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.

 

[abbey road d enfer]

Typo brain fart, honestly. I meant to write inverting,

I know, it's past bed time for us old geezers...

 

[Winston OBoogie]

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!

 

[GussyLoveridge]

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?

 

[Winston OBoogie]

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.

Oh wow, congrats to you and your wife Gus, hope your little dude gets out of ICU soon.

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?

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.

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?

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.

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?

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

 

[abbey road d enfer]

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.

Congrats, man!

 

[solkatten]

I can draw a complete schematics tomorrow so get idea of the thing

Here we go a new schematics with FB.