simple tube mic preamp

[LazyOne]

hi i was thinking of building a simple tube mic preamp.

the input transformer is a LL1538.
then i was thinking to use a 5760 tube. i don't know why but it sounds verry nice.
it has a moderate µ of 35. in practice i hope to get something like 30x
it is a twin triode so in total i would get like 900x gain (or almost 60db)
the LL1538 has a ratio of 5 that wil bring the total gain at 4500 or almost 74db.

my idea was to use standard input schematics and use 2 pads (-10db and -35db). in this way you can connect a standard line to it and use the -35db pad.
also an extra idea was to add an instrument input right after the transformer (for guitar or something).
so the input can switch to mic,mic -10db,line (-35db) ,instrument

then i was thinking to use feedback at the cathode of the first tube to make an attenuation knob and after the seccond tube a pot at the output to make an output control.
in that way it is possible to have a super clean signal from a guitar and also to distort it really bad but turn down the volume.


then the output stage. thats a bit difficult. altough the output doesn't really have to be 600ohm it maybe could be usefull for future use.
as i was thinking i wanted to use some pcl tubes but in cathode follower.
i don't want to use an output trannie (don't have money for it....yeah i'm a student of 20years old ). i've got the LL1538 quite cheap...
i prefer a balanced output.
the idea is to use 2 pcl82 (or a like) they have both a triode and penthode
the triodes can be used as a long tailed splitter and then the penthodes act as followers (at that point they'll act as triodes tough). and at the output a capacitor.

it is just an idea an you people have probably much more experience as me so what do you think.

 

[mikka]

English is too confusing.........The international language here is "Schematic" :green:

Edit: Ok: Inglish is too confusing for ME!

 

[rotation]

Hmm, some time ago i was thinking why PCL are not used more. I've seen them in tv's, some hi-fi pres, but can't remember of any pro audio application with them.
Can anyone please tell us more about this kind of tubes?
Regards!
Miha

 

[ciminosound]

PCL82 is a pentode and triode in the same bottle with a 16v heater. Similar to tube type ECL82/6BM8 which has been used in audio applications. Use the search function as I believe we've talked about the 6BM8 here before.

:sam:

 

[skipwave]

Hey Lazy,

Check out NewYorkDave's MILA preamp. Your basic description sounds like the MILA, which uses a combination of feedback and a volume control around a twin-triode. He paralleled another twin-triode to adequately drive a stepdown transformer into 600 ohms.

If you really want to drive the tubes hard, without excessive output levels, you can put a 1k volume pot after the output transformer and attenuate there. Just make sure that the pot can handle the wattage it will dissipate and that the following equipment won't be negatively affected by seeing a 1k impedance at its input.

Hope that's helpful.

Brian

 

[Guest]

Here is mine:

 

[skipwave]

Whoa! Transistor input to pentode with seperate screen supply, to MOSFET output with DC servo. I'm seeing all that at a glance.

He said simple. :razz:

Your design looks very interesting, but I have to ask, why use an input transformer to a transistor base, instead of straight to grid? Is this a requirement of using the DC servo amp loop connected there?

Thanks for sharing.

 

[Guest]

[quote author="skipwave"]Whoa! Transistor input to pentode with seperate screen supply, to MOSFET output with DC servo. I'm seeing all that at a glance.

He said simple. :razz:

Your design looks very interesting, but I have to ask, why use an input transformer to a transistor base, instead of straight to grid? Is this a requirement of using the DC servo amp loop connected there?

Thanks for sharing.[/quote]

You have very sharp eyes! :grin:

Yes, I was limited by transformers I have already.

The same about servo loop: I connected it directly to base, so no DC current ever flow through transformer/pad. Also, I used my favourite "Tango" combination of parallel source and degraded emitter follower.

 

[LazyOne]

hmm i try to search for the mila preamp but didn't find anything. anyone a schematic or so? maybe it's interesting.
the idea is to not use an output trannie (srry no money for that) thats why i wanted to try something with a pcl82 in kathode follower i think it might work.
and with a fasesplitter it can work balanced i think.

 

[skipwave]

Hmmmm, where'd MILA go?

Oh well, there are lots of other great design threads in the Microphone Preamplifiers META.
http://www.groupdiy.com/index.php?topic=16909

Take a look at this one:
http://www.groupdiy.com/index.php?topic=1948

The only working link for the design being discussed:
http://electronicdave.myhosting.net/miscimages/lineampE.gif

 

[analag]

So we have another hybrid guy in the camp. Welcome brother!
Now let's see what we have here...no parts values, the need for a less than simple PSU and my question is, is it worth it? The B+/B- then +15V/-15V scheme.

analag

 

[Guest]

[quote author="analag"]So we have another hybrid guy in the camp. Welcome brother!
[/quote]

Thank you! But I am not a hybrid guy, I eat everything that is tasty.

Now let's see what we have here...no parts values, the need for a less than simple PSU and my question is, is it worth it? The B+/B- then +15V/-15V scheme.

analag

Yes, if I have 16 channels with a tube in each, no problem to add one more power supply.

However, speaking of simple... If LazyOne needs one or two channels only, I'd recommend this one, from Skipwave's link: http://electronicdave.myhosting.net/miscimages/lineampE.gif

 

[NewYorkDave]

[quote author="skipwave"]Hmmmm, where'd MILA go?[/quote]

The original link was lost when Twin-X crashed, but MILA-1 is back online now.

LINK

 

[LazyOne]

ok, i recently got a pricelist for edcore transformers and damn they are cheap...
so i was rethinking of my idea and took another way.
i use a 1:5 input trannie that i have and on the output i'll take a 10k:600 trannie.
as a tube i stil want to use a 5670.
the total gain wil be in the order of around 60db. (almost 63db but count losses etc...).
i could use feedback to set the amount of gain with a rotary switch or something. and at the output a simple pot.
now the pot i wanted to implement just before the output trannie.
i'll try to make a schematic one of these days.. i also looked in the one bottle thread for schematics. quite interesting stuff.

i just looked at the mila again and it looks interesting. the only thing i'd probably do is making more steps (thinking about 12 steps or something).

 

[skipwave]

[quote author="LazyOne"]now the pot i wanted to implement just before the output trannie.[/quote]

That's been discussed at length here. That's a bad place for a pot. Especially if you are talking about a tube plate working into a big stepdown, like 10k:600. Try searching for "output pad" or "attenuator" or "RCA BC2B."

 

[LazyOne]

i've searched a little more and came to the mila again.
i could wire a 24 rotary switch as a gain switch. (just have to calculate some resistors...)
but then i could drop in a pot between the preamp and the sort of paralled output. a pot of say 100k log?
i don't think it would load the second half of the first tube to much.
on the second tube (parallel for output) i could use local feedback that would lower the gain of that stage but it would probably make the output drive better. altough 10k shouldnt be a problem
the advantage is that the first tube can do the most of the gain and it can be quite transparant (not to high gain= use of feedback) and i could make up by turning the output pot up. ok there are limits but it would work. and by turning the gain up i can sort of "color" the sound and turn the output pot down.
the second tube (with paralleld halves) has a little gain but has also feedback that kind of linearizes it so that it drives the output transformer well.
i'll try to put up a schematic later on the evening

 

[LazyOne]

ok i have tried to get something on paper. i do have to calculate some resistors but you get the idea and i'm just thinking if its possible
http://www.aman-nai.be/CO/scan.jpg
i hope it isn't to big.
ok it is to big changed in an url

 

[LazyOne]

btw has anyone have experience with those edcor transformers. i'm waiting on a reply now to confirm my order for some output trannies.
do they sound good?

 

[LazyOne]

ok i recently started building a version to try out.
it fits in a standard double EU rack and this is a test version so the looks are not that good probably. have to piant etc.... but first try the thing and then i'd probably let somoen build me 4 panels or so. a freind of me can do that easely for me
but for now just some pictures.

http://www.aman-nai.be/CO/PICT5822.JPG
the start by hand

http://www.aman-nai.be/CO/PICT5823.JPG
close up and yes i'm not so good at doing things straight

http://www.aman-nai.be/CO/PICT5824.JPG
after meausering and drilling

http://www.aman-nai.be/CO/PICT5825.JPG
some tools quite handy for big holes the left is just perfect for noval and the right for octal.

for the xlr connectors from neutrik i use the left thing and it then almost fits. jo just have to make way for some small round corners. after that it fits perfect and it stays there real tight and good. maybe ai will try to get a photograph of that.

http://www.aman-nai.be/CO/PICT5826.JPG
as it is now in the middle of the xlr connectors comes a jack connector but i have to go to the store monday or something like that
then the little holes are for switches (-20db, polarity and +48v phantom) and leds (one signal led probably, one clip led probably red and one led that indicates +48v i think orange or red for that one.)
i think i will go for 5mm leds. al those small 3mm things nah i do my own style .

the sideboard is a pcb that fits perfect in the eurorack casing i have with slides and then the front is fitted with some screws. sort of modular system. probably if i have the time (and the mony) there will be an eq/compressor etc.... all with tubes and not to forget a powersupply
the neat thing about the pcb is that its full of copper and i'm going to ground it and the metal eurorack is also grounded so that the sensible preamp is kind of protected from rf and other crappy things .
the powersupply i now use is borrowed from where i do some hifi amp construction to earn something. it is a choke loaded power supply that can supply like 100mA max @ 300v.

more coming soon. now doing the things where the tubes will stand on

greats LazyOne

 

[NewYorkDave]

I think the reason nobody has commented is because the circuit description is hard to follow and some parts of the schematic don't make sense. Why did you remove the feedback loop around V1B and V2 and replace it with plate-to-grid feedback around V2 only? What's the purpose of that variable shunt between those two stages? (That's about the worst possible way to implement a volume control, if that's the intention). Why is the original MILA circuit not satisfactory--have you even tried it yet?