The single most common question I’ve received over the years regarding Amp1 is can I buy a version with RCA inputs and a 1/4″ headphone jack.  The answer has always been no, because there simply wasn’t room inside the enclosure with the existing PCB.  The 2nd most common question, usually from customers putting the amp together themselves was can I buy a better volume pot for Amp1, again the simple answer is no, I’m using arguably the best volume pot in this size range.  Then they would ask can I fit in a different pot, and once again there simply isn’t room with the existing PCB.

The most recent person to ask this question was Matt, he posed very simple questions, and as always the answer was no there is no other volume pot that will fit.  This started to make me think WHY is it there no other volume pot would fit.  When I designed Amp1 the best compact pots available (with very similar performance), were the Alps RK09 and the Panasonic EVJ, I had used the Panasonic in my line drivers for years but with similar performance the Alps included a switch.  This switch simplified the design of Amp1 eliminating the need for a separate power switch somewhere on Amp1.  Ironically I recommend that people leave Amp1 on for best performance (why do we need a power switch again?).

So I need a volume pot, I don’t really need a power switch, I want RCA inputs and a 1/4″ output, this simply won’t fit!  Actually I’ve never tried to make it fit (without the PCB installed).  After thinking about it for a few days I rummaged through the parts bin and I had some decent panel mount RCA jacks, an Alps Blue (RK27) Pot and a very cheap generic 1/4″ jack.  I enlarged the input/output holes in the back and the RCA jacks mounted very nicely, same with the pot and headphone jack, enlarging the holes allowed proper mounting, although the shaft on the RK27 is too long it would need to be cut down, and the 1/4″ jack is VERY close to the pot, but all of the headphones I have on hand work.  The headphone jack will be moved down and to the right to give more clearance and leave the headphone label visible.

Amp1 rca

The PCB is sitting below the amp due to the obvious lack of space and the reason I had been saying no all these years.

Amp1 pot

I don’t like the quality of the 1/4″ jack but I have found a better quality version from one of my vendors that has terminals out the end that wouldn’t allow PCB mounting but short air wires would let it connect to a PCB.  Same goes for the Pot, there simply isn’t room for PCB mounting, if you look close the pcb terminals are sticking out the left side with air wires connecting it to the PCB.  Now the PCB is obviously way too long!  Portable1 is similar in design to Amp1 but mainly surface mount, what if I switched the resistors/capacitors and anything else that made sense to surface mount, much of this could go on the underside of the PCB.

Amp1 v2.0

After a bit of fiddling in my spare time I have an Amp1 PCB design that is 1.08″ sorter to allow clearance for the Pot and 1/4″ jack.  The RCA jacks sit above the board so they simply need new connection points.  Amp1 was designed with the high gain setting, the low gain was added for compatibility, however it’s always worked better, cleaner, quieter in the high gain setting.  Axe the low gain, switches, capacitors detract from quality, eliminate them (and the space they took up on the PCB).  What we are left with is an entirely redesigned Amp1 with the same power supply, there are  TO-99 footprints nestled into the DIP locations directly on the board.  This means that my recommended LME49720HA can be inserted and soldered directly to the board with no adapters or sockets.  The signal path of Amp1 is sorter than the original version, and the left and right channels are as symmetrical as the space would allow.

This new PCB fits the existing enclosure and mounts to the bottom panel as usual.  The two front mounting holes are gone, requiring a new hole to be drilled in the bottom panel, not too difficult.  Because of the size of the Alps RK27 the bottom panel won’t mount properly to the enclosure side panels, this means spacers will be used to “lower” the bottom to allow clearance.  This won’t be noticeable from the front as it will still sit behind the front panel, however a small gap will be visible in the rear.  The alternative is cutting a relief in the bottom to allow the “side” of the volume pot to stick through.  The pot uses the existing location on the front and once the shaft is cut down slightly the knob will fit properly and look like any other Amp1.  The difference on the front will be the 1/4″ jack, this will require a slight movement to the right, so the hole will have to be both enlarged and moved.  This can easily be done on a CNC milling machine.

I’ve been listening to this new Amp1 (just a modified stock Amp1 PCB) on the bench for awhile now (stock op-amps) and the new pot is a pleasure to use and sounds wonderful.  I’ve just started testing with the Lithium battery that I mentioned in the previous blog post, the results are simply awesome, I love this volume pot, and it’s quite convenient to be able to use RCA inputs and not requiring an adapter to use 1/4″ headphones.  About 8 hours have passed since the previous comments, I’ve since swapped to the HD650’s which sound equally as good as the K240 Monitors.  The 2nd switch is from the stock op-amps to the LME49720HA, this really brings out the top end in the HD650’s which are known for being quite dark.  The op-amps are fully “broken in” already as they were pulled from one of my other personal amps, once they warm up I will switch back to the K240’s for an update on performance.  Back to the K240’s like the HD650’s there is a new top end and level of detail that simply doesn’t exist with the stock op-amp.  Even the Alessandro MS1’s have a smooth detailed appearance and the volume pot gives a nicer range for this low impedance headphone.

After some more testing I will have a sample batch of PCB’s made and if they meet my expectations I’ll produce a new version of Amp1 with RCA inputs, Alps RK27 volume pot, and 1/4″ output jack all in the existing enclosure, by default it will likely come with the TO-99 LME49720HA op-amps soldered directly to the board, however for those who must swap them I will make it available with dip sockets instead.