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A lower powered β24

beta24

A lower powered β24

Postby amb » February 25th, 2017, 12:42 pm

Some people don't need the full 170W per channel of output power that the standard β24 provides. They also don't want the heat and electricity usage of the pure class A variant. So here is a 3rd alternative - a 40W per channel class AB variant. Using the same β24 PCBs and power supply topology, it will have every bit of the sophistication of the standard β24. Smaller heatsinks and power transformer(s) lead to a lighter, more compact package (depending on your choice of casing).

Here is a list of what changes are needed to build the lower power class AB variant. For the standard β24 documentation PDF please contact me.

  • You can use smaller heatsinks for the main output MOSFETs. I recommend a heatsink with thermal resistance of 0.5°C/W or lower per channel.
  • The output stage unregulated power supply voltage needs to be dropped in half, from ±30V to ±15V, fully loaded. This means using a 12V+12V power transformer (or 24V CT). It should be rated 200VA if using a single transformer for both channels, or use one 120VA transformer per channel. As a consequence of the reduced voltage, the 10000uF bulk capacitors' voltage rating could also be reduced from 50V to 25V. You can use higher capacitance if you wish.
  • The σ22 regulated power supply should be configured to output ±24V instead of ±36V. The recommended power transformer is thus changed to 25V+25V or 50V CT, rated 30VA if using a single transformer for both channels, or use one 15VA transformer per channel.
  • R2 and R4 should be changed to 75K ohms
  • C1 and C2 should be changed to 2.0pF (e.g., Mouser 81-RPE5CA2R0C2P1B03B or Digi-key 490-3633-ND)
  • C3, C4, C5 and C6 should be changed to 18pF (e.g., Mouser 81-RPE5CA180J2P1Z03B or Digi-key 490-3632-ND).
  • C15A-C15D and C16A-C16D may (optionally) be changed to Panasonic TS-HA ECO-S1HA272CA 2700uF, 50V caps (Digi-key P6677-ND).
  • D1-D6, D11 and D12 should be changed to BZX55C6v2 (6.2V 500mW zener).
  • D7, D8, D9 and D10 should be changed to 1N5927B (12V 3W zener).
The voltage gain for this variant is about 15, compared to 20 for the standard version.
The setup procedure is the same as the standard β24, as described in the PDF documentation.
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Re: A lower powered β24

Postby SonusBonum » February 25th, 2017, 1:27 pm

So that would be the one I have playing right now with the 200mA MOSFET bias. The only thing separating it from class A is the 1.4A bias.
It'll hardly get any hotter than 30 degrees C or 86 degrees F as shown in the video below.

https://www.youtube.com/watch?v=2oxBUo30TXM
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Re: A lower powered β24

Postby amb » February 25th, 2017, 1:35 pm

Yes, basically. I believe you intend to go to full class A, so the power supply and heatsinking are sized accordingly. For the lower power class AB version, these could be reduced to make things lighter and more compact.
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Re: A lower powered β24

Postby SonusBonum » February 25th, 2017, 1:38 pm

Yes, I will go to class A. As soon as I finished all the (4) boards I will mount the heatsinks and increase the bias to 1.4A. That will be coming week, so I will keep you posted.
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Re: A lower powered β24

Postby poseidonsvoice » February 27th, 2017, 8:16 am

Hi Ti,

Nice to see you are busy! Thanks for this smaller 40W Class AB variant. How many watts are in Class A though? <1 watt?

One of these days I need to pm/email you for the β24 design.

Thanks,
Anand.
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Re: A lower powered β24

Postby BesPav » February 27th, 2017, 8:42 am

Sophisticated one can ask for easy A/AB switch...
If case and heatsinks are already huge why not to fully use them time to time?
;)
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Re: A lower powered β24

Postby amb » February 27th, 2017, 11:05 am

poseidonsvoice wrote:Nice to see you are busy! Thanks for this smaller 40W Class AB variant. How many watts are in Class A though? <1 watt?

The amp will remain in class A operation as long as the peak output current is no more than twice the quiescent current. The quiescent is 200mA if you adjust the bias according to the instructions. Assuming an 8Ω load, this means that the amp will run in class A up to P = (0.4A)^2 * 8Ω = 1.28W peak. But you are free to increase it as long as your heatsinks are big enough to handle the additional thermal load on a continuous basis.

BesPav wrote:Sophisticated one can ask for easy A/AB switch...
If case and heatsinks are already huge why not to fully use them time to time?

The whole idea with this variant is to allow a smaller, lighter build... :)
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Re: A lower powered β24

Postby daniele86 » February 27th, 2017, 12:09 pm

Wonderful!
I like more the possibility to change from class A to AB with few steps than the idea to make a smaller, lighter build..Anyway thanks Ti!

There is also the possibility, for who made the class A version, to easily check the differences between A and AB version. ;)
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Re: A lower powered β24

Postby poseidonsvoice » February 27th, 2017, 12:41 pm

daniele86 wrote:Wonderful!
I like more the possibility to change from class A to AB with few steps than the idea to make a smaller, lighter build..Anyway thanks Ti!

There is also the possibility, for who made the class A version, to easily check the differences between A and AB version. ;)


Indeed, I quite agree. The other point is that my speakers are 96 dB efficient at 2.83V/1m with an impedance minima of 4.9 ohms. I don't listen to much more than 100 dB peaks at the listening position, and with my current amp, I have found that I don't use more than 3-6 watts most of the time. Given that, I would like to extract as much class A performance as possible. So shooting for 5 watts, it looks like 500mA of bias might work out. P = (1A)^2*4.9 ohms = 5 watts.

Best,
Anand.
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Re: A lower powered β24

Postby SonusBonum » February 28th, 2017, 12:18 am

Well... I can tell for sure that in this particular AB mode you don't need more heatsink then as shown in the video above. They played "livingroom level" for the whole weekend (48+ hours) and the hottest parts turned out to be the D7-D10 diodes. What may be an interesting thought though...: A "standby" mode, that puts the amps back to AB while not being used after a given time. In that way you keep the juice flowing while the energy bill stays in control. I never turn of my equipment.
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