Well, i'm getting trebbly mono sound from the amp, and strangely, pulling the power out of the left or right opamp results in total loss of sound. I must have wired something wrong--perhaps an incorrect assumption on how this breadboard works (i'm still very much a newbie).

If someone could please take a look at the attached pictures and spot what i've done wrong it would be greatly appreciated.

Some possibilities:
- grounding of the input and output plugs. Does the ground plug go to the vgnd rail or to the ground on the breadboard?
- could the channel crosstalk be happening through the vGnd rail?
- opamps don't appear to be oscillating--they're not hot etc.
- are pins 6 and 7 on the opamps really supposed to be jumped with no resistor?

The circuit schematic.

My ugly breadboard job. Click here for a bigger image.

[Edited by elconcho on 06-26-2003 at 04:38 AM.]

//- are pins 6 and 7 on the opamps really supposed to be jumped with no resistor?

yes,short jump,no resistor.

//- grounding of the input and output plugs. Does the ground plug go to the vgnd rail or to the ground on the breadboard?
- could the channel crosstalk be happening through the vGnd rail?

yes,vGnd rail is the amp board ground.

What values should i use for R1 and R2?

Or somehow use a BUF634 and TLE2426 together?

[Edited by elconcho on 06-26-2003 at 09:09 PM.]

As for the rest, I have trouble figuring what you wired wrong but I would suggest that you simplify your breadboarded circuit first.

The led doesn't need to be there.
The pot has got to go.
Wire the power supply. Make sure it works good.
Wire the first part of the left channel (using just one opamp (cmoy like)). Make sure it works.
Add the second opamp in the feedback loop. Make sure it works.
Then do the same for the right channel. Make sure it works.

You had a good idea to separate the power supply from the amplifying part tho.

Try to make everything as linear as possible when wiring, it takes more space and time to do but it does simplify the troubleshooting. I like to proceed in modules (like previously exposed) and generally I get great results.

Keep in mind that a cheap headphone pair and a good DMM are your best friends when breadboarding something like an A47 amp. I learned the hard way never to trust blindly my breadboarded prototypes... I saw a nice pair of HD 540 go up in smoke... So be sure to test with something cheap (a $2 pair of headphones from Wal-Mart or The Dollar Store is all you need) and measure every voltage first making sure that nothing is badly wired...

Hope this helps!

I did start with the power supply first and it was splitting the voltage nicely. I then went ahead and did all 4 amps at once--probably not the smartest idea. My next step was to pull everything but the power supply out and do one ear only.

I'd lose the pot but this amp produces waaay too much power at full volume to test without something reducing the power. I do use some garbage headphones for testing. Maybe I could throw in a 4.7k resistor in place of the pot.

As for the modified power supply circuit, should I start with my existing circuit and add the buffer or the 2nd one posted (the one by Tangent). Ie, do i need the Schottky rectifier and not need R1 and R2? What values should I use for R1 and R2?

Sorry for the millions of questions but I'd be lost without this message board.

Thanks again.

[Edited by elconcho on 06-27-2003 at 12:50 AM.]

For the power supply, I'd start with your current power supply and simply add a BUF634 at the out of the TLE2426 to buffer the virtual ground. This should work nicely. If in case of doubt, you can try Tangent's schematic, this is sure to work and the value of the resistors R1 and R2 doesn't matter so much. A value of 10K should be good, altough higher would save some battery life. The important thing is that you need to match those carefully to make sure they give the exact half voltage for the virtual ground. Since a TLE2426 is basically doing this job, I would keep it and ditch R1 and R2 altogether.

Don't worry about the questions, I am a graduate student / teacher in software engineering and I too had quite a few questions when I did my first attemps at those little things.

Hope this bit was helpful!

The amp seems to be working off of the TLE2426 based power supply ok. The sound is a bit light and bass isn't as defined as it was with my old CMoy. Hopefully adding a buffer will give it some more umph.

To be clear on the TLE2426 / BUF634 combo--should I send out from the TLE to the in on the BUF?

Great! I'm glad to hear it.

"To be clear on the TLE2426 / BUF634 combo--should I send out from the TLE to the in on the BUF?"

Exactly, this way the reference voltage will be buffered by the BUF634 and the TLE2426 will always be idling.

For the bass, it's probably that the opamps are lacking current to produce significant transients in the lower spectrum. To improve this you could buffer the opamps output by putting some BUF634 between the output and your headphones. But this would absolutely require you to buffer your power supply. And it would be near to impossible to drive on batteries.

Hope this helps!

I also got a few OPA2227P's and OPA2228P's. I believe the 2228 is for a gain >5. I don't think this amp has a gain >5 (can someone verify?). The 2227's seem to be working fine. The amp seems less harsh at high volumes with this opamp compared with the 2132, but that could just be my own wishful listening.

As for putting BUF634s between each channel output and my headphones, I've read that this could cause a DC offset voltage going into my headphones (very bad for them). See this posting on using the BUF634 in a cmoy. Would it be possible to achieve the same results putting the buffer between IC1b and R4 as in the photo below?

I'm not worried about power consumption, I plan on using this amp at my desk only. It seems to work well off of a 15V wall wart that I have that can supply up to 900mA. The voltage ripple doesn't seem to cause this amp to oscillate like it did in my old CMoy.

[Edited by elconcho on 06-30-2003 at 07:17 PM.]

The gain of that particular topology is 3. The calculation is quite simple: (R2 / R1) + 1. In this case (10000 / 4700) + 1 = 3.12. To get a higher gain, simply change either R1 or R2 to the value you look to get. For low impedance phones (16 to 32 ohm), 3 to 5 is good, for medium impedance phones (33 to 120 ohm), 6 to 15 is good, finally for harder to drive phones, 16 to 25 would be good. I suggest you use Tangent's calculator to find the appropriate resistor values: Electronic calculator

Hope this helps!

Larger Image
PDF

I have to figure out how to map it to protoboard, but I'll post that information once I figure it out.

http://www-s.ti.com/sc/ds/opa4228.pdf

How would I go about raising the gain? I've only messed with gain on the CMoy, not on a two stage amp design.

[Edited by elconcho on 07-04-2003 at 12:28 PM.]

I'm planning to use a pair of 2227s in my A47.

[Edited by ahall on 07-07-2003 at 03:38 AM.]

http://people.m...kie/a47/a47.gif

56k warning - 800k GIF ahead. I managed to fit the A47 circuit, with two dual opamps, and a slightly modified META42 power supply on a Radio Shack 276-150 protoboard. No provisions for ground or power connections to off-board components, but there's room. Round resistors are standing up. Power rails go down the sides, except for the inputs at the top.

Oops...just noticed something...be sure to jump input from caps to +in on the first opamp. And output to input between the amps. And feedback jumpers on the second opamp... So this is why PCBs go through so many revisions.

[Edited by stereth on 07-10-2003 at 07:59 PM.]

The amp is done. I wrote up a small article on it. It can be found at:

http://feistworks.com/ben/a47/