[new design] transformer balanced DAc output buffer

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Your DC bias won't work.

Also 0.01uFd and 100K form a 170Hz high-pass, pretty bass-less.

You don't have any low-pass (except transformer), which could upset some downstream devices.

It isn't Class-A as claimed, not with that chip-amp in there.

Use two 3055 as emitter followers, one per output, hang a 1:1 transformer across the emitters. That will be pure Class-A no chippy-stuff, and pure push-pull.
 
> will adjust Vref for .5 of V+

No, look at what you have. Emitter returns to V-. Emitter resistor is much smaller than collector resistor. Base bias is going to be much closer to V- than to ground. There are several ways to do it, none I like. To get collector to around V+/2, the emitter resistor will be dropping around 0.75 V, base bias near -13.65V. But then a 2.4V peak (4.8V p-p) signal out of the opamp will clip like hell.
 
now with something like the original schematic, are you trying to keep the coupling cap at 0VDC when no signal is present( don't know the correct term for this)? would the coupling to the transformer pull the signal to below 0vdc? In emitter-follower mode is the intention to be similar to totem-pole topology like MosFETs are usually arranged in when you have to swing across zero? Could you in fact use Jfets to totem pole the transformer (if you wanted to..) sorry, these are just ideas that I have but don't have a clue how to begin to execute them.

I'm sorry for such NOOB questions, but i've not dealt with this before. I'll tell you though, reading literature on this stuff all day isn't nearly as helpful as reading a few posts of comrades working through issues.

thanks! :thumb:
 
Wouldn't this be a better way to go but I am not sure I am following this thread properly
txbufferJLM.jpg

Just vary R3 & R4 equally to get the gain you need. The whole transistor output is also now in the feedback loop to keep THD low.

Sorry if I have missed the point :)

Joe
 
I think that Calrec's way of using the LL1517 is one of the smoothest sounding transformer-outputs around:

http://img.photobucket.com/albums/v172/gyraf/LL1517_output_stage_calrecUA8000.gif

It drives the transformer with the rated negative source impedance.

Jakob E.
 
"totem pole" is where you have a Pch whose source is V+, drain connected to the Nch drain to form the output, Nch source to ground. when actuated, either MosFET will pull up or down, usually swinging to the rails depending on how hard you switch the gates.. this might be an idea to use to drive transformers pretty hard.. but i don't have a clue how musical it could be since there will be short times where both FETs are on.. causing heat and small amounts of overshoot or shorting...? heh sound like the definition of class A..
 
Hey butta

congrats!

Have u seen the output buffer ckt I was hoping to use for a PCM4104 dac - would be interested to hear ur opinions on juicing that up some (just went with datasheet implmentation mostly)...

peesh
 
i posted it in one of other the dac threads u started....

http://notapplicable.co.uk/audiolab/adda/opa1632-op_buf.jpg

some info from datasheet too:

Each differential output is typically capable of providing
6.15V full-scale (differential) into a 600Ω output load. The
output pins are internally biased to the common-mode (or
bipolar zero) voltage, which is nominally VCC/2. The output
section of each D/A converter channel includes a
single-pole, switched capacitor low-pass filter circuit. The
switched capacitor filter response tracks with the sampling
frequency of the D/A converter and provides attenuation of
the out-of-band noise produced by the delta-sigma
modulator. An external two-pole continuous time filter is
recommended to further reduce the out-of-band noise
energy and to band limit the output spectrum to
frequencies suitable for audio reproduction. Refer to the
Applications Information section of this data sheet for
recommended output filter circuits.

An external output filter is recommended for each
differential output pair. The external output filter further
reduces the out-of-band noise energy produced by the
delta-sigma modulator, while providing band limiting
suitable for audio reproduction. A 2nd-order Butterworth
low-pass filter circuit with a −3dB corner frequency from
50kHz to 180kHz is recommended.

goodday!
 

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