Several months back Radio Shack began clearing out their Auvio HD Radio tuners. These components provide HD Radio reception (both AM and FM) for your home stereo system. Its well laid-out front panel features a large backlit LCD for tuning information, a detented rotary tuning control with a divot for your fingertip to allow easy knob spinning, and other control buttons. It even has an optical output in addition to the standard analog outputs! It includes an IR remote and has provisions for rack mounting (!), going so far as to include the rack ears in the box. The units started out at just below $100.00 but had their price reduced to $39.95. That was pretty good, but customers could take an additional $10.00 off with a “$10 off a $40 purchase” coupon they were giving away with codes on their in-store receipts. After checking stock on-line I visited the last store in my area that had them available and bought their last two units. I then purchased a third tuner from their website. The only tricky part came with getting a filler item to take the price over $40.00. When I bought the two tuners in store that was no problem as the total was over $40.00. Purchasing the tuner on-line would provide free shipping, but when I added a 99-cent pack of resistors to get the price up I invoked a $12.00 shipping charge! A complaint call to their website sales team gave me the price I wanted for both items and still gave me free shipping.
When I hooked the tuner up to my office stereo I was impressed by the unit’s look and feel during use, but its sound left me dry. There was limited treble and the bass, while there, lacked any kind of punch. A quick online search yielded many forum posts complaining about the same issue. Supposedly the optical output yielded normal-sounding audio, but two of my amps only have analog inputs. I listened to the tuner occasionally for several weeks but found that I couldn’t listen long due to its lifeless sound. I had a hunch that the analog outputs had the wrong resistor/capacitor combination on the driver for the analog jacks, so I spent time searching in vain for a schematic for the unit. I had resigned myself to the prospect of having to reverse-engineer the circuit’s audio stage when I finally stumbled across a web forum post by user ‘vintageaudioexchange’ at the AudioAsylum forum. This promised to be a simple swap that would yield immediate improvement, so I ordered three of the Burr-Brown OPA2604AP chips from Digi-Key. The post’s author claims the part cost him around fifty cents—I’d like to know where he got his op-amps. Digi-Key charges over $5.00 each for these little buggers.
Last weekend I decided to open up one of the tuners and try the mod. As you can see in Photo 1, the unit is what’s fondly called a ‘gutless wonder.’
The power supply is hidden beneath a metal box for shielding; the board to the box’s right holds the op-amp to be changed, shown circled in Photo 2.
The stock op-amp is a JRE 4558, a low-dollar part that’s used to good effect in many products. Not so here! I unsoldered the chip and tried to install a machine-pin DIP-8 socket. Unfortunately the row-to-row hole spacing is too far apart to use the better socket, so I installed a common, cheaper friction-fit socket before inserting the chip. While I was at it I re-touched the solder joints for the antenna inputs. The circuit board-mounted RF coax jack was the cheapest part I’d ever seen and was not attached to the back panel with the customary nut and washer. The panel hole is bigger than the jack itself although the jack is not centered in the hole. This creates a problem when a standard coaxial antenna cable hangs off the unit’s backside– the jacks’ bracket tends to bend. The jack can be seen in Photo 3.
Mounting a jack from the chassis in this manner is typically done to isolate the jack’s ground from the unit’s power and data ground connections. However, unless the cable is light and the user carefully attaches and removes that cable you get a mechanically unsound connection. Even if the jack is of higher quality the stress point is still moved to the PC board itself, which in my opinion is a poor idea. I tested with a volt-ohm-meter and found all grounds to be tied together. I scoured my “hell box” and found some appropriately-sized nuts and washers, shown in Photo 4.
Since the PC board is mounted a short distance from the back panel, it would take two nuts to make a solid mount. One of the nuts needed to be thicker than the other; this one would go on first and be screwed against the jack’s bracket. This filled the space between the jack and back panel. (I could also have used a thinner nut and screwed it back off the jack to the point where it touched the inside of the case’s back panel.) Washers were another story. The hole in the unit’s back was bigger than a standard washer for that size of connector. I found a few larger washers with solder tabs and used tin snips to cut the tabs off to make them appear like standard washers. The modified washer then went onto the connector from the outside of the back panel before attaching the second nut. This yielded a solid connection that protects the jack in case the cable gets jerked or pulled.
After re-connecting the unit I crossed my fingers and powered it up. While doing my research I’d found another thread where people were discussing op-amp swaps in audio equipment, and a poster cautioned that the resistor/capacitor filter values for the new op-amp might need adjustment since the new chip’s bandwidth exceeds the specs of the old part. I can report that this was unnecessary. There is a significant improvement to the sound—better high-end, improved low-end, and the sound is less fatiguing for extended listening. The original poster said he’s not the type to claim the difference “was like a blanket was lifted from the speakers,” but I will. The difference is that noticeable. I can now happily listen to my new acquisitions without constantly be bugged about their sound quality.
Additionally, I read that Best Buy sold a similar unit under the “Insignia” brand, the NS-HDTUNE. This tuner is reputed to have the same internal circuitry, so the op-amp swap should work on this tuner as well. These are still available as of today on the Best Buy website as an “outlet” item for $79.95; they won’t last long at that price.
I should also mention that, contrary to the logical extension that the “HD” in HD Radio stands for ‘High Definition” as it does in HDTV, it does not. It stands for “Hybrid Digital,” which is a trademark of its progenitor iBiquity Digital Corporation. A brief history of the format’s development can be found here.