[ask] Is there a way to mod a CMOY amp to have more power? - diyAudio
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Old 26th December 2014, 05:16 PM   #1
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Default [ask] Is there a way to mod a CMOY amp to have more power?

Hello everybody.

Stumbled upon this forum while doing some googling to find answers to my question.

So I got a simple DIY CMOY amp from a friend. Just an average project, nothing special. I kinda like the sound. Problem is, it's not very powerful.

There's no problem if I use it with IEMs, or easy to drive headphones, like my AD700, but when I tried it with my everyday headphone, the HE400, this amp falters badly. Somewhere around 12 o'clock of the volume knob, it started to clip north and south. Between 1 o'clock to 5 o'clock (max), I got no volume gain whatsoever, it's like the pot malfunctioned at that region. I suspect it got not enough current to run HE400 properly. HE400 is 35ohms, 93dB/mW.

This is some pictures of the amp in question:

Click the image to open in full size.

It can be used with a 9V battery, but I'm using an external power supply, 9V 2A.

Click the image to open in full size.
Click the image to open in full size.

This is the close-up of the board. It's a simple DIY project using breadboard, so it's a bit of rat's nest on the other side.

My question is, are there any ways/mods I can do to make it output more power or current? I can handle soldering gun, but I can't make head or tail out of electronic circuits. So I'm hoping for simple 'idiot's way' answers, i.e. 'change this capacitors to that type of capacitors', or something like that.

Sorry for my English, it's not my mother tongue.

Thank you in advance.
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Old 26th December 2014, 05:25 PM   #2
Mooly is offline Mooly  United Kingdom
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The 9 volt supply is one of the limitations here. It means the opamp can only swing around -/+4 volts at best, and into 35 ohm, a lot less than that.

Things you could try would be increasing the supply. The opamp can take up to 36 volts in total but make sure any caps and other parts are OK at that voltage. You could also add a small series output resistor to the headphone feed (or increase what is there) along with an increase in supply.
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Old 26th December 2014, 07:14 PM   #3
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Quote:
Originally Posted by Mooly View Post
The 9 volt supply is one of the limitations here. It means the opamp can only swing around -/+4 volts at best, and into 35 ohm, a lot less than that.

Things you could try would be increasing the supply. The opamp can take up to 36 volts in total but make sure any caps and other parts are OK at that voltage. You could also add a small series output resistor to the headphone feed (or increase what is there) along with an increase in supply.
Hello, thank you very much for the answer, but it's kinda 'greek' to me, as in I don't understand much.

What I (kinda) can understand is:

- Instead of using 9V, it's better to use higher input voltage. Will 24V be okay? I think I can get my hands on a 24V power supply easily.

- I have to make sure the capacitors can handle 24V. Which other parts I should check the voltage limit?

- I'm guessing the power indicator LED will fry at 24V? I tried measuring the voltage while it's lit, it's at 3V. So I should get a resistor to drop the voltage to 3V from 24V?

- I don't really understand this part:

Quote:
You could also add a small series output resistor to the headphone feed (or increase what is there)
So I should add resistor(s) to the amp audio out?

Thank you very much, and sorry, I don't have much knowledge in electronics other than soldering cables.....
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Old 26th December 2014, 07:45 PM   #4
Mooly is offline Mooly  United Kingdom
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24 volts DC would be fine. The caps to check are those large electrolytic can type ones, the voltage will be marked on the wrapper. All the others take way over 24 volts.

Your CMOY appears to have two transistors and possibly a couple of diodes ? near them. If you can point me to the circuit diagram for this (there are lots of CMOY variations) then I can tell you exactly what they are and whether it would work with no other changes. I think they will be to derive what is called a "virtual ground" from the single rail supply.

Adding a resistor can make the load the opamp sees easier to drive... but I think tbh your main problem here is a lack of supply, 9 volts just isn't enough.
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Old 26th December 2014, 07:48 PM   #5
Belaji is offline Belaji  United States
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The really quick try is to just use two 9V batteries in series which will double your voltage. And all you need to do is change how the "new" battery connects to the old 9V single battery connector. (Just make sure all the capacitors can handle 18V.)
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Old 26th December 2014, 07:54 PM   #6
jcx is offline jcx  United States
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the circuit diagram would help - likely there are current limit R that don't play well with low Z, low sensitivity phones
the O2 project amp doubles up those op amps for more output current
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Old 26th December 2014, 08:34 PM   #7
HotIce is offline HotIce  United States
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Use balanced/inverted output configuration, to almost double the output swing.
But at that point it's no more a cmoy
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Old 27th December 2014, 04:56 PM   #8
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Quote:
Originally Posted by Mooly View Post
24 volts DC would be fine. The caps to check are those large electrolytic can type ones, the voltage will be marked on the wrapper. All the others take way over 24 volts.

Your CMOY appears to have two transistors and possibly a couple of diodes ? near them. If you can point me to the circuit diagram for this (there are lots of CMOY variations) then I can tell you exactly what they are and whether it would work with no other changes. I think they will be to derive what is called a "virtual ground" from the single rail supply.

Adding a resistor can make the load the opamp sees easier to drive... but I think tbh your main problem here is a lack of supply, 9 volts just isn't enough.
Quote:
Originally Posted by Belaji View Post
The really quick try is to just use two 9V batteries in series which will double your voltage. And all you need to do is change how the "new" battery connects to the old 9V single battery connector. (Just make sure all the capacitors can handle 18V.)
Quote:
Originally Posted by jcx View Post
the circuit diagram would help - likely there are current limit R that don't play well with low Z, low sensitivity phones
the O2 project amp doubles up those op amps for more output current
Thank you for the answers.

I don't have the diagram, and I lost contact with my friend who built it, so asking him for a diagram is a no go.

I took a picture of the back of the PCB. Because it's a simple DIY with a breadboard, it's a rat's nest. I don't know if anybody can make a meaning out of it:

Click the image to open in full size.
Click the image to open in full size.

The picture has been flipped, so left stays left, and right stays right.

The capacitors it's using are 220uF 16V, so I guess 24V is a no go, unless I replace the caps. I tried plugging a 12V 1A power supply, and true enough, it got wayyyy better. It still clips near the max volume, especially on low frequencies like kicks, but it sure is better and more resilient to clippings on lower volumes. The IC got a bit warm to touch. Not hot, just warmer (with 9V, I can't feel any temperature difference)

I scrounged at my tidbits box, and found some capacitors that are 470uF 25V. Can I just use them to replace the original capacitors?

Quote:
Originally Posted by HotIce View Post
Use balanced/inverted output configuration, to almost double the output swing.
But at that point it's no more a cmoy
It's greek to me, I don't really understand...
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Old 27th December 2014, 05:55 PM   #9
HotIce is offline HotIce  United States
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In the usual configuration, you have one opamp getting signal on the + input, and feedback on the - input, generating an in-phase output, with the other side of the output being GND.
Now if instead of GND, you use another opamp, with the input signal fed to the - input, and feed its output in replacement for GND, you double the output swing, given same rail voltage.
But, unless you go SMD PCB, you are unlikely going to fit this into a typical cmoy case.
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Old 27th December 2014, 06:49 PM   #10
Mooly is offline Mooly  United Kingdom
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Its not possible to figure out exactly whats what from the picture. The transistors form a virtual ground... its a standard circuit, its just the detail that is difficult to fill in.

A bit of googling from the CMOY site... I'll bet its this variation, 3rd picture up from the bottom.

Virtual Ground Circuits

Your two caps should share the supply equally. If you measure the actual voltage across each then you should see 6 volts (running on a 12 volt supply). Does that mean you can run it on 24 volts as it is As long as it all works correctly then yes. If a fault occurred that allowed either cap to see more than 16 volts then you would damage the cap... and they can go pop if there is enough current available from the supply

So its up to you. I would advise changing those 4.7k resistors to perhaps 10k although it should work just fine as it is, just a bit wasteful of power.

The heating of the IC is normal and you could be feeling the other parts getting slightly warm too.

Edit... the CMOY site if you are interested,
http://tangentsoft.net/audio/cmoy/
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