Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 17th December 2011, 04:06 PM   #1
dadod is online now dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Default Class i and siblings

Here is quick simulation of an amp with Edmond non switching auto bias OPS.
I appreciate comments and suggestions, specially VAS improvements.
Thanks to Edmond Stuart who is author of this excellent output stage.
I will try to simulate the same IPS with Class i OPS next.
Damir
Attached Images
File Type: jpg Non switching EdmondOPS amp-sch.jpg (156.0 KB, 1680 views)
File Type: jpg Non switching EdmondOPS amp-sim.jpg (255.8 KB, 1591 views)
File Type: jpg Non switching EdmondOPS amp-sim2.jpg (196.7 KB, 1346 views)
  Reply With Quote
Old 18th December 2011, 04:28 PM   #2
dadod is online now dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Default Class-i OPS

And now I simulated almost the same IPS connected to the Class i OPS.
Distortion is higher then with Edmond OPS, but it could be improved with better( I suppose) VAS as input resistance of the Class i OPS is highly non linear.
Damir
Attached Images
File Type: jpg Non switching Class-iOPS amp-sch.jpg (169.4 KB, 1307 views)
File Type: jpg Non switching Class-iOPS amp-sim.jpg (247.3 KB, 1213 views)
File Type: jpg Non switching Class-iOPS amp-sim1.jpg (204.0 KB, 188 views)
File Type: jpg Non switching Class-iOPS amp-sim2.jpg (216.9 KB, 183 views)
  Reply With Quote
Old 18th December 2011, 07:54 PM   #3
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default Class-I

Hi Damir,

Indeed, the input impedance is highly nonlinear. Most likely, that's the cause of much more distortion. See below for the test circuit and its Zi as of function of the output voltage (Actually, it's the output current that alters Zi). Zi varies from 60kOhms to tens of MOhms.

Why improve the VAS (i.e. lower Zo of the VAS) to compensate for deficiencies of the Class-I OPS? Isn't it a waste of time time, as the OPS itself can easily be improved (by means of my AB2 OPS, as shown in your previous post)?

In the next few days, I will show that the nonlinear Zi of class-I is not the only downside of this OPS.

Cheers,
E.
Attached Images
File Type: jpg Class-I-Sch-Zi.jpg (133.8 KB, 750 views)
File Type: jpg Class-I-Zi.jpg (135.9 KB, 718 views)
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 18th December 2011, 08:17 PM   #4
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default Autobias-II (AB2)

And here for the sake of comparison, the input impedance of the AB2 OPS:
Now, Zi varies between say 1 to 10 MOhms. Needless to say that this variation has far less impact on the distortion, as a typical VAS has far less difficult to handle such a variable load.

edit: Zi simmed at the DC level. Above 10kHz it drops considerably (of course).
See for more info on geometric mean biasing: http://home.tiscali.nl/data.odyssey/AutoBias_II.html

Cheers,
E.
Attached Images
File Type: jpg AB2-BJT-Zi-sch.jpg (160.5 KB, 512 views)
File Type: jpg AB2-BJT-Zi.jpg (118.1 KB, 445 views)
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 18th December 2011 at 08:47 PM.
  Reply With Quote
Old 19th December 2011, 09:33 AM   #5
dadod is online now dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Lower distortion with Class-i OPS.
Separate emitter resistors in Class.i input transistors lowered distortion but it is still ten times worst then with Edmond AB2 OPS. Not sure yet how this change influences thermal stability.
Damir
Attached Images
File Type: jpg Non switching Class-iOPS amp-sim3.jpg (199.1 KB, 336 views)
  Reply With Quote
Old 19th December 2011, 10:25 AM   #6
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default Class-I

Hi Damir,

I see that you have incorporated the filter network (R63, R15 & C23) between the VAS output and the OPS input. However, this filter is only needed if you are using the OPS in a standalone environment, e.g. connected to your tube pre-amp. When embedded in a 'complete' amp, as shown by your post above, you may omit this filter and probably get better distortion figures.

Regarding the 0.33 Ohm emitter resistors (RE), be warned. Since the output impedance (without global NFB), equals RE, any mismatch between the top and bottom RE will affect the performance. Kendall has already warned us: A mismatch of 1% will increase the 2nd harmonic (of 1kHz) to -77dB. Therefore, he recommends the use of precision (0.1%?) high power metal film resistors. I don't think that enough. Also unequal PCB traces and imperfect solder joints may ruin the performance. So I also recommend a Kelvin connection to these resistors.

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 19th December 2011, 10:59 AM   #7
dadod is online now dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Quote:
Originally Posted by Edmond Stuart View Post
Hi Damir,

I see that you have incorporated the filter network (R63, R15 & C23) between the VAS output and the OPS input. However, this filter is only needed if you are using the OPS in a standalone environment, e.g. connected to your tube pre-amp. When embedded in a 'complete' amp, as shown by your post above, you may omit this filter and probably get better distortion figures.

Regarding the 0.33 Ohm emitter resistors (RE), be warned. Since the output impedance (without global NFB), equals RE, any mismatch between the top and bottom RE will affect the performance. Kendall has already warned us: A mismatch of 1% will increase the 2nd harmonic (of 1kHz) to -77dB. Therefore, he recommends the use of precision (0.1%?) high power metal film resistors. I don't think that enough. Also unequal PCB traces and imperfect solder joints may ruin the performance. So I also recommend a Kelvin connection to these resistors.

Cheers,
E.
Hi Edmond,
Two questions:
1. How critical emitter resistors are in your AB2 OPS, and how to adjust question current for different emitter resistors value(.22R)?
2. I intended to put all in one box, tube pre and OPS. Do I need the filter network in this case(how about AB2 OPS)?
Damir
  Reply With Quote
Old 19th December 2011, 12:01 PM   #8
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Quote:
Originally Posted by dadod View Post
Hi Edmond,
Two questions:
1. How critical emitter resistors are in your AB2 OPS,
Hi Damir.

In the AB2-OPS tight matching of emitter resistors is not critical, as the output impedance depends not RE. The trouble with class-i is that the feedback is taken from the RE's. At idle from both RE's, when sourcing current from the top RE and when sinking current FB is taken from the bottom RE.

In case of the AB2-OPS however, FB is taken directly from the output (as it should be). Therefore, the output impedance is much lower. This is a distinct and important difference.

Quote:
and how to adjust question current for different emitter resistors value(.22R)?
By increasing the currents through Q9 and Q12 (in post 4), i.e. set R13=R14=100R. BUT I wouldn't do that, as the minimum current through the OP devices comes close to zero. A zero current should be avoided at any time. Set for example R13=R14=91R and you will see the devastation effect.

Quote:
2. I intended to put all in one box, tube pre and OPS. Do I need the filter network in this case(how about AB2 OPS)?
Damir
Much depends on the output impedance (also at HF) of the pre-amp. It also depends on whether some HF filtering has been done in the pre-amp. The point is that the OPS (both versions) likes to see a low Zo of the preceding stage (in order to avoid instability) and don't like extreme fast transients.

Another point of concern is the DC offset. If the pre-amp is only AC coupled to the OPS (most likely) you have to define the DC operating point of the OPS by means of a suitable resistor at the input to GND. However, if this resistor is too low, the pre-amp doesn't like it (more distortion) and if it is too high you will get more offset. Somewhere in between there must be an optimal value (I hope).

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 19th December 2011, 12:18 PM   #9
dadod is online now dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Hi Edmond,
Thank you, I think I see now what I am going to do.
As AB2 looks mutch better I will use this one, with my tube preamp, and in all transistors version.
What alternative is for WHAT transistors, matching individual transistors and somehow thermaly connect them?
damir
  Reply With Quote
Old 19th December 2011, 01:49 PM   #10
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default matched transistors

Hi Damir,

Probably you mean THAT transistors (never mind). In principle, you can use any decent low noise small signal tranny (beta > 100; Ft > 100MHz and low internal Rb and Re), but they should be matched, that is, PNP to PNP respectively NPN to NPN. I guess that a delta Vbe of about 3mV (or less) is sufficient.
Regarding thermal coupling, again PNP to PNP resp. NPN to NPN.
Remember, we are dealing here with only 33mV across the emitter sense resistors at idle condition. So any Vbe offset will have a large impact on the quiescent and minimum current of the OPS. This applies to the Class-I as well as to the AB2 OPS.

BTW, Farnell has the THAT300 and THAT320 in stock, though they are a bit expensive, about 9 euro.

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  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
Class-AB meets Class-D: Yamaha's EEEngine Topology - where are this Diy Projects? tiefbassuebertr Solid State 29 14th April 2014 04:12 PM
Collection of Class B topologies <100mA Idle and Sound closest by Class A tiefbassuebertr Solid State 37 27th July 2012 08:04 AM
Can a Class AB PP amp be said to be operating in Class A at low signal levels? ray_moth Tubes / Valves 19 23rd January 2009 07:52 PM
How about a round-up of Class A kit power amps, or collectable vintage class A? Brisso57 Solid State 4 14th February 2007 10:30 AM


New To Site? Need Help?

All times are GMT. The time now is 08:13 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