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
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
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.
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.
TL071 help
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
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
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.
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.
hhhhmmmm
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
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
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.
Attachments
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
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
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.
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.Hi Mooly.......well guess what..after I installed the bleeder resistor ( 5K ) across the output, the dcv dropped down to about 3mv!. This was the case with either the tl071 or the opa134 !
Yes, pin 4 is ground and there was zero volts on it....and yes it is a single supply rail ( one bridge )
So, do you think I can now short out that output cap!
Thanks heaps..
Paul
Yes, pin 4 is ground and there was zero volts on it....and yes it is a single supply rail ( one bridge )
So, do you think I can now short out that output cap!
Thanks heaps..
Paul
Bit of lateral thinking you see. You must not remove the cap. Pin 6 of the OpAmp will be biased to around half the supply voltage with it being a single rail supply. So the cap is VITAL to stop DC from reaching the pre amp and power amp inputs and must never be ommited. I am surprised no bleed resistor was fitted really, and it's something I would consider fitting permanently. If you do use something much higher like 100K as this will present no load at all to the OpAmp.So that's good then, are we saying it's sorted, and does the OPA sound any different
nearly sorted
.....I replaced the 5k bleeder resistor with 500k, as suggested.Should I re-install the .1mfd caps on pins 7 and 4 ( they have been off for the last bit of testing.
In regards to the output cap, it is 25/470 mfd with maybe a 10 nano film across it...can I change the value down to even 5mfd (poly ), as I am trying to get maximum sound quality here.
In regards to sound difference between the two op-amps...well I wouldnt say they are worlds apart, but the opa134 is clearly more open and also more extended in the top end, and a bit more detail across the board as well. It also sounds a bit more "analogue" or musical....but the lowly tl071 isnt that bad at all.
Paul
.....I replaced the 5k bleeder resistor with 500k, as suggested.Should I re-install the .1mfd caps on pins 7 and 4 ( they have been off for the last bit of testing.
In regards to the output cap, it is 25/470 mfd with maybe a 10 nano film across it...can I change the value down to even 5mfd (poly ), as I am trying to get maximum sound quality here.
In regards to sound difference between the two op-amps...well I wouldnt say they are worlds apart, but the opa134 is clearly more open and also more extended in the top end, and a bit more detail across the board as well. It also sounds a bit more "analogue" or musical....but the lowly tl071 isnt that bad at all.
Paul
Hi,
All sounds good so far. Put the 0.1 caps back directly across pins 4 and 7. That's for ensuring the supply is at as low an impedance as possible right on the OpAmp itself. The output cap (the one from pin 6 to the RCA socket) I wouldn't change the value of it without seeing it and knowing what it works into although 470 mfd sounds huge for a coupling cap working into high impedances such as a preamp input. The larger the cap the more extended the low frequency cutoff point. Bypassing with a film cap is fine and the value isn't critical 0.1, 0.047 etc.
Do you understand the values, 0.1 mfd =100nf=100000pf and 0.01mfd = 10nf = 10000pf. A 0.1 mfd cap may be marked 104 meaning 10 and four zeros = 100000pf. Easy
Interesting what you say on the OPA vs the TL0. I agree the TL071/2 series is pretty good and is a standard recommendation of mine, for upgrading "cooking grade" opamps. Much better sonically than NE5532 / 4 etc. For something better I have always liked the OPA604-- to me it comes closest to an "analogue" sound, maybe because the distortion spectrum is mainly even harmonic -- the right sort !
All sounds good so far. Put the 0.1 caps back directly across pins 4 and 7. That's for ensuring the supply is at as low an impedance as possible right on the OpAmp itself. The output cap (the one from pin 6 to the RCA socket) I wouldn't change the value of it without seeing it and knowing what it works into although 470 mfd sounds huge for a coupling cap working into high impedances such as a preamp input. The larger the cap the more extended the low frequency cutoff point. Bypassing with a film cap is fine and the value isn't critical 0.1, 0.047 etc.
Do you understand the values, 0.1 mfd =100nf=100000pf and 0.01mfd = 10nf = 10000pf. A 0.1 mfd cap may be marked 104 meaning 10 and four zeros = 100000pf. Easy
Interesting what you say on the OPA vs the TL0. I agree the TL071/2 series is pretty good and is a standard recommendation of mine, for upgrading "cooking grade" opamps. Much better sonically than NE5532 / 4 etc. For something better I have always liked the OPA604-- to me it comes closest to an "analogue" sound, maybe because the distortion spectrum is mainly even harmonic -- the right sort !
happy
Hi Mooly,
Thanks for the tip with reading those caps....I normally just measure them with my dvm before they go into the cct.
For the output cap(s) I might try a 10mfd back gate electro I have lying around bypassed with a 10 nano wima mkt, and see what happens !
Just for your info, my amp is based around Peter Daniels lm3875 gainclone with a 10k Acoustic dimension stepped attenuator, cables are DH Labs and speakers are NHT st-4.
I will try the opa 604 and see how that goes....once again thanks for your expert assistance.
Hi Mooly,
Thanks for the tip with reading those caps....I normally just measure them with my dvm before they go into the cct.
For the output cap(s) I might try a 10mfd back gate electro I have lying around bypassed with a 10 nano wima mkt, and see what happens !
Just for your info, my amp is based around Peter Daniels lm3875 gainclone with a 10k Acoustic dimension stepped attenuator, cables are DH Labs and speakers are NHT st-4.
I will try the opa 604 and see how that goes....once again thanks for your expert assistance.
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