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- September, 2016 - After seeing the success of the fan on the heatsinks of the JLH class-A headphone amplifier, I decided to do the same with the Gilmore headphone amplifier. I cut out a very roughly one-inch circular hole on the back of the case and bolted on a small 1.5-inch fan that is powered from the output relay circuit. The fan blows air in the general direction of the heatsinks. It's not perfect, but maybe it is cooling the inside of the case a little bit. It makes a little noise, but you can't hear it with headphones on.
Also, I finally removed the cross-feed switch from the front panel, and erased the lettering. I covered the small hole with a small, round piece of black electrical tape.
- Summer, 2016 - Put the OPA-2134 op-amps back in the B1 preamplifier's phono circuit and the OPA-2134/BUF634-based headphone amplifier, in place of the LM4562s that I put in back in 2009. I like the OPA-2134 sound better. I did not put the 100K Ohm resistor back in place of the 47 KOhm one that I put in the OPA-2134/BUF634-based headphone amplifier to reduce the DC offset. Is should be fine as is, but I will put it back some day.
- Summer, 2016 - Bought a small, USB-powered fan on line and built a small box with an on/off switch, some weights, and resistors to reduce the 5 VDC USB voltage to about 2.5 VDC to eliminate the fan noise. The fan sits behind the JLH headphone amplifier at my work (described here) and it reduces the temperature of the exposed heatsinks from fairly warm to down-right cool. Hopefully, this will increase the life of the little class-A amplifier.
- October, 2015 - Having tired of the dark-sounding IC-based portable headphone amplifier (described here), that I had been using to listen to music at work, I built a discrete JLH-variant to replace it (described here). It sounds fantastic, like the improvement from the opamp-based home headphone amp to the Gilmore amp, or from the opamp-based preamplifier to the Pass B1 Buffer.
- September, 2014 - The hum in the Gilmore headphone amp (described here) was NOT gone! I took off the top while it was humming and began poking around inside. I found that if I scratched the power leads that run between the power supply and the right-channel amplifier board, I could hear the scratching noises in the headphones - the wires were microphonic! How could this be? Somehow the power was leaking into the signal. On each amplifier board, right where the power lines connect, I had two 100 uF capacitors, one from the (+) rail to ground, and the other from the (-) rail to ground. However, the one from the (+) rail to ground connected to the ground right at the output connection for the amp. Could this be where the hum was getting into the signal? These capacitors were not in the original design, I just thought they would be a good idea. So, I removed all four from both amplifier boards, and I have not been able to make the hum happen again, even after two hours (or more) of continuous play. At last!
- January, 2013 - Disconnected and removed the cross-feed circuit in the Gilmore headphone amp (described here), in the hope of curing a weird intermittent hum in only one channel. I never really liked the cross-feed sound, and I wanted it out of the signal path. It sounds much clearer now, and I hope the hum is gone.
- August, 2011 - Back to the OPA-2134/BUF634-based headphone amplifier again (described here). When I bypassed the first opamp circuit (see previous) I failed to remove the 1.5K ohm series resistor and the 47K ohm resistor to ground that preceded the second circuit. So, I removed those. I also reduced the gain resistor value in the second opamp stage from 1.62K ohm to 862 ohm because the gain was too low. The DC offset is now back to between 5 and 8 mV to DC, but Oh Well.
- April, 2011 - I just can't leave that OPA-2134/BUF634-based headphone amplifier alone (described here). This time, I used wires to bypass the first opamp circuit immediately after the little 100pF ceramic capacitor in the input sequence (1uF - 47K - 1K - 100pF). I also removed the crossfeed circuit, because I found I don't like the sound of it in the Dynalo headphone amp (takes away the "punch" and some of the bass - just makes it sound flat and lifeless). Because the first opamp stage was giving 6 dB of gain, I had to reduce the R2 resistor value in the second opamp stage from 3.83K ohm to 1.62K ohm to double the gain in that stage. It still has less gain than before, but now it plays at 3/4 volume instead of 1/2, which should give better performance. Also, by eliminating the first opamp stage, the DC offset is now down from between 5 and 8 mV to between 1.5 and 3 mV DC.
Also, I made some tiny wooden standoffs from dowels that I glued under the mega heat sinks on the BUF634's. Now they are solid and stable, so I don't have to worry about movement causing the IC leads to eventually break.
- October, 2009 - I replaced the 100 KOhm Noble volume potentiometer in the second (B1 buffer) preamplifier (described here) with a 25 KOhm Noble volume potentiometer, and the 50 KOhm Alps Blue volume potentiometer in the Gilmore "Dynalo" headphone amplifier (described here) also with a 25 KOhm Noble volume potentiometer. I think it improved the sound a bit in both, especially the preamplifier (just wishful thinking?), but at least these two components now have the more-or-less correctly-valued volume potentiometers in them.
- October, 2009 - I finally decided to clean up and "re-cap" my old Sansui AU-317 integrated amplifier that I bought new back in 1978. (see here).
- August, 2009 - I got four LM4562 opamps as "free" samples from National Semiconductor. The LM4562 is supposed to be one of the better sounding opamps around, and because it is a dual opamp, I can use it to replace the OPA2134 opamps I use in various places (the OPA627, also an excellent opamp, is a single, and is harder to use as a replacement). However, the LM4562 has a BJT front end (rather than an FET front end like the OPA2134, OPA627, and other popular opamps), so it is harder to use without generating lots of very deleterious DC offset. I used the new opamps as follows:
- Replaced the two OPA-2134 opamps in the OPA-2134/BUF634-based headphone amplifier (described here) I had to reduce the R1 resistor value from 100 KOhm to 47 KOhm on each opamp stage to get the DC offset down to a reasonable value (between 5 and 8 mVDC). I also put some serious, mega heat sinks on the BUF634's. They get fairly warm, and I want them to last.
- Replaced the two OPA2134 opamps in the phono circuit in the second preamplifier (described here). To do this I had to de-solder the OPA2134 opamps and install sockets. I put the new opamps in the RIAA /gain portion of the phono curcuit, and left the OPA2134's in the subsonic filter circuit. The LM4562's produce about 4 to 5 mVDC offset in the gain stage, but the big 1 uF capacitors filter that out.
- July, 2009 - Built a second Cmoy OPA2134-based portable headphone amplifier, this time in a red Altoids tin box.
- June, 2009 - I got my hands on an old oscilloscope for a short time (a 1982 Heathkit 4550, 10 MHz, poorly calibrated I'm sure) and I used it to measure all of the my gear, along with my function generator kit that I built a while ago. I measured 1 KHz, 10 KHz, and 100 KHz square waves through each one (see here). As a result, I made the following improvements:
- The B1 Buffer preamplifier power supply I built back in November, 2008 was oscillating because of too little capacitance between the two three-pin regulator ICs. I increased it from 100 nF to 100 uF with Panasonic 35V FC electrolitic capacitors, and that cured it.
- On the Gilmore "Dynalo" headphone amplifier's power supply, I removed the connectors for the return sense wires, because I am not using these anymore. Maybe I should be, but I am not 100% sure how to do it properly with the pre-regulator configuration of the power supply.
- On the B1 Buffer preamplifier I also added 470 KOhm 1W resistors from the volume potentiometer's wiper to ground to provide DC safety in case of pot failure.
- In the smoothing circuit of the power supply for the second power amplifier (described here), I added 0.1 Ohm 5W resistors in-line on the two (+) traces between the smoothing capacitors in the middle of the circuit. This reduced the ripple in the power supply output from 22 mV to 15 mV on the +42V rail, and from 11 mV to 7 mV on the -42V rail.
- May, 2009 - Replaced the VAS and output transistors in the Gilmore "Dynalo" headphone amplifier (described here). I ordered new transistors, and with the leftovers from the original build, I was able to select better-matched sets. I epoxied the sides of the transistors to some stout heat sink chunks (left over from trimming the new amplifier's heat sinks) and used thermal grease between the tops of the transistors and the heat sinks. I perforated the top of the case above the heat sinks, and I moved the fan to the underside of the case top and built a little sheet-metal diverter to direct the air flow to the heat sinks. I then re-calibrated the DC offset to less than 1 mV on each channel.
- December, 2008 - Upgraded the power supply in the Gilmore "Dynalo" headphone amplifier with bridge shielding, more capacitance, and a tracking pre-regulator (described here).
- November, 2008 - Replaced the OPA627-based preamplifier circuit in the second preamplifier with a FirstWatt B1 Buffer circuit, and upgraded the power supply with bridge shielding and serial regulators (described here).
- September, 2008 - Replaced the ESP P88 preamplifier circuit in the original preamplifier with a FirstWatt B1 Buffer circuit. It sounded so good, that I decided to build another one for the second preamplifier.
- August, 2008 - Built a true Cmoy OPA2134-based portable headphone amplifier in an Altoids tin box (described here).
- November, 2007 - Replaced the ESP P3A amplifier boards in the second amplifier with ESP P101 amplifier boards (described here).
- September, 2007 - Replaced the ESP P3A amplifier boards in the original amplifier with ESP P101 amplifier boards (described here). It sounded so much better, I decided to build a new set for the second amplifier.
- May, 2007 - Added an output muting and relay circuit to the Gilmore Dynalo headphone amp by redoing the power supply board.
- April, 2007 - Bought (on ebay), cleaned, repainted, and upgraded a 1978 Sansui TU-217 tuner with all new electrolytic caps, and increased the capacitance of the power supply (described here).
- May, 2006 - Re-built the OPA-2134/BUF634-based headphone amplifier with a better power supply (described here).
- April, 2006 - Replaced the OPA-2134-based P88 board in the second preamplifier circuit with a dual mono, OPA627-based preamplifier circuit I designed myself (described here).
- February, 2006 - Built a OPA-2134-based headphone amplifier, with the BUF634-based buffer from the DAC, in a small Radio Shack plastic project box for use at work for MP3's on my work computer (described here).
- January, 2006 - Built a Gilmore "Dynalo" headphone amplifier for home use in the DAC case (described here).
- June, 2005 - Built a OPA-2134/BUF634-based headphone amplifier for home use in the DAC case (described here).
- May, 2005 - Built opamp-socket adapters to replace the OPA-2134 opamps in the "original" second preamplifier with OPA627 opamps (described here).
- April, 2005 - Built a "portable" Cmoy-style OPA2134-based headphone amplifier (described here).
- Spring, 2005 - Upgraded the original preamplifier (described here) and original power amplifier (described here) with lessons-learned from building the second set.
- January, 2005 - Built a set of speaker stands (described here).
- Fall, 2004 - Built the second pair of AR.COM speakers (described here).
- Summer, 2004 - Built the second power amplifier, using ESP Projects P3A (described here).
- Summer, 2004 - Built the "original" second preamplifier, using modified ESP Projects 05 (power supply), 06 (phono pre), 99 (subsonic filter), and 88 (preamplifier) as described here.
- January, 2004 - Added a BUF634 output buffer to the DAC (described here)
- November, 2003 - Added a relay and turn-on delay circuit to the original preamplifier's output.
- November, 2003 - Bought (on ebay), cleaned, and upgraded a 1992 Rotel RCD-965BX CD player with OPA627 opamps in the output buffer, and a Kwak Clock (described here).
- Feb, 2003 - Built the DAC
- Fall, 2002 - Rebuilt and tweaked the (original) power amplifier.
- Summer, 2002 - Built the first (original) power amplifier, using ESP Projects P3A.
- Summer, 2002 - Built the first (original) preamplifier, using ESP Projects 05 (power supply), 06 (phono pre), 99 (subsonic filter), and 88 (preamplifier).
- Spring, 2002 - Built the first (original) pair of AR.COM speakers.
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