[tbdoah]

Here are the parts for a Meta that I'll be using for amping 4P's w/ a 4P -> 4S cable, DT770Pros, and eventually DT831s.

AD712JN or AD823AN?
C1 - 0.22uF Panasonic polypropylene
C2/3 - 1000uF Panasonic FC
C4 - 10 ìF 16V
EL2001 (one per channel)
27mm ALPS "Blue Velvet", 50K
R1 - 1k ohm
R2 - 475K
R3 - 220K
R4 - 2.2K
R5 - 3.3K?
R6 - 3.3K?
R7 - 100
R10 - Doubled 100s?
RCA/mini In
Mini Out


Thanks a lot for all your guys help on this board

.

-T

 

[mclaren20]

shouldnt you go for a 1/4 out?

you never know when you'll get some phones with that on them, and the 1/8 >> 1/4 plug is so much more commen. oh well, just a suggestion.

edit

hell man, do both.

 

[myself, aka me]

Your resistor values are strange.

For a gain of 5 try these:
R3 - 470R
R4 - 2k2
R5 - 2k2
R6 - 220k

I don't remember what the DT831 is, but if it has a high impedance you could build a gain switch in.

I think you can shove a resistor in eitehr R8 or R9 [I have forgotten which is outside the loop] and there you have it - built in 4P -> S cable. 1/4 out is good. You're a T-boarder, probably have a D66, in which case you'll just use that quarter out. Stacked EL2001s are good too - I didn't 'cause i'm not skilled enough to solder stacks, I fear i'd burn 'em with the iron. haha. Wall powered? Battery powered? I think the AD823 is good for batteries, but you can do better with a wall chip. Perhaps a pair of AD843 is what you're after, but it needs lots of volts. [and a browndog adapter].

 

[guzzler]

stacking buffers is really easy, definately do it!

g

 

[jeffreyj]

Tough call on the setup for this one. The ER-4P's with the P->S cord have the following specs:

100R impedance
98dB SPL at 1mW
3.0Vrms max. input (90mW)
30mA max. current.


The ER-4P's without the cord, though, have a very different set of specs:

27R impedance
107dB SPL at 1mW
0.75Vrms max. input (21mW)
28mA max. current


Assuming that the typical portable device can output 0.8Vrms without distorting, using the ER-4P's with the converter cord will require a voltage gain of 5 while using them natively will require no voltgage gain. In both cases, though, 1 buffer should be more than enough to supply the maximum amount of current the headphones can take.

From the above data, it is obvious that the cord that converts the P's to S's merely puts 75R resistors in series with each channel (not quite worth $65, eh?), so mclaren20's suggestion was a good one

 

[guzzler]

the advantage of stacking buffers is not just for the current, if that were the case, one buffer would suffice for most phones. Stacking them decreases the output impedance, provides (possibly) better transient response and allows each chip to run cooler. However, you pay the price with increased current draw...

g

 

[tangent]

Stacking buffers also improves slew rate. Look at the SR vs. load or current graphs in any buffer's datasheet. This accounts for the lower THD one measures when stacking buffers.

 

[jeffreyj]

buffers, schmuffers!

Assuming we are driving the native ER-4P's, how much slew rate do you think need to assure "distortionless" amplification of a 2Vpp (0.7Vrms) signal? Well, the equation for determining this in V/uS is:

S=(Vpp*Pi*f)/10^6

Plug in the unadulterated numbers and you get a relatively unimpressive. 0.125V/uS?!

A 20kHz sine wave will be faithfully replicated by an op-amp with the above slew rate driving the above peak-to-peak voltage.

Now I know that having merely adequate slew rate is not enough - the above equation does not indicate how the driver stage has to work very hard as it approaches slew limiting, and that distortion may very well begin to creep in despite the signal not exceeding the slew rate. I agree that it's best not to push one's luck, so multiply the required slew rate by 10x and you ought to be well and truly covered.

g. - I don't think having a super-low output impedance is terribly important when driving headphones because they have much more mechanical damping intrinsically than speakers; a similar discussion took place on Headwize recently and the conclusion was that a damping factor of 2 is more than adequate.

 

[Dreamslacker]

Do definitely stack 2 layers of buffers for each channel. The power side buffer can remain as a lone piece.

 

[tbdoah]

Hrmmm, good idea on putting 75 resistor in R8 or R9 would save me $69 bucks

.

So, I should stack buffers, 2 deep I'm guessing? So 6 in total.

Actually me, I'm thinking of using both battery power and a wallwart, but even after reading tangent's DC section, I'm still a bit dazed on how to wire a wallwart and battery with a switch. I'm pretty sure it's a closed circuit, and then i put the spst switch inbetween the input terminals on the board and the power sources?

Thanks for your help

 

[JMT]

Just get youself a switched jack, no need to wire on an additional switch. Check out this thread here.

 

[guzzler]

you only need 5 buffers, the power has no advantage from being stacked...

jeff; i sort of skimmed that thread on Headwize, must admit it's way over my head

the thing is though, most folk have reported an improvement (there could always be that dreaded placebo effect at work though!) from stacking buffers, and it's not a huge amount extra so unless battery life is a real issue, it sort of makes sense to do it! each to their own though!

g

 

[tangent]

Quote:

there could always be that dreaded placebo effect at work though!

You blame placebo effect when you can't measure a difference. You most certainly can measure an improvement from stacking buffers.

 

[tbdoah]

Thanks JMT for info on that switch...

Anyway, about that 75k resistor, how would I put it in?

 

[JMT]

75R, not K!!

Just put it in line with your output, either tombstoning them in OR/OL or on the output jack solder tabs.