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need schematic for UNbalanced OUTPUT STAGE for THAT1510
Old 30th June 2009
  #1
Gear Head
need schematic for UNbalanced OUTPUT STAGE for THAT1510

Hi,

I have a StudioProjects C1 mic condenser and an Audigy 2 Value sound card.

I made a mic pre with THAT1510 , i used the fig. 4 schematic from THAT IC datasheet recomandation. I used 45V for Phantom power (5x9 V 6LR61 batteries ) and -15 ,+15 V (20 X 1.5V C batteries) for that1510.

The problem is that i have no gain control (non that i'v noticed) : I used a linear POT 2k5 in series with 10R rezistor . If i rotate the POT there is no change in gain, just some clips/scraches . Even if i dissconect the POT and leave 1510 pin 1 and 8 with almost infinite resistance betwen them there is no gain or loss . I'v cvadruple cheked the XLR , soldering and PCB layout --- no fault. I tried changing R1 & R2 from 10K to 1K but no noticible gain or loss; It feels like there is some gain from THAT1510, a few dB ....

the outup of the 1510 is in series with a 47uF cap. I used the preamp in the mic input of the Audigy but there is to much white noise and the usefull signal is to low -- like -20 dB voice and -55dB white noise
If i use the preamp in the line input of the Audigy the signal is to low - i have to yeal very loud to get a recording wtih -3db peak wtih the line set at 100 volume recording.If a speak normal volume at the microfon i get a voice recording of -35 dB and the white noise is at -70 dB .
The mic cable has a lenght of 3 meters and the output cable is 1 meter.
The white noise is from the sound card - it has -75 dB (line in) even if no cable is pluged in the Audigy 2 sound card.

If i use the -10dB pad on the microfon i can hear it right away, about 3 times less usefull signal volume !!!

Why can't i change the gain, why no gain ???!!! do i have to use a line output Op Amp even if i plug the preamp in the mic input ?
If i need an OpAmp can someone help me with a simple but efective (low noise) schematic for unbalanced output ?
Attached Thumbnails
need schematic for UNbalanced OUTPUT STAGE for THAT1510-schematic-that1510.jpg  

Last edited by kategra84; 30th June 2009 at 04:56 PM.. Reason: clarification
Old 30th June 2009
  #2
Sounds like there's an open in your gain resistor path, maybe your pot is bad. The datasheet states that you get 0 dB of gain with infinite resistance, and you say when you turn the pot, nothing changes, so it sounds like either your pot is open, or the line that runs to it is. If you have a meter, probe between pins 1 and 8 on the THAT chip and see what the resistance is. If you see an open, then it may be one of your resistors or a wire. If you see the 2.5k at the pins, and it changes when you turn the pot, then your THAT chip is probably bad. That should be all you need to fix to get the results you're after. Some people put a pot on the output line as well so they can control the overall level and gain seperately. This is good when you want to run a pre hot for color, but it's probably not necessary for a cleaner amp like this where you really don't get much color either way.
Old 30th June 2009
  #3
Gear Head
i mesured the resitance betwen pin 1 & 8 and it' s 2k5 and lower as i turn the POT .... and i hear artifacts when i rotate the pot
Old 30th June 2009
  #4
Hmmmm, strange. So the whole circuit is passing audio and you hear noises if you turn the pot, but it doesn't affect the gain? Judging by your levels it definitely sounds like the amp is working, but is just at minimum gain. And you say the resistance is correct at the gain control pins?

Is your reference grounded? Not sure exactly what affect it would have if it wasn't, but it's definitely related to the gain circuit internally. I know the schematic says it should be connected, just asking.

Other than that, it's hard for me to say without looking at it myself, but my next step after double checking to make sure everything looks just like the schematic would probably be to try a spare chip.
Old 30th June 2009
  #5
Gear Head
Yes the reference is grounded ;

i'll try to let it looose to see what happens

Last edited by kategra84; 30th June 2009 at 06:50 PM.. Reason: Yes the reference is grounded At first it wasn't -- is wrong
Old 30th June 2009
  #6
Gear Head
Thank you aclarson !

I am using a test board and when i tried to ungroud the referance pin i noticed that the pot cable wasn't soldered good on pin1 it just was over the pin but no contact ;

and i also had a short betwen pot cable and pin 7 so that where the artefacts came from ;

The mic pre now works like a charm , incredible GAIN , i can't even use full gain cuz i hit peak values only from moderate whispering.
Old 30th June 2009
  #7
Cool, glad to be of service, it's usually something simple like that.
Old 9th October 2011
  #8
Gear Nut
 
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In thumbnails of #1 post
C1 C2 C3
how they work in this configuration?
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