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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

class A  Line level buffer
class A  Line level buffer
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Old 22nd February 2011, 10:25 AM   #11
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
my newest headphones are 95dB/mW

2.2Vac from a CDP will give ~120dB at the ear hole.
I doubt I'll need any extra gain. I suspect I'll be in attenuation mode !
But yes, to the idea. A switchable X1 (+0db), X2 (+6dB), X4 (+12dB) gain setting would be ideal. The difficult part may be adjusting the compensation so that each gain option operates as stabily as the best.

I was going to ask:
1.) would converting the lower 139 to a complementary and becoming a push pull ClassA reduce the performance?
2.) would using the dual soic8 as a two channel single ended ClassA output stage cause overheating? Rth j-a = 145C/W
I'm thinking to reduce the opamp bias slightly from 5mA to 4mA and reduce the voltage from 34Vdc to 24Vdc (this reduces opamp temperature by ~22Cdegrees)
24Vdc still gives ~+11dB of overhead above maximum signal.
regards Andrew T.

Last edited by AndrewT; 22nd February 2011 at 10:37 AM.
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Old 22nd February 2011, 01:53 PM   #12
Bonsai is offline Bonsai  Europe
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Join Date: Jul 2003
Location: Europe
Interesting points Andrew

1. No I don't think this will reduce the performance. I basically did this design because good performance can be obtained with a very simple circuit. I think a complimentary class A should work very well, but it will probably be more complex due to the bias requirements. The loaded follower aproach used in this design is simple and very stable.
2. No, I if I understand you correctly, this should work quite well. Likewise, my constant current load on the op-amp outputs is probably a little heavy handed - I think 3mA will also work fine.

Re the point about the gain, the reason I propose this is that if you apply 2x to 3x gain to the headphone stage, I find the volume pot will typically sit between a quarter to to about 10past for most listening - so its setting is similar to where you set it when listening over the speakers.

That said, 3V pk into 32 Ohm headphones is incredibly loud - there's plenty of headroom if you listen at normal levels.
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Old 19th March 2011, 03:54 PM   #13
robmil is offline robmil  United Kingdom
diyAudio Member
Join Date: Jun 2004
Location: London
Hi Bonsai

thank you for your brilliantly laid out information. As an aside...
What software do you use for the documentation. It looks absolutely first class!

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Old 19th March 2011, 04:52 PM   #14
Bonsai is offline Bonsai  Europe
Account disabled at member's request
Join Date: Jul 2003
Location: Europe
Hi Rob - thanks.

I use . . . . Microsoft Word - nothing special. I am using MS Office and in my version there are some document templates or themses - I just selected one of them and stuck with it.

I'm busy working on a class A headphone amp based on the above BTW. Shown be able to publish something in a week or two.
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