New Posts  All Forums:Forum Nav:

cmoy upgrade(s) - Page 2

post #16 of 42
1/27/12 at 10:30pm

 

Quote:
Originally Posted by drtturnip View Post

 

Ok. School me.


It is not uncommon for chip op amps to have way more than 36 parts per channel on the chip, plus whatever you hook them up to (feedback resistors, caps, etc). That is definitely not a simple design when you can build quite nice tube or SS gear with about 5 or 6 parts per channel. 

 

Op amps are readily available and easy to use which is nice, but opening the door to simple discrete circuits allows so much freedom that is not offered with op amps. 

 

Ooh, and for a second project the CHA-47 is pretty awesome.

Reply
post #17 of 42
1/27/12 at 11:05pm

 

 

 

Quote:
It is not uncommon for chip op amps to have way more than 36 parts per channel on the chip, plus whatever you hook them up to (feedback resistors, caps, etc)

 

 

Well not the njm4556 or equivalent...

 

Plus the internal transistors are generally direct coupled and in such a small package that capacitance and inductance are a non factor. That can't be said for ill designed circuit boards or wiring. The external components... one input capacitor and three resistors. Lets get one thing straight. When it comes to signal path audio degradation in a properly designed circuit the single most insidious culprit is the capacitor. Evil little vixen.

 

 

 

Quote:
...opening the door to simple discrete circuits allows so much freedom that is not offered with op amps.

Much more freedom for error.

 

Reply
post #18 of 42
1/27/12 at 11:06pm
Thread Starter 


thanks for the replies!
i am using an iem to test it-Brainwavz M2, as i dont have decent headphones as of now,with my only hd 202 not working properly.also,using common 5% resistors,have not matched them .have used 47 ohm resistors in the opamp output for this purpose too.the input capacitors are 0.47uf polypropylene.
virtual gnd circuit uses 470uf 25v nichicon caps,4.7k resistors.
opamp circuit uses default 100k to gnd for the bias resistor,10k in the feedback loop,and 1k to gnd.and 47ohm in output

Quote:
Originally Posted by drtturnip View Post

Have you measured the output for dc voltage? If so then you have too much dc offset caused by a bad opamp or unbalanced circuit. If you aren't going to drive inefficient headphones I would lower the gain. Here is something I did that lowered the dc offset and gain. The gain you have now would probably pick up noise until it is shielded in a metal enclosure.

 

 

i had used jaben's guide for testing
http://www.ahkang.org/audio/Building%2520the%2520Jaben%2520CMOY%2520diy%2520kit.html

and the offset reads 0.015v.supply reads 4.3 and 4.4v(using a "cheap" multimeter)
am now using a aluminium can to house it and there is lesser interference.i guess that is the issue?i also stuck a small metal piece between the polyester caps and that helped reduce crackle too.do i connect the casing to ground or just insulate it?


and thanks for your thread.i had seen it while digging,but overlooked it.

Quote:

Originally Posted by mono View Post

 

We may be putting far too much thought into surajspai's topic, on my second read it seemed like the goal is not to upgrade a CMOY amp but to build a different amp as the next project, A47 or CMOY2.  If member support is important if/when things go wrong, I'd search the forum to see if there are any current topics with lots of posts, other members having the same type of amp out and fiddling with it makes for easier/faster peer support during a build.

i did search,tried adding resistors to ground,0.1uf capacitors between vcc to gnd ,vee to gnd like this thread says,

http://www.head-fi.org/t/263547/cmoy-using-joshatdot-pcb-with-parts-list/75#post_3416702
didnt work either.guess the casing was needed?


and yes,i wanted to perfect this circuit and and try a better one to use a a portable amp.i know it sounds silly,but i am just starting with audio diy and right now i am a noob.i admit i havent done enough homework or experimented enough.
 

Quote:
Originally Posted by mono View Post

Is that a TL072 opamp?  If so, [EEK!] using something else with higher current or paralleling two of them like in an A47, or paralleling two of something else especially, is the next upgrade.  None of the old popular opamps for CMOYs do very good driving a load (cans), it's just sort of the simplest circuit that can be thrown together with an opamp and work without much development. 

 

Put the POT in the circuit or temporarily put a few dozen KOhm resistor between each channel and ground to see if some of the noise goes away (then remove when you add the POT). 

 

CMOY2, if it's this design linked, is just using an opamp as ground channel to do away with the TLE railsplitter, or the buffer chip in an A47, or a dual rail power supply.  There are many ways to more or less get the same job done, with minor audible differences sometimes.

 

http://www.sgheadphones.net/index.php?showtopic=3220


yes,that picture shows the tl072,as it was the first run and i didnt want to risk the opamp.i have the op2132pa though(also a smt ad823),and am using it now.

http://imageshack.us/photo/my-images/35/dsc03330im.jpg/

if i use the pot(alps rk097 10kohms),the noise lowers but is still there(without the casing).

 

Quote:
Originally Posted by nikongod View Post

If you have access to an oscilloscope make sure that your amp isn't oscillating. 

 

The hiss could be coming from your computer (computers are neutoriously noisy), the amp its self, or oscillation. 

 

If the amp gets quieter when you cover it, it is probably picking up RF somewhere. Cover it :) 

 

What gain did you use? What headphones do you have?

hiss is still present,mild though,with the casing on using a opa2132pa.i can check it up in college using a oscilloscope though.no change in opamp temperature(does not get warm or any such thing) .

is this because i am using iem's?will it go away when used with higher impedance headphones?i used the standard gain of 11(tangent's schematic),so i have to lower this?have no headphones as of now,as my sennheiser hd202 distort very badly.

 

Quote:

Master the cmoy before you move on. You will learn a lot.

sure will!


 


Edited by surajspai - 1/27/12 at 11:23pm
Reply
post #19 of 42
1/27/12 at 11:28pm

If your voltages measure correct and you get sound change out the op amp. That tl072 will give you grief. Then regroup.

 

Reply
post #20 of 42
1/27/12 at 11:40pm
Thread Starter 

like i said,am now using the recommended opa2132pa.the tl072 was only to see if i could get any sound out of the circuit.will try out the resistor matching and also different values for the resistors.


Edited by surajspai - 1/27/12 at 11:42pm
Reply
post #21 of 42
1/28/12 at 12:18am

 

 

Quote:
i did search,tried adding resistors to ground,0.1uf capacitors between vcc to gnd ,vee to gnd like this thread says,

What resistors to ground? Bypass capacitors from power supply to ground nearest to the op amp are always a good idea.

 

 

I would lower the gain and try to balance the feedback gain resistance with the input resistance.

 

" the ideal value for the feedback resistors (to give minimum offset voltage) will be such that the two resistances in parallel roughly equal the resistance to ground at the non-inverting input pin."

http://en.wikipedia.org/wiki/Operational_amplifier

 

I used 150k to ground and 270k from output to inverting input. This gave a gain of 2.8. More than enough for my needs and I got a dc offset of .001 and .000. Different op amps will give different results. You can adjust the values for higher or lower gain but shoot for a parallel resistance of 100k. 

 

The stock gain of 11 seems a bit much. If you need that much gain move on to another circuit.

 

Reply
post #22 of 42
1/28/12 at 2:00am
Thread Starter 

sorry,i didnt read that properly.i thought resistors between opamp output headphone drivers.so,i connect a resistor of a few kilo ohms(say 2.2k) between ground and the output channels?bypass capacitors i am using,but i'll try this and post back.

 

and i dont need that gain,just used standard values.i replaced the resistors with your values 150 and 270k,also increased the output resistor to 75ohms,so the hiss is barely audible now.so i guess thats that for the cmoy?

Reply
post #23 of 42
1/28/12 at 10:58am

Cool. What is the dc voltage on the output now?

 

I would not use an output resistor. The only resistor from the output should be your feedback gain resistor. R4 (270k) in this schematic

 

http://tangentsoft.net/audio/cmoy-tutorial/misc/cmoy-tangent-sch.pdf

 

Are you still using iem to test with? They can be very low impedance. Try some headphones in the 50 ohm range.

 

I would also change the input cap from .1 to at least .22 .47 if you can find one that will fit. The sub bass rolls off to soon otherwise.

Reply
post #24 of 42
1/29/12 at 10:10pm
Thread Starter 

sorry for the late reply,yes,i have made changes as you said.i also removed the R5 resistor,and now am using a JVC RX700 headphone to test.
dc offset-6mv,not yet as low as you have described.hiss is absent when playing from a discrete source,but the noise has come back(i removed it from the metal casing.i saw a few pcb designs and am making a new board,with the inputs caps separated,also removed the ground layer from near the opamp.lets see if it works.

Reply
post #25 of 42
1/29/12 at 10:55pm

Are you still using the tl072? If so with a different opamp the offset will likely lower but 6mv is good. Progress. Now what noise has come back? The tl072 are weird so a different op amp may take care of everything at this point.

Reply
post #26 of 42
1/30/12 at 12:25am
Thread Starter 

no.got rid of the tl072.using only the 2132pa now.noise-the interference is driving me nuts.i bring the circuit close to a phone or a laptop,it starts to generate noise.re did the circuit on stripboard,and there's minimum interference.guess its a design issue.any suggestions to reduce interference?


Edited by surajspai - 1/30/12 at 1:58am
Reply
post #27 of 42
1/30/12 at 5:50pm

Unless it is in a grounded metal enclosure I think it would pick up noise.

Reply
post #28 of 42
1/31/12 at 11:38pm

It's like polishing a turd.    Ok, not really but the point to the CMOY design is small size, simplicity, low cost.  All the time spent, could be spent on something better which is a large part of why this forum exists, that people move on to better sounding amps.

 

Edit:  The rest I wrote previously was over the top


Edited by mono - 2/1/12 at 10:34am
Reply
post #29 of 42
2/1/12 at 12:01am

You can't polish a turd, but you can roll it in glitter   biggrin.gif

 

cheers

FRED

Reply
post #30 of 42
2/1/12 at 6:01pm

 

 

Quote:
All the time spent, could be spent on something better which is a large part of why this forum exists, that people move on to better sounding amps.

Oh that makes sense. Build something that doesn't work correctly but don't bother to figure out why. Maybe there should be a separate forum for those who know everything and don't give a crap about those that don't.

Reply
New Posts  All Forums:Forum Nav:
  Return Home