[bhjazz]

Also on the subject of soldering items like LEDs, I am much into the practice of dropping a few alligator clips between the solder joint and the LED itself. I killed plenty of LEDs before that. Heat just isn't the greatest thing. And sometimes a damaged LED isn't seen until you fire the whole rig up later.

 

[infinitesymphony]

Success (sort of)! I built the first amp using the parts I've listed on the first page, and it works. However, it distorts past anything but quiet volumes, and on transient hits (i.e. louder = more distorted). I'm using the OPA2228P right now, if that makes a difference. I should also note that I'm using resistors instead of the input capacitors, so I suspect a DC issue, especially since I heard a pop when the amp was turned off.

On the board, just below the op-amp and between the V+ and V- battery parts is a tiny hole that connects to the TLE railsplitter. What is it for?

I have a multimeter that I can use for testing purposes. What should I be testing? It would be most helpful if you could tell me which parts to probe to test for certain things, for example DC offset, quiescent current draw, battery power per channel, etc.

Update____

I did some op-amp rolling to see whether or not a different op-amp would alleviate the problem, and it did. I tried:

OPA2228
OPA2132
AD823
OPA2107
LM4562

The only ones that seemed to cause distortion were the OPA2228 and the LM4562. Does this help to explain my problem?

Perhaps it's due to the voltage gain that these op-amps require... This first amplifier has a gain of 6.

 

[holland]

Nope, doesn't help.

Take 2 mini-mini plugs. Plug one into the input, one into the output.

On the one that goes to the input, tape a resistor leg to the other end of the plug, so it shorts ground, left and right.

On the output, take your DMM, set it to DC V and measure the voltage from left to output ground and right to output ground.

Set your DMM to mA plug and partially connect the battery to the amp (one terminal). For the other terminal, put the DMM between the battery and the terminal and measure the current draw.

Little hole, looks like power V+.

 

[infinitesymphony]

Looks like I'll have to pick up another mini-mini cable.

Quote:

Originally Posted by holland /img/forum/go_quote.gif On the output, take your DMM, set it to DC V and measure the voltage from left to output ground and right to output ground.

So, connect one probe to the sleeve of the input cable (with TRS shorted) and the other to the sleeve of the output cable?

Quote:

Originally Posted by holland /img/forum/go_quote.gif Set your DMM to mA plug and partially connect the battery to the amp (one terminal). For the other terminal, put the DMM between the battery and the terminal and measure the current draw.

I'm using two 9V batteries in series... Would completely connecting one battery and half-connecting the other accomplish the same thing? This might be a silly question, but does the amplifier need to be turned on for the current measurement?

Quote:

Originally Posted by holland /img/forum/go_quote.gif Little hole, looks like power V+.

The little hole can't be V+, because that's just to the right of it.

 

[holland]

Quote:

Originally Posted by infinitesymphony /img/forum/go_quote.gif So, connect one probe to the sleeve of the input cable (with TRS shorted) and the other to the sleeve of the output cable?

Yes. You can do without it, but it guarantees that there's no stray stuff on the input that will get amplified.


Quote:

Originally Posted by infinitesymphony /img/forum/go_quote.gif I'm using two 9V batteries in series... Would completely connecting one battery and half-connecting the other accomplish the same thing? This might be a silly question, but does the amplifier need to be turned on for the current measurement?

Yes, it needs to be on. Yes, it should. You just need to break the circuit for the power and insert the DMM into it.

Quote:

Originally Posted by infinitesymphony /img/forum/go_quote.gif The little hole can't be V+, because that's just to the right of it.

That's an illogical assumption. You can just measure it to find out for sure. From the trace, it looks like V+ as it runs to pin 8, the switch on the pot and powers the LED. It looks like a probe point. In the time it took you to post you could have stuck your DMM in there and found out for sure.

BTW, it's the switched "on" V+, if it helps any.

 

[Joshatdot]

The tiny hole is a VIA, a pathway from top of the board trace to bottom of the board trace. You should solder a cut resistor/cap leg in there.

 

[infinitesymphony]

Quote:

Originally Posted by holland /img/forum/go_quote.gif That's an illogical assumption. You can just measure it to find out for sure. From the trace, it looks like V+ as it runs to pin 8, the switch on the pot and powers the LED. It looks like a probe point. In the time it took you to post you could have stuck your DMM in there and found out for sure.

Hahah, good point. I'm still a little green about when and where to use a DMM, but I'm understanding more. I just don't want to fry anything.

Quote:

Originally Posted by Joshatdot /img/forum/go_quote.gif The tiny hole is a VIA, a pathway from top of the board trace to bottom of the board trace. You should solder a cut resistor/cap leg in there.

Do you think that might be affecting the functionality of the CMoy? If so, I'll solder in a resistor leg.

Quote:

Originally Posted by holland /img/forum/go_quote.gif You just need to break the circuit for the power and insert the DMM into it.

I turned on the amp and used the DMM to complete the loop between the second battery and its adapter, and the reading was around 18V. I'm using two 9V batteries, so isn't this the reading I should expect?

 

[holland]

Quote:

Originally Posted by infinitesymphony /img/forum/go_quote.gif Do you think that might be affecting the functionality of the CMoy? If so, I'll solder in a resistor leg.

It's not necessary to solder a leg and won't affect functionality. You can if you want, or not.

Quote:

Originally Posted by infinitesymphony /img/forum/go_quote.gif I turned on the amp and used the DMM to complete the loop between the second battery and its adapter, and the reading was around 18V. I'm using two 9V batteries, so isn't this the reading I should expect?

Yes, for voltage, but what you want to measure is current, so set it to milli-amps, amps, or whatever your meter has. You need the milliamp scale. The current draw should be the close to the quiescent current of the opamp spec. If you are having troubles with headphones, then measure the current without the headphones then plug in the headphones and see how much the current changes. It should still be in the same ballpark, very low. OPA2228 should be around 7.4mA.

 

[infinitesymphony]

Gotcha...

I got a reading of 8.4 mA with no headphones connected, 9.9 mA with source and headphones connected. Are those readings high enough to cause problems?

 

[infinitesymphony]

I ordered two mini-mini cables from Monoprice (my first order with them

), so I should be able to test the amp's internal DC offset within a few days. It still seems strange that all of the other op-amps worked at high volume levels without any audible distortion, but the OPA2228 and LM4562 distorted like crazy with just a slight turn of the knob...

 

[NelsonVandal]

I don't know about OPA2228, but LM4562 has very low output current, and it's not sufficient for low impedance phones. You have to use it with some kind of buffers. LM4562 can be cranky. Measure the current draw with this opamp, and measure the DC-offset.

 

[holland]

You can test DC offset now and see where things are.

The current draw seems a bit high, but I don't have an OPA2228 (nor a 2227 for that matter). Keep your DMM on it for a while and see if there current draw bounces around. For the DC offset, you can measure now and see if it bounces around too. Those are indicators of oscillation, but I don't think I've ever heard of the OPA2228 oscillating, it's not overly fast. Also, does the chip get hot?

I believe the OPA2228 puts out current that's similar to OPA2132, so if the OPA2132 works without distortion, so should the other.

BTW, what do you mean by distortion?

 

[infinitesymphony]

Just to be clear, the DC offset test is the one that involves probing the ground portions of the input/output mini cables? If so, I'll be able to do that as soon as the cables arrive.

At that point, I'll test current draw and DC offset for both op-amps again.

I checked for excessive op-amp heat when I first turned on the amp, since I'd remembered from Tangent's guide that high heat is a sign of something wrong (bad orientation, power supply problem, etc.). But none of the op-amps have heated to anything close to hot.

The distortion sounds like clipping. At a very low volume, it's not audible, then as the volume is turned up, it slowly creeps in with bass drum hits. A little further up (still not at a normal volume), the whole thing is just massively distorted and unlistenable.

Also, pictures!

I gave the 470 uF a little room so that it could be bent over the TLE2496. This way, it can easily fit into a typical mint tin.

My soldering iron is still a bit too hot for this purpose, hence the burn marks and some not-so-shiny joints, but it seems to work well enough. I haven't cleaned the board with isopropyl alcohol or nipped the joints down yet, so it should look better after that step.

 

[Joshatdot]

what are all your resistor values & locations again?

 

[infinitesymphony]

Quote:

Originally Posted by Joshatdot /img/forum/go_quote.gif what are all your resistor values & locations again?

They're also in the first post, but here they are separated from the list:

C2 - 332
RLED - 10K

R2 - 100K
R3 - 332
R4 - 1.5K
R5 - jumper

These are the same values used in amb's Mini3 for a gain of ~5.54. I bought extra 10K and 1K resistors to make a gain 11 amplifier if this configuration proved problematic, or wasn't enough to drive certain headphones.

The diffuse blue LED from your parts list looks very nice, BTW. A 10K resistor for RLED seems to work well; it's more than bright enough with that amount of resistance.