Mic Tube Preamplifier Power Supply transformer/transformers question

[kagliostro]

Whith the help of Butterylicious I have a project for a Tube Mic Preamplifier

(  http://www.el34world.com/Forum/index.php?topic=8860.0  )

I've some problems about the PS transformer/transformers

I do not succeed to find of one enough small (in current abilities and more in dimensions)

the voltage I need are for filament (6.3v 600mA or 12v 150mA EDIT 300mA)

the phantom power (48v 10mA)

the plate voltage (330v EDIT 305v 10/15mA - two tubes 12ax7 & 12AU7)

EDIT Hi Voltage transformer 330v-0-330v = 435v rectified

For the filament I've a 9v (for PCB) transformer and think to use a LM317 to have a filament current (not voltage) supply

For 48v I've a 48v (for PCB) transformer and think to use a  TL783C circuit

about 330v (which is the voltage Butterylicious established) I've some problems

I'm not able to find a transformer so small to be put in my box (the requested current is small and the transformer can be small, but not able to find one - I've necessity for a low profile, max near 5 or 5,5 Cm)

This is the question:

If I use two little transformers connected back to front (PCB low profile transformers)

220v / 48v - 48v / 220v

after rectify I can obtain only near 308v

if I put a doubler I obtain 440v (min I think) that seems to me too much for a bleed resistor (or not,  considering low consumption ??)

if I use two transformers

110v+110v / 48v - 48v / 110v+110v

and in one of the 110v output I put a tripler I think I can have the requested 330v

But I'm not sure about what is best to do considering that the circuit have a low consumption and that I don't want to have a noisy supply

also can I take the 48v in the middle without affect phantom with noise ?? (need only 10mA max)

Please can give me help to decide about that ?

Many Thanks

Kagliostro

[nateflanigan]

Hi, I can't exactly answer all of your questions but I can help a little...

Stepping down the voltage, splitting it over to a low voltage supply and another transformer then stepping it back up for a high voltage supply is totally possible.  The Gyraf pultec uses a set up like that, I have built one and it sounds great, and very quiet.  I can't say how that would jive with phantom but with enough filtering I guess it'd be fine.

How small of an enclosure are trying to use?  I built  the MILA (search for it at "The Lab" over at prodigy pro), it's a single channel tube pre that by all accounts is pretty similar to the UA 610.  I had alot of fun building it, and it was really cheap too.  I built it in a box that's about 12"x12"x4", also very quiet.  Still haven't gotten around to adding phantom though.

[kagliostro]

Hi nateflanigan

my box is 60 mm tall (2.36")

I know the Gyraf G9 project

http://www.gyraf.dk/gy_pd/g9/g9pd.htm

and his way to obtain voltages with two transformers

but that project has a lower B+ voltage than those of Butterylicious

Kagliostro

[bibi]

Hi Kagliostro,

I use this one :  http://www.musikding.de/product_info.php/info/p1361_Toroidal-transformer-230V----250V--9V--24V.html

It should suit your needs just fine and it's available in Europe.  The owner of the shop is very easy to deal with and answers for all the products he sells.
The voltage for the phantom +48 needs to come from the 24V winding then through a doubler-- easy to do.  If you need a schematic for that let me know...

[kagliostro]

thanks bibi

that is an interesting transformer

but may be I've not explicated correctly my need

in the project Butterylicious used a 330v-0-330v transformer

http://www.el34world.com/Forum/index.php?action=dlattach;topic=8860.0;attach=13612

than edit then the rectified voltage was 430v

he used two bleed resistor in series to obtain a high filtered 305v

so, as I can understand, the question is if a doubled 220v will give a 440v that can be bleed by the resistors till 305v without put noise in the circuit due to the doubler

I'm not so experienced and in my mind doubled and tripled voltages are noisy or they are more noisy than the same voltage obtained without a doubler or a tripler circuit

Am I totally wrong ???

don't know, can give me info about that ?

THANKS

Kagliostro

[bibi]

I wouldn't worry much about finding a 330-0-330 transformer.  If you need to drop less voltage then you can increase the capacitance to compensate.  You might find the voltages aren't exactly as in the schematic anyhow.
In terms of doubling voltages, I meant the phantom power supply (or 48 volts) to power the condenser microphones.  This supply comes from the 24VAC supply on the transformer. 
 

[sluckey]

The transformer bibi suggests has a 250v winding. Just put a full wave bridge on that and you will have 350vdc (unloaded). Should be easy to get down to 305 with a simply CRC filter.

[kagliostro]

for bibi

I have understand you about the doubled 24v to have the 48v for phantom

and I'm not looking to a 330v-0-330v transformer, I think that a 0-330v will be fine the same way

thanks sluckey

the answer question is, if I use a doubled 220v to obtain a rectified 440v and then use the bleed resistors

in terms of operation is the same as to have the original project supply ?

( http://www.el34world.com/Forum/index.php?action=dlattach;topic=8860.0;attach=13612 )

Kagliostro

[sluckey]

if I use a doubled 220v to obtain a rectified 440v and then use the bleed resistors in terms of operation is the same as to have the original project supply ?

You could do that, but why complicate things. Just use the 250v winding connected to a bridge to give 353vdc. Remove C8 and R16. Connect the bridge positive output to the junction of C9 and R17. Notice that juction has 355v on it? Now R17 and R bleed should give exactly what you want. If not exact enough, just change the value of R bleed up/down to suit.

[RicharD]

I agree with Slucky.  the bridge rectifier is the way to go.

Question:
How far are you into this build?  I have been focused on mic pres for the past couple of months.  After a number of clues from PRR, I think I finally had an epiphany.  The next design (which I have not posted yet) I think is gonna be the good one.

There are some issues with the design I came up with for you.  I'm certain the circuit will work and will sound OK.  I'm shooting for sounds great.

Please let me know where you are sitting with the project.  Tell me what you have purchased and I'll work around that.  Also let me know what you've built.  Pictures will be helpful.

-Richard

[kagliostro]

Hi Richard

I've near all parts for building the pre

The case was a big PC Hub and I got some aluminum plates to obtain a rack mount and new back panel , inside shields and mounting stuff

I've the input and output transformers (from RS those you indicated to me)

http://it.rs-online.com/web/search/searchBrowseAction.html?method=browseSubRange&Ne=4294583802&N=4294571284&productNum=2106352

http://docs-europe.origin.electrocomponents.com/webdocs/0d72/0900766b80d72893.pdf
http://it.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6676035

(it was a very short wait - only few days - for the input,  but a very very long wait to get the output, real a pain)

I've turret board - Tube sockets - tube shields

some transformers for phantom and filament - caps - resistors - pot - LM317 - TL783c - heat sinks

BUT at the moment I can take every new way project without problems

only I'll prefer don't change the input and the output transformers if possible and not strictly necessary

the other stuff I can use in other way

So I'm here "hanging from your lips" and waiting directives on what to make

 

have a nice day

Franco

[RicharD]

Cool!  We can totally work around what you have.  I'll crunch some number tonight and get you a new schematic.  The circuit I'm currently experimenting with uses octals (6SL7 & 6SN7) but it can easily adapt to using a 12AX7 & and 12AU7.  I'm starting another thread about my current circuit and hopefully PRR will chime in.