Have i broken it?

[Mr Bryan]

Hello, I have reciently made the zen i/v converter for my cd player. During the testing stage something must have happened as now I am having a problem.

The out put from the dac, in the (left)channel i was using for testing stops working when connected to the zen i/v, but the other(right)channel is fine fine when conneted to the same circuit. and testing with the oscilloscope i can see an output from the left channel dac o/p but it stops when the zen i/v is connected.

Also if i reconnect the cd player to its original state the is output on both channels.

surgestions...

[Mooly]

So what you are saying is the loading of the zen I/V (have you a link to the circuit) kills the output from the left DAC but works with the right DAC. Also the original I/V stage of the player still works when connected as per original.

Sounds odd as I/V convertors usually present a low impedance so I can't see how the DAC could work with one and not another. Is there any kind of DC offset present on the DAC output (different offest L to R) that the Zen may not like but the original can cope with.

[Mr Bryan]

hello thank you for your responce, I will check tonight if there is a dc offset like u surgest.

I did have it working but managed to knock the breadboard which resulted in the loss of signal. Originally i thought i had a problem with the zen i/v but after trying it on the right channel it was fine.

But i am unsure why if i dammaged the left channel of the dac why it works with the origanl set up.

the dac i have is pd6376 datasheet is http://www.datasheetcatalog.org/data.../UPD6376GS.pdf

[Mooly]

I think that's all you can do, to compare L and R from the DAC... and looking at the data sheet perhaps compare both outputs terminated initially with nothing (open) and then loaded with say 1 k and just see if there are any differences in DC conditions and output on a scope.

[Mr Bryan]

ok , there is 1.6v dc offset onn the l and r outputs from the dac. when connenting the zen i/v the l channel drops to 0v. r channel remains the same. this is with using the same i/v circuit.

I have also noticed that the output of the left channel of the dac is clipping the top and bottom of the wave on peaks. but the right channel is fine.
is there any thing i can do or have i broken it?

[Mooly]

Hmmm... well it does sound a bit like you have zapped the internal opamp in the DAC tbh. Yet I have doubts...

The data sheet page 13 show the DAC working into a low pass filter. It is audio coming out of the DAC

Nelsons I/V do-dah is something altogether different I imagine. The conversion from I to V requires a virtual ground to work into. The I/V process has already been done in the DAC via the internal opamps !

[Mooly]

Have you a link to Nelsons circuit and article.

Does that make sense... the I/V process is already done in the DAC... all that's needed is the filter on the output.

[Mr Bryan]

interesting, I did have it up and running and the right channel is fine, so i guess i have dammaged the dac in some way,

So you are saying the dac has an internal i/v .

You live and learn i guess , this was my first venture into modding cdplayers, back to the second hand shop then i guess. But i dont like to give up on things though i had so many plans for this player!

[Mooly]

I'm going off the data sheet. That shows the output of the DAC connected to a simple opamp filter to remove HF artifacts. That filter has a high input impedance so doesn't load the DAC much. An I/V convertor appears essentially as a near zero impedance and the current generates a voltage output from the opamp stage.

Look at fig 13 here. This shows the classic arrangement. The fact there is no "input resistor" is correct. The input works into a "virtual earth" so I'm assuming from the title of the Zen (I/V convertor) that it too has this low impedance input.

Current-to-voltage converter - Wikipedia, the free encyclopedia

[Ken Newton]

You have been put on the right track by Mooly. The data sheet shows an internal I/V converter op-amp type circuit. Therefore, your DAC chip is a voltage output type, not a current output type. Which means, among other things, that an external I/V circuit, such as the Pass zen converter cannot be used. The zen circuit presents about a 12 ohm load to the DAC's internal I/V op-amps, nearly a short circuit.

If you are saying that the left channel now clips even with connected back to it's original state then yeah, the DAC chip is likely trashed. The mystery to me isn't so much why is the left channel did not work, but rather why the right channel did.

What might you do to improve a CD player which utilizes this DAC chip? First, replace those 10uF electrolytic caps, which are shown A.C. coupling the DAC to the output filter op-amps, with Black Gate non-polar electrolytics of the same or greater value, voltage, and having exactly the same lead spacing. 10uF film caps. are physically large and may be problematic to use there for that reason. Second, you could try other op-amps in the filter circuit. easy if they're socketed, more trouble and risk if they are soldered. Third, build an external diy DAC kit. Which might also give you the opportunity to use you Pass zen I/V. Be sure then that the DAC chip of any kit is a current output type.