An amplifier to take to University - A small low power integrated amplifier

[monty78pig]

OK, so I'm hopefully going to be heading off to University to study Electronic Engineering and want to bring a bit of HiFi with me, but don't think I can bring a bi-amped pair of Monitor Audio RX6 speakers with their 19" control amp, and power amps with me along with me, I doubt they would fit in a medium sized cardboard box...

I've got a very good ESI 24bit/96KHz USB audio interface that will work well with a miniature silent PC, as well as a pair of Gale Gold Monitors that I've seen hit surprisingly loud levels with only an SE EL84 tube amp at 4 watts. With this in mind (as well as the fact that I'll be living in a small room in a hall where loud music is considered undesirable) 3 watts of RMS power should be enough for what I need.

I've got a decent 16V single winding transformer lying about that I pulled out of a cheap mini system a few years back, which I'd like to use for the power supply as it'll be getting thrown out if I don't use it.

I would have liked to use MOSFET's for this, but I need to get as much voltage swing as possible on the output as I'm only using a 16.6V single supply, so a couple of compound pairs seem like the best bet. I've also added a voltage divider for the headphone output, the values are quite low, but I wanted good damping on the phones as I do most of my listening on the 325is anyway. For this reason, I have made the quiescent current a slightly high 100mA, so I can use my cans in what is basically class A mode.

I'm going to have 4 inputs (FM radio, audio interface etc.) mainly for the reason that I can do it with a 3 way 4 pole rotary switch I have lying around. All the devices used are ones I already have to bring costs down and use up parts, this is basically the amplifier equivalent to the fry-up people have before they go off on holiday. I'm pretty sure I can fit this thing in a pretty small enclosure.

Anyway I'm not all too experienced in discrete amplifier design, I calculated my values to be sensible, but if I've left anything out then I would be very grateful for some advice...

Schematics attached.

[Bigun]

Originally Posted by monty78pig

I'll be living in a small room in a hall where loud music is considered undesirable...3 watts of RMS power should be enough for what I need.

rule 1: students aren't supposed to be worried about what is undesirable

rule 2: students never have enough RMS power

live on the wild side.

[dangus]

Yes, buy a rack mount semi-pro power amp. 250 to 700 watts per channel should be adequate. Then some 15" two-way (semi)pro speakers with a sensitivity approaching 100 dB. Then you will win the stereo wars, and probably get invited to a lot of parties.

On the other hand, if you really don't want much power, try a T-amp or YDA148 amp. $20 will get you one of each (including shipping from China), if you're undecided. Add an input selector switch and a 12V to 14V power supply, and you're done. The YDA148 chip includes (optional) active anti-clipping features, but requires heatsinking for 4 ohm loads. I'd still recommend a sensitive pair of speakers.

[sakis]

under this approach you may as well go with the ''Pro " idea like having pro amp of 700 W and 15" 2way speakers together with a couple of stands , a couple of cables , a traktor or hercules controller and one laptop .... Then you may as well start your own PA business even before going to the university ,,,Trust me this will teach you a holly lot more in business than a universit
kind regards
sakis

I started my own business way before university ... The first small club i constructed for a costumer which included small PA system Dimmers lighting and so on was done when i was 16....at the age of 18 i was already working in the biggest club of Athens making 3 times the money of an average salary....

[jaycee]

I think it'd be worth you buying a more usable transformer, so that you can get a symmetric supply and lose all the coupling capacitor hassle. A 2x15V 100VA transformer only costs ~£20. Your cheap mini system transformer is only likely to be 30-50VA at most.

Consider the more usual LTP topology for the input and you could probably do without the regulation, which would get you a bit more swing.

[monty78pig]

Thanks for the suggestions everyone. Somehow I don't think I'm going to be able to fit a pair of two way 15" 500W speakers on a medium sized desk top, let alone their amplifier , and at that sort of sound level I don't think anyone could hang on to their hearing for more than a few minutes at that level!

100dBW running at 200W RMS would work out to be a jaw dropping 123dB , louder than a jet aircraft taking off, and I don't want that in a small room , and I'm pretty sure anyone in adjacent rooms won't appreciate it all that much either... I can get enough RMS watts - but I can never seem to get enough dBW. After all the building will have it's own central heating .

A split supply LTP topology would be better, but I don't have any more money for components and transformers, so have to use the ones I already have and if I don't use them then they will probably end up getting thrown out by my Mother...

I've made a few improvements based on what was said, firstly I've taken away the regulation to get some better peak-to-peak on the output. In order to get the adequate supply rejection to do this, I've put a CCS in the VAS to increase open loop gain (also increasing linearity ).

I've also added a shunt regulator into the front end to get better supply rejection as it has a low (but linear) open loop gain and would suffer from supply ripple. It also allows me to bias the input with a constant voltage. I've added a pot so I can adjust to get the best output symmetry when I test the amp to get symmetrical clipping.

I've also changed a few of the transistors to BC337/327 because of the need for extra driving capability of the output devices.


With the improvements I now have a minimum RMS output of 5.5 watts per channel! (Assuming a +20V rail with worst case conditions into a 6 ohm load), that means 98.4dB of RMS sound output with the speakers I have!

Schematics attached.

[jackinnj]

You can cheat and get a couple of the development boards from Texas Instruments for some of their Class-D amplifiers.

[jaycee]

As an engineering exercise using only parts you have to hand, you've done quite well here. Build it and see how it sounds, especially if it'll cost you nothing.

I think the only thing I would suggest now is leave room on your PCB for some TO247 sized output transistors. You might find the TO-220's are a bit small. Also should you decide you really need more supply voltage, you only have to change the transformer and output devices then. The pinout is the same so you can still use your TO-220 devices, just bend the leads a bit.

[tvrgeek]

I would just get a little digital switching amp USB powered and run everything from your laptop. Not DIY, not creative, but you need your time to study mechanics, calc, chem, and the other wondrous fun classes. Grab something like the tiny Warfdales.

[dry joint]

If you're going to study EE, you'll collect spares so buying a transformer is sensible. A high powered headphone amp would do fine, the Panda kit with better driver and output transistors would give you a very detailed high bandwidth amp with potential for modding or even us as a headphone amp later.