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Old 11th September 2012, 06:06 PM   #1
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Default How do I Improve bass handling in a headphone amp?

I have built a grado ra1/chu moy stlye headphone Amp. I'm using a dc to dc converter to get a voltage boost and proper balancing rather then dealing with dual 9v batteries. I have added a opamp voltage power rail splitter and changed the psu caps to 470uf the Amp measures very well but it's bass shy compared to my headphone outputs on my pro audio interfaces. how do I measure the headphone handling so I know what to adjust in a diff headphone amp design so I can have a similar amp to my audio interfaces when I'm on the go?
here is my voltage splitter

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here is what I'm getting out of my fast track pro

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Frequency response (from 40 Hz to 15 kHz), dB +0.05, -0.04 Excellent
Noise level, dB (A) -100.3 Excellent
Dynamic range, dB (A) 100.3 Excellent
THD, % 0.0010 Excellent
THD + Noise, dB (A) -92.2 Very good
IMD + Noise, % 0.0031 Excellent
Stereo crosstalk, dB -86.2 Excellent
IMD at 10 kHz, % 0.0035 Excellent

Last edited by doulos24; 11th September 2012 at 07:00 PM.
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Old 12th September 2012, 07:29 PM   #2
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Alright well I built a headphone load box but I'm still not seeing the measurements I'm looking for anyone know how to measure frequency response under headphone load?

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Old 12th September 2012, 08:32 PM   #3
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How about a fake load (102 Ohm?) and measuring voltage across it using a voltage divider as input to your sound card, then use some frequency plotting software.
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Old 13th September 2012, 01:27 AM   #4
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that might work thanks
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Old 14th September 2012, 12:46 AM   #5
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I don't think that FR measurements with a fake load will help you w.r.t. the bass.

Almost all headphone outs on audio interfaces have a high output impedance, such as 20, 50 or even over 100 ohms. This reduces the damping factor which has a couple of negative effects (FR variations, higher THD ..). Since most dynamic headphones have a resonant frequency around ~80 Hz the headphone driver impedance also rises and has a peak there.

The output impedance of the amp and the load impedance work like a voltage divider. This will cause a bass boost. (Actually, the output impedance will attenuate the resonant frequency less than other frequencies where the load impedance is lower.)

If you still want to increase the output impedance instead of using an EQ: just put a resistor in series with each headphone driver.

Last edited by xnor; 14th September 2012 at 12:49 AM.
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Old 14th September 2012, 03:34 AM   #6
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Better to use a genuine dual-rail supply than one of these voltage divider 'virtual earth' arrangements, especially one built with a 741.

The 741 doesn't have a lot of slew rate to keep up with whatever opamps you're using for the amplifier proper and it doesn't provide much current. Both of these are considerations. You need twice the current of a single channel.

The '3rd. channel' designs are pretty much discredited. Even when properly implemented they contribute crosstalk and distortion. You'll get much better results with a proper dual-rail supply, even if it's only a couple of rechargeable 9V NiMhs.

You can build a dual-rail supply with a single AC wallwart. 12V 500mA or better will do. Preferably a little bit more than 500mA.

Use 2 NJM4556 opamps in parallel (1 dual) for each channel of your cmoy.

Put a 16 ohm resistor across each channel when doing your RMAA measurements, this will show the design at its worst likely performance into headphones. Few headphones are < 16 ohms, 16 ohms is not uncommon though.
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Old 15th September 2012, 02:30 AM   #7
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Quote:
You'll get much better results with a proper dual-rail supply, even if it's only a couple of rechargeable 9V NiMhs.
not according to my measurements I tried a tle2426 and a max1044 I'm using a ad817 50ma output current not a lm741 I just needed a general opamp for the schematic

Quote:
Put a 16 ohm resistor across each channel when doing your RMAA measurements, this will show the design at its worst likely performance into headphones. Few headphones are < 16 ohms, 16 ohms is not uncommon though.
I used 56 ohm as my headphones are 55 ohm I also have 300 ohm phones

Last edited by doulos24; 15th September 2012 at 02:33 AM.
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Old 18th September 2012, 09:04 PM   #8
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I found a way to get the data I'm after

built these today

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Old 18th September 2012, 09:55 PM   #9
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Quote:
Originally Posted by doulos24 View Post
not according to my measurements I tried a tle2426 and a max1044 I'm using a ad817 50ma output current not a lm741 I just needed a general opamp for the schematic

I used 56 ohm as my headphones are 55 ohm I also have 300 ohm phones
1. It's highly inconsiderate to ask for advice on the basis of a misleading schematic.

2. Your measurements are almost certainly wrong. Read this page of this blog:- NwAvGuy: Virtual Grounds & 3 Channel Amps.

3. 16 ohms is generally taken as a standard, being the lowest impedance commonly encountered in 'phones.

4. Your picture is unnecessarily big, wastes bandwidth and is so blurred as to convey no useful information whatsoever.
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Old 18th September 2012, 10:23 PM   #10
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Quote:
1. It's highly inconsiderate to ask for advice on the basis of a misleading schematic.
the diffrence in the 741 and the other op amp was .02% so not really a factor

Quote:
2. Your measurements are almost certainly wrong. Read this page of this blog:
they are are consistent with RMAA and when using voxenngo span and room eq wizard with an outside signal generator


Quote:
4. Your picture is unnecessarily big, wastes bandwidth and is so blurred as to convey no useful information whatsoever.
as for the bandwidth thing the pic is hosted offsite same bandwidth

no useful information? in ear plugs with omni mic elements attached so I can measure the headphones as i wear them.

Last edited by doulos24; 18th September 2012 at 10:27 PM.
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