*Project* - CMOY Headphone amp

[p4radox]

Penski beat me to it in getting his CMOY amp up and running first. But I'm going one better and building my own...

Here's the main page from which I got the original plan and schematic:
http://tangentsoft.net/audio/cmoy-tutorial/


The reasons for making this amp:

-I like electronics
-I've studied the stuff involved in this sort of circuit and can understand what's going on (hopefully...) so it'll be good to put theory into practice.
-I have a big pair of Sennheiser earphones which I'd like to make louder when I'm away from my HiFi using my iRiver H120
-It's something to do
-It's cheaper than buying one prebuilt (see later)

The circuit is as follows:

A quick summary:
One or 2 batteries in series are used with a potential divider to creat a "dual" power supply for the op-amp.
The Op amp is fed directly from the line-in, and the gain is controlled by the feedback loop containing R4.

Unfortunately the OPA2132PA Op-amp was only available at one shop in the UK, and the rest of the stuff was far cheaper elsewere so I ended up paying postage twice. It came to £22 in total. (Okay, so maybe a pre-built one from the USA would have been a better option...).

Here's all the stuff when it arrived:

Items clockwise from the top-left:
-Extruded aluminium container
-Coupling capacitors (to isolate the op-amp from any unwanted DC-offset)
-Strip-board (what I'll be building the final circuit on)
-Op-Amp and power LED
-A big bag of resistors
-Bread board for prototyping
-Power switch
-Power capacitors
-2 stereo jacks
-9V battery connectors
-Volume control potentiometers

The aluminium container is gorgeous: it's really light and well made, but smaller than I expected. I'll have to modify the plan to only use one battery rather than 2. Also the power switch is pretty big. I'll probably downsize to a mini toggle-switch I've got lying around from my last project. The stereo jacks are pretty poor quality, I should have bought more expensive ones. They'll do for now though. I also bought the wrong potentiometers. I needed a single dual-potentiometer to handle both channels. So I went and bought one from that popular high street electronics store.

The prototyping stage:

I put all the stuff in a breadboard. Amazingly it worked first time! The only thing that went wrong initially was that the volume potentiometer when turned clockwise increased the volume in the left channel but decreased it in the right. This lead to some funky sound effects when playing with the volume, but it wasn't really what I was after. Some swift re-wiring dealt with it no problem anyway.

First impressions are that THIS THING IS LOUD. I'm certain it could make mincemeat of my Sennheisers if I unleashed it fully. It also seriously amplifies any noise at the input. The only noise-free source I've managed to find is the line-out from my Audigy 2. The background "hiss" from the iRiver really is quite loud when amplified.

I tuned down the gain by swapping out R3 for a 3.3k, which besides preventing me destroying my head at high volumes allows full 360 degree use of the potentiometer.

Tomorrow I'll be planning on how I'm going to fit all this stuff into that tiny container, and perhaps starting the soldering of the finished product.

 

[p4radox]

Oh, and a warning. Make sure not to accidentally short-circuit a 9V battery. I almost melted this one (and almost melted my hand picking it up...):

 

[p4radox]

Good luck, you may want to look into a "rail splitter" instead of using a voltage divider. It will use less power and you only need 1 battery then.

Also, that Hammond enclosure looks pretty small, will you fit the PCB plus 2 x pp3's?

Once I get my class D amp working in it's mint tin, I'll add some pics to the thread.

EDIT: When you buy another pot, go for a dual gang log one. Linear will work, but not very well.

I've already bought the dual pot, but I think it's a linear one. It seems to work okay though. I assume the log one you mention is simply a logarithmic resistance scale?

The enclosure is very small, yes, but I'm confident that with some careful design work I'll be able to fit it all in. Only room for one battery though.

I'll look into the rail-splitter that you mention.

I'm also thinking about whacking in some slightly bigger coupling capacitors to improve bass response.

 

[p4radox]

Thanks for the responses and input.


markiemrboo - thanks for the advice on the log pot, I'll give that a shot later on.
What's the advantage of the low-pass filter? I understand how they work, but why do I need to filter out high input frequencies?
Also, what exactly is the purpose of a supply bypass capacitor? Are the 220uF capacitors not the same thing, if they're to go from V+ and V- to ground?



MikeHunt - I was looking at some more advanced power supply designs. This one seems pretty simple:

I've got a pile of 741s lying around somewhere which I guess I could use as the buffer.

I'm still not really clued up on the advantages of this design are over the simple resistor-divider. This part of the tutorial page tries to explain the disadvantages of the resistor-divider:

Because the op-amp's two inputs are always equal due to op-amp action, 1 mV is forced across R3. The op-amp must then put 10 mV across R4 to keep the op-amp inputs equal. As you can see, this puts 11 mV of DC across the load; if the load is 32 Ω at DC (such as a pair of Grado SR-60s), 0.34 mA is forced through the load. This current can only come from the rail splitter, which looks like two parallel resistors to the load. Ohm's law tells us that since the current is 0.34 mA and the resistance is 2.35 KΩ (two 4.7 KΩ resistors in parallel), the voltage at the midpoint of the divider is forced ~0.8 V away from the ideal midpoint.

In this particular situation, then, a 9 V battery would split to about +3.7 V and -5.3 V instead of the ideal +/-4.5 V. Different op-amps, headphones, and resistor values will give a different split. Therefore, it is best to simply realize that this offset will be significant with low-impedance loads, and it will increase as the load impedance goes down, rather than calculating offset and trying to counteract it somehow.

^which doesn't make much sense...


randal24 - The container is a Hammond extruded aluminium case, 80 X 54 X 23mm with removeable plastic ends:

 

[p4radox]

Okay, so I was noticing a bit of distortion on big drum hits at high volume, which I think might be clipping. (Is this clipping? I'm new to this audio jargon...). So I figured I'd follow MikeHunt's suggestion and try out a different power supply design. I had a 741 Op amp going spare so I created this:

A separate power supply built on a separate bread-board (which links to the main one seen in the previous photos). And voila, my clipping issues are solved. This should improve the battery life - ie: clipping occurs at a much lower battery voltage now.

 

[p4radox]

Not a problem. I've used that method on a simple preamp with good results.



It one way to help try and stop RFI from spoiling things. RF can be modulated in to something that is audible. On unity gain stable opamps you can put a small capacitor in parallel with the feedback resistor, R4 in your schematic, to limit the bandwidth as the frequency gets higher. An OPA2134 is unity gain stable.

Unfortunately I think that the filter will be affected by source impedance, which would be the potentiometer. This would mean the filter would change with volume. I'm not sure how to combat that. I guess you would just choose a value which gives a high enough -3dB point at highest impedance?



The general idea is something like this:

Traces, wires etc are not ideal. There is a small amount of resistance, inductance and capacitance which, when current is drawn by the circuit, can create voltage drops. This means you get ripple on the supply / ground.

Capacitors store energy which can help fill in this ripple.

An opamp will have something called PSRR, or Power Supply Rejection Ratio. It's a measure of how much of the supply voltage fluctuation actually makes its way in to the output. Generally these figures are pretty good at lower frequencies, but degrade as the frequency of the power supply fluctuations gets higher.

Larger electrolytic capacitors aren't so good with higher frequencies, so you tend to put smaller value capacitors, with much better high frequency performance, to do this job. A ceramic (NP0 / C0G) or film type is good here.

The opamps - input is connected to ground by a resistor. If ground is varying, this can cause a signal to appear at one input and not the other. This will cause the signal to be amplified by the circuits gain. Not something particularly desirable!


Bit of a mess, but I hope it helped somewhat. There's probably loads of articles out there which explain it much better than I can!

Thanks for the in-depth reply, that's really helpful

So would a capacitor in parallel with the feedback resistor be a better option than the low-pass filter at minimising RFI interference? I assume RFI is radio frequency, ie: >~100MHz? According to a quick back-of-envelope calculation, a 0.15pF capacitor in parallel with the feedback resistor should attenuate the gain by -3dB at 100MHz. Does this sound reasonable?

In terms of the bypass capacitors: should I add a 10nF capacitor over the existing 220uF one?

Thanks again!

Experimenting is half the fun.

Basically that quote with the jargon was saying that using a simple voltage divider (2 resistors) can result in some DC offset on the output.

This is more of a problem with speakers, as they are low impedance (4-8 ohms), and if the worst case DC offset can kill your speaker, best case reduce sound quality.

With headphones it's less of an issue, but after building any amp, weather for headphones or speakers, the first thing I do is check of DC offset on the outputs.

Do a google search for "DC offset" for more info. Basically, a rail splitter should eliminate or vastly reduce DC offset.

Cheers

According to my multimeter there's no DC offset at either output.

I added up how much it was going to cost me to make one... well, two... I would break one while making it.

I've bought one form the US like Penski's Much cheaper.

Don't remind me...

Anyway, I'm really enjoying making this, much better than getting someone else to do it for you!

 

[p4radox]

Quite possibly.

The OPA2134 only has a bandwidth of 8MHz! -3dB at about 100KHz is probably good to aim for. With your current resistor values this would be around 160pF. That sounds like it'd be a non standard value, so try for something close to that I guess.

Lol, you can tell I'm pretty new to this audio stuff...

So having a cut-off frequency at 100kHz should filter out anything higher than audio frequency then? If the bandwidth is only 8MHz then why is it even necessary to filter out radio frequencies which start at 100MHz or so?

Thanks again!


Anyway, I've been busy today with uni work so no real progress was made on the amp itself. However, I've now drawn up what I think is a final design for which I'll need a few more capacitors:

please ignore the poor quality.

I've also been looking at how to wire up the front panel. I can hopefully fit the power switch, power LED, volume knob and two audio jacks all on one end of the container.

 

[p4radox]

Oh, and apparently some headphones like to be driven with an output impedance of 100R. Perhaps a future tweak could be to add some kind of switch on the back to toggle a 100R resistor, in series with the output, in and out of the circuit. I think that resistor was shown in the original schematic, but it's gone now

I suspect this is something to do with power matching, although I'd have thought simply adding a resistor in series with the ouput would not simply set the output resistance to that value...

That tactic was mentioned in the tutorial I was orininally following as a method for reducing noise. I tried it yesterday and it didn't make much difference. However I just tried it again now and it reduced the slight background hiss by loads! Using a 220R has almost completely eradicated the hiss. I don't know why it made a difference now and not yesteray, but one of the other modifications I made must have made a difference. Either way, the background hiss on the Vaughan Williams .ogg I'm listening to at the moment is louder than the amp's hiss, which is awesome!

Oh, and thanks for that link.

 

[p4radox]

Disaster! I think I've destroyed the Op Amp. I don't know what's wrong with it, but no matter how many times I rebuild the amp, I get very little gain and a stupid amount of distortion.

I'm going to keep trying to fix it for a little longer, but I'm afraid I might have to buy another one...

 

[p4radox]

Phew. Another rebuild seemed to fix it. There must have been a mistake somewhere. Perhaps I'm just getting tired, but I seem to be making more mistakes now...!


Anyway, I tried bigger coupling capacitors but they made little difference to the sound at all, and were physically bigger too. So I'm back with the 0.1uF ones.

I also put those 10nFs in the power supply, no problems there.

Tomorrow I start designing the final board for use in the amp its self.

 

[p4radox]

A short update:

I've now got the amp fully working again (still in the prototype stage) with a couple of modifications. I implemented the star-grounding system that markie recommended, and also added a few capacitors to improve the high frequency response and to hopefully reduce RF interference. I also got hold of a 10k logarithmic potentiometer so the volume control now accurately reflects the dB scale.

One thing I forgot to do was get a bigger cap to replace the 220uF one in power supply. Does anyone know if simply using two 220uF caps in parallel will have the same effect as adding a single bigger one, in terms of aiding the "big hit" response? I only ask as I have two 220uFs from the original rudimentary power supply I built.

I've also designed the final layout for the strip-board that's going in the enclosure, and made a 'mock up' of it in the actual stripboard I'll be using:

It looks extremely cramped: that's because it is. The finished product will look a lot neater: all component legs will be insulated and the wires will be the minimum length possible to reduce clutter. I'm confident that I can make this work...

 

[p4radox]

Out of curiosity, did it make any difference to the hiss?

With capacitors in parallel you add the uF, but the voltage stays the same. You will get lower ESR too. So yeah, two 220uF per rail = 440uF per rail.

Neat! Can't easily make it out myself, but still.... the planning is immense!

I didn't notice any difference to the hiss to be honest, but I'm not a true audiophile really!

Re the caps: that's what I thought.

The planning was essential, it only just fits on the board at all!



Mike - That's a nice looking amp! I've no idea what tripath means, but I'm sure it was pretty satisfying to make!



Dr. Em:
That Class A looks awesome! I'd be scared to even go near it, never mind lick it!

 

[p4radox]

Okay, I finally finished soldering it up. First things first, a few comments on the whole process:

-It was incredibly difficult. I knew it was going to be fiddly, but there was the odd moment when I could have done with an extra pair of hands to hold everything. Even pinning things down with blu-tac wasn't enough when I made a mistake and had to remove a couple of components.

-I've realised that my Antex 21/25W soldering iron is pretty shoddy. The end is too fat and I think there is some sort of residue on the tip so only a small part gets hot enough.

-Stripboard is horrible to work with. The copper strips tend to peel off if a component is soldered then removed. It was also very weak where I had to cut the strips, and frequently started to peel away.

-I tried extremely hard to insulate all bare metal legs/connections, but found that the heatshrink shrank too much axially and ended up leaving the odd part exposed. Even when using insulated wires, the plastic insulation would shrink when the wire was being soldered in.



Despite all the problems I encountered I manage to fit everything in, with only one minor deviation from the hand-drawn plan on the previous page. I was really pleased with this, my hard-work in the planning stage appears to have paid off!

Having soldered it all up I was certain it wouldn't work first time - it was just too much to ask for such a complex circuit on such a small board to work properly first time around. I was right. The LED didn't light up on the first try.

A bit of fiddling with the multimeter showed that there was 9V across the main star ground point and the battery's 9V rail which was obviously not correct - it should have been closer to 4.5V. The LED was not lighting because it was connected over the main ground rail and the 9V rail, thus no voltage across it. A bit of knife action on the back of the board revealed that there was a short from one rail to the other. This was easily cleared.

The LED then lit up, and I was greeted with fully amplified sound! Sheer delight such as this I had never known before! I was expecting hours of laborious debugging work before it even approached a working state!


Anyway, enough geeky gabbling, here are some more photos to keep you all happy:

Stripboard prepped for soldering. Notice the knife cuts to isolate different parts of the circuit:

The finished circuit! Took me about 3 hours to get to here:

Check out how cramped it is! In particular, note the densely populated top-end, between the 2 coupling capacitors. All the resistors are vertical to save on board real-estate!

The next big challenge is fitting the damn thing into that little aluminium box. I've drawn up some plans for the front panel, and it's very, very tight.


Finally: another warning not to short-circuit 9V batteries. I don't know how this one happened, but it melted through 3 little plastic bags.

 

[p4radox]

That is literally as wide as it could be. There's one spare row down one side to sit on a shelf on the inside of the box and one spare row down the other side which I'll have to shave a bit off in order to get the battery in.

edit: it sounds immense. The bass is absolutely fantastic and voices are very, very crisp. That's about as well as I can describe it, not having trained ears.

 

[p4radox]

There's no hiss, even at max volume.

 

[p4radox]

Lol, I haven't finished this one yet! I'll be making a start on the container tonight hopefully.

Re the resistor insulation: that's what I did. What I mean in my post above was that the actual wire insulation I applied was shrinking significantly while soldering, so in some cases I added another layer of heatshrink over the top.

I'd also been advised that a drill bit was the best way of using stipboard, but as you said, there wasn't enough room for that!

 

[p4radox]

One more update for today.

Just some proof for all you naysayers and doubters: it does indeed fit in that tiny little enclosure:

 

[p4radox]

Just finished my first one (in terms of circuit anyway, still need to find a case) going by the reference design as found on the net and it works perfectly first time

Going to try the suggestions posted here for my second amp a bit later

Good to hear someone else is giving it a shot! How's it going so far?

The best improvements I made were placing a 200ohm resistor in series with each channel output which reduced background hiss immensely, and changing the power circuit to the 741 buffered version which stopped distortion at high volumes.

 

[p4radox]

Okay, I finally found the time to get it finished!

To be honest I made bit of a mess of the front panel, the holes aren't quite lined up and there are a couple of scratches which the flash from the camera exaggerates.

I also ended up with a volume knob that's too big really, but it was the only one in stock at the shop I got it from.

But overall I'm really, really pleased with how it's turned out. It makes a massive difference to the sound of both my iRiver and my little 256mb flash player. I'm also very pleased that my planning paid off and I actually managed to fit the damn thing in that little box!

Mike - you've interested me in Class D amps now. Having read a little I think it might be my next project. In the meantime I've got some exams to pass, so it'll have to wait unfortunately.

 

[p4radox]

It wasn't R5 that improved the sound for me. R5 is within the feedback loop, and I got best results without it. I added the 200R directly in series with the output.


As for the 741, are you using this power circuit?

Try ditching R3. It gave me loads of trouble with uneven rails.