A Bride for a Beast? - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 27th November 2009, 02:37 AM   #1
diyAudio Member
 
Johnloudb's Avatar
 
Join Date: Feb 2008
Default A Bride for a Beast?

I'm building a JFET preamp with a Transformer based volume control at the output. I'm not saying this is a "bride" to Nelson's beast amp, but it is clearly inspired by his design. As always, many thanks to Pass for sharing these great DIY designs/ideas! I think this might turn out quite good, if I can get the transformer right.

Click the image to open in full size.

Here is the modeled response of this preamp. Doesn't look like it can provide the 32 mA of current needed below 100Hz. The transformer primary is 100mH and it will have a switch to provide an optional +6 dB of gain. To get 1mH takes 75 turns. So, 100mH is 7500 turns. And the secondary requires 14.500 turns! Threading that much wire through a toroid is going to be a real pain.

Might need more turns or more JFETs for this. I don't want a high output impedance. The JFETs are LSK170C but I plan on using BF862 JFETS.

Primary current with 1 volt peak applied:
Click the image to open in full size.

Frequency response at output of transformer:
Click the image to open in full size.

I don't think that the simulated response is really accurate though. Shouldn't it have more bandwidth? The toroid is powdered iron #26 mix with a bandwidth from DC to 500kHz.

I don't know much about transformer design so any helpful comments are welcome!

I might consider an EI type transformer core if I can find a good one. As I understand those transformers should be laminated to reduce eddy current loss and distortion. But, aren't toroids better, but the more difficult way to go?
Attached Images
File Type: png Beast_Preamp_Schem.png (6.3 KB, 325 views)
File Type: png Beast_Preamp_Iout.png (3.3 KB, 323 views)
File Type: png Beast_Preamp_Response.png (3.4 KB, 332 views)

Last edited by Johnloudb; 27th November 2009 at 02:46 AM.
  Reply With Quote
Old 27th November 2009, 08:27 AM   #2
diyAudio Member
 
Join Date: May 2007
Hey John,
Don´t think this is the best of ideas. At 20Hz your 100mH has a reactance of 13ohm! Place a descent transformer at the input instead. Probably a 1:2 transformer with at least a primary inductance of 100H would do. You can the dump all except one to three pairs of FETs and add a Slagle AVC at the output.
If going the "transformer after" route I´d advise you go for a commercial TVC like Silk. Think it can take 6V and also has a +6dB winding.
  Reply With Quote
Old 27th November 2009, 02:36 PM   #3
diyAudio Member
 
Johnloudb's Avatar
 
Join Date: Feb 2008
Okay, thanks, that would work. But I'm kind of set on doing my own transformer. 10 JFETs in parallel is fine with me. And, I'll do some experiments and raise the impedance of the transformer. Air core transformers are more linear ... maybe I'll just wind a bunch of wire on a spool to get a higher primary inductance. Take more wire but it'd be easier than treading a toroid. The coupling coefficient for air core is like 0.4 I think, so would mean a higher impedance at output, but still less than a commercial volume control I think.

Any concerns about that?

If that's also a bad idea, I may put a good step up transformer at the input, like you mention, and put a 1k pot at the output for volume control. I would only need the gain for LP playback, so I would have a switch to bypass the transformer.
  Reply With Quote
Old 28th November 2009, 11:50 AM   #4
diyAudio Member
 
Join Date: May 2007
Simple idea: Why not increase your phonostage gain to match your other sources?
  Reply With Quote
Old 28th November 2009, 06:26 PM   #5
diyAudio Member
 
Johnloudb's Avatar
 
Join Date: Feb 2008
You mean build a new phono preamp. That could be an option since I plan on upgrading my phono cartridge to MC which means a new phono preamp. I'm planning on doing my own.

I'm going to wind some wire on a spool today or tomorrow, and see what I'm in for. Then maybe start looking into phono preamp options.
  Reply With Quote
Old 28th November 2009, 06:59 PM   #6
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Why did you load the output with a capacitor?

  Reply With Quote
Old 29th November 2009, 12:10 AM   #7
diyAudio Member
 
Johnloudb's Avatar
 
Join Date: Feb 2008
Hi, I found it flattened the high frequency response and extended it. Don't know why. Still, I don't really trust the spice model of the transformer. I haven't really looked into the generic model too much. I tried to edit the process model in Spice and got an error message saying, you can't do that. I'll do some real world experiments once I roll a transformer see what happens.
  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


Similar Threads
Thread Thread Starter Forum Replies Last Post
The beast OTL daly41k Tubes / Valves 0 18th January 2009 03:09 PM
The Beast... sklimek Tubes / Valves 5 29th June 2005 04:04 PM
Making a beast of an amp sbattersby 115 Instruments and Amps 29 16th May 2005 03:28 PM
taming the gc beast ss1 Chip Amps 6 29th May 2004 05:54 PM


New To Site? Need Help?

All times are GMT. The time now is 11:58 AM.


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