Hi all
I seemed to have had 1 or 2 AD BUF03 buffers fail on my Theta DS Pro Basic 2 DAC output stage. As many of you may know, these are now obsolete and so are the BUF04 pin equivalent.
I have been thinking of trying the BUF634 from Burr Brown but these have a possible DC off set of 100 mv vs 1mv on the BUF04.
It is a DC coupled ouput stage and I am not sure if my ECI 3 is DC coupled or not. I would obviously prefer to avoid the possibility of a highish DC offset.
Does anybody have any suggestions for a replacement, or how I may make the BUF634 work. (could one trim the off set?) BUF03/4 has a specific set of pins for this.
Regards
Guillaume
I seemed to have had 1 or 2 AD BUF03 buffers fail on my Theta DS Pro Basic 2 DAC output stage. As many of you may know, these are now obsolete and so are the BUF04 pin equivalent.
I have been thinking of trying the BUF634 from Burr Brown but these have a possible DC off set of 100 mv vs 1mv on the BUF04.
It is a DC coupled ouput stage and I am not sure if my ECI 3 is DC coupled or not. I would obviously prefer to avoid the possibility of a highish DC offset.
Does anybody have any suggestions for a replacement, or how I may make the BUF634 work. (could one trim the off set?) BUF03/4 has a specific set of pins for this.
Regards
Guillaume
first question - do you know why the BUF03 units failed?
We wouldn't want to replace them to have the same problem 😉
May want to look at discrete designs to replace?
mlloyd1
We wouldn't want to replace them to have the same problem 😉
May want to look at discrete designs to replace?
mlloyd1
You may use AD843. This opamp in unity gain mode sounds like BUF03. BUF634 sounds worse than BUF03 and AD843
The usual cause of a BUF-03 failure is heat due to a high rail voltage, that is above 15volts. A discrete design would have to go a long way to beat the BUF-03, they were excellent.mlloyd1 said:
If you search, you can find a unit or two still on the shelf.
Opinions on the Sound Quality the BUF634?
Any opinions on BUF634 good or bad would be appreciated!
Any opinions on BUF634 good or bad would be appreciated!
My friend has a Theta DAC which employs two BUF03 for the last buffer output stage. The right channel failed. I bought two BUF03 IC from the PRAudio. My friend passed this IC to a repair shop and get it done. After few weeks time, the DAC failed the right channel again. I asked my friend I can take it home and take a look at it.
After checking the data sheet and signals around the BUF03, the +/-15Vdc are good. There's signal at the input but nothing at the output. The DC at output seems good. Then, I had the IC replaced with the spare one. It started working good. Then, I noticed that there's one 100uF 25V tantalum capacitor at the -15Vdc bypass changed color from yellow to dark brown. I had it removed from circuit board and measured the capacitance. It read 0uF and there's burning sign at the side. I had it replaced with a 10uF tantalum in parallel with a Sanyo 100uF 20V OS capacitor.
By the way, the BUF03 has a small heat sink at the top. I added some heat transfer compound before putting in the circuit board. I hope the DAC will keep working from now. The temperature at the IC is around 45 deg C.
Merry Christmas and a Happy New Year.
Johnny
After checking the data sheet and signals around the BUF03, the +/-15Vdc are good. There's signal at the input but nothing at the output. The DC at output seems good. Then, I had the IC replaced with the spare one. It started working good. Then, I noticed that there's one 100uF 25V tantalum capacitor at the -15Vdc bypass changed color from yellow to dark brown. I had it removed from circuit board and measured the capacitance. It read 0uF and there's burning sign at the side. I had it replaced with a 10uF tantalum in parallel with a Sanyo 100uF 20V OS capacitor.
By the way, the BUF03 has a small heat sink at the top. I added some heat transfer compound before putting in the circuit board. I hope the DAC will keep working from now. The temperature at the IC is around 45 deg C.
Merry Christmas and a Happy New Year.
Johnny
I just came across this thread, and hope it is okay to ask followup questions here.
I am having ongoing issues with my Theta Pro Basic DAC.
Several months ago, one channel was at -14 volt output. The other channel was working. The power supply was at +15.4/-15.1 volts. Replaced the BUF03 and the two tantalum caps bypassing the buffers. It ran for several months.
A short time ago, both channels stopped working and are both at -14 volts output. The power supplies remain at +15.4/-15.1 volts. The inputs to the buffers are at 0 volts (as should be) at rest. Checking with a scope there is a small (~10 mV) of very high frequency (>200 MHz) noise apparent, but it may be just induced noise from the environment – it's very low level.
Replacing the BUF03s again without other fixes seems unlikely to resolve this for the long term – has anyone seen this kind of problem with the BUF03s, and/or low level high frequency noise causing BUF03s to blow?
My apologies if this was the wrong place to post this. Not having access to the schematics is making troubleshooting fairly tiresome, any assistance would be greatly appreciated.
cheers, Paul
I am having ongoing issues with my Theta Pro Basic DAC.
Several months ago, one channel was at -14 volt output. The other channel was working. The power supply was at +15.4/-15.1 volts. Replaced the BUF03 and the two tantalum caps bypassing the buffers. It ran for several months.
A short time ago, both channels stopped working and are both at -14 volts output. The power supplies remain at +15.4/-15.1 volts. The inputs to the buffers are at 0 volts (as should be) at rest. Checking with a scope there is a small (~10 mV) of very high frequency (>200 MHz) noise apparent, but it may be just induced noise from the environment – it's very low level.
Replacing the BUF03s again without other fixes seems unlikely to resolve this for the long term – has anyone seen this kind of problem with the BUF03s, and/or low level high frequency noise causing BUF03s to blow?
My apologies if this was the wrong place to post this. Not having access to the schematics is making troubleshooting fairly tiresome, any assistance would be greatly appreciated.
cheers, Paul
I am very fond of the BUF03 and use the AJ mil spec version. It's a unique unity gain buffer being fet input and bi-polar output, as it has no overhaul feedback and runs hard into class A (needs big heat sinks).
Very high fet input impedance, nice low bi-polar output impedance at 2ohms at 70mA, DC offset nulling pins and will drive my low impedance Sennhiser's direct no problems.
I think it's by itself and nothing sounds like it. Very reliable so long as you heat sink it well.
Ebay has many in Asia and would be hard to make fakes that work, as nothing else has that package configuration and without feedback.
Cheers George
Very high fet input impedance, nice low bi-polar output impedance at 2ohms at 70mA, DC offset nulling pins and will drive my low impedance Sennhiser's direct no problems.
I think it's by itself and nothing sounds like it. Very reliable so long as you heat sink it well.
Ebay has many in Asia and would be hard to make fakes that work, as nothing else has that package configuration and without feedback.
Cheers George
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I have no personal experience with 03's, but 634 sounds worse than 04. 634 has frequency extremes raised and less detaail in midfrequences. Not awful difference, but real.
It is using the same (dirty) power rails for the opamp as for the buffer. Far better to clean up the opamp's rails by some low-pass (RC or LC) filtering.
Oh No! I will be using an improved Sulzer regulator as power supply.
But for the preamp, I haven't made up my mind between this option and Walt Jung's AD744/AD811 preamp.
But for the preamp, I haven't made up my mind between this option and Walt Jung's AD744/AD811 preamp.
AD744 +AD811 (using Walt Jung's trick of bypassing AD744 output stage) is old but very good (both sound and measurement-wise).
I've had one for years and still keep as backup when I'm experimenting with other line stages.
Make sure to put heat sink on AD811, it's output stage runs "rich" unlike many other opamps.
mlloyd1
I've had one for years and still keep as backup when I'm experimenting with other line stages.
Make sure to put heat sink on AD811, it's output stage runs "rich" unlike many other opamps.
mlloyd1
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What would you consider more modern or distinctive? With ICs, of course.
Valves are old, but are still considered great.
Valves are old, but are still considered great.
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Gary Galo replaced the AD744 with the AD825 on his Adcom 565 upgrade. Perhaps that is worth doing.
I don't know if there are other chips worth trying.
I don't know if there are other chips worth trying.
Yes, the AD811s run quite hot.
I don't think you need sinks on them if you use lower supply voltages.
At least none of mine have self destructed on 13-14VDC.
There are plenty to try. The OPA1642 is certainly one of the newer and better sounding JFET op-amps out there.
the AD744 (and maybe one or two other AD parts) are unique in that you have access to the signal prior to the output stage so you can avoid the output stage nonlinearities, presumably to use some output stage that makes different "audio-preferred" compromises. the other (newer) parts don't have this option.
which is better?
finding out which you prefer is part of the joy of DIY...
🙂
mlloyd1
which is better?
finding out which you prefer is part of the joy of DIY...
🙂
mlloyd1
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