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β24 in pure class A

beta24

β24 in pure class A

Postby amb » July 25th, 2010, 2:31 am

The β24 was designed to be a 170Wrms per channel (into 8 ohms) class AB power amp, but for those who don't need that much power but want a pure class A variant, it could be done (but as expected of a pure class A speaker amp, it will run HOT).

The target output power is 30Wrms per channel (into 8 ohms), and it would require as much heatsinking (or perhaps more) as the standard version. The output stage heat dissipation will be around 85W per channel (170W total) at idle (Each MOSFET will dissipate about 10.5W). Thus, despite the lower output, the amp will not be smaller in size and only slightly less in weight.

Various operating points and circuit parameters were modified to optimize for the class A variant. Even though I did not actually build one, I ran enough iterations of this in PSpice simulation to be happy with the results. Based on my previous experience with PSpice and the parts models for the β24, β22 and other amps, I am confident that the amp will work as designed.

The voltage gain for the class A variant is about 15, compared to 20 for the standard version. Just a hair over 1Vrms input will be able to drive the amplifier to full 30W output. In fact, if the PSU is robust, this amp should be capable of delivering over 40W into 8 ohms before clipping. Like a pure class A amp should, the maximum heat dissipation occurs when the amp is idle. It would actually runs cooler when playing loud.

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

  • 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 350VA if using a single transformer for both channels, or use one 200VA 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
  • R25 and R26 should be changed to 562 ohms.
  • VR2 and VR3 should be changed to 500 ohms.
  • R45, R46, R47 and R48 should be mounted to the heatsink as they will get warm.
  • 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 setup procedure is mostly the same as the standard β24, as described in the PDF documentation, with the following additional step:

Adjust the output stage quiescent current to 200mA per bank as instructed. Then, after the two 22 ohm 10W test resistors are removed (and the unregulated power rails connected directly), turn on the power and slowly adjust VR2 and VR3 clockwise until you read about 280mV drop on R45, R46 and R47, R48, respectively. This corresponds to 1.4A of quiescent current through the output stage. The MOSFETs will begin to get hot. Re-adjust VR2 and VR3 periodically until the readings stabilize.

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Re: β24 in pure class A

Postby johnwmclean » July 25th, 2010, 9:51 pm

It’s great to know there’s a aberration from the established build. I hazard there’s a fair few β24’s that are no where near utilising the full measure of the rated 170Wrms per channel and could be operating glorious class A! :lol:
Why is the word 'Abbreviation' so long?
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Re: β24 in pure class A

Postby nattonrice » July 26th, 2010, 8:57 am

This is very excellent!
I just tapped the sinks and test fitted everything.
Ti, you weren't kidding when you said you got creative with fixing the devices =P

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Re: β24 in pure class A

Postby Nebby » July 26th, 2010, 9:55 am

nattonrice wrote:Ti, you weren't kidding when you said you got creative with fixing the devices =P

What are you talking about? :?:
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Re: β24 in pure class A

Postby MASantos » July 27th, 2010, 4:42 am

This is really awesome!

Ti you for got to mention it will also work as a room heater during the winter! :cool:
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Re: β24 in pure class A

Postby nattonrice » July 27th, 2010, 5:36 am

Sorry...
In the original posts on the b24 when he got the modushop case Ti said that he would have to get creative when mounting the output devices and resistors.
I had just finished tapping the heatinks on the same case for a b24.
I was half asleep so I assumed it made sense hehe.
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Re: β24 in pure class A

Postby Nebby » July 27th, 2010, 6:51 am

Ah, no apology required. I was just curious because it was so vaguely worded :)
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Re: β24 in pure class A

Postby Olli » July 27th, 2010, 4:05 pm

May I ask the advantages of pure class A over the original class AB?
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Re: β24 in pure class A

Postby amb » July 28th, 2010, 1:34 am

Olli wrote:May I ask the advantages of pure class A over the original class AB?

It would take pages to fully answer this, but the short answer is as follows:

Pure class A means that the output transistors never turn off over the entire signal waveform swing, so there is absolutely no switching distortion (aka crossover distortion), and the high operating currents help the output devices achieve better linearity. This is not to say that class AB can't perform excellently and sound great -- careful design can minimize the distortions to very low levels (such as the use of cascoding, and other techniques), but class A still holds an edge, at a huge cost in terms of poor efficiency and wasted heat, not to mention much lower output power per unit weight or size or cost.

To understand class A (and other amplifier classes) here are something you could read:
- Naval Electrical Engineering Training Series - Amplifier Classes of Operation
- Nelson Pass' A40 amplifier article
- Nelson Pass' article about class A, class B and the transition between them

There are also many other good sites that cover this subject, you could find them by doing some web searches.
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Re: β24 in pure class A

Postby mattcalf » July 28th, 2010, 2:01 am

Olli wrote:May I ask the advantages of pure class A over the original class AB?


Was going to ask this exact question.

Thanks for the great reply Ti.
amb wrote: Like a pure class A amp should, the maximum heat dissipation occurs when the amp is idle. It will actually run cooler when playing loud.
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