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Blochead Diamond Buffer + Gain Stage - Page 2

post #16 of 22
2/21/10 at 11:34pm
My suggestion: Ditch the opamp. Use a CineMag C series microphone transformer instead.

se
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post #17 of 22
2/22/10 at 2:14am
Quote:
Originally Posted by rjm003 View Post
Hi P-A,

I browsed your schematic a couple of times as I was researching this circuit. All told I've seen half a dozen variants of the basic LH0002. As I see it the "improvements" fall into three categories:

1. Addition of JFETs to increase the input impedance.
2. Revising the input stage to allow more flexibility in setting the bias current in the output stage.
3. Replacing the resistors in the input stages with current sources of various levels of complexity to improve linearity.

I read your design as being mostly "3", with a two nested current mirrors instead of just one...

As well as your design, there's Sijosae's, the PPA, the BUF634, LH0033, revised Millett, ... and more. It's hard to tell sometimes whether the project author has actually developed the circuit themselves or just cut and paste the buffer section from somewhere else.

Can you tell I'm getting confused yet?
I have got inspiration from a current feedback IC and what you'll see are current sources with cascodes.
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post #18 of 22
3/3/10 at 10:44pm
Thread Starter 
An update:

I have seen the light and realized that feedback and class AB are like peanut butter and chocolate. Or maybe frosting on a brownie. Or frosting on a mushroom. I'm not sure the correct analogy, but class AB basically needs negative feedback to get the distortion under control ... not because the distortion is especially high vs. a single ended follower stage, but because the distortion is so much nastier.

Bias it into class A and we can talk about running open loop. So as a line stage the design as presented still has a lot of merit - it will easily stay in class A for any reasonable output load. As a headphone amp, the bias current would have to be ratcheted up an order of magnitude, and the whole power supply circuit bulked up in proportion.

So... I will mod. my circuit to wrap the buffer into the feedback loop, or if I'm really clever I might be able to make this open loop / closed loop by with a simple jumper.

Or... I might just make a Szekeres Mosfet with an op amp front end.
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post #19 of 22
3/6/11 at 4:35pm
Thread Starter 

Continued and revised here:

 

http://www.head-fi.org/forum/thread/542577/rjm-audio-sapphire-headphone-amplifier-preamp-prototype-boards-available#post_7316350

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post #20 of 22
4/7/11 at 5:10pm
Thread Starter 

This design eventually came to fruition. It took just over a year. The board area was doubled and the circuit values were considerably tweaked, but the basic circuit stayed pretty much the same.

 

Please see the web page http://phonoclone.com/diy-sapp.html for more details.

 

DSC_7999j1000.jpg

 

 

 

 


Edited by rjm003 - 4/7/11 at 5:25pm
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post #21 of 22
4/8/11 at 3:58pm

Very nice. 

 

Which feedback connection do you prefer?

 

Do you ever listen to just the buffer, by bypassing the opamp?

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post #22 of 22
4/9/11 at 10:37pm
Thread Starter 
I've only listened to the open loop configuration so far, it sounds so good and measures so well I am not really motivated to try it any other way. Perhaps later.
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