Q: I live in USA and I can't buy AC to AC wall transformers anymore. What can I do?
A: This is a brand new problem as of 2016! Triad abruptly stopped making AC to AC transformers. Mouser only carries triad. As of late 2016 while I'm writing this Jameco (www.jameco.com) still has a full line of AC-to-AC transformers. The other solution is to just separately box up a 60Hz torroidal transformer from Mouser or Digikey with a 2.1mm power plug coming out to plug into the ODA. I don't recommend going below a 16Vac secondary or 2amps, but 18Vac (or two 9Vac secondaries in series) is fine (for the +/-12.5Vdc rail build).
Q: I live in Europe and I can't buy AC to AC wall transformers anymore. What can I do?
A: The best solution is to just separately box up a 50Hz torroidal transformer with a 2.1mm power plug coming out to plug into the ODA. I don't recommend going below a 16Vac secondary or 2amps, but 18Vac (or two 9Vac secondaries in series) is fine (for the +/-12.5Vdc rail build). Farnell and RS Components both have torroidal transformers and they all run at 50Hz. I can't get 50Hz tranformers here in the states, Mouser and Digikey only stock 60Hz transformers. A 60Hz transformer can overheat if run on 50Hz (appears as an applied over-voltage), but the reverse works just fine, a 50Hz transformer can run on 60Hz.
Q: Can I buy a parts kit from you?
A: I'm sorry but no. There are just too many parts involved. There is a good chance of loosing one or more of the surface-mount parts in the rug (I've done it many times!) or having a semiconductor or other part be bad right out of the bag. Mouser Electronics & Digikey pack up everything in well labelled bags and they are good about replacing defective parts. I highly recommend buying at least one additional of all the smaller surface mount parts to solve that "lost in the rug" problem. All of the surface mount parts together are only around $35 of the parts price.
HOWEVER - two exceptions. I have the two parts in the BOM that Mouser doesn't stock, the LT1963A and LT3015 LDO voltage regulators, on hand to sell to prevent having to place a separate order to Digikey. Please contact me. If Mouser is out of stock of any parts (or any parts are discontinued) I should have those on hand too to sell.
Q: Why does Front Panel Express charge so much for the front and rear panels?
A: I wish I knew! The only other company I've found that can make panels is CAM Expert (dotcom), which charges 20% less, but the quality of the work seemed a little better through FPE. FPE also runs the occasional 20% off sale on their Facebook page, I've noticed.
Keep in mind that FPE charges quite a bit less if the panels do not have any text engraving, just holes. I've also posted FPE CAD files for the front and rear panels with no next. They also charge quite a bit less if text engraving has no color fill (just bare aluminum).
I have also posted just the measurements for all the front and rear panel holes. The case comes with blank front and rear panels. If you have a drill press or access to a milling machine you can just drill/engrave the panels yourself and save that $100 or so that FPE would charge for the set.
If anyone out there knows of a company anywhere in the world that can make panels for less, PLEASE let me know!
Q: I'm going to put the ODA board in my own custom designed case. Is there anything I need to be aware of?
A: Just that the metal rear panel is used to heat sink the voltage regulators. So if you use a non-metallic case you need to use a separate heat sink on those regulators.
Q: Do you ship outside of the USA?
A: I do. The US post office here has the lowest rates. Their "first class international parcel" is the lowest rate, which is $13.50 for small to medium boxes and $22.50 for larger boxes (actual cost, no markup by me). This service goes by ship. Some typical shipping times I've seen are 3 weeks to Australia and 2 weeks to South Korea. Part of that time is also going through customs on both ends.
Q: I've built my ODA but no sound comes out!
A: Remember that your ODA has a headphone relay with a 3 second turn-on delay to prevent turn-on thumps and allow the parts to thermally stabilize (for super-low DC output offsets). You won't hear any sound at all coming out - or be able to make any electrical measurements on the ODA output - until that 3 seconds is up. Also remember that your new ODA has an input select switch. Try the other switch position.
Q: Can I plug headphones into both of the front panel output jacks (the 3.5mm and the 1/4")?
A: Yes you can! That is a great use for the amp's high current output capability in fact. Just make sure the total parallel impedance of the two headphones does not go below 15 ohms. So for example two 32 ohm headphones would be 16 ohms in parallel and OK. But a 32 ohm headphone and a 16 ohm headphone would not be OK since the parallel resistance is 10.7 ohms.
The same comment applies to using a Y cable on the output to power two headphones.
Q: Isn't the 2 amps specified in the BOM for the power transformer ridiculous overkill, especially since the maximum current output is 420mA per channel and the quescent (idle) chip current is around 30mA?
A: It turns out that the double half wave power supply in the O2 and ODA charges the filter capacitors in short high-current spikes that are around 2.5 times the average current draw. So to keep the transformer core out of saturation - which can cause excessive heating and create electrical noise - the secondary really does need to be rated at 2 amps. The transformer manufacturers call this "de-rating the secondary current for use with a capacitive input half wave rectifier".
Q: NwAvGuy believed 100% in objective measurements. He wrote that if an amplifier measures well, it will sound good. Do you follow that same approach?
A: Only about 50%. :) I tend to believe that it is also possible for an amplifier to measure well but sound bad, which really just means there is a measurement somewhere that matters which hasn't been discovered yet. I believe that in the end the "how it sounds" test is most important, although I would agree with NwAvGuy that blind testing (eliminating "sighted bias") is the only proper way to do that.
But I do believe that great measurements are a good place to start looking for a good sounding amp. An amp measuring extremely poorly in any given area is not likely to be a winner on listening tests.
Q: Texas Instruments is discontinuing the LME49990 op-amps in 2016. Can I use other op-amps in the gain stage?
A: I am stocking those LME49990s, see the question above about buying parts no longer stocked at Mouser. It should be possible to use other op-amps in place of the LME49990, with a couple of things to note. I specifically chose the LME49990 since it is rated to drive all the way down to a 600 ohm load at low THD+N levels, which allows it to drive the 1K pot and feedback network in the ODA. The 1K pot is part of the "special sauce" that keeps the ODA background noise extremely low, as it keep Johnson noise low. You will need to make sure any other op-amp also is able to drive at least 600 ohms with low THD+N. The OPA827 is one that can and so is the OPA1688. I haven't tested these op amps in the ODA though, so it is possible they may oscillate. You should test the temperature of any alternate op amp. Normal should be in the 90F - 100F range. Oscillating may be in the 140F+ range. Note also that the OPA827 and OPA1688 are FET-input op amps. These may have more input voltage noise, but usually will have less current noise than the LME49990.
Q: Can I put the ODA PC board in a box other than the specified metal B4-080 case that has slots for the PC board?
A: You can, the PC board has holes in all 4 corners for mounting, but there are a few issues you will have to deal with. The rear metal panel is used for heat-sinking the two voltage regulators that are flush with the rear edge of the PC board. You would have to come up with some way to use external heatsinks for these. The rear panel also has holes for the AC input jack and RCA inputs. You would have to removely panel-mount those and run wires back to the PC board holes.
The front panel has a few PC-mount parts that have relatively short shafts to go through the metal case's 2mm thick aluminum panel. All these parts - 2 push button switches and one 5 position rotary switch - have lots of pins and the circuits involved can oscillate or pick up noise if the parts are panel mounted and wires are used to connect to the board.
The best solution here is to probably use the metal front and back panels specified for the ODA, but then use your alternate case material for the sides. But even here you will have a tricky alignment issue in getting the board mounted at just the right height and X & Y for the parts to properly stick through the panel holes. With the original metal case all of this is taken care of automatically when the board is slide into the slot and the panels attached.
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