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what class A amp for 4ohm speakers? DOZ?
what class A amp for 4ohm speakers? DOZ?
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Old 30th September 2017, 06:23 PM   #1
filthyone is offline filthyone
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Default what class A amp for 4ohm speakers? DOZ?

Hi people,
I want to build a class a amplifier for my 4ohm speakers, but i can't decide which one. I neen around 10-15 watts (for casual music listening).

There are lots of interesting circuits around, but i don't know which one would work better with the lower load impedance. I was leaning towards the zen amplifier because of it's apparent simplicity (i have already built the bride of zen preamp and i simply love it). But now i'm leaning towards the ESPs' Death of Zen, which seems an even better approach. I tend to prefer BJTs over MOSFETS, and i already have the output bjts for DOZ.
Do you think it will cope with the 4 ohms? I think i need to use a lower supply voltage, maybe 25 volts DC single supply, but increase the bias current to maybe 4 Amps. I already have two YUGE heatsinks (4,5 Kg of black anodised aluminium each) so heat management won't be a problem. I also have lots of 2N3055s that i would like to use if possible (i know they are not the best out there), and six 33.000 μF / 63 Volt sic-safco capacitors, never used but i reformed them and they are ready for some serious CRC buisiness (they all measure a bit over 40mF!). I should only need a suitable big transformer.

Does anyone have to suggest any other circuit that would work better with my speakers? Any specific advice on building Doz?

Thanks for any input!
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Old 30th September 2017, 08:19 PM   #2
bwaslo is offline bwaslo  United States
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what class A amp for 4ohm speakers? DOZ?
No circuit suggestions (sounds like mine would be more complicated than you might like).

But if it helps, for 15W of Single Ended class A at 4 ohms you'd need to idle each channel with at minimum
sqrt(2*15W/4ohms) = 2.74 Amps
when using current source or inductor fed Class A.
With inductor fed SE, the supply voltage would need to be at least
sqrt(2*15W*4ohms) = 11V
and with current source fed SE you'd need at total of 22V supply (+11 and -11, each idling at >2.74A).

For resistor fed Single Ended, like basic Zen, idle current would have to be well above the 2.74A and the total supply voltage would need to be well above 22V also (depending on the resistor).

With push-pull operation the idle current can be half that much, 1.37, and total supply voltage needs to be at least 22V.

Those all assume ideal operation, of course, with real parts you'll need some extra of supply voltage and idle current.
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Last edited by bwaslo; 30th September 2017 at 08:23 PM.
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Old 30th September 2017, 09:43 PM   #3
HI-FI PRO is offline HI-FI PRO  China
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I have to dissagree with you. For push pull idle current must also be at 2.74A or higher to ouput 15w rms. (push pull output devices only conduts one at a time per cycle)
lets say push pull or so called ocl amp idles at half of this. This means the upper voltage will only swing V=IR

1.37*4=5.48v and lower voltage swing -5.48v. Because of crochoff lawcurrents entering node must equal to current leaving node, to output 1.37a there must be 1.37a in series accross transistor. and current is fixed you can now calculate maximum voltage avalibe to speaker or directly calculate power from P=I*I*R

For your voltage requirement, we know 2.74a results in 15w rms, this means v=ir
you must have supply voltage of 2.74*4 or more volts. =11v
thefore to be practicall voltage supply should be at least +-13v and best +-15v
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Old 1st October 2017, 12:55 AM   #4
bwaslo is offline bwaslo  United States
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what class A amp for 4ohm speakers? DOZ?
Don't think so. Say you have a push pull amp with 1.37A idling through its outputs. At idle no current goes out the output. At positive peaks, the top output's current doubles, so it conducts 2.74A, while bottom output is right about to cutoff (0A). So +2.74A peak goes to the 4 ohm load. At negative peaks, the opposite happens (top device is at 0A, bottom is at 2.74A) and -2.74A peak goes to the 4 ohm load. And the peak voltage on the load is 11V,

2.74A peak for a sine wave means its rms current is 2.74*0.707 = 1.94A.
Sine wave power in a resistive load is I^2 * R = (1.94^2)*4 = 15W.
In voltage terms, the 11V peak is for a 7.75Vrms sinewave and V^2/R is your 15W again.

When the load isn't resistive, more current would be needed, but if we're talking design for a 4 ohm load, only 1.37A idle current per lung is all that is (theoretically) needed. Remember that in push pull, the current through the load is the DIFFERENCE between the positive and negative device instantaneous currents! Your Kirchoff's law says the currents entering a node have to equal the currents that leave, so when one polarity output device doubles and the other polarity cuts off, that double current has to go out the load.
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Last edited by bwaslo; 1st October 2017 at 01:00 AM.
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Old 1st October 2017, 01:10 AM   #5
HI-FI PRO is offline HI-FI PRO  China
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If your assuming idle is half of max current then for se transistor like death of zen idle current only needs be be 1.37a.
So if one device cuts off then no current will be supplied from that device, idle at 1amp then max class a is 1*1*speaker resistanc. assuming your speaker has enough voltage supply to swing.
Only one device will conduct during uper cycle or lower with death of zen or any ocl amp.
Same power formula should apply to single transistor se amp cap or transfomorer couppled.

If you don't belive me, do a lab test with oscillie scopes and current meters. and a high power load reisstor or speaker.

Last edited by HI-FI PRO; 1st October 2017 at 01:17 AM.
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Old 1st October 2017, 03:54 AM   #6
koja is offline koja  Canada
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check out First Watt product line up. the amount of generated heat and different sonic qualities i.e. suitability for particular speakers and choice of music are all a consideration. F4 offers the unique opportunity to entertain different preamps for swinging voltage. I myself use it in a bridged/balanced configuration. F5 drives clarity in highs (probably not the best choice for forward sounding FRs and such but good for speakers with less pronounced top), controls bottom really well, drives low impedances and has lower heat output than others. Aleph30 has an outstanding mid range but poorer bottom (might still be good with a bass reflex or a similar speaker) and into 4 ohms will generate a ton of heat (I would not use it there). these are some I have experience with. I also use ACAs in a multi amped system as the power is only ~6W/ch.
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Old 1st October 2017, 05:48 AM   #7
HI-FI PRO is offline HI-FI PRO  China
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My view on sound quality is best to have high quality schematic.
Tried poor schematic Class A vs class ab.
Class AB with very good schematic sounds best. Then you can good schematic+ class a.
My impression on class a vs ab is only minimal diffrence. There is a diffrence but not very big. This is especially true with poor quality schematics. Good scheamtic you hear a bit more from ab to class a.
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Old 1st October 2017, 04:08 PM   #8
filthyone is offline filthyone
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Thanks all,
now, aside from the conjecture over single ended and push pull idle current...

Do you think it is a good idea to double the outputs of DOZ, just to be on the safe side? I mean two 2N3055 per side, with a 0.1 ohm balancing resistor. (I have lots of them, i could even triple them)!
Will it stress the driver transistors more, the same, or less? My intuition tells me that it ought to be about the same stress on the BD139s, just better dissipation capability on the output. Am i wrong?

I'm talking about this schematic
http://sound.whsites.net/p36-fig4.gif

Last edited by filthyone; 1st October 2017 at 04:11 PM.
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Old 1st October 2017, 08:58 PM   #9
HI-FI PRO is offline HI-FI PRO  China
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Absolutely you can doubble the outputs and put 0.1ohms, the stress on bd139 stays the same.
However make sure you buy geniune bd139 from trusted stores. Allot of fake from china and aliexpress.
Fake bd139 has low power rating and low votlage rating.

You can always put some heat sink cooling on drivers. or use somesort of high power to-220 transistor like tip41c, if they do get hot.
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Old 2nd October 2017, 09:42 AM   #10
filthyone is offline filthyone
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Thanks a lot! Will do...
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