[Tomo]

Hey,

After been inspired with countless people, I decided it might be time for me to upgrade. I never really use my secondary system because I rarely listen outside of my own home these days. (My apology for the lateness.) In fact, it was so dusty I had to replace the rubber things for the Ety4S.

It is a simple opamp-buffer combo. No nothing. For the lack of BUF634, I don't even have rail splitter or ground driver for now, but I have less than 0.01V differential between rails after few hours of operation. I consider that satisfactory.

Usually when I build anything, I experiment. This time I used OSCONs really really close to the opamps and buffers. No Tantalum, No Ceramic. I am using Panasonic HFQ 2200uF for reservior. OSCONs require some time before organic-semiconductor stuffs to settle from thermal agitation during the soldering. But it already sound good.

I am currently running the amp using stock OPA134. I will be inserting OPA627 within next few hours.

I am surprised to notice that OPA134 + BUF634 gives much less harsher sound.

I will try to post the picture later. You will be proud of my wiring

T

 

[Tomo]

Hi,

OPA627 and BUF634 are absolutely heavenly with Ety4S. I think they make good combination. K240M would probably sound too laid back with the amp though.

I think I am gonna enjoy the sound today. I will delay testing other opamps.

Tomo

 

[zoboomofo]

Schweet! I just got my BUF634s yesterday! I too have been inspired by the recent activity here, and at Headwize. I'll be making a Burr-Brown amp: OPA627+BUF634, and then an Analog Devices amp: AD8610+EL2001 next. Can't wait for pics. I have no real idea how I'm going to lay these out yet. I'm thinking a portable rig for the AD amp, and a larger home version for the BB amp.

 

[Tomo]

Hey,

Here it is.

Small Circles near opamps are Sanyo OSCONs (4.7/16) And input is coupled with a pair of AB Carbon Resistors. Rest should be self explanatory.

Note this amp schematics is almost identical to Kevin's Lead Battery Power Portable Amp. (G=2 and it has no DC-DC convertor.)

Whapping Hi-Rez for Your Pleasure,

Tomo

 

[zoboomofo]

Nice tidy layout. Hi performance, and exotic parts seem to be the norm nowadays. Very exciting stuff to get into when it's available. AB = Allen Bradley?

 

[Tomo]

Hey,

Yea, they are Allen Bradley. It is supposed to be the cheapest of all HIFI Carbon resistors. But then it sound so good I keep on using it.

All the resistors are matched at least to 2% including Carbon Resistors. The board came with ground planes on both side. I think it is glass fiber board.

Really simplistic but it is pretty good. It pairs wonderfully with Ety4S I think. But then I wouldn't be sure it would go with every other headphones out there. I think it is a bit on laid back side. So I wouldn't pick very laid back amp to go with it. (Or I am just being paranoid)

T

P.S. The case is a mess. But then none of my amps have good looks. They are either scary looking or extremely mediocre. Everything was recycled from terminated projects. So I guess I can't ask more.

 

[Tomo]

Hi,

I used RMAA 5.0 to measure the performance of this amplifier.

Noise level -93.6dBA
Dynamic level 92.6dBA
THD 0.0018%
IMD 0.022%
Crosstalk -91.4dB

at 24bits 48KHz.

This is very identical to the characteristics of Audigy Card. I think all this say is that this amp is better than Audigy analog stage.

...

T

 

[ppl]

Tomo Since you did not use a virtual ground driver or a DC to DC convertor what did you do with the ground just tie it between the rail capacitors?

 

[Tomo]

Hi,

That is so.

T

 

[ppl]

Ah! you do realize that what you have in effect is a Large value output Coupling capacitor.

 

[Tomo]

Hi,

Really? Why so?

T

 

[ppl]

Well because the output from the Amp must form a complete loop back to the Power supply. The Ground is the midpoint Between the positive and negitive rails. in order for the load current to return to the supply it must pass through the rail capacitors. Since the Output Current is AC it will flow through the rail caps.

Try a Virtual ground driver ether based upon Buffers like we used to do or a high Current CFB type op amp set for unity gain to maintain a low output impedance all the way down to DC and eliminate the rising impedance as frequency is lowered that the caps alone will provide.

 

[Tomo]

Hi,

Ooo. I see what you mean. Headphone impedeance is in series with a capacitor which in turn is in parallel with the battery. Then again I do have eight 4.7uF OSCONs, two 0.47uF MKP, and two 2200uF HFQ.

4.7uF OSCONs should offer about 0.2ohm at 100KHz and there are four per rail so 0.05ohm. I think I get a little less than 1~2 ohms at 10KHz; so 0.125~0.25ohms. From HFQ, I should get less than 1 ohms at 10KHz. I think HFQ would stay below 10 ohms to quite low frequency ie 10Hz.

These capacitors are placed on both rail so their impedance become paralleled/halved.

This seems good. However, this isn't all. Remember the inductance of the circuit. Lets pick the worst case. Lets say I have 10uH. Z = wL So I get about 10 ohms at 100KHz. Halve that for having two rails. I get 5ohms.

So overall, supply impedance stay near 5 ohms. It isn't all that small but headphone impedance is 100 ohms (Ety4S). So it isn't all that big deal. (32 ohms is too small though) I guess there is virtue for high impedance headphones. Hail to thee, honored K240M!

T

P.S. Then again it doesn't hurt to have impedance below 1 ohm. I probably should try ground driver some day.

 

[ppl]

Hi tomo> the Impedance of the capacitors at high frequencies are going to Keep the Impedance low. My concern is at low frequencies. Now look at your Amps Output impedance it is extreamly low while the Impedance on the ground is alot higher. Since your phones are a three wire device this would be the same as placing a resistor in series with the Com. Headphone ground allowing crosstalk. In addition capacitors have a static charge and this in essence is floating the Entire ground of the Amp Between a Voltage source that is not stable since unlike a conventional power supply or even a VGD the ground must rely upon this floated charge for current. theroy states that on short Duty cycles the available AC current is only Limited by the max ripple current rating of the cap's. since in your case this is at least 1 or more Amps @ 120 Hz. you are safe since with a pair of BUF-634's 600 mA is all you can output anyway.

The real problem is the ground being supplyed current from a capacitor the Potential is changing in response to the load demand and this puts lots of noise upon the entire ground of the amp. this can be picked up by the inverting input.

I would do some distortion tests under load and see what kind of distortion you actualy get. i would also try a VGD and see if Distortion Improves. The use of a High current op Amp for the VGD ( I use LT-1210) will make the Impedance at low frequencies quite low.

 

[Tomo]

Hi,

I am kinda approximating using the data for 47uF/16V OSCON. It's impedance rises to about 10~100 ohms (oops) around 10Hz. But it drops below 0.1ohms at 100KHz. 4.7uF has about 0.25ohms at 100KHz.

You see that I underestimated the values for 2200uF.

Ripple current is like 1~2A. Crazy.

Interesting. Let me do current load characterization. (I will do it for 32ohms, 100ohms, 300ohms and 600ohms)

Lets make fair assumptions:

Wrms=Irms^2*R

And, we need about 20mW for ample volume.

I(32ohm) = 25mArms
I(100ohm) = 14mArms
I(300ohm) = 7mArms
I(600ohm) = 5mArms

Lets double that for safety.

Anyway, it is uber-clear that I can't drive Ety4S or Grado directly off of an opamp. But then it doesn't tell me much about the caps stuffs. Cap has max ripple current of 1~2A. I don't see how 0.01~0.02A can cause serious problems in dynamics.

Just an idea but you can potentially feed them both to non-inv and inv input. Considering CMRR, we may be able to forget about it. ... But then I am getting noise level below -90dBA. ...

Thanks for induging me ppl. I like to being convinced about stuffs.

T