Universal hybrid triode-mosfet input/drive stage - 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 21st August 2014, 12:39 AM   #1
diyAudio Member
 
Join Date: Jan 2014
Default Universal hybrid triode-mosfet input/drive stage

Hi all,

While still waiting for hardware parts to be delivered to my mailbox for my first full amp build I've been pondering over many circuits and topologies. After many cycles I came up with a topology as attached.

I know my way around toobs, but have no actual experience with mosfets, so I am not sure if it will work, although it runs fine in ltspice.

This is how it works.

The core of the stage is the bottom triode U1 (here 6SL7) acting as the signal voltage gain stage. R1 is the cathode resistor. No bypass cap needed because of active loading.

The magic of this circuit is the top mosfet M1 (here IRF820) together with the resistor and cap network R2-R6 and C1,C2, which actually serves four purposes at once.

1. Together with C1 and R2 it forms a mu follower or quasi-CCS active load for the bottom triode, maximising gain and minimising distortions.
2. Resistors R4-R6 and pot U3 form a voltage divider between B+ and GND, providing a fixed reference voltage for the gate (and hence stage) output.
3. Cap C2 cancels B+ ripple at the mosfet gate, which actively cancels ripple at the stage output..
4. Furthermore, the IRF820 provides a ultra low impedance and high current delivery capability, with no additional resistance between B+ and the stage output.

Besides the above, this stage has the following nice properties:
- Can be used with almost any small signal triode. Adjust R1, R2, C1 as fits. Usually one set of values fits all hi mu triodes, and one set fits all medium mu triodes.
- As a source (or mu) follower, the mosfet is sonically transparent.
- Compensates for tube ageing and sample variations. Anode and output voltage fixed by circuit, anode current settled by triode.
- Uses only tiny valued caps, free to choose your fave pio/foil/teflon... caps for reasonable bux.
- Virtually perfect PSRR.
- Can be used to RC or DC couple almost any following stage, at any desired (fix) output voltage.

Now, as I am not firm with mosfet technology, can anyone comment on the network? Is an enhancement mosfet ok (ltspice sims ok) or do we need a depletion type? Are the currents (SD, Gate) sufficient for stable operation? I chose large value R4-R6 to minimise C1 and C2. Are there any other more suitable fets up to the task?

Thanx for looking,

GB
Attached Images
File Type: gif Uni_stage.gif (36.7 KB, 224 views)

Last edited by goldenbeer; 21st August 2014 at 12:46 AM.
  Reply With Quote
Old 21st August 2014, 03:51 AM   #2
diyAudio Member
 
Join Date: Jan 2008
if you're going to add silicon then why not just use a pnp as a constant current source as the current is low.
by the way any more then 200v is overkill as the tube will only put out 100 v p-p
because the tube has 96v - v R2 across it

Last edited by stocktrader200; 21st August 2014 at 03:57 AM. Reason: typo
  Reply With Quote
Old 21st August 2014, 05:46 AM   #3
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Quote:
Originally Posted by goldenbeer View Post
...After many cycles I came up with a topology as attached...
Interesting. Can you post the LTSpice model with necessary .lib and .inc ?
  Reply With Quote
Old 21st August 2014, 05:56 AM   #4
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
looks to be a gyrator load? a search on this term will bring a lot of results at diyaudio. I used it already, and like it a lot indeed - in my last prototype I drove the G2 of a 6W6 directly from the source output.
__________________
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
  Reply With Quote
Old 21st August 2014, 07:35 AM   #5
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
Great to see more hybrid designs. The hybrid mu-follower idea hasn't been used nearly as much as I would have expected.

see also here: http://www.fetaudio.com/wp-content/u...9/Mu-Stage.pdf

and: http://phonoclone.com/diy-twi.html
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 21st August 2014, 09:20 AM   #6
diyAudio Member
 
Join Date: Jan 2014
Thanks guys for your replies,

@stocktrader200
With the 6SL7, this circuit swings 130 Vpp. With the 12AX7 dropped in (no other change required) up to 170 Vpp.
Of course you can lower B+ (adjust R4-R6) with identical performance, or change R4-R6 to yield a higher anode voltage and load line. Just as you like.
The IRF820 is able to deliver big power on demand, depending on the following stage's requirements. This circuit can drive following output stages into A2 regions easily. It's a always a good idea to connect the drain to B+ directly to minimize output impedance for that.

@Erik
Yes, lots of wisdom around this forum - Great reading!
The circuit is not technically a gyrator, since it is not self contained(?) but it has the properties of a gyrator load, along with a fixed output dc voltage.

@Bigun,
thx for the links, I'll have to do some more reading.

GB
  Reply With Quote
Old 21st August 2014, 11:32 AM   #7
diyAudio Member
 
Join Date: Feb 2010
Location: Helsinki
This has been my plate load for.. well, everything, for a good time now. It provides excellent results.

Make a balanced pair with this plate load for both triodes, and a powerful CCS in the tail. Cleanest of the clean.
  Reply With Quote
Old 21st August 2014, 03:42 PM   #8
diyAudio Member
 
Join Date: Jan 2014
@Bigun, I read your links and it is kind of funny I came up with the same topoly as Alan Kimmel in his article.

Following up with that, I made some changes and this is the latest schematic. (attached)

I added a protection zener and modified the voltage divider circuit, adding a seperate R7.

Using the 6SL7 as shown, the stage now has a (simulated) gain of 65x (mu = 70), 136 Vpp max output swing, and a PSRR of -70dB
Attached Images
File Type: png 21.08.png (11.4 KB, 120 views)
  Reply With Quote
Old 21st August 2014, 03:48 PM   #9
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
Well, they say great minds think alike, so you are in good company with Alan. What will you use this circuit for, what output stage will it drive ?
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 21st August 2014, 04:03 PM   #10
diyAudio Member
 
Join Date: Jan 2014
Quote:
Originally Posted by Bigun View Post
What will you use this circuit for, what output stage will it drive ?
Well, as it is, you could direct drive a 2A3 using a 2.35k cathode resistance (2x4.7k paralleled), straight into A2 territory as far as the bulbs will go.... (or is that "glow" )
  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
Question: Class AB Hybrid with small signal triode and Power Mosfet output? danielwritesbac Tubes / Valves 19 9th February 2013 06:21 PM
Universal hi-end hybrid output stage for DAC audiodesign Digital Source 16 22nd October 2012 05:07 PM
2 stage Stereo/4 Stage Mono Triode Preamp with independent drive control & inverters sampleaccurate Tubes / Valves 10 9th March 2011 10:31 AM
Recommend an input stage triode please zigzagflux Tubes / Valves 20 16th April 2008 03:42 PM
Mosfet drive in Vas stage and use Bipolar output YUTK Solid State 0 26th May 2005 05:50 AM


New To Site? Need Help?

All times are GMT. The time now is 05:07 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