passive I/V and transformer

[D.A.R.R.Y.L.]

Hi All,
I read about a passive i/v stage and it's possible benefits.
Just connect a resistor from dac output to ground and the connect a wire from pin-side resistor lead to rca.

Output is very low. I read somewhere that this signal can be boosted with different types of outputs stages (tube, opamp of course, and transformer)
I have not yet got the tube skills and opamps are already present. But how to implement a transformer after this dac output with pasive i/v and before rca. Is there still need for output caps?

[Hattori Hanzo]

But how to implement a transformer after this dac output with pasive i/v and before rca. Is there still need for output caps?

It's dead easy. Just take a look at e.g. the wiring diagram of the Sowter 8347. Experiment with I/V resistor values but don't go too high. No need for output caps but make sure that no DC is applied to either primary or secondary windings of your transformer!

[ezkcdude]

Those transformers are expensive, though. I contacted Sowter about a suitable transformer for the PCM1794, and they recommended 9762, which costs ~$120!

[D.A.R.R.Y.L.]

Won't any transformer with suitable gain do?

[poynton]

Originally posted by D.A.R.R.Y.L.
Won't any transformer with suitable gain do?

Hi.

I bought a pair of audio transformers off EBAY just for this purpose. Nice mu-metal screened cans. There is still 1 available on Ebay listing no. 7605809903
http://cgi.ebay.ie/Audio-Transformer...QQcmdZViewItem

They are 1 + 1 : 2 + 2 ratio.
Low dc resistance and depending on how they are wired 75 to 600ohm impedance.
suitably connected they should give a reasonable voltage step-up.

I think they are available from Farnell or RS.
Part No. is A262A2C ( Oxford Electrical Products)


Andy

[D.A.R.R.Y.L.]

Isn't a 1:1 useless, because there is no gain?

[poynton]

Originally posted by D.A.R.R.Y.L.
Isn't a 1:1 useless, because there is no gain?

Hi.

Yes, 1:1 would be useless - no gain

But parallel the input and series the output gives 1:4

Read the spec :
http://www.farnell.com/datasheets/9126.pdf

Andy

[D.A.R.R.Y.L.]

Do not see this in spec-sheet 1:4
Just see 1+1:6,45

(please note I am a novice with this type of circuitry)

[poynton]

Originally posted by D.A.R.R.Y.L.
Do not see this in spec-sheet 1:4
Just see 1+1:6,45

(please note I am a novice with this type of circuitry)

The spec sheet is for 3 types of transformer.

Look at the centre spec. for the type I bought. [although this was only given as an example]

If you bought the other type you would get 1 : 12.9 ratio !!

Andy

[dddac]

The voltage output by using a "simple" resistor, is not low per definition. It entirely depends on the DAC chip you are using. a 1541 will end up with 25mV or so, a PCM63 does 100mV, a TDA1543 (running at 8volt) can do almost 2 volt etc etc...

If you are low, you will get very good results with the sowters. 120$ expensive? a normal amplifying stage will cost much more. How much did your DAC cost and your speakers ?

Any way, I have had very good results with 3 projects where I used the Sowters (PCM63 DAC and 2 upgrades for Sony SACD playaers)

You can read about this projects, with published circuits at my web site click here to go to my Site

Oh, almost forgot: transformers will only work properly if the input impedance of the next stage is high. A 10k inputs will transform back to 25 Ohm, which will lower the output of the DAC again ! I use 200k Volume pot for hat reason