Improving a Flea-Powered Headphone Amplifier

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This headphone amp upgrade adds four relay-selectable inputs, a numeric display, a phono-preamp stage, and an auxiliary output.

Thanks to Aren van Waarde (and Dr. Smith, a.k.a. G40EP) for his article on the G40EP. Mr. van Waarde’s challenge and built the headphone amp---Accepted!. After powering up the circuit, I was amazed what I heard!

This amp is so simple and clean—no magic—with a straightforward circuit (a little bit different from TDA2003 manufacturer suggested topology) that works very well.

I must confess that the circuit did not hold my attention at first look. After two weeks, I decided to search for the parts in my junk boxes and put every thing together. The components are easy to find, with neither one “white-fly” nor esoteric “audio grade” component. In a few hours of heat and some smoke, the circuit was working very well, to my surprise. In fact, this simple circuit surpassed my expectations.

So, I decided to add the following improvements:

  • • Multiple selectable inputs
  • • Phono (MC) preamplifier with RIAA passive network
  • • Volume control
  • • Visual indication to selected input (a LED seven-segment subminiature display)
  • • Auxiliary output (to feed my single-ended “A-class” 47 triode mode amplifier)
  • • A reasonably fine-looking cabinet


Above: Fig. 1: G40EP (exterior).


Above: Fig. 1: G40EP circuit.

CIRCUIT FEATURES





You can analyze these improvements separately in five parts:

1. The G40EP, a TDA2003-based low- gain amplifier (see the article for more details)

2. Power supplies: two separate single +12V DC to each G40EP and one dual ±12V DC for the phono stage.

3. The phono stage itself.

4. The MUX and digital section.

5. The cabinet.

G40EP:

This is the same circuit as in the article. But, due to the difficulty of finding R1/R101 (130-ohm I mounted a 270-ohm in parallel (135-ohm which worked fine. Basically it’s a TDA car audio power amplifier operating at low gain so the quiescent point leads the amp to work at “A-class” in a very linear region of its transference curve. See Fig. 1 and Table 1.

POWER SUPPLIES

As I said before, I used three power supplies. Two of them are identical (Table 2 and Fig. 2), and a symmetrical (positive and negative) one for the phono stage (Table 3 and Fig. 3). These small transformers rated to 500mA are so simple, like all the circuitry discussed here.

For the positive ones I used transformers rated to 110V AC/15V AC, single secondary coil. For the phono stage power supply, I used an 110V AC/15+15V AC center- tapped. In the symmetrical power supply for the phono stage, all transformers are in parallel with the mains, S0 and F1 circuit.

PHONO STAGE

I have found many interesting circuits at the ESP website. While searching for an RIAA passive network preamp, I encountered Mr. Elliot’s web pages, which included an interesting topology that I changed by substituting the original Burr-Brown chips to the common TL074 (Texas Instruments). My objective was to simplify the original circuitry that uses four single audio grade op amps from TL family (quad in a 14 DIP package).

(below) TABLE 1: CIRCUIT PARTS LIST:

Component--Attributes --Designation


(above) TABLE 2: POSITIVE POWER SUPPLY

The result was very good: No PCB layout, everything was mounted in a perf-board (pad pattern, instead of track pattern) with ground plane provided by aluminum foil (used for cooking and glued with glue-stick inside the cabinet and correct grounded in a single point). The schematics I used are in Fig. 4. See also Table 4.

At Mr. Elliot’s website, you can find more information about RIAA ac curacy and other significant parameters. If you want to improve the phono pre amplifier according to your needs, drop a note to us and let us know what you did. The phono circuit has deep clean bass and affordable mids (voice is very clean), the highs are very honest, too.

You will soon note the huge differences of recordings among your vinyl records.

(below) TABLE 3: SYMMETRICAL POWER SUPPLY

Component---Attributes--Designation


(below) TABLE 4: PHONO STAGE PARTS LIST

Components—Attributes--Designation



Above: Fig. 2: Positive power supply.


Above: Fig. 3: Symmetrical power supply for phono stage.


Above: Fig. 4: Phono stage.


Above: Fig. 5: Mux and digital section.


Above: Fig. 6: Connections diagram.

MUX AND DIGITAL SECTION

I needed to use my headphones with the following different sources: a Philips CD-60 CD player, a Pioneer VSX-4800 tuner, a Marantz SD-535 dual tape deck, and a turntable (Garrard B2-FG and/or a Thorens TD-166). So I decided to build a relay circuitry to do this and more. Small numbers around the selector switch were not an option, because I needed to see the selections from a distance. Figure 5 and Table 5 show the MUX and digital section.

I used three small sealed reversible relays with two sections NO-NC each, to do the job, commanded by the S1 rotary switch and the “OR logic” provided by some diodes in a simple and old-fashioned way. I used the same approach to command the D19 (seven-segment CC display). Depending on the S1 position, D19 displays “P” for phono input or “1,” “2,” or “3” for auxiliary direct inputs.

Note: If the switch is in the P (phono) position, all relays are displayed “off” in the schematics. This is to facilitate construction, regarding normally open (NO) and normally closed (NC) contacts, depending on the kind of relays you are using.

D19 is an LED seven-segment common-cathode (CC) device. I used a subminiature one (10mm x 7mm). As notice, I used a LM78L12 TO-92 small regulator, because I supplied the power for this perf-board from unregulated +12V DC (in fact, 18V DC unregulated—taken before U1 or U2) from the G4OEP power supply. You can choose any one unregulated source to feed the MUX; left or right will work fine. I experienced no audible interference.

CONSTRUCTION

The cabinet is a simple MDF box with Live faces, no bottom end, covered with Curupixa (pronounced Kooroopy shah, a Brazilian wood) darkened foil. I machined all holes in the MDF box from the inside before mounting. After machining, I covered the box and the wooden foil with contact glue (beware of poisonous vapors when using), let them dry for ten minutes, and carefully applied the foil over the box with a metallic, smooth, rounded mass to eliminate air bubbles between the MDF and the covering.

I then applied glue to a ground-plane made from aluminum cooking foil and covered the inner box surface with glue from a glue-stick. Some sanding work and carnauba-wax application guarantees a natural opaque finish.

I chose the chicken-head style knobs to give a vintage look to the G40EP headphone amplifier. I also built a pair of acrylic round washers (30mm diameter, 2mm thickness, and 5mm in length) to put around the knobs, just for aesthetics; my intention was to illuminate with LEDs from the backside when it’s powered-up.

The small white square in Photo 2 is the display, showing the selected input from the back-panel. Finally, a grounded 2mm aluminum plate covers the bottom side of the MDF box. Small rubber feet glued on this plate sustain everything. Table 6 lists the final parts.

Figure 6 shows the system connections (one channel shown). I hope this little challenge encourages other DIYers to build their own units and share the results.

TABLE 5: MUX AND DIGITAL SECTION PARTS LIST

Components---Attributes---Designation


TABLE 6: MISCELLANEOUS PARTS

Components---Attributes---Designation


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Also see: Single-Ended, Vacuum Tube, Class-A Headphone Amp (a DIY project)

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Updated: Tuesday, 2015-04-21 20:16 PST