Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Biasing op amps in class A does it worth it?

Status
Not open for further replies.

hentai

New Member
Hi!
I've been reading about some experiments with op amps having their final stage working in class A by adding a current load from the negative supply rail to output, thus forcing the output stage to run single ended. From what i've seen most belive its a great tweak for the sound but what about the op amp is this safe? Did any of u tried it?
Thx
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I seem to recall this been used years back on a specific opamp?, I'm SURE Audiguru will remember exactly which one? :D

Considering almost everything you play, CD's etc. will usually already have been through hundreds of opamps without any such mod it seems a total waste of time!.
 

eng1

New Member
This eliminates the cross-over distortion, but has some drawbacks. When choosing the current you must consider the characteristics of the op amp: maximum output current, maximum power dissipation and the voltage levels of the signal. Also.. the load impedance will depend on the output impedance of the current source (FET?).
I would look for low distortion op amps, but this is just my opinion.
 

hentai

New Member
hmm mb ur right but still couldn't this be used in low level preamps (like phono stages, mic preamps...) or little headphone amps? Most ppl who tried it said it brings an improvement to the sound... or its just placebo effect? :D
 

Nigel Goodwin

Super Moderator
Most Helpful Member
hentai said:
hmm mb ur right but still couldn't this be used in low level preamps (like phono stages, mic preamps...) or little headphone amps? Most ppl who tried it said it brings an improvement to the sound... or its just placebo effect? :D

Probably so - the only way to test it is by blind testing, which would probably show no difference.
 

audioguru

Well-Known Member
Most Helpful Member
The old LM324 and LM358 opamps have a high amount of crossover distortion. National Semi who invented them recommend biasing the output in class-A to eliminate the crossover distortion. A Japanese company copies them and shows curves of the amount of crossover distortion. BUZZZ!

Some audiophiles must have seen it and tried it with opamps that have only 0.00008% distortion. They think that biasing the output in class-A makes their excellent opamps have even less distortion. Ridiculous.
 

Attachments

  • LM324 crossover distortion2.PNG
    LM324 crossover distortion2.PNG
    24.7 KB · Views: 119

Nigel Goodwin

Super Moderator
Most Helpful Member
audioguru said:
The old LM324 and LM358 opamps have a high amount of crossover distortion. National Semi who invented them recommend biasing the output in class-A to eliminate the crossover distortion. A Japanese company copies them and shows curves of the amount of crossover distortion. BUZZZ!

Some audiophiles must have seen it and tried it with opamps that have only 0.00008% distortion. They think that biasing the output in class-A makes their excellent opamps have even less distortion. Ridiculous.

Told you AudioGuru would remember the number! :D
 

hentai

New Member
Some audiophiles must have seen it and tried it with opamps that have only 0.00008% distortion. They think that biasing the output in class-A makes their excellent opamps have even less distortion. Ridiculous.
hahaha i guess ur right
 

Hero999

Banned
I though low distortion op-amps always had class ! outputs anyway.
 

audioguru

Well-Known Member
Most Helpful Member
The old LM324 and LM358 were the first low power opamps. They made them use less current by biasing the ouput transistors in class-B. BUZZZ!

All other opamps, even newer "low power" ones bias their output transistors properly in class-AB so there is hardly any crossover distortion.
 

Sceadwian

Banned
Keep in mind, people that love tube amps think that the distortion they impart on the signal is an improvement in sound. Acoustical perception is a very relative thing, and a very expensive industry.
 

Hero999

Banned
I said low distortion not low powe and my above post did contain a typographical error. Am I right in saying that most low distortion op-amps have class A output stages?
 

audioguru

Well-Known Member
Most Helpful Member
Opamps don't use class-A output stages because they would melt and would have a high output impedance if they did.
They all use low impedance class-AB except for the old LM324 and LM358.
 

Hero999

Banned
I know that's not true, the rail rail CMOS omps like the CA3130 have class-A output stages.
 

audioguru

Well-Known Member
Most Helpful Member
I forgot about the old CA3130 opamp with a Cmos class-A output stage.
With its max supply voltage of only 15V its supply current is a very high 10ma when its output is linear.
Its output has very high distortion when it drives a load and a load reduces its voltage gain.
Its output impedance is about 500 ohms.
 

Hero999

Banned
cadstarsucks said:
Actually most amps you run into today in all your battery operated toys are class D.
I don't believe that. I have never seen a class D amplifier in piece of consumer electronic equipment before.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top