Aspen Headphone Amp

[Bigun]

Originally Posted by keantoken

..Or we could just use one hole for one lead, and put a line of holes for the other lead. Same thing as the slot, really (Are you sure most PCB manufacturers don't do slots? It would be cooler).

Changes have been made to my schematic.

So, possible features:

1: Switchable crossfeed.
2: "Low bias" mode for battery operation (?)

Aren't I missing some? Perhaps a "line out" for use as a preamp?

- keantoken

I'm sure pcb guys can make anything you can draw, I can only guess, from a position of ignorance I have to say, that for a low cost pcb it's usually better to stick to simple, and one drill size would be even better (but not possible in this case).

Crossfeed - yes we need this
Low bias for battery - don't know, if the supply is clean a battery only helps with portability - assuming the source is also portable. I don't think we've optimized this design for portability - there are many other designs out there intended to be very portable.
Line Out - this is a good idea, not sure if impacts the pcb or is simply up to the constructor to add connectors and source selection switch ?

[AndrewT]

Originally Posted by AKSA

Now, the biasing. How about three 4148 diodes to set the output stage bias at around 40mA?..............
And if we assume a max output of 12Vpp into 32R (that's 590mW) then the max current ever drawn by the load would be 6/32 = 188mA, and with 40mA bias the Class A zone would run out at 2.56Vpp, which is 25mW, plenty loud in fact if we assume 100dB/watt, around 70dBA sound pressure at the cans........

that's not how I would do it.
I thought headphones were more like 90dB to 100dB per milliwatt not per Watt.

40mA bias allows upto 75mA of ClassA output current. Is it a push pull output stage?
Max power into 32ohm phones ~90mW
90mW gives ~110dB from 90dB/mW phones. That is more than loud and we are still in ClassA.

However, looking at peak current values gives somewhat different results.
12Vpp = 6Vpk. This voltage into a full range (no crossover) driver may draw double the current that a nominal resistive impedance would draw.
Peak maximum current draw could be 6Vpk / 32ohms * 2 <=375mApk.

I would design for that level of ClassA output. It will be enough for quality listening even when the average level is 30dB below the peak ClassA limit.
I would also design the output stage and the driver to achieve that 375mApk output when driven into ClassAB.

Is my logic/understanding correct?

[AKSA]

Andrew,

Thanks for your comments.

I think you are dead right about db/milliwatt. My mistake.

This would make things better rather than worse, I'd say.

We cannot go too high on dissipation because things will get too hot. With a 24V single rail and 40mA bias we still have to dissipate just under half a watt per output device, and this is about all we can tolerate with a small driver (4793/1837) and enclosure.

However, with up to 25mW output in Class A, this should be more than enough.

Cheers,

Hugh

[AndrewT]

Originally Posted by AKSA

This would make things better rather than worse, I'd say.

We cannot go too high on dissipation because things will get too hot. With a 24V single rail and 40mA bias we still have to dissipate just under half a watt per output device, and this is about all we can tolerate with a small driver (4793/1837) and enclosure.

However, with up to 25mW output in Class A, this should be more than enough.

agreed.

[AKSA]

Now, slots might be difficult, I say just select the correct pin spacing for a cap of that size and VW rating....

What's Switchable Crossfeed? It sounds very painful, can we do it?

Now, if 40mA is our chosen bias, what would low bias be? 10mA, 20mA??

If we use a single 4AH battery of 24V, the 40mA would exhaust 25% of the battery charge (1AH) after about 9 hours, taking account of LTP and VAS currents. That would be enough for two generous listening sessions. I say we don't need it.... but hey, this is a consensus design, so over to you. Might be that the best thing to do is build a ratsnest prototype, and try it out at different bias settings for subjective assessment.

Cheers,

Hugh

[iko]

Originally Posted by AKSA

Might be that the best thing to do is build a ratsnest prototype, and try it out at different bias settings for subjective assessment.

I now hear the voice of reason somewhere in the distance

[keantoken]

I also think someone should listen to it... Without that, our endless simtweaking is useless.

I've already got a project going...

- keantoken

[keantoken]

Or we could call it "class B" mode. Maybe we can get away with very low bias?

Or perhaps, we can put schottky's on the emitters for a kind of "non-switching" class B as was shown by PaulBysouth and Kenpeter in my Allison thread.

Now c'mon guys... All these exotic ideas and no one is willing to ratnest it and see what we can do? We're developing a product. We shouldn't shun a good idea because we can't predict its audible affect.

If no one else can, I might as well, if someone is willing to send me some spare parts...

- keantoken

[iko]

Hey, I'd gladly help but my hands are really full with something that needs to move forward, so, sorry, not this time.

[ostripper]

By KT - Without that, our endless simtweaking is useless.

A good point. The "sim" will assure that we have no fire or magic smoke , but won't tell us sqaut about what the final sound will be. The japanese Op's and my MJE's sim within 5% of each other (slightly different Vbe's)but I KNOW they sound different , either as a driver stage in a larger amp or as the main OP's on this one. I guess one could even try some TIP30/31's, dual supply , class/a or B , even ksa 992's as the input differential. This circuit will plot in a very similar manner with all the device I have mentioned.

I have gone "full circle" .. built an amp , changed input devices or voltage stage components .. THEN gone back (after hearing something I liked) and saw what was happening with my loop gain/UG/square wave to correlate "sound to sim". The simulation just gives one more "opinion" as opposed to just changing components and hoping for the best.
OS