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-   -   class A output stage without feedback for a DAC? (http://www.diyaudio.com/forums/solid-state/224434-class-output-stage-without-feedback-dac.html)

iceyo 27th November 2012 01:31 PM

class A output stage without feedback for a DAC?
 
http://firstwatt.com/graphics/m2_simp.jpg
the pic above is from first watt M2

Hello everyone. I am DIYing a resistor-ladder DAC. it's similar to MSB platinum DACs.

I am a FPGA enginner, have no experience in analog circuit design.
Here's the thing. I have built a voltage divider to generate the signal voltage. But I do not want to follow an opamp as the output stage. I would love to try the most musical solution, just like first watt does.

Therefore, I am asking for help.

1.Pure class A, no feedback.
2.MOSFET probably.
3.signal input is 0-5V. signal 'zero' is 2.5V. final output is about 2.5V peak to peak.

I have no idea what MOSFET to choose, or how to calculate the values of nearby resistors and caps.
Please feel free to give any suggestion or idea. Thanks!

Fast Eddie D 27th November 2012 04:40 PM

Transformer coupled? What transformer do you intend to use?

Purists typically don't like audio transformers and designers avoid them unless they offer a significant advantage. What advantage does this transformer coupled circuit offer over a typical modern class A buffer?

Use an active biased class A emitter or source follower or just (GASP) use an op amp.

Or if you're just trying to be cool, use a cathode follower buffer. There are even modern lower voltage tubes, and PC mount tubes. But using these devices might cost you snob factor.

iceyo 28th November 2012 12:28 AM

Thank you, Eddie.

What I intended to show the figure is the simplicity. It is a Nelson Pass's work. I need no transformer.

Since a DAC analog output does not consume large power, so I probably go for a source follower. The problem is I do not know how to design this. To be specific, which MOSFET, values of the resistors or caps, slew rate and etc.

Fast Eddie D 28th November 2012 08:23 PM

Quote:

Originally Posted by iceyo (Post 3260004)
Thank you, Eddie.

What I intended to show the figure is the simplicity. It is a Nelson Pass's work. I need no transformer.

Well you showed a circuit with a transformer. I thought you wanted to use that cirucit. I would never use a circuit like that for a buffer.

Quote:

Since a DAC analog output does not consume large power, so I probably go for a source follower. The problem is I do not know how to design this. To be specific, which MOSFET, values of the resistors or caps, slew rate and etc.
So just to be clear, you are familiar with digital circuit design but not analog circuit design, right?

I need to know what your input impedance requirements are for the buffer, and what kind of power supply you have for the buffer.

There are a lot of buffer circuits available here and on a google search. Which one is for you depends on your answer to the above questions.

I'm not a SPICE guy, and I have never used schematic software either. I come from way back when transistors were new in hi fi equipment. That being said, I like to use op amps extensively in my designs. Despite the woo, op amps can outperform typical discrete circuits for many audio applications. Simple discrete line level buffers seem kind of snobby to me when you can make an excellent buffer with a couple buck$ worth of parts and a cheap op amp.

Bigun 29th November 2012 12:56 AM

Hi iceyo,

I am not clear what your plans are - is it a commercial DAC in a box, or do you have a DAC and want to interface to the DAC chip ?

I believe a transformer is a very valid option and I don't agree with Fast Eddie on it not being the choice of purists - there are many purists who build vacuum tube amplifier for whom transformers are the purest of all approaches.

Sowter make a transformer specially for coupling to DAC chips which many people say is a very musical solution.

If you already have a DAC with line output then you are asking about a power amplifier. If this is your first project then I would suggest picking a design where the parts values are known and published. If you want a Nelson Pass design, there are many and you should go to the Pass Forum, not this one, and ask there for advice - you will get good support on building Nelson's published designs there.

Good luck - DIY is fun !

6L6 29th November 2012 01:07 AM

Quote:

Originally Posted by Fast Eddie D (Post 3259283)
Purists typically don't like audio transformers and designers avoid them unless they offer a significant advantage.

Purists in the US, perhaps. But it's completely the opposite in Japan. Just sayin'.

Designers don't like them because they are usually quite expensive. But sometimes they are the proper answer to the question.

iceyo 29th November 2012 01:53 AM

Quote:

Originally Posted by Fast Eddie D (Post 3261422)
Well you showed a circuit with a transformer. I thought you wanted to use that cirucit. I would never use a circuit like that for a buffer.


I'm not a SPICE guy, and I have never used schematic software either. I come from way back when transistors were new in hi fi equipment. That being said, I like to use op amps extensively in my designs. Despite the woo, op amps can outperform typical discrete circuits for many audio applications. Simple discrete line level buffers seem kind of snobby to me when you can make an excellent buffer with a couple buck$ worth of parts and a cheap op amp.

Thank you. There is no DAC chip. the D to A module is built from a resistors network (R2R). I can choose an opamp as output buffer, but I do want a dicrete one. I have 5V,+12V,-12V on board. I think the input impedance of a MOSFET is large enough.

I am a SPICE guy and I respect experience, but I try to be a know-how guy. I will simulate the circuit.

abraxalito 29th November 2012 02:01 AM

Quote:

Originally Posted by iceyo (Post 3259002)
Hello everyone. I am DIYing a resistor-ladder DAC. it's similar to MSB platinum DACs.

I am a FPGA enginner, have no experience in analog circuit design.

If you haven't discovered already, one of the major problems with R2R DACs is glitching. That's a reason why DAC chips for audio don't use R2R in an unmodified form. You've chosen a very steep path to become an experienced analog engineer, I wish you a lot of luck :p

Vincent Brient has done this incidentally - http://www.vishaypg.com/foil-resisto...dy/totaldac_1/

iceyo 29th November 2012 02:28 AM

Quote:

Originally Posted by Bigun (Post 3261782)
Hi iceyo,

If you already have a DAC with line output then you are asking about a power amplifier. If this is your first project then I would suggest picking a design where the parts values are known and published. If you want a Nelson Pass design, there are many and you should go to the Pass Forum, not this one, and ask there for advice - you will get good support on building Nelson's published designs there.

Good luck - DIY is fun !

Thank you. Copying one's circuit and make it sings will not satisfy me. I start to learn basic circuits and simulate.

iceyo 29th November 2012 03:25 AM

Quote:

Originally Posted by abraxalito (Post 3261858)
If you haven't discovered already, one of the major problems with R2R DACs is glitching. That's a reason why DAC chips for audio don't use R2R in an unmodified form. You've chosen a very steep path to become an experienced analog engineer, I wish you a lot of luck :p

Vincent Brient has done this incidentally - Vishay Precision Group - Foil Resistors - Case Study: Developing a Multi-Channel DAC for High-End Audio Applications

thanks.
1. I am not trying to be an analog expert, just for a deep understanding of a voltage buffer circuit.
2. Clearly, you misunderstand the R2R's problem. glitching can be eliminated by clock and registers. As your link says, the problem comes from the accuracy of the resistors.


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