Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

01a DHT Preamp
01a DHT Preamp
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
Thread Tools Search this Thread
Old 13th May 2012, 10:07 AM   #1
mogliaa is offline mogliaa  United Kingdom
diyAudio Member
mogliaa's Avatar
Join Date: Dec 2009
Location: London
01a DHT Preamp
Default 01a DHT Preamp

Hi everyone,
After playing with DHT preamps (26, 71a, 4P1L, 46), I ended up staying with a CX301a version of it. I liked its warm sound and tone so I decided to stay with it. It's been over 2 months so far that I haven't returned to my precious 26 preamp (I will probably revisit my 26 preamp based on latest updates from Andy Evans).

As I'm working on a version of the 4-65a SE amp by Michael Koster, I started investigating options and ways of improving the 01a as will be the first valve of my amplifier.

Using curves plot with my curve tracer and Dmitry's composite model, I got a very well matched spice model for the 01a.

I modelled in LTSpice two versions of the gyrator loaded preamp with filament bias and Rod Coleman's superb filament regulator:

1) Version 1: Using Anatoly's PNP-FET Gyrator
2) Version 2: using classic cascoded-FET gyrator with mu-follower output.

Interesting results came out of this first simulation test. I think I haven't managed to optimise (again) Anatoly's gyrator despite have managed to keep good VDS (38V) to keep the CoSS low. I get slightly worse THD (0.03% versus 0.016%) compared to the MOSFET version.

Looking at output impedance, version 1 has 10K @1kHz whereas the version 2 can get as low as 734 ohms but impedance raises significantly at low frequencies (70k version 1 versus 80k version 2). This can be reflected in the frequency response where version 1 performs better than version 2.

At high frequency both preamps perform very well up to 1MHz despite version 2 performing slightly better.

At this point, it looks like if output capacitor of version 2 is increased in value to improve the LF rolloff, it seams that overall performs better than version 1 as THD and output impedance are better?

Have I missed anything here?

Has anyone tried both circuit and compared sonic differences?

Thanks for the help
Attached Images
File Type: png 01 Preamp1 Output impedance.png (21.7 KB, 741 views)
File Type: png 01a Preamp 2 Frequency response.png (19.0 KB, 715 views)
File Type: png 01a Preamp 2 Output impedance.png (29.6 KB, 706 views)
File Type: png 01a Preamp version 1.png (52.7 KB, 734 views)
File Type: png 01a Preamp version 1 Frequency response.png (19.1 KB, 702 views)
File Type: png 01a Premp version 2.png (42.3 KB, 382 views)
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
  Reply With Quote


01a DHT PreampHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
HQ-DHT (All tube DHT amli) 26-45-2A3 quanghao Tubes / Valves 44 20th January 2013 09:58 AM
01A tube amp Schematic??? jmillerdoc Tubes / Valves 4 29th December 2009 01:39 PM
For sale - several audiophile DHT tube amp/preamp projects (UK) andyjevans Swap Meet 9 16th December 2009 03:49 PM
Preamp with DHT dejanm Tubes / Valves 9 29th October 2007 08:15 AM
Can I use a 01A in a headphone amp? sbelyo Headphone Systems 27 28th April 2006 11:33 AM

New To Site? Need Help?

All times are GMT. The time now is 11:49 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio