#26 pre amp - Page 125 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th October 2011, 02:07 PM   #1241
iko is online now iko  Canada
diyAudio Moderator
 
iko's Avatar
 
Join Date: May 2008
Location: Toronto
In case people are interested, this is my filament supply for my 26 preamp. My aim was to get as ripple free constant current through the filament as possible, a high impedance looking from the filament to the power supply, and a low impedance looking to the bias resistor/ground.

For what it's worth, here's a simulation of the current for various values of L2: 1mH, 10mH, 30mH, 50mH, and 100mH (1R DCR, 50pF parallel capacitance). The green line is for 1mH, and the rest go lower in dB as the inductance gets higher.

The third plot shows the voltage across the filament; changing the dB values positive would give the line rejection of this circuit.

I am aware that reality and simulation are not the same; the simulation results are here only for illustration purposes.

The exact values for R3 and R6 should be chosen in real life via measurement. But good values to start with would be so that the parallel value of R3 and R6 be equal to sum of Q1 base-emitter voltage drop (approx. 0.65V) and the voltage drop across the LED (in my case approx. 2V), divided by the desired current through filament supply (in my case I aimed for about 1.045A).

I cannot stress the importance of using large heat sinks for the power mosfets. And yes, the large voltage drop across the mosfets is necessary, so that they can do their job properly.

Edit: oh, I forgot to mention, 100R-220R gate resistors for the power mosfets M1 and M2 may be employed if necessary.
Attached Images
File Type: png psu-filament-26-3b.PNG (38.5 KB, 559 views)
File Type: png psu-filament-26-3b-current.PNG (26.9 KB, 538 views)
File Type: png psu-filament-26-3b-linerej.PNG (20.5 KB, 513 views)

Last edited by iko; 5th October 2011 at 02:22 PM. Reason: Omission
  Reply With Quote
Old 5th October 2011, 02:14 PM   #1242
diyAudio Member
 
merlin el mago's Avatar
 
Join Date: Sep 2009
Location: Catalonia - Europe
Thanks for sharing Iko
  Reply With Quote
Old 5th October 2011, 02:24 PM   #1243
работник
diyAudio Member
 
Rod Coleman's Avatar
 
Join Date: Jul 2004
Location: Warwickshire UK
Quote:
Originally Posted by ikoflexer View Post
And yes, the large voltage drop across the mosfets is necessary, so that they can do their job properly.
This is the first reason why MOS pass devices are not used in my 26 filament supply! There are other problems with them. But 60W of waste heat for 2x 1.5W filament (stereo) is too much for most of us.
  Reply With Quote
Old 5th October 2011, 02:39 PM   #1244
iko is online now iko  Canada
diyAudio Moderator
 
iko's Avatar
 
Join Date: May 2008
Location: Toronto
Right, Rod. I could make it more efficient, and it would be quite easy to redo the circuit with power bjts, but the performance would not be the same. Even so, lowering the input voltage to 25V (about 10V across the mosfets) would still give a very reasonable 90dB reduction in current ripple. But yeah, it's just another option.
  Reply With Quote
Old 8th October 2011, 07:17 PM   #1245
mogliaa is offline mogliaa  United Kingdom
diyAudio Member
 
mogliaa's Avatar
 
Join Date: Dec 2009
Location: London
Slowly making some more progress on the supplies. Today I finished the two LT filament raw supply boards to deliver 18V with minimum ripple. Using PSU designer and recommendations from Rod Coleman, I came about with a circuit that delivers 18V and 30mV ripple at full load (1.05A).

I will stack the boards in the supply chassis to save some space. Now I need to work on the HT supply. That will be interesting!

BTW: I found that the noise I have was the HT bench supply. Will see what happens when I get my final HT.
Also did a THD test today and got THD=0.03% @ Vi=1Vrms and Vo=1.84Vrms with a CX-326... nice!

Cheers,
Ale
Attached Images
File Type: jpg IMG_0785.jpg (577.0 KB, 477 views)
__________________
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
http://www.bartola.co.uk/valves/
  Reply With Quote
Old 9th October 2011, 09:53 AM   #1246
diyAudio Member
 
merlin el mago's Avatar
 
Join Date: Sep 2009
Location: Catalonia - Europe
Congrats Ale, very neat work, this circuit is the PSU for Rod Coleman heaters?

Felipe
  Reply With Quote
Old 15th October 2011, 10:46 AM   #1247
mogliaa is offline mogliaa  United Kingdom
diyAudio Member
 
mogliaa's Avatar
 
Join Date: Dec 2009
Location: London
Yes Felipe. You can get very clear instructions from Rod Coleman to design your raw power supply circuit to use Rod's boards. Mine have to be 18V as I'm using filament bias.

Have breadboarded the HT PSU with AZ1 and CLCLC smoothing stage succesfully. Now will crack on the Salas shunt regulator.

Also finished the pre-amp chassis with a nice wood touch. See attached....

Cheers,
Ale
Attached Images
File Type: jpg IMG_8345.jpg (81.7 KB, 358 views)
File Type: jpg IMG_8349.jpg (68.6 KB, 186 views)
__________________
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
http://www.bartola.co.uk/valves/
  Reply With Quote
Old 15th October 2011, 10:53 AM   #1248
diyAudio Member
 
merlin el mago's Avatar
 
Join Date: Sep 2009
Location: Catalonia - Europe
Very nice construction with Andy's touch, do you have space inside the box for Salas HV reg? I also have AZ1 and I will do CLC before the Salas HV that I have.
  Reply With Quote
Old 15th October 2011, 12:11 PM   #1249
mogliaa is offline mogliaa  United Kingdom
diyAudio Member
 
mogliaa's Avatar
 
Join Date: Dec 2009
Location: London
Not at all, the Ll1660 use most of the space. Either way, I haven't built a complete separated chassis for LT and HT supplies. Salas regulator will be on the supply chassis
__________________
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
http://www.bartola.co.uk/valves/
  Reply With Quote
Old 15th October 2011, 12:24 PM   #1250
massimo is offline massimo  Italy
diyAudio Member
 
Join Date: Dec 2002
Location: Milan, Italy
Quote:
Originally Posted by mogliaa View Post
Salas regulator will be on the supply chassis
Not the best solution IMHO. PS chassis should contain LV and HT raw supplies only.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 03:25 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2