mkoijn

That1510, 1646 preamp
« on: April 23, 2008, 08:26:35 AM »
Hi,

If somebody could have a quick look at my schematic to see if I've made any major blunders I would greatly appreciate it.  it is at the following url.

http://www.violini.de/

The front end is pretty much taken direct from the THAT1510 demo board application note. and the back end I took from the Seventh Circle T15 schematic, but I dropped the fine gain trim as I need repeatable gain settings.

Thanks

Albert


jaygunn

That1510, 1646 preamp
« Reply #1 on: April 23, 2008, 09:58:22 AM »
What's the benefit of the NPN in the phantom power circuit?

And no phantom switch...on all the time?
John Gunn
Chapel Hill, NC

mkoijn

That1510, 1646 preamp
« Reply #2 on: April 23, 2008, 10:02:12 AM »
According to THATCORP 1510 demo datasheet, the npn provides a softstart when phantom is switched on.  

Any other comments?

rlaury

That1510, 1646 preamp
« Reply #3 on: April 23, 2008, 01:24:15 PM »
This looks good except that I don't see a need for the OPA604.
It will just add it's offset BACK into the the balanced output driver
that you've worked so hard to remove by setting up the servo's.


RonL
When all you have to work with is a hammer,
everything looks like a nail.

mkoijn

That1510, 1646 preamp
« Reply #4 on: April 23, 2008, 01:33:41 PM »
Thanks Ron,

Yes, this is the bit I was not sure about, I recall there was some discussion that the 1646, DRV134, SMM2142   really need to be driven by a buffer opamp. But as I could have a straight connection from the 1510,  maybe it's not needed?

sonicmook56

That1510, 1646 preamp
« Reply #5 on: April 23, 2008, 06:19:16 PM »
The input of the 1648 output amp needs to be driven from a low impedance source, or the common mode will suffer.

Without looking at a datasheet, I suspect you can drive the 1648 from the output of the preamp chip.  Please don't do anything without looking at the datasheets.

you could keep the buffer if you wanted to add a trim pot between the output of the preamp IC and the input of the buffer, but you would loose your repeatable gain unless you used a switched resistor array.

~B

mkoijn

That1510, 1646 preamp
« Reply #6 on: April 25, 2008, 04:13:01 AM »
Thanks sonicmook56,   I looked at the datasheets, but being a newbie to electronics, I wasn't certain what parameters were relevant. There is a "minimum resistive load" for the 1510 which is 2k0 and the "input impedance" of the 1646 is 5k0, but what is the output impedance of the 1510?

According to this post from mediatechnology,
http://www.groupdiy.com/index.php?topic=18995&postdays=0&postorder=asc&start=45
a low source driving impedance is essential for good noise performance of the 1646. It seems for optimal noise performance the driving impedance should be lower than 20R.
 

Thanks again

Albert

radiance

That1510, 1646 preamp
« Reply #7 on: April 25, 2008, 05:48:45 AM »
Quote from: "rlaury"
This looks good except that I don't see a need for the OPA604.
It will just add it's offset BACK into the the balanced output driver
that you've worked so hard to remove by setting up the servo's.

RonL


I'm trying to understand the servo concept but I don't get this.
Are you saying that after the DC offset is removed by U2b, it's added back by the OPA604? Or does the OPA604 adds some DC offset of his own?

My understanding is that once you've taken care of DC offset by adding a servo circuit at the begin of the "line" you don't have to add more servo circuits down the "line" . Any thoughts?

Sorry for hijacking your thread Albert
"Knowing that you are dreaming, however, does not automatically guarantee full rationality.
Then again, being awake doesn't ensure good thinking, either." -  Lynne Levitan

mkoijn

That1510, 1646 preamp
« Reply #8 on: April 25, 2008, 05:56:51 AM »
Quote from: "radiance"
Quote from: "rlaury"
This looks good except that I don't see a need for the OPA604.
It will just add it's offset BACK into the the balanced output driver
that you've worked so hard to remove by setting up the servo's.

RonL


I'm trying to understand the servo concept but I don't get this.
Are you saying that after the DC offset is removed by U2b, it's added back by the OPA604? Or does the OPA604 adds some DC offset of his own?

My understanding is that once you've taken care of DC offset by adding a servo circuit at the begin of the "line" you don't have to add more servo circuits down the "line" . Any thoughts?

Sorry for hijacking your thread Albert


Thats quite alright radiance, I'm interested aswell. My understanding is not that the OPA604 adds back in the already removed dc offset, but that it adds some of it's own, although I think the dc offset of the OPA604 is very small.

radiance

That1510, 1646 preamp
« Reply #9 on: April 25, 2008, 09:50:28 AM »
Ok, I see. But suppose we need that opa604 there as a buffer (for whatever reason). Do we need to ad a servo there as well just to get rid of the opa604's own DC offset?

Take for example this summing circuit from Fred Forssel
 http://www.forsselltech.com/downloads/schematics/Summing%20Buss2.pdf

In this case the  summing opamp (JFET990-2) has a servo circuit by means of an OPA604. Suppose we want to ad a buffer after the fader (not shown in the pic). Do we have to ad a servo circuit by that buffer opamp as well?
"Knowing that you are dreaming, however, does not automatically guarantee full rationality.
Then again, being awake doesn't ensure good thinking, either." -  Lynne Levitan


mkoijn

That1510, 1646 preamp
« Reply #10 on: April 25, 2008, 01:06:31 PM »
No, I think that the dc offset can be trimmed out with a pot connected to terminals 1 and 5 of the OPA604.

radiance

That1510, 1646 preamp
« Reply #11 on: April 25, 2008, 01:17:41 PM »
Should have read the data sheet of the 604 better before asking questions  like these...:oops:
"Knowing that you are dreaming, however, does not automatically guarantee full rationality.
Then again, being awake doesn't ensure good thinking, either." -  Lynne Levitan

Samuel Groner

    Z├╝rich, Switzerland
  • Posts: 2935
That1510, 1646 preamp
« Reply #12 on: April 27, 2008, 03:59:36 AM »
I second that the OPA604 buffer is pointless (or rather disadvantageous as it will increase distortion and offset). Besides this the schematic looks fine to my eyes.

Samuel

mkoijn

That1510, 1646 preamp
« Reply #13 on: May 01, 2008, 06:09:57 AM »
Thank you very much Mediatechnology for your very comprehensive reply, this is very helpful  and I will incorporate your suggestions into the design.

regards

Albert


 

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