Battery powering the Muffsy Phono Stage
The Muffsy Phono Stage – Before
“There is nothing clever about this, but it’s effective and is even quieter than the already quiet adapter & on-board AC/DC power supply powered version.”
If you haven’t heard of the Muffsy Phono Stage you should check it out here . In essence, it’s a kit option of an open source phono stage design – the CNC Phono Stage that has a pretty wide following in the diy audio community such as on Audiokarma.org for example. The CNC itself is based on some application notes from National Semiconductor for the LME49720 op amp.
Anyway, the nice things about the Muffsy version are – i) It’s a full kit minus AC Adapter & Case, ii) specifically designed to fit in a nice, small, cheap case readily available online (the 0905), and iii) has great build instructions and support, if you need it, from from the enthusiast behind ‘Muffsy’.
It a bit of fairly simple soldering and you end up with a quality phono stage. Anyway I built one (and then a couple more as it happens).
The phono stage is powered by a separate AC-DC low ripple power supply, which fits in the same box.
I had a few issues with hum initially (which is how I know the support is so good), but it sounded great.
I also have a Rothwell Rialto phono stage which retails for about £400 and which HiFi Choice magazine said a few years ago: “This unit sounds utterly sublime.”
To my ears, both are frankly great, but if the Rothwell has the edge, it sounds ever so slightly quieter. ( less hiss, needle up, same photo stage gain, same pre-amp settings)
So – why not see if powering the Muffsy by battery makes any difference – why not indeed!
There is going to be two parts to this – modifying the phono stage and building the battery box.
The Phono Stage
The phono stage has an AC socket and a single way switch, and a separate board for the PSU. Out comes the AC socket, switch and power supply.
Because the phono stage requires a +v/0v/-v dual rail supply, we’re going to feed the phono stage with a shielded three core cable and use a DPST switch on entry (to ensure both + & – voltages can be cut). Then we need to fab a three core cable for inside the enclosure. I decided to braid 22awg wire, and cover in a copper shield, tied to 0v, just for good measure.
One of the cool things about the Muffsy Phono Stage is the backplate, designed for the 0905 enclosure. I wanted to reuse this so I needed my new DPST switch and my dual rail DC supply to fit in the pre-cut holes. No problem with the SPST round rocker – there is a DSPT version of the same thing. For the 12mm power socket – I found GX12 ‘aviation plugs’. Genius, and they look good too.
Apologies for the lighting but hopefully you get the gist. The GX12 in the backplate bottom left presents 3 wires – +12v/0v/-12V. Common (blue) goes straight to the choco block at the rear of the case, red and black go via the DPST switch to same. The cable is routed as far away from any signal paths as possible, and is shielded.
(Yes, I l know it’s a different version from the main image – but I said I have a few of these!!)
I said I don’t do this for a living! Sometimes the patio furniture becomes your workstation on a sunny day during ‘lockdown’.
3 single core braided 22AWG, adhesive copper tape covered for a shield (and then later covered in insulation tape)
The Battery Box
Not that tricky.
Bought a very inexpensive pine trinket box on the ‘bay. (By the time I’d stained it, varnished and waxed it, it would probably have been cheaper to buy a pre-made diamond encrusted box but that’s not really the point).
Two 12v SLA batteries . And a charger .
My cunning plan was the box would either charge the batteries (in parallel) or power the phono stage (with +12v/0v/-12v). But given the capacity of the SLAs, it didn’t need to do both at the same time.
So even I managed to work out that I’d need a 4P3T switch, some sockets – my GX12s and power-in from the charger, and some cable.
The charger had a couple of croc clips to charge the battery and that wasn’t going to work. So I broke into the charger (literally – somewhat destructively as it was so cheap it was all glued together) and soldered a replacement cord with DC barrel pin from my collection of old wall warts that matched the barrel socket I took out of the phono stage…
It’s all a bit messy and there is some permanent damage but since it was going to sit on the floor anyway – I’m happy enough.
So here is what’s needed circuit wise:
Essentially, the batteries connect to the four terminals of the ‘OFF’ setting of the switch. Then, thrown ‘right’ (as you look at the diagram), the switch connects ‘-‘ and ‘+’ to create the common 0v, and the left battery’s ‘+’ terminal forms +12v and the right battery’s ‘-‘ terminal forms -12v.
When charging, the switch has a couple of jumpers on the ‘left throw’ (again as you view it from the diagram) that place the batteries in parallel.
In this way, there is no danger of the circuits ‘crossing’.
(You might read some stuff about not charging batteries in parallel. They are probably, right, but it possibly won’t end badly. YMMV. I guess the advice is – Don’t just follow blindly what some guy on the Internet says, especially when he admits to not being an expert at this…)
Assembling the Box (and supply cable)
Basically, you need a couple of holes for the in/out, and one for the toggle switch. (And if you like, add a couple of status LEDs across the in & out (with appropriate resistors) to give some visualisation of the box’s state. (i.e. charging or suppling).
The results
It’s hard to A/B something when you’ve hacked A to pieces, but I’m pretty sure it’s sounds less hissy now at high volume, needle raised. (Even though it was really quiet before).
[EDIT: I’ve A/B’ed it against the Rialto – it’s way quieter – needle up, same phono stage gain, same pre-amp settings. Very happy!]
[Btw – I suspect my original issue with the Muffsy was the AC/AC adapter; I bought a cheap one rather than the one ‘Muffsy’ recommended – go figure!]
I’ve not used it for long enough to comment on battery life (not that that matters with the always-available charger. Nor whether, if the batteries have slightly different characteristics (which they will), there is any issue with either charging or with the phono stage because of voltage variations on the separate rails. (although the PSU it replaced had slight variations anyway presumably due to the Resistor and Cap components not being 100% matched).
What I did think, after the fact, is that since my Rothwell Rialto uses 24v DC from a wall-wart, I could have incorporated this as an option by placing the batteries in series through another switch option. I might have a think about that as a ‘future upgrade’, and how I could ensure ‘off’ before ‘on’ as I cycle between options. If I can’t do it passively, I could always use an Arduino and Relay Board….