DC offset OPA 134

[rdatman]

The kit talks about pin 6 with reference to ground on OPA 604, but what is shipped is an OPA134.

Can you forward to me the updated instructions for this op amp... (I can't get even .1V adjustability out of the pot I have installed. It reads 104 on the side so that should equal a 100k ohm multiturn.)

[rdatman]

3 out of 4 have an offset of (-).4uV or less-- one has a -(3) VOLT offset. Cannot find the source...

Solved 1/2 of the problem, there was an omission about installing two 1k ohm resistors. That was the problem with regard to the multi-turn pot not working.

I just had ordinary carbon film resistors for this purpose, my guess is this an appropriate use of such a low quality resistor?

The last OPA134 opamp seems to me to be malfunctioning. Based on what I read from the schematic, it seems that the voltage would originate from that opamp directly since you have the signal folding back into the output.

It reads (-) 3Volts now. at both the -V in and pin 6.

[tpryan]

Solved 1/2 of the problem, there was an omission about installing two 1k ohm resistors. That was the problem with regard to the multi-turn pot not working.

You should only install R38 or R39, not both, depending on the type of op-amp used at U4. For the OPA134, only R38 is useful. Installing R39 as well will only increase current consumption needlessly. You should also read the "offset voltage trim" section of the OPA134 datasheet, available at www.ti.com.

I just had ordinary carbon film resistors for this purpose, my guess is this an appropriate use of such a low quality resistor?

Carbon films are fine for this application.

The last OPA134 opamp seems to me to be malfunctioning. Based on what I read from the schematic, it seems that the voltage would originate from that opamp directly since you have the signal folding back into the output.

U4 is connected as a unity gain follower, which means that the output voltage at pin 6 should be the same as the input voltage at pin 3. What voltage do you measure at U1, pin 6?

[rdatman]

The voltage is the same on -V and pin 6, (I said that in the previous post)-- that voltage is (-) 3V.

I also get that voltage on pin 4 of the THAT 15XX.

In the schematic you supply the control circuit for the voltage offset shows both resistors PLUS the 100K pot. I didn't realize that it is a universal design. Got it now.

I listened to the other mic pres which I built-- all work flawlessly except for the one with offset issues.

I am wondering if anyone has replaced the 134 with the 627 ( I realize that DC offset is not pin compatible, otherwise it should work?

This is in my mind the finest opamp I have ever heard except for the LME49710 series, which I like very much. The 134 is not as nice.

[tpryan]

I also get that voltage on pin 4 of the THAT 15XX.

Pin 4, or pin 6? U1, pin 4 should be -B (-18v or so). U1, pin 6 is the output. See the data sheet at www.thatcorp.com for more information.

What voltage do you measure at U2, pin 1 and U2, pin 7? Let us know what you find.

This is in my mind the finest opamp I have ever heard except for the LME49710 series, which I like very much.

LME49710 in the DIP package will work fine in place of OPA134 and is much cheaper than OPA627, but you may want to reduce the supply voltage to +/-17.

[rdatman]

So you are confirming that the 627 will work? I have 12 sitting around, and love the sound in another dual servo mic pre I modded, the MP20...

The pinout for the THAT 1512 is not listed in the spec sheet but I'm pretty sure it is the -V (#4) pin that we're talking about....

If you're holding the board so that the opa 134 is lower left hand and the 1512 is to its right, the pin I am talking about it the one that traces to pin 6 of the OPA134-- it is the lowest left hand pin on the THAT chip. That seems to make sense since it is looking at the output of the opa134.

It sounds to me like the opa 134 is malfunctioning, but before I start replacing it (it is not socketted I'd like to have an educated guess what is going on.)

[tpryan]

So you are confirming that the 627 will work?

I haven't tested it, but any op-amp that is unity gain stable will probably work fine. You should have a scope available to test for ringing or oscillation, however.

The pinout for the THAT 1512 is not listed in the spec sheet but I'm pretty sure it is the -V (#4) pin that we're talking about....

http://www.thatcorp.com/datashts/1500data.pdf - Table 1

DIP package pinout: http://en.wikipedia.org/wiki/Dual_in-line_package

If you're holding the board so that the opa 134 is lower left hand and the 1512 is to its right

Output of U4 (pin 6) drives input of U3 (pin 4). U4 is OPA134, U3 is THAT 1646, not 1512.

It sounds to me like the opa 134 is malfunctioning, but before I start replacing it (it is not socketted I'd like to have an educated guess what is going on.)

Too soon to tell without knowing the voltage coming out of U1, pin 6 and going into U4, pin 3, but now I'm wondering if you haven't swapped the 1512 and 1646?

[rdatman]

I misremembered, it is 16XX not 15XX op amp that we're talking about. There are no errors (in locations) on any of the boards that I built, the problem is I don't have the device in front of me, it is in my garage and my computer is in the recording studio.

(And for the third time ) that voltage is -3 Volts-- negative 3 Volts.

[tpryan]

The voltage is the same on -V and pin 6, (I said that in the previous post)-- that voltage is (-) 3V.

I also get that voltage on pin 4 of the THAT 15XX.

If that's the case, you should also see -3V at the -18V test point measured in step 29, as well as all the other -B points in the circuit. That's a power supply problem, not an offset problem. If that's what you're seeing, make sure R33 is 150 and not 1K5.

[rdatman]

All of the Voltages seem to be correct, I would have posted any anomalies that I found. The voltages vary slightly from +/- by a few tenths of a volt otherwise what I have is 17.56 (positive side) and (-) 17.52 (negative side).


The only places I see incorrect voltages is on the opamps and resistors nearby.

U1:
1 -.77V
2 -.18V
3 -.18V
U2:
2 -.77V
3 -.77V
U3
4 -.3.23V
7 -1.92V
U4
2 -3.23V
6 -3.23V

Those are the negative voltages on the opamps other than those that are supposed to be negative and are correct. I ran across the problem on final assembly of the control circuit for the DC offset.

It was then that I found an extreme reading, and contacted you.

Keep in mind that I built 4 units three are perfect and working fine. I have triple-checked the resistor color codes, and polarity on caps that are polarized and the diode polarity. Those are NOT possibly the issues.

What could be the issue in my best estimation is a damaged opamp.

[tpryan]

I'm afraid I still don't see the exact voltages at U1, pin 6 and U4, pin 3. Please let us know what you find at those two test points. Make sure R26 is fully clockwise when making the measurements.

[rdatman]

I'm afraid I still don't see the exact voltages at U1, pin 6 and U4, pin 3. Please let us know what you find at those two test points. Make sure R26 is fully clockwise when making the measurements.

Those essentially read 0V.
U1
pin 6 reads -10mV
U4
pin3 reads -42 mV

I hope that helps. Like I tried to tell you the pins that are supposed to read +/- 17.6 volts do. The ones that should read 0 volts do the others are listed as I found them. Also the 16XX That chip gets very hot...

[rdatman]

Reading with pot fully clockwise.
U1
1 -.67V
2 -.14V
3 -.14V
5 +16.7V (1 volt low)
6 -15.9V
8 -.67V
U2
2 -.67V
3 -.67V
U3
1 -1.31V
2 -2.5V
4 -5.6V
7 -2.5V
U4
2 -5.6V
3 -15.86V
6 -5.6V
When I was testing before the pot was fully faded b/c thats the method described in the biasing voltage instructions. These readings are very different.

[tpryan]

I suspect U3, the 1646, though U4 may be a problem as well. Before you remove either, carefully cut the trace connecting U4, pin 6 and U3, pin 4 with an X-acto knife. Then measure the voltages at U4, pin 6 and U4, pin 3 again and let us know what you find. If you don't care to do surgery on the board yourself, send it in and we'll get it going.

[rdatman]

I scraped across the trace with a sharp edge and disconnected it.

Voltages as follows:
U1:
1 -.68V
2 -.14V
3 -.14V
5 0V
6 0V
8 -.67V
U2:
2 -.67V
3 -.67V
Unchanged
U3:
1 -.65V
2 -1.46V
4 -1.25V
7 -1.43V
8 -1.16V
U4:
1 17.37V (low)
2 -1.9V
3 0V
6 -1.9V