Happy new year everyone! We’re back, again, and as always I haven’t been regular with my blog posts. Anyway, I don’t think that matters all that much because I post what I need to when I need to so it’s all okay. Today has been a special day! Actually, there’s been a few significant days recently that I just didn’t write anything for but that alright because that just means I have all the more to write about here!

Funding

So in the past few weeks the first issue I encountered was that money became a bit short, which meant that I wasn’t actually able to purchase the parts I needed to start experimenting with the design and drawing up plans. To tackle this, I decided to try and set up a GoFundMe page (which can be found here) and share that around some online communities where people who might be interested could see it and take a peek. Of course, anxiety decided to take over not long after I set it up and I only posted it on one subreddit. Despite messaging the moderators beforehand and getting the all-clear to share it, the post ended up being removed with no reason why, and messaging the mods asking why didn’t seem to get any response so that just somewhat added to my worry. Anyway, I looked at other places to post it including a subreddit dedicated to sharing GoFundMe campaigns but I couldn’t bring myself to share it as the majority of the posts there were people asking for things for medical bills or to literally just have enough money so their family didn’t starve and I felt I would have an overwhelming guilt if I posted it there. Needless to say my page received zero attention except for a £10 donation from a friend overseas as a Christmas present which was very nice. I might share it around some more places sometime in the near future, though.

Anyway, more recently my parents had switched moods from “we’re not paying for that you’ll have to find your own funds” to “we’ll pay for anything college related” so I guess that’s a win? Needless to say, they sent me £70 the other week which was enough to cover the costs of what I needed to start building the power supply for the amp, which is great because I got all those parts ordered on Saturday night and they arrived today so I’ve been working with those and seeing how it all works together!

Amplifier Design

So in the past couple of weeks without having money I decided that I would instead do as much designing and research and redesigning and theorising as I could to streamline the design process as much as possible so when I did finally acquire funding for this project I would (hopefully) need to make minimal modifications to the final design. This might sound like it makes sense and should have been the first thing I’d do, however in the past I’ve usually chucked something together from components after a rough initial design, seen how it performs, and modified it from there. This extra downtime however has meant that I’ve been able to read and refine by searches and designs sufficiently and have gone down to details I wouldn’t have otherwise for the initial. Below is my complete design for the amplifier, from the power supply to the output drivers. Hopefully the extra design effort has been worth the time it cost.

I am aware that image looks like tiny garbage, the proportions aren’t very portrait blog friendly. Fortunately, the only part we need to focus on right now is the power supply.

Power Supply Design This is the power supply design in all it’s glory. I’ve decided to go with a regulated supply rather than a switch mode power supply as SM supplies are considerably more complex to design and also require specialised transformers for certain parts which would end up generating more electrical noise than I would like which might end up coming through on the audio signal. At the top left, I have the mains connection which has an 800mA fuse and a switch connecting to the primary coil of the transformer. On the secondary side, there are two 6A glass fuses on the positive and negative ends of the transformer. Said transformer is a 160VA twin output, which has two 0-12V rails, however I can connect the first 0V with the second 12V to give a centre tap on the transformer which allows me to use that as a ground reference, with ±12V rails. These then go into a full bridge rectifier to convert the 24VAC into ±12V DC, which is then in turn decoupled by the two 4700uF electrolytic capacitors and passed into the two LM338 linear voltage regulators to go from ±12V to ±9V. The issue with these regulators is that they have to be able to supply around 4.5A of current, which means a power dissipation of 13.5W. The heat sinks I bought to go with the regulators and the output drivers have a thermal resistance of 6.8K/W, or for every watt of power that has to be dissipated, the temperature of the heat sink will increase by 6.8°C. Assuming this to be perfect heat transfer, the theoretical maximum temperature the regulators will reach with these heat sinks is 91.8°C, which is comfortably below the 125°C temperature limit set within the regulator. To test how effective these heat sinks are, I set up the circuit shown below and left the power running for 20 minutes or until the regulator throttled the output power due to thermal limits; whichever came first. Other than the fact that I just realised now after about a day of testing that 12-9≠4 but in fact 3, and that the power dissipation will be 13.5W, not 20W, the main issue I encountered was a hardware limit of my equipment.

My bench supply can only provide 5A of current, so as such I decided to only draw 4A of current to stay safely within the limits. My dummy load that I used is constructed of 10 0.22Ω 7W resistors in series to give a total resistance of 2.2Ω. Strangely enough, despite having a combined power rating of 70W, I found the resistors to easily reach in excess of 200°C without active cooling during testing, however there was little current drift during that time so I suppose that their resistance manages to stay quite constant even at such high temperatures.

Anyway, once I had constructed my dummy load I found the resistance to be closer to 2.3Ω which meant a current draw of 3.9A at 9V. This wasn’t an issue however, as it wasn’t the dummy load I was testing. Due to the current limit on my bench supply, I chose to have a higher potential difference dissipated across the regulator. This allowed me to account for the current limit and still dissipate the same power. For anyone reading who hasn’t spent two years studying electronics thus far, first of all I would like to apologise for how confusing all of this is. Secondly, I increased the voltage across the regulator as the equation for power is

Power = Voltage * Current

P = V I

As such, given that the planned 3V * 4.5A = 13.5W wasn’t achievable, I instead had to increase the voltage to account for the decrease in current. To calculate this new voltage, I did the following:

3 * 4.5 = 13.5

Actual current = 3.9A

P = VI

=> V = P / I

= 13.5 / 3.9

=3.46V

To dissipate this new 3.46V across the regulator, I increased the input voltage from 12V to 12.46V. This is why the above diagram show an input voltage of 12.46V.

Needless to say, the results of almost all of my tests proved fruitless, as the thermal resistance of the heat sinks is rated it 6.8K/W as stated above, however that is with the use of a transfer medium such as thermal grease. Unfortunately, I don’t have any thermal grease on my person and don’t have the funds to purchase any for the next week or so. As such, this meant that the actual thermal resistance of the heat sink ended up being much higher consequently the regulator throttled in all my tests (both the initial 20W tests I did using my wildly incorrect maths, and my more recent 13.5W tests). I plan to order some thermal grease as soon as I have the money so I can continue testing, however in the mean time there’s less than 5 minutes left until next year and I am very tired so I’m going to tie up the end of this blog post here and go and get a glass of water because I am quite thirsty. I’ll see you all in the new year!