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 10th April 2009, 07:38 AM   #1
diyAudio Member
 
Join Date: May 2007
Default Schade84

Instead of foolishly using the triode drivers Ri for feedback(high driver THD), this must be a better way to do it. Used ca 10% local NFB as proposed by Schade. Surely the circuit can be fine-tuned. I get indications of below 1% THD both in the driver and total circuit at 4W/8ohm.
Attached Images
File Type: png schade84.png (60.8 KB, 1015 views)
  Reply With Quote
Old 11th April 2009, 12:47 PM   #2
diyAudio Member
 
Join Date: May 2007
Comparision and two possible modifications to make the first circuit acceptable with respect to THD and clipping.
Attached Images
File Type: png rhsc45.png (64.6 KB, 843 views)
  Reply With Quote
Old 11th April 2009, 02:09 PM   #3
diyAudio Member
 
Join Date: May 2007
Had made the last two circuits too complicated. Removed the cap in the feedback.
  Reply With Quote
Old 11th April 2009, 07:01 PM   #4
diyAudio Member
 
jerluwoo's Avatar
 
Join Date: Feb 2006
Location: Ashland,Ky U.S
I bought a Motorola phono console a while back that used this feedback in its EL84 se output stage. It was setup like circuit 3 above. It sounded horrible.
  Reply With Quote
Old 12th April 2009, 07:54 AM   #5
diyAudio Member
 
Join Date: May 2007
I am impressed Motorola used 6C45 ! So you probably didnīt look close enough!

The interesting thing is that when you go from circuit 1 to 2, distortion is cut almost in half when the local NFB ala Schade is adjusted to give the same total gain as in circuit 1.

Forgot to add the simplified circuit from post #3:
Attached Images
File Type: png rhsc45.png (25.3 KB, 658 views)
  Reply With Quote
Old 12th April 2009, 12:35 PM   #6
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
The change from 1 to 2 is simply adjusting the source impedance of the driver. Swap a pentode in there, or perhaps an ECC81 like the RH84 and the impedance is dominated by the plate impedance of the driver again.
cheers,
Douglas
__________________
the Tnuctipun will return
  Reply With Quote
Old 12th April 2009, 02:12 PM   #7
diyAudio Member
 
Join Date: May 2007
Hi Douglas,

Mentioned the Ri in post #1.

Maybe you missed the point. Distortion is halved from 1 to 2.

The method used in 1 only works with pentodes.

Look at the point where the feedback resistor returns to the EL84 grid. If we had infinite gain from the pentode this would be a virtual earth point. Due to finite gain we just have a very low impedance. A triode driver will have gross problems.

With a shunt/Schade inverse-feedback as in 2 and 3 we a have a definded input impedance of in, this case, ca 47k. Now we can find a triode driver that works OK!
  Reply With Quote
Old 12th April 2009, 02:46 PM   #8
Svein_B is offline Svein_B  Norway
diyAudio Member
 
Join Date: Oct 2005
An interesting variation of the shunt feedback Lars.

Inserting a serias resistor before the EL84 will make the load on the driver more "benign". The voltage swing required will however increase by a factor of about 4 due to the voltage divider represented by the 47K and Rfb (Rfb being in effect about 470K/33).

So it seems that whatever way we do it, there is a very delicate balance between feedback ratio, gain, and distortion.
What do you recon is the ovarall gain of the modified topology, and what would be the (simulated) output impedance?


Atached loadlines is my rough estimate of the two variants using 12AT7/ECC81.

Svein
Attached Images
File Type: gif ecc81-rh4.gif (61.0 KB, 580 views)
  Reply With Quote
Old 12th April 2009, 03:25 PM   #9
diyAudio Member
 
Join Date: May 2007
I prefer to call it "Schade" as this variation of shunt feedback around a output tetrode was proposed by him.

I choose the shunt feedback to give exactly the same total gain and output impedance is on par. THD is halfed in the Schade.

The variation recommended by you and others, is IMHO no good enough and with about double the THD compared to the Schade. A triode driver does not work so good under those circumstances.

Started the thread as I couldnīt see the benefit of the earlier variation when using triode drivers.
  Reply With Quote
Old 12th April 2009, 04:24 PM   #10
diyAudio Member
 
Join Date: May 2007
Normalized the three variations with identical workingpoints and identical gain. Ua of drivers just above 150V on all three.
Attached Images
File Type: png rhsc45.png (68.5 KB, 1204 views)
  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 01:40 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2