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Russ White 2nd December 2007 03:23 PM

THAT transistor headphone amp (250ma discrete opamp) design sanity check.
2 Attachment(s)
Hello Folks,

I was looking at my Mouser catalog the other day and noticed that they carried THAT corp matched transistors. I figured these would be a great input diff-pair + mirror and current mirrors for a driver stage.

The design goal is a headphone amp that can drive loads down to say 32-ohms (the average headphone is around 90-ohms). I figure a maximum of 250ma is very reasonable for the purpose.

I want the board to utilize SMT component as much as possible. I also don't want to bias the devices very high (no external heatsink) so I am thinking around 6-8ma. Using 3 pair of output devices allows the amp to still operate in class A for most of the volume range at which you would listen to headphones, yet still very good THD even when it goes into A/B.

Bias is adjusted with R22.

I chose the FZT1051A/FZT1151A from zetex for output devices because of the high current gain and low saturation voltage and good power capability for the task.

The circuit is designed for +/- 15V rails, but that is actually somewhat flexible.

I tried to isolate the power supply for the input/VAS/driver sections from the power transistors with R20/R21. This was to reduce supply ripple at those critical points.

R1/R2 would actually be a multi turn trimpot(20 ohm) to null the output offset.

Simulation gave me different results with the "gear" integration vs the "tap" integration. With gear the miller comp cap (C9) could be as low as 33pf and the amp would stay stable (in simulation) in trap it had to be at least 75pf. I will probably actually use 100pf for some safety margin.

It simulates very very well, but I just wanted to be sure it looks sane. Before I go buy parts and test it.

I have attached a PDF of the circuit.

Thanks for taking a look!


Russ White 2nd December 2007 03:33 PM

More details...
2 Attachment(s)
The THAT transistor part I would use is: 340S14-U

It is a SOIC14 part.

Here is the spice project which you can use with LTSpice.

Russ White 2nd December 2007 03:55 PM

Sim results
With 1V input 4vpp output 20khz, 32-ohm load:

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 2.000e+00 1.000e+00 179.88 0.00
2 4.000e+04 7.479e-07 3.739e-07 -58.92 -238.80
3 6.000e+04 5.004e-07 2.502e-07 -101.83 -281.72
4 8.000e+04 4.467e-08 2.233e-08 32.37 -147.52
5 1.000e+05 1.239e-07 6.197e-08 -89.18 -269.06
6 1.200e+05 2.436e-09 1.218e-09 -135.74 -315.63
7 1.400e+05 1.010e-08 5.050e-09 -100.76 -280.65
8 1.600e+05 4.799e-10 2.399e-10 -150.25 -330.13
9 1.800e+05 3.181e-09 1.590e-09 3.94 -175.95
Total Harmonic Distortion: 0.000045%

CBS240 4th December 2007 04:39 AM

Hi Russ

I have used this very part, 340 SOIC 14, and they work very well. The only thing is the Hfe, which I measured at about 88. You might consider a CFP or constant power arrangement for the input differential. These transitors don't like lots of current. My best results were with Ic between 0.8mA to 2mA. Changing the simple current mirrors to Wilson mirrors would keep a constant voltage on both THAT transistors. If you do this, go for low noise devices. I like and They have high gain and bandwidth, are cheap and also available from Mouser. They could be used as Q's 5, 16, and 17 as well. I measure Hfe for these at close to 400 with a DMM.:)
Another advantage is that I was able to use the THAT arrays without any degeneration. The price you pay for with Hfe, I guess, but the essence of a current mirror is not gain, it is equality.

KSTR 4th December 2007 06:53 AM

Hi Russ,

looks good, I'd say, epescially the output diamond buffer. Maybe you want to consider HF degeneration of the LTP (emitter Rs, shunted at the emitters with a coil for AF) to get an additional pole/zero in the open-loop gain. I also second CBS's comment on the Wilson mirrors but then you will need to lift the emitter of Q5 a bit (that is, lower it actually, e.g. with a bypassed resistor or a diode drop).

- Klaus

Russ White 4th December 2007 02:08 PM

2 Attachment(s)
CSB240, and KSTR thanks very much for your input.

I have been experimenting in SPICE with your suggestions.

I also decided that probably one of those THAT300 devices per channel is OK, the mirrors for the output buffer can be degenerated sufficiently the matching should not be too crucial. I used 10R which should be enough don't you think?

Here is the result. :)

4vpp 20khz 32ohm load (about 62ma peaks) output devices biased at 7ma.

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 2.000e+00 1.000e+00 179.82° 0.00°
2 4.000e+04 5.317e-07 2.659e-07 81.75° -98.06°
3 6.000e+04 1.589e-07 7.947e-08 -122.71° -302.52°
4 8.000e+04 4.046e-08 2.023e-08 50.80° -129.02°
5 1.000e+05 5.966e-08 2.983e-08 -77.56° -257.37°
6 1.200e+05 3.747e-09 1.873e-09 8.69° -171.13°
7 1.400e+05 9.068e-09 4.534e-09 -71.58° -251.39°
8 1.600e+05 3.267e-09 1.634e-09 95.26° -84.55°
9 1.800e+05 4.653e-10 2.327e-10 125.60° -54.22°
Total Harmonic Distortion: 0.000028%

Here is the new circuit.

Russ White 4th December 2007 02:14 PM

2 Attachment(s)
And here is the simulated freq response.

jcx 4th December 2007 04:32 PM

2 Attachment(s)
you might want to use a realistic input, load impedance, especially for estimating stability

I've played the earlier circuit and found next to no phase margin when I added 1 nF to the output to sim cable load C, and upped the Vin source to a few KOhm (being lazy I only used the "simple" Middlebrook test not the Tain loop gain probe - the loop gain probe may give different results)

I'd add 100 Ohm series with 100 pF from in+ to gnd and change the sim Vsource to include the reasonable midpoint resistance of the Volume pot

since it appears you're willing to play with the mirror/VAS, I think the ef buffer between them is a good deal - huge open loop gain boost - can be arranged for good symmetry, allows reasonable values of mirror Q degen to reduce their noise contribution

C10,11 just seem like a bad idea, C2 is too big, the closed loop gain should probably be higher, smaller area, faster, lower C Qs in the driver add a little to the phase margin in my sim
C1 has a hard time helping when LED incremental resistance is likely 10s of Ohms, its better for ps rejection to split R14 and bypass the midpoint, if you want to filter diode noise and isolate the 2 sides RC filters from the diode to Q bases are better

with these fast Q there seems to be no problem using Cherry's output stage enclosing compensation, I always like to try 2 pole to max audio band loop gain

V4 is the added "Middlebrook" test AC source, plot V(out)/V(fb)
or go for the full loop_gain_probe in the examples/Educational/LoopGain file

Russ White 4th December 2007 04:57 PM

Wow that look very nice. :)

Let me play with it for a while.

I try running a transient sim with "trap" integration and it was not stable. With "gear" it was stable.

Russ White 4th December 2007 05:12 PM

Also the bias was way too hot, the Qs for the Diff Pair need to have the same drop as the drivers. Thats why I used the same Qs.

V10 and C11 let HF AC through at low impedance to better drive the bases of the drivers. Interstingly adding them back in makes the amp stable in "trap" mode.

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