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Old 11th September 2019, 06:01 PM   #21
IVX is offline IVX  Ukraine
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so you don't need +/-8VDC rails at all, right?
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Old 11th September 2019, 06:13 PM   #22
xrk971 is offline xrk971  United States
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Originally Posted by RickTH View Post
At 2,90$ , even I would take a crack at soldering them but it wouldn't solve the cooling problem.
Shame on Texas Instr for not putting them in a DPAK (TO252) for proper cooling , or even a TO39 , so they can ramp up the price threefold for a metal can.
There is a thermal pad on the belly of the OPA1622, and once soldered to a pad on PCB with vias to the other side with more copper, the heat is a non issue. They are warm, but not hot when playing. My IR thermometer says 40C (PCB and IC) when playing loud.

VSON10 was surprisingly forgiving to solder with paste and hot plate/hot air on top. Main trick is to use paste very sparingly. Use loupe to check for bridges on pins and mop up with a copper braid and iron if needed.

The OPA1622 has a mute pin that is very handy. Connect to GND to mute output. This can be used with either a passive delay (via RC charge time constant) or an MCU/relay.

I think I paid around $5 ea for 10.
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Last edited by xrk971; 11th September 2019 at 06:18 PM.
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Old 11th September 2019, 08:55 PM   #23
drizdraz is offline drizdraz  United States
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Originally Posted by IVX View Post
so you don't need +/-8VDC rails at all, right?
I already have +/-8V to use (this HP amp is going into existing product). So cheapest solution is to use what's available, rather than LDO'ing lower or SMPS'ing higher, both which require extra cost and real estate.

I browsed your website and ASR review -- really amazing what you've done for $50! Wow. I think you need to increase the price to $200, otherwise you will go broke. Or repackage the little DAC in a big fancy box, and charge $10,000. Maybe I should ask you to help me with some offshore manufacturing

Are u in Shenzhen?
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Old 12th September 2019, 12:50 AM   #24
PRR is online now PRR  United States
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Originally Posted by drizdraz View Post
...I wonder about ear health listening to headphones with peaks over 100dB SPL. 100mW into 32 ohm cans roughly translates into >115dB SPL. That's freaking dangerous! Seems like 100mW is plenty for everything but perhaps 8 ohm IEM users? ...
IEMs are usually high-efficiency, don't need as much power to deafen you. (Idle hiss can be more the problem.)

I once *designed* for 120dB SPL into "any" headphone. This was a Special for in-room recording monitoring of live bands. I expected to use >100dB SPL only a few seconds a night, but I wanted to know any distortion was the recording path, not the headphone amp. Insane for normal listening. (And yes, I've gone deaf, though not from that amp.)

My survey led to 7Vrms behind 29 Ohms.

Agreed that 114dB SPL is ample for any non-insane purpose. Which leads to 3.5Vrms or 5V peak. Using available +/-8V supplies allow for high-drop buffers (Darlingtons). And at some point you have to let the User decide his own self-abuse (but you also put a warning in the manual!).

The 29 Ohms semi-equalizes power between low-Z and hi-Z phones. (The hi-Z typically need less power.) It also cuts-down idle hiss in 32r and down (IEMs). And is cheap protection for the amplifier (shorts happen).
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Old 14th September 2019, 12:55 PM   #25
Pano is offline Pano  United States
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Interesting. 29 ohms seems lower than i expected. Can't remember what I've got in series with my TI chips, will have to check.
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Old 14th September 2019, 07:48 PM   #26
PRR is online now PRR  United States
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> 29 ohms seems lower

That's a large-signal impedance, to taper max power over the wide range of impedances.

Small signal impedance may be different if you show that a given headphone gives a nicer response that way.

Roughly it tapers from 200mA @ 300r to 400mW @ 30r, reflecting the fact that most hi-Z designs are also efficient, and 7Vrms in 32r is way more power than we could want.

It's been over a decade since I had this spreadsheet. Now looking again, it appears that 7Vrms behind 29r will not drive ALL headphones to 120dB SPL, though the ones out at the 3 Watt line were "special" and not something for portable gigs. (Also "all" these models are discontinued, but the physics set some limits that do not change.)
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Old Yesterday, 01:44 AM   #27
sgrossklass is offline sgrossklass  Germany
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Just as a side note, 29 ohms outside the feedback loop is not something I'd consider generally acceptable... some IEMs will show an appreciable frequency response deviation even at 1 ohms (and, I imagine, may be tuned to specific nonzero output impedances in some cases), and even the world of full-size headphones is anything but exempt from this kind of shenanigans, e.g. the trusty Sennheiser HD558/598 should ideally be seeing around 10 ohms at the very most, and that's for a 1 dB deviation threshold (still perceptible but not dramatic terrain, possibly still too high for perfectionists).
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Old Yesterday, 04:07 PM   #28
PRR is online now PRR  United States
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The large-signal resistance can be inside the NFB, even put in the collectors of a p-p emitter follower.

Without it, an amp which is Loud Enough at 600 Ohms *may* at 32 Ohms blow your brains out too easily.
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