Headphone amplifier drop-in replacement for Phonoclone 3 and VSPS300

[rjm]

rev 14 boards ordered (14s1 variant, mounting hole spacing 2.75 x 3.55 inches to match ver. 10b)

Yellow boards with black silk screen. Oooh...

[kffern]

Originally Posted by rjm

@kkfern


If you measure the voltage across R12, R13 , the current V/R is the current though the output devices Q5 Q6. I'd appreciate it if you could report that number to me.

Hi RJM, I get 8.6 to 8.7 mV across 1.15 resistors. I don't know why I got much higher voltages before. It has been running for months though. A lot of the time with music. Through junk phones. I did blow a pair of decent earbuds though during early testing. I think it was too loud for a while.

kffern

[rjm]

That's 7.5 mA. Pretty much what I get. It will vary a fair bit on startup while the transistors come up to temperature.

It's less than I calculated because of "reverse thermal runaway", the output transistors cool as the drivers heat up. The new board revision mounts the transistors on the shared headsink so I anticipate that the output bias current should be 3x higher (so ~10x more class A output power).

Transient class B output power is defined by the voltage rails and output load, and for 16 ohm earbuds is about 3 watts.

The input is AC coupled, but for safety during testing (off transients can occur if you are poking test probes in the wrong places) please keep 50-120 ohms in series with the output if using low impedance headphones. The resistor can be temporarily rigged at the output headphone jack for example.

Also worth checking for high voltage transients on the output during startup/shutdown. (I will have a look at mine too. I use 300 ohm phones so never ran into a problem.)

/R

[rjm]

Latest BOM for the Sapphire Amplifier. Updated quite a bit with "helpful" notes...

Excel xlsx format, use Office 2007 or later, Open Office, Google Docs, or *download the free compatibility patch from MS* for older versions of Office.

[rjm]

The ordering web page is updated with information for the Sapphire project.

here

[rjm]

So I checked for any dangerous transients on turn on / shut down and could find nothing significant.

There is a small "thump" on turn on, but the measured DC on my Fluke meter doesn't move more than a few tens of mV. Switching off there is no sound through the headphones, but there is a short, slowly trailing voltage spike of <0.5 V for both channels as the capacitors discharge. Nothing here that would damage headphones.

DC offset voltage when powered up is very low, <100 mV, typically 20 mV.

[jaycee]

Using a good quality FET input opamp pretty much gets rid of any offset.. i could only measure perhaps 2mV on my circuit. edit - I guess this only applies if you have the diamond buffer in the feedback loop.

After the headphone death incident, I've decided I will be putting a protection circuit in! At the very least, a small relay wired between the two supplies - if one goes missing, the relay opens. I might even add DC detection. Can you tell how upset I am about losing my cans yet

[rjm]

Originally Posted by jaycee

Using a good quality FET input opamp pretty much gets rid of any offset.. i could only measure perhaps 2mV on my circuit. edit - I guess this only applies if you have the diamond buffer in the feedback loop.

After the headphone death incident, I've decided I will be putting a protection circuit in! At the very least, a small relay wired between the two supplies - if one goes missing, the relay opens. I might even add DC detection. Can you tell how upset I am about losing my cans yet

Once out of testing, losing one half of a bipolar supply would be extremely rare... However, to prevent damage you need to limit the output current to 250 mA.

I'm trying to think of the most elegant way to deal with it, but I wonder if a 30 ohm series output resistor might be the easiest and simplest solution. The other possibility is fuses on the secondary windings.

30 ohms output resistance would prevent 16 ohm headphones from ever seeing much more than 4 volts, limiting the output current to 250 mA.

...

Actually FET input opamps generally have higher output offset voltages for low impedance input circuits than bipolar types, though that's not cast in stone. As you say though, the output offset drops out if the buffer is included in the feedback loop. The point is even open loop, the offset is negligible and there is no advantage in improving this spec further than 50 mV or so.

R

[kffern]

Happy to report that the Sapphire and Phonoclone work fine while powered off the same supply. I have the Saphire running through the preamp tape out.

Following up on my listening comments, the sapphire does sound a bit bass light compared to the DAC1. I'm not sure whether it doesnt go as loud or low, or the DAC1 just has a meatier bass. Then again through my modded Lenco/Micro Seiki TT the bass sounds just right. If you've always wondered what those lyrics were, this is the HP amp for you.

Kffern

[rjm]

I agree, and it's interesting to hear you come up with these observations on your own.

Well, OK, actually I don't agree. The Sapphire seems bass shy, because there is no distortion of low frequency reproduction to generate higher, more audible, overtones. No "bloom" if you will. The amount of slam on tap is nothing short of frightening however. But yes, I can see exactly why you'd feel that way.