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Old 31st July 2009, 09:39 PM   #1
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Join Date: Jun 2008
Location: Cnaterbury, Kent, UK
Question Class A headphone amp Gain Question

Hi, I'm in the middle of designing (by means of combining others ideas) into a USB headphone amp.

For the analogue stage I'm using designs by Rod Elliot (http://sound.westhost.com/). Basically his Stereo width controller (project 21) inputting into Minimalist Preamp (Project 37A) into DoZ Headphone Amp (project 70).

My question is quite simple in that the input to the stereo width is fed from a DAC I/V stage at 2V RMS. The preamp has a gain of 3.2 by default and the Power amp a gain of 13.3.

I may be way off the mark but I assume this would mean the amp would be trying to output 85V RMS.

What I want to know is if there'd be any detriment to lowering the gain of the preamp to around 2, then have a pot to adjust volume then the DoZ gain at around 3-4. I just want to check the severe reduction in gain wont affect things too much.

Rob
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Old 1st August 2009, 12:12 PM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Hi,
your headphones may have an absolute input voltage limit somewhere between 1Vpk and 5Vpk.
Your normal peak listening level is likely to be less than 10% of this.

You do not need any gain from a 2Vac source through a pre-amp/power amp combination. You need attenuation.

Use a buffer around your volume control and use a buffer to drive your headphones.
Total gain =+0dB, compared to your proposal of +32.6dB

If you have a variety of sources then a headphone buffer that can be adjusted to +12dB gain as well as +0db would increase your options when using a low output voltage source.
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regards Andrew T.
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Old 1st August 2009, 01:04 PM   #3
Gordy is offline Gordy  United Kingdom
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Join Date: Nov 2006
I think Andrew has provided good info there. However some headphones, especially the older high-impedance designs, can be a little low on sensitivity. I would recommend designing so that you can drive about 50 mW into anything from 30 to 300 Ohms (and initially disregard the 120 Ohm feed-out resistor that is noted in Mr. Elliot's article. You can always add it later if needs be).

You do not need the preamplifier that you mentioned. I would use a 10K potentiometer after the Width Controller and then feed it straight into the amp.
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Old 2nd August 2009, 02:02 AM   #4
Artie is offline Artie  United States
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Hey Rob; I'm a bit of a newbie here myself, but here's an alternate idea: Go ahead and use Rob Elliot's PS and preamp, but then mate it with Greg Szekeres class A amp from HeadWize. Its a super simple design thats gotten good reviews, and it has slightly less than unity gain. It, and Rob's preamp, could be a match made in heaven.

Just a thought.

Artie
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Old 2nd August 2009, 05:38 PM   #5
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Thanks people, I'm now looking at an attenuator for the DAC source and a preamp/attenuator combo for a line/general input. Severly limiting the gain on the preamp though to around 2 ish.

Quote:
Originally posted by Gordy
I would recommend designing so that you can drive about 50 mW into anything from 30 to 300 Ohms (and initially disregard the 120 Ohm feed-out resistor that is noted in Mr. Elliot's article. You can always add it later if needs be).
That's why I was gunning for the class A beast headphone amp rather than an opamp buffer of some description. nice sound + plenty of power into any load.

I'm not sure about ommiting the 120R current resistor. As the headphones are plugged in the outputs of both channels will be briefly shorted to ground :S
I'm worried either a) the output transitors melt pretty much instantly pumping current into a short circuit or b) it will shorten their life c) the output eletrolytic will die.
I think it's safer to leave the resitor in though once everything working and I understand a bit more then I'll tweak.


Quote:
Originally posted by Artie
Go ahead and use Rob Elliot's PS and preamp, but then mate it with Greg Szekeres class A amp from HeadWize.
Artie
Thanks Artie I'll have to check that out. I'm building the amp on a seperate pcb to the low level stuff so perhaps I'll build both and run some listening tests.. and so begins the path to good sound

Thanks for the help people
rob
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