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Old 8th June 2008, 10:38 PM   #1
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Default One box dedicated headamp/preamp.

I had earlier started a thread on the topic of current amplification before I/V. This thread is maybe not the best way to approach my question so I will go for the whole story so one can see where the I/V thread fits in. Here it goes...

I would like to build a DAC/Headamp/Preamp in one box with short leads(no ICs). It will be based around the Twisted Pear Buffalo DAC and use it's internal volume control. The headphone amp(hopefully, preamp as well) will be a tube or FET based current amp. I am going to try this transconductance thingy that the FirstWatt amps do for Lowthers and the like. I want a trans-conductance amplifier to drive my HD650 as I think it would exceed any other type of headamp out there! I have always loved SET amps for there "rightness." I am not into audio speak so I will do my best. Those who have heard great 300B amps know that tubes can be quite transparent as well and posses a magic that I have not heard in the transistor realm. Mr. Pass's FirstWatt amps seem to address that quest for "magic" so I here to get help.

First off, I don't care if it gets made with dog poo so long as it sounds good! I prefer tubes, others prefer FETs and because I have never heard a good SS amp does not imply one does not exist. My bias towards tubes is obvious which can be attributed to never hearing a totally SS amp, CD output, preamp, or whatever to blow my skirt off. As soon as a tube replaces an opamp, traditionally things sound better. Maybe not technically perfect, but natural. I am not talking tubey, bloated, round... I am talking open, glorious, effortless, alive. If these requirements can be met with FETs, I am all ears. I need SET sound, detailed and musical.

Secondly, this will be an exploration into something I think just might work and something that will work. That is right, I want to build this thing so that the current output of the DAC stays as current through amplification(trans-conductance amplifier) and "creates voice coil acceleration in direct proportion to the input signal" for my HD650 as Mr. Pass states. Little ore no feednback is preferable. If this works I would like to use the current out from the DAC to drive a current amp for the full range speakers and if at all possible, a sub.

Finally, if this will never work I want to use the current output of the DAC with the best I/V for "natural" sound. I can just use my preamp I have like any normal person would. Again, tubes, FETs, transformers, even FET opamps like Twisted Pear's IVY(my standby if my crazy ideas fail) will be considered. I would like to explore why I should not amplify the DAC's current output to have considerably more current out and then use a passive resistor I/V? Anyone asked that before? Is there a good reason why not? This way a resistor could be selected for wide bandwidth and better slew rate with much higher current levels resulting similar output voltage levels as when the resistor is used and then voltage amped. What the benefits might be are questionable but one might be that the additional current might allow you to drive a preamp with 10k input impedance. I don't know for certain if it would work, why not try?


Thank you for everyones' time in advance,

David


P.S. I really really want to find out if the DAC's current could be output to a trans-conductance amplifier directly to my cans! That would be too cool for school.
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Old 9th June 2008, 12:03 AM   #2
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http://tubecad.com/2007/02/blog0097.htm
http://tubecad.com/2007/02/blog0098.htm
http://tubecad.com/2007/03/blog0099.htm


I found a guy that is thinking like me! These links might clarify what I am trying to do. I want to use my HD650 though and I thought the benefit of this circuit was that uneven impedance does not affect performance.
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Old 9th June 2008, 07:37 AM   #3
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Hi David,

You could give this one a try. I'm using it with the HD650 as well and it is the best one I've ever heared. I build my test version out of my junk box as it only uses 3 fets and 8 resistors...

Szekeres VE as published by RJM :
http://www.geocities.com/rjm003.geo/...o/diy_sze.html

You can have a look at my version as well:

http://www.diyaudio.com/forums/showt...31#post1534031
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Old 9th June 2008, 08:34 AM   #4
juma is offline juma  Germany
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Try this one, it will make you smile

- R4 and R5 set the bias (100 mA is deep in Class A - you can measure it as a voltage drop over R6 and R7)
- R1 sets the DC offset
- Gain = R8/R9 + 1
- Q3 and Q4 need some heat sinking (2-4 watts per MOSFET)
- It's good to thermally couple Q1 and Q2

It's very stable, you can try it on perfboard.
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Old 10th June 2008, 01:42 AM   #5
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That is great guys but both seem to be voltage amplifiers, right? The experiment would be to mimic the way Nelson Pass designed the FirstWatt for Lowthers, but for headphones. The main difference being that it never gets converted to voltage until the cans themselves.
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Old 10th June 2008, 03:09 AM   #6
Ipanema is offline Ipanema  Malaysia
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Default current mirror = current amplifier

Hi,

How about using a current mirror to amplify the current? True current output if you don't mind the noise amplification as well.

Regards.
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