4/8 ohm Stereo chip for Headphone amp? - diyAudio
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Old 8th April 2010, 06:06 AM   #1
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Default 4/8 ohm Stereo chip for Headphone amp?

I got a TDA 7377 that I'm thinkin of using for a headphone amp.

Can you use a chip designed for 4 and 8 ohm speaker loads for a headphone application?

I know this chip is over-kill, I will run it at a min. voltage.

The chip is 2x20w @ 4ohm or 4x6w @ 4ohm, actually there is lot's of ways to wire the bridgeable 4 channel amp.

It takes a regular 12V power supply, not building a portable amp so power consumption is not a concern.

I've got all the parts I beleive. I could build a moderate stereo amp, or a kick-butt headphone amp.

Since the ohms increase with headphones, I assume the wattage used by the chip would be less?
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Old 8th April 2010, 06:39 AM   #2
Mooly is offline Mooly  United Kingdom
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Yes it will work, but as you say is overkill and not ideal.

You will need to pad the outputs with series resistors to bring the level at the phones down (and also the noise from the chip), just as in a normal socket on a "normal" amp.
Suggest something around 120 to 330 ohms for starters.
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Old 8th April 2010, 02:22 PM   #3
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I guess I could use a variable resistor of sorts, to try a few values.

When I "pad" the output, do I connect the resitor to ground, or in series with the speaker?
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Old 8th April 2010, 05:12 PM   #4
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I've used the following and it seems to work great:

Headphone Adaptor for Power Amplifiers
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Old 8th April 2010, 06:46 PM   #5
Mooly is offline Mooly  United Kingdom
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For simplicity I meant just in series, which is what the majority of amps with a headphone socket do.

Redshifts187's link is great... that's a better way as it provides a more constant and known output impedance, and you have said power consumption isn't an issue.

So why not try both methods and see what you think
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Old 8th April 2010, 08:39 PM   #6
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Thanks uys, I will try both.

I thought I would use this chip because it sounds good and thought by downsizing the load to headphones it would maintain or raise the fidelity, as it can drive the headphones effortlessly.

It has
0.03 THD at 2ohms
0.02 THD at 4ohms
0.?? THD at headphone impedances.

Is this an accurate assumption, or am I making the fidelity worse?
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Old 9th April 2010, 06:16 AM   #7
Mooly is offline Mooly  United Kingdom
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Basically correct
It's not just about pure numbers as to how something will sound anyway, re the valve amps that sound totally effortless and musical despite having distortion perhaps approaching 50 to 100 times the levels you quoted. It's the type of distortion, the spread of the harmonics etc that really colour and define the sound... and often we like that... and prefer the amp with the higher levels.
Class b output stages and long tailed input pairs tend to generate odd harmonics the amplitude of which rises rapidly with increasing frequency which isn't good, however your chip being so lightly loaded this is hardly an issue.
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Old 9th April 2010, 07:34 AM   #8
wwenze is offline wwenze  Singapore
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Quote:
Originally Posted by TechnoBeaver View Post
Thanks uys, I will try both.

I thought I would use this chip because it sounds good and thought by downsizing the load to headphones it would maintain or raise the fidelity, as it can drive the headphones effortlessly.

It has
0.03 THD at 2ohms
0.02 THD at 4ohms
0.?? THD at headphone impedances.

Is this an accurate assumption, or am I making the fidelity worse?
THD vs output power graph, starts high and gets lower as power increases until somewhere in middle where it starts to increase again.

Background noise, speaker amps tend to have more of it than headphone amps.

That's why the resistor in series is needed - to waste that noise.
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Old 9th April 2010, 03:08 PM   #9
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To bad my TDA7377 Datasheet does not have any graphs.
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