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Old 24th February 2007, 06:16 PM   #1
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Default Rail switching amplifiers and supply voltage levels

In a rail switching amplifier (which some on this board call Class G, and others call class H - I mean an amplifier that has multiple voltage levels [usually 2 or 3] to switch to depending upon the amplitude of the output signal), the voltage levels of the supply tiers are usually equally spaced. As an example, QSC Audio's RMX1850HD uses +/- 44V, and +/- 88V. Some of their three tier designs use +/- 50, 100, and 150V. I didn't look at all the schematics, but I would assume that all of their rail switching amplifiers have rails equally spaced (meaning the higher voltages are integer multiples of the low voltage rail).

==> For a rail switching amplifier, on the average, is the maximum efficiency achieved with the rail voltages being equally spaced? Is there good reason for this, or simply convention because no other methods have been tried? Are there some well known rail switching amplifiers that do not have voltage rail levels equally spaced?
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Old 24th February 2007, 06:32 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I would expect the voltage increment between the different supply voltages to be completely irrelevant.
I suspect equal increments may have more to do with buying in ready wound transformers.
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Old 24th February 2007, 07:03 PM   #3
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Hi AndrewT,

Yes, I thought that as well: use the same voltage levels to save costs on transformers (I looked on several transformer makers websites, and their ready-wound ones come with equal secondary voltages, so buying these for an amp would save cost and complexity).

I ask this question because I would like to try and make a rail switching amplifier (some day), and I have two power transformers: one that would give ~+/- 75V, and another that would give ~+/- 40V. I was wondering that, because the high voltage level is not twice the lower voltage level, would this be a disadvantage (or maybe an advantage)?
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Old 25th February 2007, 02:48 AM   #4
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i doesn't really matter much what the higher/lower voltage ratio is. the whole idea behind class f/g/h designs is lowered heat dissipation at low power and idle.

the idea actually came from radio, where certain AM radio transmitters used "adaptive bias", where the carrier wave idled at let's say 1kw, but the actual average RF power was increased when modulation was present, so that a 1kw transmitter was actually running an average power of 2kw when modulated. this greatly reduced cooling requirements, especially cost effective for radio stations because the transmitter runs 24/7 mostly unattended.

the only way to do this with audio amplifiers is to have either variable or stepped/switched rail voltages. one shceme for this uses a variable power supply with a predictive method of changing the rail voltage, another just switches in a higer fixed rail voltage when the amp nears clipping at the lower rail voltage. i think the Carver M-400 cube used the first method and class g amps use the second method ( class h amps do too, but class h is a copyrighted term and a registered trademark)
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Old 25th February 2007, 03:10 AM   #5
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Default Re: Rail switching amplifiers and supply voltage levels

Quote:
Originally posted by rtarbell
In a rail switching amplifier (which some on this board call Class G, and others call class H - I mean an amplifier that has multiple voltage levels [usually 2 or 3] to switch to depending upon the amplitude of the output signal), the voltage levels of the supply tiers are usually equally spaced. As an example, QSC Audio's RMX1850HD uses +/- 44V, and +/- 88V. Some of their three tier designs use +/- 50, 100, and 150V. I didn't look at all the schematics, but I would assume that all of their rail switching amplifiers have rails equally spaced (meaning the higher voltages are integer multiples of the low voltage rail).

==> For a rail switching amplifier, on the average, is the maximum efficiency achieved with the rail voltages being equally spaced? Is there good reason for this, or simply convention because no other methods have been tried? Are there some well known rail switching amplifiers that do not have voltage rail levels equally spaced?
Equal rail voltage spacing lets the designer to choose same voltage secondaries as well same voltage rating capacitors as well, which inturn decreases cost...

Unequal rail voltage lets the designer to get maximum voltage swing as permitted by the maxVCE of transistor...
For example...
For a 200V Transistor...In 2 Step class H with +-64, +-128V equal rail spacing, the Max working imposed rail voltage under safe condition is 64+64+64=194 which is under 200V, but max output swing is limited to 64+64=128peak under no load conditions

In unequal spacing such as +-35V , +-120V lets the user to get max voltage swing of 35+120=155V peak under no load condition from the same 200V transistor and the transistor is still safe as the max voltage across it would be 35+35+120=190V

Equal spacing is low cost
Unequal spacing has high cost + better reactive load handling+ max voltage swing obtained....

Choice is yours...
Kanwar
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Old 27th February 2007, 01:42 PM   #6
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Thank you all for your replies, MOST informative!

Workhorse: you made a very valid point about the breakdown voltage of the output transistors, and the maximum Vce across each one in a rail switching output stage. All this time, I just assumed that the maximum Vce in the output stage would be the difference of the two highest supply rails. This is not true: only one of the highest supply rails might be used at any given time, not both.
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Old 28th February 2007, 07:13 AM   #7
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I'm fixing a Class G amp from Luxman that uses un-equal rails.

first rail is 74V and second rail is 102V.

They seem to use a simple zener for switching the rails over(looks like when zener falls out of conduction high rails transistor switch gets thrown into saturation)

If you want a peak at the schematic, just say so (if it's ok by the moderators)

BTW I seem to see a lot of people inter-changing the letters G and H for the same type of degin. From what I understand, Class G is a rail switching anp and cCass H is a rail modulated amp(power supply tracks colleter voltage +5V or more extra).
Is this correct? It's hard to find information on Class G and H.
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Old 28th February 2007, 10:07 AM   #8
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Quote:
Originally posted by ifrythings
If you want a peak at the schematic, just say so (if it's ok by the moderators)

BTW I seem to see a lot of people inter-changing the letters G and H for the same type of degin. From what I understand, Class G is a rail switching anp and cCass H is a rail modulated amp(power supply tracks colleter voltage +5V or more extra).
Is this correct? It's hard to find information on Class G and H.
Post the schematic, that wont irritate the mods...

Update your knowledge bank....

Linear switched Step=G
Switched Square Step=H
Modulated supply Tracking=TD
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Old 28th February 2007, 11:50 AM   #9
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ifrythings:

Thanks for your info; by all means, post the schematic if you can! (Unless of course you might get into copyright trouble...)
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Old 1st March 2007, 07:05 AM   #10
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Here's the the schematic of theLuxman I was talking about.

Quote:
Workhorse

Update your knowledge bank....

Linear switched Step=G
Switched Square Step=H
I'm lost now.... acording to this,your info is way different. (Or I'm not understanding it properly?)

Or is that info wrong???

Info is from http://www.wikipedia.org(Go almost to the bottom of the page.)
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