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-   -   Scaled-down Bridged Zen for Headphones (http://www.diyaudio.com/forums/pass-labs/741-scaled-down-bridged-zen-headphones.html)

paulb 14th September 2001 11:26 PM

I've seen the discussion on a bridged Zen. What about a scaled-down version for driving 32-ohm headphones? It should be straightforward to calculate the required power levels and scale the resistor values accordingly. Using a bridged version is essentially the equivalent of each amp driving 16 ohms.
It could be driven from the Zen balanced-out preamp (was that the Bride of Zen, I think?).

rickcr42 15th September 2001 02:25 AM

Why go to the extra expense of a bridged zen ?The transformer and heat sink cost alone would be way high for a headphone amp. Just add a resistor voltage divider to the output for headphones,mabe say 100 ohm/1 watt from each output to the corresponding leg of the phone jack and 10 ohm/1 watt from each of those legs to the ground lug of the jack.Check the HeadWize Library FAQ section for the specifics

Rick

djk 15th September 2001 10:00 AM

Headphones have a common ground.Most bridge amps cannot drive a common ground.

paulb 15th September 2001 02:45 PM

Oops, right, the common ground. So that's what a brain fart feels like....

Okay, just the scaling-down part. Take the Headwize Szekeres buffer, make it common-source to get some gain, use a current source instead of a resistor,...suddenly it's a Zen. I don't want to just build a big amp and chop down its output; that really would be a big waste of power supply, heatsink, etc.
So I'll restate the question: has anyone tried scaling down the Zen to lower wattage / higher load impedance?

Thoth 15th September 2001 03:20 PM

How much power are you interested in putting out? 1W?

I've never done this, but here are some ideas:

The starting point would be the Zen Revisited amp. You should be able to cut the idle current (increase R1), lower the gain (increase R11), and be done with it. Lowering the supply voltage (25V) should lower the heat output still more. You should also be able to cut the output capacitance in half, but this isn't necessary.

Going still further, cutting down on the input filter capacitance (5000uF), and going to a larger inductor (4mH/18ga) would probably decrease ripple. Going to a double-PI filter (3 caps and 2 inductors) would decrease the riple even more. Use PSUD2 ('http://www.duncanamps.com/psud2/index.html') to check the ripple at the required load.

My understanding is that the distortion should drop with the lower gain.

[Edited by thoth on 09-15-2001 at 11:04 AM]

GRollins 15th September 2001 11:13 PM

Paul,
The Zen would be a fine choice, but what about the SOZ? A small version would be a cinch, and it's already scalable.
Amps are stable into higher loads, you need not worry about a 32 ohm load on a nominal 8 ohm output. Many designs actually give better performance into higher loads.

Grey

paulb 15th September 2001 11:38 PM

Yes, I have thought about the SOZ too. Something about me has a problem with a resistor instead of a current source, but in the interests of starting with simplicity maybe I should give it a whirl. I've seen some reports that the resistor version "sounds better". Hmmmm.

djk 16th September 2001 05:05 AM

Are we still talking headphones? The SOZ is a bridge amp too. http://www.passlabs.com/images/projects/sozenf1.jpg You cannot ground either output.

paulb 16th September 2001 03:10 PM

You could just use one of the outputs. If balance is important, a dummy 32-ohm resistor on the other output would be easy (remember, we're only driving maybe 50 mW into the phones).
Since the SOZ drives its load differentially (equivalent of 4 ohms per side), I think I could scale all the resistors by a factor of 8. This should reduce the power output by the same amount. If I go for the 5W version voltages, it would mean power of 0.625 watts - should be plenty for headphones.
Comments?

Thoth 16th September 2001 05:54 PM

The 'problem' with the SOZ seems to be that that it's a balanced amp. Stereo headphones have a TRS (Tip-Ring-Sleave) connector. The tip and ring supply the signal to the headphone drivers. The sleave is the common (not necessarilly ground).

I think that if the power supplies for the two amps are separate and isolated (amp grounds not connected together or to earth ground), and the undriven input is connected to amp ground, then the outputs associated with the grounded inputs could safely be connected together to form the common.

There is a potential safety issue here, in that the common is usually at ground potential. This headphone amplifier will have a floating common. It might be possible to connect this floating common to earth ground, but I 'm not sure.

Will this work?


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