AD844 as a common base stage in the I/V converter

[Pedja]

I posted already the suggestion for such stage but now I can confirm it works and it works fine. I tried it in my TDA1541A non-o/s DAC and it was better than the other things I tried which include a few classic opamp I/Vs and passive resistor I/V followed by the tube stage. Application is similar to that suggested by Kuei for the OPA660.



If used with JFET with Idss of 10-12mA, values between 200 and 500 Ohms for VR1 should be fine. Trimming should be redone after a few minutes of warming up.

Vuki had reported a successful implementation of this stage (without current injection) in the AD1865 DAC.

More info on my TDA1541A DAC page.

Pedja

[Pedja]

Hello diyAudio,

Just wanted to say – though in the meantime I did have certain number of hits on my DAC page, I am completely disappointed by the reaction of The Forum. Ok, I know it happens that the people sometimes stay silent about the things they are in fact interested for, but…

Some interesting things I have dug lately.

http://www.stereophile.com/printarchives.cgi?840
I might be out of time, but I’ve never before heard for (or listened to) Ayre. In the review it was mentioned that the AD844 is used without feedback and the spectral content of the measured distortion is similar to those graphs I uploaded.

Another interesting case. Krell KAV-CD250 comprises AD844 and the trimmer (btw the same BI 67 you might note in my DAC) is aboard. The fact the trimming is done here and not downstream (I don’t see a coupling caps, hence it should be done somewhere) may and may not be related to the fact the AD844 is used as a common base since the PCM1702 puts out a bipolar (+/-) current. But if it is used this way, I’d expect to see the trimming circuit exactly here. The pictures I found are here:
http://www.webmedia.pl/maciejza/tuning/krell.htm
http://www.webmedia.pl/maciejza/tuni...ell/krell9.jpg
Unfortunately I don’t have any further info about this player. Fyi, the AD844 used like a classic I/V opamp is good but not spectacular.

Best regards,
Pedja Rogic

[Peter Daniel]

Here's ML 30.6 DAC view:
http://madrigal.harman.com/image_lib...30_6DAC_lo.jpg

Did you compare your stage against Kuei's OPA660 version?

[Pedja]

No. I’ll compare it to some discrete (single end, not symmetrical) common base stages next weeks but OPA660 should wait a bit. Circuits are similar but in fact it will be easier to make a new board for the OPA660 than to change this I have with AD844.

Nice pic of ML, this is even better , but as I can see it uses the classic opamp IV.

Pedja

[jan.didden]

Originally posted by Pedja
No. I’ll compare it to some discrete (single end, not symmetrical) common base stages next weeks but OPA660 should wait a bit. Circuits are similar but in fact it will be easier to make a new board for the OPA660 than to change this I have with AD844.

Nice pic of ML, this is even better , but as I can see it uses the classic opamp IV.

Pedja

Pedja,

What is that R151 bent over backwards over that chip? Looks like a power R used to heat/temp stabilise the chip? What chip is that?

Jan Didden

[jcarr]

Jan: Looks like Texas Component's TX2352. Essentially a Vishay S102 sans molded packaging.

PDF download link here:

http://www.texascomponents.com/Tx2352a.pdf

hth, jonathan carr

[jan.didden]

Jonathan,

I suspected something like that, looks like a Caddock. But the intriguing thing is, what is that chip it tries to hide??

Jan Didden

[Pedja]

Hi Jan,

I meant originally that both OPA627 and AD826 can be used exclusively as a classic IV opamps.

As about the secret chip, I found this:
“The Reference DAC modules are based on the highest grade of the new 24-bit, multi-bit Burr Brown1 1704 DAC and OPA627AP I-to-V converters.”
http://www.marklevinson.com/products...-6&details=yes
And in fact it looks like it is BB.

But for some reason, John Atkinson claims:
“The original UltraAnalog DACs have been replaced by new 24-bit DAC modules designed and built by Madrigal.”
http://www.stereophile.com/printarchives.cgi?159

Pedja

[Peter Daniel]

Originally posted by Pedja


But for some reason, John Atkinson claims:
“The original UltraAnalog DACs have been replaced by new 24-bit DAC modules designed and built by Madrigal.”
http://www.stereophile.com/printarchives.cgi?159

And those are the pictured modules, featuring 1704 DACs.

I've built a DAC using paralleled 1704 K DACs, but the non-oversampling TDA1543 sounds better to me.

If it wasn't for all that NIGC controversy I would be bulding a DAC using 1541S2 chip right now

[Pedja]

Hi Peter,

From the Strereophile’s test, one (me for example) could realize that ML invented something regarding the DAC chip and swapped the previous one with some their own module. Actually (if I am not again mistaken), the module comprises BB DAC chip and output stage, just like the previously used UltraAnalog's DAC (I guess) incorporates the DAC (strictly speaking) and the IV stage.

Don’t know about the PCM1704, but while we are at BB, compared against the SM5842/PCM63K, the TDA1543 non-o/s lacked some dynamics (and this was probably related to the fact the later used a resistor only I/V) but many other things, let’s say musicality, were on its side (I think I wrote this already somewhere).

Pedja