Low power 2 W @ 8 ohm, 4 - 16 ohm usable simple, class A SE - any recommendations?

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pelanj

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Joined 2008
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#1
I would like to build some low power amplifiers to power midrange and HF horns. My priority is absolutely zero hiss and hum with 110 dB/W.m drivers. A PCB or kit form would be great. I will need 4 channels in total

I was thinking about the Death of Zen headphone amplifier without the output resistor. That one looks like it fulfills most of the needs - the 4ohm operation is not mandatory.

I would like to have SE class A, since most of the time the amps will be in the milliwatt range.

I would like to avoid chip amps and class D unless there is a proven stellar performer.

I would prefer single supply with a coupling capacitor - the value will be selected to protect the drivers and also may be a part of the crossover for the HF horn (or the only crossover for it).

Edit: It needs to have enough gain to reach full power at consumer HiFi level.

Thanks for any suggestions!
 
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#2
You are probably aware that the JLH 10-15W class A amplifier design is a shoe-in for your requirement and is widely used for it. Where lower power is required, the output transistors can be much smaller (e.g.TIP41 can provide up to 8W @ 8 ohms impedance) and the supply voltage lower.

There are already a number of cheap Ebay kits which supply from a watt or so for headphones up to 8 watts output and checking these out will show you how simply it's done even if you want to build it completely yourself with your own source components .

For that matter, you could scale down the DOZ design similarly, though I don't recall seeing any examples of this.
 
#4
The 120R resistor fitted to the output of the DOZ headphone amplifier is for protecting the headphones from an accidental "serious amount of power". Headphones are high impedance and this is an effective way to limit power to them but inappropriate for low impedance speakers where much higher currents are expected.

If you want to build the DOZ design, use the amplifier just as the DOZ power amplifier design is presented. They are the same basic design, after all.
 
#6
Ian, I thought the DoZ headphone amp difference to the DoZ is lower power supply, lower quiescent current and cheaper transistors. I can do exacly that, thanks.

Rallyfinnen, I guess I need to read through the threads here to find out the differences.
 
#9
Have you considered tubes?

I use 3 Icon Audio 5w EL84 SET amps for just this purpose.
Could have built but had the cash and not the time..

They each drive a pair of

111db/w Raal Lazy Ribbon Tweeters.

Vitavox S2 compression driver upper mid horns - 110db/w.

Mid horns JBL2482 drivers at 111db/w.

I use them active. X/O etc done by WAF Najda DSP (now sadly not available).
No hiss, no hum.

Something like that circuitry would be good.
 
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#11
cumbb: I would prefer choke loaded. Since they will be used from 400 Hz and 4000 Hz, the chokes do not need to be too large. My DSP has balanced outputs, a good point. Any links? I do not feel comfortable enough to design by myself. Maybe time to start using Spice again🙂

Speedysteve7: Yes, my backup plan is a set of Chinese FU32 based amps. I have one and it is really great - nice sound and completely silent. I would like to hear some of these SS class A amps, I only know tubes and commercial SS amps. I wonder if they can match the tubes🙂

I use JBL2445 and Beyma CP385ND, both are around 110 dB/W.m.
 
#12
Psu 20 - 30 dc about?-:

- any BjT (sounds much much better than MosFet or JFet), hFE < 90;
- thick film resistor (sounds much much better than wirewound); (only one at collector: 10 - 22 Ohm; or one at collector and one at emitter (total of two; op-amp - self-balanced;-) 10 - 15 Ohm - depends on psu and heat sinks;
- trimmer: the mid at base; the other two: one each collector and emitter. Voltage at base about 1/2 voltage psu.
Output at the collector.

Unbalanced: Do not forget the caps; input and output.-)

Like this;-)
 
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#15
Would anyof these work: MJ15003, 2N3055, 2SC5200, TIP35C, TIP3055?Or any suggestion for another common type? These are from a local part dealer. I would prefer TO247 or TO220, if it could handle the current.

The bias current will be roughly 1/2 PSU voltage divided by the resistor value?

Trimmer 1k?

Would it work with an inductor instead of a resistor? 10 mH should be enough for 400 Hz and up I think. I need to fire up the Spice again🙂
 
#16
Never use a TO-3 type. Sounds ugly;-P

2SC5200 and other TO-3P sound fine and creamy, but a little bit inanimate and overcast;-)
Rough: The smaller the better.

Trimmer: 10 k or 5 k - should suffice.

Current = voltage/ohm - I hope;-)

10 mH - just try;-)
Any threads are in here. The most used a transformer of a microwave oven. > 100 mH. The more the better;-)
 
#17
pelanj said:
I would like to build some low power amplifiers to power midrange and HF horns. My priority is absolutely zero hiss and hum with 110 dB/W.m drivers.
Click to expand...
Should be good enough to present no issues on an unregulated supply, I assume? Since we're not talking excessive power I would consider going regulated though, similar to a headphone amp. Doesn't have to be anything fancy, a cap multiplier with a suitable Darlington may get the job done if you allow for enough dropout in idle.
pelanj said:
Edit: It needs to have enough gain to reach full power at consumer HiFi level.
Click to expand...
Please define "consumer HiFi level" in terms of Vrms max. There's a big difference between 300 mVrms and 2 Vrms! (About 16 dB to be exact. One would require 22 dB of gain, the other just 6 dB.)

Most people using very sensitive drivers have issues with too much gain rather than too little. The combined gain of preamp and power amp - more often than not anywhere from 42 to 46 dB - results in preamp noise / hum being amplified excessively. A good conventional preamp might be somewhere around 4 µV of noise (20-20k), plus 26 dB of power amp gain that's 80 µV out, or 19 dB SPL @ 1 m for your drivers. That's at least 12 dB more than I'd like, and with a good preamp no less (a mediocre one could be 6-8 dB worse still).

What is the preamp setup going to be like? I wouldn't go all passive what you've got multiple power amps to drive, there should be at least a buffer of sorts.

Your drivers are so sensitive, in fact, that no small amount of standard 26-30 dB power amps would still give some faint hiss on their own. (Even 30 µV = 10.5 dB SPL @ 1 m.) This is basically headphone territory @101 dB SPL / Vrms / m.

Are there any benefits to be had by going for current driving with these particular drivers, or does the horn make it a moot point? If the former, I would suggest a combination of some series resistance (maybe 8.2 ohms or 15 ohms for ~6 dB of padding right there, depending on driver) with a voltage divider ahead of the power amp stage for the rest of attenuation required, ideally 10+ dB. This would require more voltage swing from the power amp (equivalent to an 8 Wpc) but not any more current, and your coupling capacitor value would halve, which is always welcome since you'd still be looking at around 25 µF in what probably is a large film cap. The application would be looking even more like a powerful headphone amp then.

pelanj said:
Would anyof these work: MJ15003, 2N3055, 2SC5200, TIP35C, TIP3055?Or any suggestion for another common type? These are from a local part dealer. I would prefer TO247 or TO220, if it could handle the current.
Click to expand...
These would all do just fine. Honestly you could probably get by with a 4 A type like TIP41C/42C, too, at least in something like a JLH...
pelanj said:
Would it work with an inductor instead of a resistor? 10 mH should be enough for 400 Hz and up I think. I need to fire up the Spice again🙂
Click to expand...
Very good idea if you're not going to rely on a pre-tested concept.

Not like I generally think very much of SE Class A amps, which more often than not seem carefully designed for minimum efficiency and maximum distortion (read: making a hobby out of shooting yourself in the foot), but at least at these kinds of levels it's not like the power bill and heat output is going to kill you right away, and the parts count tends to be low (which does not equate to lowest cost, given that things like heatsinks and transformers cost good money these days).
 
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#18
I am driving the amps from a DSP with +12 dBu maximum input and output level, output impedance is 500 Ohm (Thomann DSP 4x4 mini). The signal source is RPi with Allo Boss DAC or Chromecast Audio set to 2 V output. In front of the DSP, there is a passive volume control.

I think there will be low distortion at the levels I am using. Cold class B sounds really awful with this power, I managed to replace one with class A tube amp, it works well, but I need it elsewhere.

So I just buy some parts, use my lab supply and scope and try to find out if this is a dead end or not. The inductor will be definitely tried out, I will definitely find somethat would not saturate at low levels in my parts bin..
 
#20
For zero hiss at 110dB sensitivity you'll need to pay attention to low noise design principles,

Note that 110dB at 1m is 130dB at 10cm, ie about 0.9µV will be at the threshold of hearing at 8 ohm and 10cm from the horn. That's 6.5nV/sqrt(Hz), or a 2.7k resistor's voltage noise.


I guess it depends what "zero hiss" actually means, but I'd take it to mean inaudible in a quiet room with head close to the
speaker (not necessarily pressed to it, that's a foolish thing to do). 10cm seems a plasible range of interest.
 
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