Marsh headphone amp from Linear Audio

[Telstar]

Originally Posted by RNMarsh

Using worst case load (33 Ohms) shows the 2nd and 3rd harmonics are in the neighborhood of -90 or under .01% that I said in the article you would get if you did not match transistor compliments. But with matching you can get well below -100. Thx for trying various devices for the DIY-er who might not find the ones I used.... however, I bought all transistors over the Internet for 10-15 cents each. It appears the topology is pretty immune to the specific transistor part..... another plus. -RNM

But what would be your preferred output transistors in this circuit? (low Z HP around 40 Ohms)

[jan.didden]

Originally Posted by homemodder

Power handling, inductance and accuracy.

There are drawbacks too but one makes these decision based also on the functionality of the resistor.

Power handling is the same whether you have two parallel or two series: each takes half of total power (assuming equal resistors).
Accuracy also the same.
Induction is double in series and half parallel, but capacitance is double in parallel and half in series.
Voltage coefficient effect is half in parallel.

So it all depends. If you are concerned about voltage effects go for series - each only sees half the voltage.

jan

[magnoman]

Originally Posted by janneman

Voltage coefficient effect is half in parallel.

So it all depends. If you are concerned about voltage effects go for series - each only sees half the voltage.

jan

Jan,

The above conflicts I assume a typo and you mean Voltage coefficient is half for the series case.

Thanks
-Antonio

[simon7000]

When you place two resistors in series the inductance increases if they are inline. If you double them back to form a "U" then the inductance decreases! So using two in series can result in lower capacitance and inductance!

However an important issue is thermal matching. Due to tempco there is some third order distortion present. If the feedback voltage dividers are run at vastly different power levels then the thermal lag between the distortion can result in a small level of 9th harmonic distortion. So using a 4R/R divider offers a bit of an improvement.

[jan.didden]

Originally Posted by magnoman

Jan,

The above conflicts I assume a typo and you mean Voltage coefficient is half for the series case.

Thanks
-Antonio

Yes a very awkward statement of me, sorry. What I meant to say is that voltage coefficients effects are half because the voltage is half; the voltage coefficient itself, - the change in resistance per volt across the resistor - is a property of the resistor and doesn't change with the circuit.

jan

[jackinnj]

Speaking of output transistors -- the MJE200/210 hfe will vary from ~150 to 210. I found that the 2sb649ac/2sd669ac varied somewhat less. These have a higher ft than the On-Semi devices.

I think that the -3dB bandwidth which Richard stated in the article (25kHz) is extremely conservative as mine goes out quite a way...

[RNMarsh]

Originally Posted by jackinnj

Speaking of output transistors -- the MJE200/210 hfe will vary from ~150 to 210. I found that the 2sb649ac/2sd669ac varied somewhat less. These have a higher ft than the On-Semi devices.

I think that the -3dB bandwidth which Richard stated in the article (25kHz) is extremely conservative as mine goes out quite a way...

Are you refering to Open Loop BW? That 25kHz was for Open-loop BW.

[RNMarsh]

Originally Posted by janneman

Slightly OT:

Spend an afternoon with a friend who had a whole stack of Richard Marsh pre- and power amps.
Sounded pretty nice to my ears!
Technical build was immaculate.

jan

I find the largest limitation is not amps and preamps but the source material. The HD downloads and a great DAC give the best sound I've ever heard to date. Really lets you know just how good your amps and preamps are capable of sounding. With this headphone amp, you will hear the closest sound to a master possible.

[ at the present time, i use Senn HD800 and HiFiMan HD500 mostly... a few others are here but not used as often]

[RNMarsh]

Originally Posted by homemodder

True, I cant hear distortion at .01 levels but on paper this amp still has high distortion, this is just 1 volt output. Lets not even indulge with what can be achieved with opamps.

The circuit values are not optimized for high voltage output... just for headphone levels. And for low Z (at least down to 30 Ohms) which few if any op amps can do by themselves. Today we dont need much more than a couple volts rms to drive anything to full rated output. Using similar topology and only 20db global negative feedback, levels of under .001% at 22 v p-p output was obtained. Note: this was in my TAA article back around 1980! But that was into a high Z. Levels of this amp are in the -110-120 dB range into 30 Ohms and dont forget -- DC coupled without servo and no drift.

In case anyone is wondering, I have used many sim programs and have MicroCap as my favorite. However, this circuit was not developed using any sim programs. Sim programs are great learning tools and useful for some things, but after awhile you just dont need it anymore if you've done enough circuits. This one just isnt that complex.... very refined, yes.

Other than that --
Thx,
Richard

[magnoman]

Originally Posted by janneman

voltage coefficients effects are half because the voltage is half; the voltage coefficient itself, - the change in resistance per volt across the resistor - is a property of the resistor and doesn't change with the circuit.
jan

Jan,
Yes I know what you meant, thanks.
But if you consider the 2 series resistors as one equivalent R, then the voltage coefficient for this R is effectively half of each of the individual's.

Originally Posted by simon7000

When you place two resistors in series the inductance increases if they are inline. If you double them back to form a "U" then the inductance decreases! So using two in series can result in lower capacitance and inductance!

Simon,
How does doubling back decrease the capacitance?
It would seem to me that doubling back would add a small lead capacitance as the potential extremes are closer to each other.

Thanks
-Antonio