cal gamma dac op-amp tl071

[Chilli6565]

Hi all,

I am using the great little cal gamma dac. I would like to change out the the two tl071 op amps for something better, the one replacement which looks like it will "drop in " is the BB opa134 op amp....just wanted conformation that it will drop in with little or no mods needed.
Are there any others which would be suitable as well?

thanks

paul

[Mooly]

Depends how they are used. If the offset null feature is required then the OPA isn't suitable without modding the board.
Just because there are OpAmps in a device they are not always used in the audio side, they could be in the PSU or a servo etc. Just beware
And although the same warning applies, I like the OPA604 sonically. See if pins 1 and 5 are used or not with your TL071.

[Chilli6565]

Hi Mooly.......thanks for the assistance.I should have given a bit more info, there is one tl071 for each channel. I traced the pcb back from the rca center pin on the back of the dac and it goes to pin6 (out ? ) on the op amp.I have had a good look at pins 1 and 5 and they are definatley not connected...so can I now use the opa134 or opa604??. Also there is a DC blocking cap in series( electro ) bypassed with a film just before the output...can i safely short them out of the cct???

thanks again
Paul

[Mooly]

Probably O.K. to use. From what you say they are just output buffers with possibly a filter network in the feedback loop. No matter, there should be no real problem swapping them.
As to shorting the output cap, first measure on the most sensitive DC volts range on your DVM from the signal ground ( The outer of the RCA socket ) and pin 6 with no signal. Anything more than say 5 or 10 millivolts and I would say no, although a lot depends on what you feed it into. If you feed it into an AC coupled preamp then no problem unless the offset is high. Any DC voltage on a volume control (such as a passive preamp DC Coupled one) and the control will sound noisy as you turn it. An offset can also cause loud "thumps" as you switch between sources on preamp.
Measure the voltage and tell us what it is, both with the TLO71 and whatever you fit.
Two more things, if the output from pin 6 goes directly to the RCA ( via just the cap ) it may be wise to add a 68 ohm or thereabouts in series from pin 6 to isolate the replacement OpAmp. The TL071 is unconditionally stable really, the others are not and cable capacitance could upset it. The other thing is to add a 0.1 mfd ceramic or polyester cap direct on the IC from pins 7 to 4.

[Chilli6565]

Hi Mooly,
Good and bad news!....I soldered in the .1 mfd caps, no prob. opa 134's arrived today...installed them no prob. I also changed out a few 25/1000 mfd cheap electros for panna fm's ( like them a lot )... I think they are de coupling caps?.
It fired it up great and it is sounding really nice already.

Thats the good news!....the bad news is I seem to be getting 3.4 volts dc! at the output rca ( no load ).....how can that be when it is after the blocking caps, I even tried another meter!!.
I checked at pin 6 ....3.4vdc and even the same at pin 3 (+ in ?).

Sorry, I didnt do any measurements with the tl071's

Is my dac possessed !!!

Paul

[Mooly]

O.K. It sounds like either you have done something untoward or, more likely as the output is AC coupled is that it's oscillating. Remember I mentioned that the OpAmps may not be 100% unconditionally stable. Have you used sockets ? Have you any small value caps on hand in the 22 picofarad range ? Anything really, even up to 0.1 mfd just to see if it stops it. Connect between pins 2 and 6 before you do anything else.

[Chilli6565]

hope this helps

[Chilli6565]

and this

[Chilli6565]

thanks for that info Mooly....since your last post I have removed the op-amps altogether from the sockets ( which I installed ) and re did the measurements....they were pretty much the same !....3.5 vdc on pin 3....2.5vdc ( which is a bit lower than b4 ) on pin 6 and 9 vdc on pin 7 which is correct ( I think ). I have checked all my solder connections where I have replaced caps.
I even used another power supply to see if that was affecting things...nope,still the same.
I havnt done your last mod as I thought now that the problem may be elsewhere?

Paul

[Mooly]

Hi,
Have got your message
This may sound strange but just looking at the picture, does it just run on a single supply rail. Does pin 4 of the OpAmp go to ground ? What is the DC voltage on pin 4 ?
You say it works OK. Does the output coupling cap from pin 6 have a "bleed" resistor on the output side. If not a DVM would read a DC voltage at the output as the end of the cap is "floating". If you are not sure add a resistor - 1k to 1 meg across the RCA output and remeasure.
If we are back to suspecting an oscillation problem have you tried putting a TL071 back and re-checking. Or are they to badly chewed up, I know what double sided print with plated through via's is like to work on. Even a 741 etc would be OK to check.
For the "filter" around the OpAmp to work the OpAmp (obviously ) has to be present. Perhaps not as obvious is the fact that with it not present the circuit will still pass the signal undistorted, but unfiltered and this may be confusing things as well.