SE to Balanced tube preamp idea - Page 2

hollow_man · 2nd January 2009, 12:05 PM

Now getting closer - how about this?

EC8010 · 4th January 2009, 04:56 PM

Why have all those expensive FETs down the bottom? The bipolar transistor rules OK. You might do better to hold the gate of the upper FET at a negative voltage so as to increase Vds and avoid some of the "tetrode" negative resistance problems sometimes seen at low Vds with high voltage FETs.

hollow_man · 4th January 2009, 05:38 PM

I already had a bunch of the K247s, so it is ok. Thanks about the point regarding the mosfet Vds; I have revised the schematic. Is the 4.7u capacitor wired correctly?

hollow_man · 4th January 2009, 05:57 PM

I notice that the CMR follows a single-pole law, the pole located roughly at 30Hz, decreasing 20dB/dec thereafter. This is not very nice, is it?

EC8010 · 5th January 2009, 09:17 AM

I'd ideally like to see more voltage across that upper FET and less across the lower. The lower ones only need 5V or so. The capacitor looks fine, but it doesn't need to be so big. FETs are high impedance devices - scale those resistors up by a factor of 20 and scale the capacitor down by a factor of 20.

hollow_man · 5th January 2009, 09:45 AM

Will do, cheers. A smaller mosfet with less capacitance helps push the CMR 1st order pole up, so a IRF820 will help here.
I ordered the 6N6s too dirt cheap, so the design will be on its way soon. I need to get the regulated tube power supply design right, as the power supply rejection of the LTP is virtually nil.

Miles Prower · 5th January 2009, 10:31 AM

Quote:

Originally posted by hollow_man
TThe cascode CCS you suggest seems to work very well; only 0.03dB differential imbalance is produced. But out of pseudoscience or just biased intuition, I was thinking of not using bipolar transistors in this design.

Cascoded BJTs make excellent active tail loads for LTPs. I've done a couple of projects already and used cascoded BJTs for active tail loads in both. They definitely do a good job here. Put enough voltage across them, and any sort of "varactor" effects become unimportant. AC balance is excellent, and they sound very good too.

hollow_man · 5th January 2009, 10:48 AM

The nice thing about this topology is that the mosfet may be ditched in favour of a bjt and vice-versa. The bjt seems to yield 1MOhm//15pF as opposed to 15M//100p of the mosfet. I guess it is wait until construction time to see which sounds best.

hollow_man · 6th January 2009, 11:24 PM

With regards to the power supply, this is what I'm working on.
Line regulation is useful around here.
I hope I'm not boring people by re-inventing the wheel...

hollow_man · 6th May 2009, 07:34 PM

Got the pcb today...relay volume control, balanced signal path, regulated ps etc

4th January 2009, 04:56 PM	#12
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	Why have all those expensive FETs down the bottom? The bipolar transistor rules OK. You might do better to hold the gate of the upper FET at a negative voltage so as to increase Vds and avoid some of the "tetrode" negative resistance problems sometimes seen at low Vds with high voltage FETs. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

5th January 2009, 09:17 AM	#15
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	I'd ideally like to see more voltage across that upper FET and less across the lower. The lower ones only need 5V or so. The capacitor looks fine, but it doesn't need to be so big. FETs are high impedance devices - scale those resistors up by a factor of 20 and scale the capacitor down by a factor of 20. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
balanced tube preamp	Freddie	Tubes / Valves	29	18th March 2010 10:50 AM
Tube Preamp Idea	mattthegamer463	Tubes / Valves	14	8th February 2009 01:56 AM
Balanced tube preamp for class D power amps.	jane	Tubes / Valves	5	13th December 2006 08:41 AM
balanced stereo tube preamp for a recording studio	zdpinet	Tubes / Valves	12	15th July 2002 04:53 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

4th January 2009, 05:57 PM	#14
hollow_man diyAudio Member Join Date: Nov 2004 Location: Athens	I notice that the CMR follows a single-pole law, the pole located roughly at 30Hz, decreasing 20dB/dec thereafter. This is not very nice, is it?

5th January 2009, 09:45 AM	#16
hollow_man diyAudio Member Join Date: Nov 2004 Location: Athens	Will do, cheers. A smaller mosfet with less capacitance helps push the CMR 1st order pole up, so a IRF820 will help here. I ordered the 6N6s too dirt cheap, so the design will be on its way soon. I need to get the regulated tube power supply design right, as the power supply rejection of the LTP is virtually nil.

5th January 2009, 10:48 AM	#18
hollow_man diyAudio Member Join Date: Nov 2004 Location: Athens	The nice thing about this topology is that the mosfet may be ditched in favour of a bjt and vice-versa. The bjt seems to yield 1MOhm//15pF as opposed to 15M//100p of the mosfet. I guess it is wait until construction time to see which sounds best.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)