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Old 25th August 2010, 09:52 AM   #3021
danrap is offline danrap  Belgium
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Today I've try to mount a double opa627 on the cs4398 dac, leave on its place the ugly ne5532 as buffer.
To me, the opa627 sounds better than the monacor ltr110.is it possible?
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Old 25th August 2010, 11:24 AM   #3022
anbello is offline anbello  Italy
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Quote:
Originally Posted by Ryssen View Post
You could connect them on the DAC board with the ground where the 2 caps meet (104) like on this photo.

Click the image to open in full size.
Hi Ryssen,
can you post a picture of the back of this board?

Thanks
Andrea
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Old 25th August 2010, 11:39 AM   #3023
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Hi Andrea

Maybe I can help here -

Click the image to open in full size.

The op-amps are dual types and specified in the schematic as OPA2604. In the picture I have OPA1612.

If you like I could email the schematic.

Rich
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Old 25th August 2010, 01:34 PM   #3024
danrap is offline danrap  Belgium
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You use this on the LM-DAC?
I don't know that opamps are soldered back to the board, I'm thinking that the pcm1798 use the output stage of the dac..

R+R-L+L- pins, can be used to connect e.g. to a trafo?

@Andrea
I sent you a pm

Last edited by danrap; 25th August 2010 at 02:02 PM.
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Old 25th August 2010, 03:50 PM   #3025
anbello is offline anbello  Italy
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Hi RichLund,
thanks a lot for the photo, i would like to have the schematic via email.

@danrap
I have replayed to your PM.

Ciao
Andrea
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Old 25th August 2010, 04:24 PM   #3026
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Andrea - I thought it might be easier if I uploaded the schematic to a file server. Anybody else that needs it can have it:

LM-DAC3.doc
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Old 25th August 2010, 08:02 PM   #3027
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Tried to get the adapters to work but had mixed results.

With Ryssen's help I managed to get sound from the first adapter but only in one channel. Fearing i may have damaged the chip I tried to remove it but wrecked the pads on the PCB. Removing 28 pin smd chips is a nightmare... However the sound from the one channel was very good!

The second adapter, which originally i had populated with op-amps and decoupling components, produced no sound even with components removed. So may also have a damaged chip.

I have continuity from the chip pins to outputs and continuity to ground.

I have spare PCM1798 chips and will order some more boards to try. I built the boards like this:

Click the image to open in full size.

Click the image to open in full size.
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Old 25th August 2010, 10:37 PM   #3028
Ryssen is offline Ryssen  Sweden
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Are you sure there was contakt on all the soldering points,the ground and so on?


I did some SMD soldering this evening,thats a small chip...

Click the image to open in full size.
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Old 26th August 2010, 09:54 AM   #3029
anbello is offline anbello  Italy
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Quote:
Originally Posted by RichLund View Post
Andrea - I thought it might be easier if I uploaded the schematic to a file server. Anybody else that needs it can have it:

LM-DAC3.doc
Thanks RichLund
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Old 26th August 2010, 03:31 PM   #3030
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Quote:
Originally Posted by Ryssen View Post
Are you sure there was contakt on all the soldering points,the ground and so on?

Hi Ryssen

As it happens a couple of contacts seemed questionable. I will try to reflow the joints later.


I received this interesting email from Bill Whitlock of Jensen Transformers regarding suitable transformers for current output DAC's:

Hello Rich,

Thanks for writing. I get lots of inquiries about transformer outputs for
various DAC chips - and I know this idea is very popular among audiophiles -
but I have never seen anyone ever publish performance data (linearity,
noise, etc.) after such a modification. The chip manufacturer never intended
such use, otherwise specifications like "output voltage compliance" would be
on the data sheet. I've inquired of application engineers about this
"uncharted territory" use of these DACs and received the following typical
response from Bob Gonzalez of Texas Instruments in January 2010:

"The PCM1794 (and its replacement, the PCM1794A) was designed to feed into
the summing junction of an opamp powered with dual supplies and having its
positive input at common as shown in figure 24 of the PCM1794A datasheet.
Whereas the PCM may operate with a non-zero compliance voltage it has not
been characterized for that configuration and therefore no compliance spec
appears in the datasheet. I hope this helps. If you have further questions
please email or call me at 972-644-5580."

For example, for the DACs you mention, full-scale output current is 7.8 mA
pk-pk. Therefore, a 10 Ω load on this output would produce only 78 mV pk-pk
or 27.6 mV rms full-scale signal. Since the impedance ratio of any
transformer is the square of its turns (voltage) ratio, a turns ratio of 1:4
would make a 10 Ω load at the DAC output look like over 160 Ω at the
transformer output and still raise the full-scale output voltage to only
61.7 mV rms. The only way to get more output voltage is to raise the load
impedance seen by the DAC. For example, a load of 507.5 Ω at the DAC output
would produce 3.96 V pk-pk or 1.4 V rms at full-scale (still a bit short of
the industry norm of about 2 V rms). If the DAC chip could tolerate this
kind of output voltage swing (i.e., "compliance"), no transformer step-up
would be required at all. Then a simple 1:1 transformer could be used to
convert the differential output to single-ended, if desired. But, since the
DAC manufacturer doesn't specify "output voltage compliance" at all, I
believe there's a high risk that it would work badly, if at all.

I hope you will understand that I'd rather not have one of our transformers
put at risk of being called "the problem" when such applications work badly.
We have a 35-year reputation as makers of the world's highest performance
audio transformers, but even the finest transformers can produce
disappointing results when used improperly. In any case, we do not make a
suitable 1:4 step-up transformer for this application (we make a 4:1
step-down transformer, but it is a gross oversimplification to believe that
all one has to do is interchange primary and secondary to make an equally
high-performance step-up transformer).
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