A single supply, 2 transistor DAC-IV stage.

[Brian Guralnick]

Here is my first bread board draft.

The power supply is meant to be floating compared to the DAC supplies.

The virtual short is between DAC I+ & DAC I-. As for the DAC VG (virtual ground), It may be useful to connect to so some DACs VG's, or the DAC's may perform better without it.

For the DAC's with Voltage out, add a series resistor on both DAC Vouts (+/-) & connect them to the DAC I(+/-) on this circuit.

The outputs still need to be buffered. I making a 4 JFET solution which connects right to this design & it will create both Balanced & Unbalanced outputs.

[Brian Guralnick]

Here are the first tests.

Test equipment:

Lousy BK Precision 3011 function generator.
Good BK Precision 1786 DC Regulated Power Supply.
Great Tektronix TDS 3054 DSO

The scope was operated from battery pack & it was floating.
The DC supply was also floating.
The function generator's ground pin was cut & it was also floating.

For these readings, the source current was at 6.666ma p-p.
A triangle waveform was chosen to verify linearity.
The output voltage is a little overkill, to fix this, R1 & R2 just need to be lowered to 2 Ohm, or 1 Ohm making a terrific virtual short.

Sorry for the squished/scaled scop shot, forum maximum size was reached...

[Brian Guralnick]

Same as the last shots, except, the drive current is now 0.666ma p-p.

[Brian Guralnick]

Now, to make this design using matched transistors.

I've chosen Analog Devices MAT-04 device.
http://products.analog.com/products/...?product=MAT04

Having 4 transistors will give 2 for the left channel & 2 for the right. In my next drawing, I'll also add an output gain trim & balance.

Now for the Voltage follower / unbalanced conversion stage, if I could only find 4 matched N-Channel J-Fets.

Comments?

[Brian Guralnick]

hatasa Wrote: (on my dual transistor page)
diyAudio Prophet
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Hi Brian,

I would like to know is it possible to connect with the +/-15 supply.

And I would like to ask is the Vout in balance mode if I connect the GND to VGGND?

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1. Yes, as long as this supply is floating. The problem here is the location of the DAC's 0 current. My DAC_VG is balanced right between the 2 transistors BASEs. If you were to power this design with my 0v at -15, the DAC-VG will be at around -14.3 volts.

2. With the +/-15v supply & balance calibration, the GND should be fine for the frame ground of output. Yes, this output is balanced.

My additional JFET buffer solution also does a sum inversion improving further the output SNR & it can then be used for both balanced & unbalanced. It also created a new shield GND for balanced.

Using just 2 JFETS, 1 for + & 1 for -, or none at all, now rely's more heavily on you amp's / pre-amps CMRR.

For pure minimalist design, if you want to drive a pre-amp directly, no buffer, in place of the MPSH10s, use the dual transistor LS325A, & shrink R3, R4, R5, R6 so that the circuit will draw 100-200ma across v+&v- instead of the 20ma in the current design. Also, increase C1, the gain BW of this new configuration with C1 at 15pf will be in the 100MHz range.

[promitheus]

how would someone use this for an unbalanced input ?

[Brian Guralnick]

Which DAC are you using.

Typically, the DAC - input should go to your DAC common, the DAC I output should go to the DAC + input. My DAC VG input should not be used. Remove the C2,C3,C4,C5. R1 & R2 may be replaced with 1 resistor.

A good minature way to test this circuit with an existing DAC PCB would be to power it with 2, or 3 x 9v batteries.


After analysing many dual transistor data sheets, I've determined that Analog Devices' MAT02 dual transistor will incredible preformance & balance in this circuit. In fact, over 20x beter than anything else.

http://products.analog.com/products/...?product=MAT02
Expensive though.

[promitheus]

I have the PCM 1704 DACS. they don´t have a balanced output and they have a current output , so I need a I/V converter.

[Nicke]

promitheus

Why don´t you try the one Jocko suggested recently.
Haven´t built it myself(yet) but it looks interesting.
Jocko´s I-V stage

Nicke

[Brian Guralnick]

Originally posted by promitheus
I have the PCM 1704 DACS. they don´t have a balanced output and they have a current output , so I need a I/V converter.

This is an I/V converter.

I assume you mean a Burr-Brown PCM1704. I have never used the component, but I took a quick look at it's data sheet.

For this DAC, I would connect my DAC I- to the ADC's AGND. The DAC's current output should go to my DAC I+.

Set up the PCM1704's (Ref DV, Servo DC, &, BPO DC) so that the Iout from the DAC is biased at the Agnd. Just use they typical application circuit in the data sheet.

Now, the PCM1704's output current on the spec sheet is +/- 1.2mA. This means it is 2.4mA p-p. My R1&R2 @ 5 ohm should be perfect the way they are.

Remember, the output of my ciruit is balanced, even though the output of the PCM1704 is not.