Author Topic: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier  (Read 46800 times)

0 Members and 1 Guest are viewing this topic.

Offline Groucho2005

  • Regular Contributor
  • *
  • Posts: 84
  • Country: es
Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #175 on: January 29, 2016, 10:05:33 pm »

You have to differentiate a bit. Theoretically, a Class-A amplifier can have up to 50% efficiency. A Class A amplifier with complementary bipolar or MOSFET output devices comes very close to that number.

Not true for an equivalent design to the one shown in the SIT datasheet.  The efficiency of a simple class A design is no better than 35% at full load, and much worse at low outputs or under quiescent conditions.  Efficiency can be improved by using active loads rather than a simple resistor or transformer coupling, but even then you are unlikely to reach 50% efficiency.
I suppose you missed my use of the words "theoretically" and "up to". Also, efficiency obviously relates to the maximum power output.
Lastly, transformer coupling is much more efficient than using active loads.
 

Offline gadget73

  • Regular Contributor
  • *
  • Posts: 71
  • Country: us
Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #176 on: February 03, 2016, 12:47:19 pm »
That's the one thing about Class A amplifiers I always remembered ....  If you want them to run as cool as possible, you had to run them at full volume.

 That sounds like a myth to me? Can you link to an analysis to support that.

Some amplifiers will switch in and out of Class A operation depending on output demands.  At low levels it runs a room-warming Class A bias level, with some crank it switches to a more sane AB1 bias level and the heat drops accordingly.  A straight Class A amp chugs out essentially the same heat with the volume at 0 or 11. 
 

Offline Brumby

  • Supporter
  • ****
  • Posts: 8601
  • Country: au
Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #177 on: February 03, 2016, 01:10:32 pm »
That's the one thing about Class A amplifiers I always remembered ....  If you want them to run as cool as possible, you had to run them at full volume.

 That sounds like a myth to me? Can you link to an analysis to support that.

Best I've come up with so far...

http://www.analog.com/library/analogDialogue/archives/40-06/class_d.html

 

Offline coppice

  • Super Contributor
  • ***
  • Posts: 4175
  • Country: gb
Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #178 on: February 03, 2016, 03:41:05 pm »
That's the one thing about Class A amplifiers I always remembered ....  If you want them to run as cool as possible, you had to run them at full volume.

 That sounds like a myth to me? Can you link to an analysis to support that.
It comes directly from the definition of class A. A pure class A amp draws a constant amount of power. At zero output it all ends up as heat in the amp. At maximum output some ends up in the load, so less ends up as heat in the amp.
 

Offline Brumby

  • Supporter
  • ****
  • Posts: 8601
  • Country: au
Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #179 on: February 03, 2016, 03:49:47 pm »
That's what I was talking about!  Thank you.  :-+
 

Offline FM

  • Newbie
  • Posts: 1
  • Country: us
Re: EEVblog #837 - Reverse Engineering A Valve Headphone Amplifier
« Reply #180 on: July 06, 2016, 08:30:54 am »
If my really quick look is correct, it looks like you can remove the valves and jumper pin 2 (control grid) to pin 3 (cathode) and get a much lower power, halfway decent headphone amp.
Can anyone confirm that method to bypass the tubes? 
It would lower the power by a lot?
I wish another video was done on this amp about bypassing the tubes.

This amp gets criticism for the tubes not doing much, but if bypassing the tubes doesn't lower the power output, it's a lot of power for the price, and bypassing the tubes means no need to buy replacement tubes when the stock ones wear out.  And it gets rid of the tube distortion, of course.

I have one and like it, but it'd be great to be able to bypass the tubes.  If the power output would be the same. 
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf