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Class B design completion
http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=6335
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Author:  indr [ 09 Sep 2018, 05:28 ]
Post subject:  Class B design completion

Image
I like this simplistic class B design.
Someone posted a video without details of parts.
Sound quality is of distinct flavour, goes well with electronica.
Need to make it work with best possible components.
With tl082 and tip31/32, nothing is coming out.
More iimportantly, will it work with mosfets?
I prefer mosfets' tubey sound )
x

Original video link without showing components
[YouTube]https://m.youtube.com/watch?v=QHoOnP4cMDw[/YouTube]
Another one corroborating the design, again without showing actual circuit
[YouTube]https://m.youtube.com/watch?v=5OjTz3SjAXA[/YouTube]

Author:  Geek [ 09 Sep 2018, 14:48 ]
Post subject:  Re: Class B design completion

Hi,

The "dead zone" with bipolars is about 0.7V while enhancement-mode MOSFETS can be about 4V. Lots more "crossover baggage" to overcome.

Cheers!

Author:  indr [ 09 Sep 2018, 19:52 ]
Post subject:  Re: Class B design completion

Sorry, the scheme was not uploaded properly, here it is

Author:  indr [ 09 Sep 2018, 19:55 ]
Post subject:  Re: Class B design completion

Geek wrote:
Hi,

The "dead zone" with bipolars is about 0.7V while enhancement-mode MOSFETS can be about 4V. Lots more "crossover baggage" to overcome.

Cheers!


Thanks, that means 4 zeners of 4v each - base-base )

Author:  Yungman [ 10 Sep 2018, 01:28 ]
Post subject:  Re: Class B design completion

What is the input reference to? to 0V ( ground)? If so, measure the output with a volt meter. Make sure you have 0V.

What is the purpose of this? You know you have tremendous dead zone in the crossover region, it's going to sound terrible. There are plenty of circuits to set up bias to avoid crossover distortion. Even if you close the loop with the output transistor, you cannot get rid of the crossover distortion.

You need a more complete schematic show how you input the signal, what is the power supply. You cannot just draw some sketches like this.

Author:  indr [ 10 Sep 2018, 03:45 ]
Post subject:  Re: Class B design completion

Yungman wrote:

You need a more complete schematic show how you input the signal, what is the power supply. You cannot just draw some sketches like this.


Its not a complete circuit, its called, Topology !
I needed best possible components.
Someone already passed a vital info regarding mosfet.
Without asking for input and powersource.
As standard input is understood to be with quite good op impedance, say a analog radio or mp3 player, and standard supply would be 12-0-12 v.

Author:  Yungman [ 10 Sep 2018, 04:03 ]
Post subject:  Re: Class B design completion

I took the liberty to add one transistor to make it to class AB. You might have a much better chance to get better sound:
Attachment:
opamp amp.jpg

This is just a quick drawing, you have to try it and optimize it.
I added Q1 to create a bias spreader to bias Q2 and Q3 on at idle. Normally, you adjust to get 25mV across R7. But that might make the transistor hot, so you might want to go down to like 10mV.

Normally, Q1 has to be clamp together with one of the output transistor, this will give thermal compensation to keep the idle current stable.

Gain of this amp is R3/R2 + 1 = 10 + 1 = 11.

C1 is to stabilize the amp, you can decrease the value to get better frequency response as long as it does not oscillate.

I put down the DC voltage of a few points. You check those point to make sure it is correct.

If changing to MOSFET output transistor, increase value of R4 to increase the bias spreader voltage to bias the two MOSFET on.

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