[rogerlike]

Hi there, I have a META42 I built back in 2002.

I just replaced an AD8620 with 2x OPA627BP on a Brown Dog adapter and I'm now getting a lot of hiss (source disconnected, volume knob independent). I've also since destroyed the AD8620 so I cannot put it back in for reference.

Any quick ideas? Should I decide to finally build a PPA?

Thanks a lot

 

[tangent]

Basic Troubleshooting for Headphone Amplifiers

 

[rogerlike]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif Basic Troubleshooting for Headphone Amplifiers

Hello Sensei Tangent! Thanks for the link, I'm going to try and hunt down the problem. I was hoping that maybe changing op-amps and introducing hiss was a known problem, but it sounds like I've probably damaged something in the process of changing the op-amps :/

 

[tangent]

Added noise is indeed not an expected symptom.

The amp may not be broken. Going through the steps, especially the supply current check, may help illuminate things.

 

[rogerlike]

My META42 is class-A biased with the jfet cascode, quad 2002 buffers for each channel, double 2001 virtual ground buffers. Dual OPA627BP op-amps

Iq seems to be 41.0mA w/ power supply at 12.5v

This seems quite reasonable I think?

I'm getting about 1.1mV left and 1.3mV right DC offset on the outputs.

I tried adding 200ohm (all i had around) at R9 and then at R8 (normally jumpered) and the hiss remained the same in both cases.

Nothing seems to be getting hot or even warm on the amp board. The virtual ground voltage seems to be correct.

R3 = 475
R4 = 4.75k
R5 = 2.74k
R6 = 475k

Giving a gain of 10.3

I suppose next I should trace a signal through the amp.

Does this make sense?

Thanks so much

edit*
I've increased the gain to 17 and the noise went up, and decreased it to 5 and the noise went down. Does this help in detirmining where the noise is?

 

[n_maher]

Unless I'm understanding something incorrectly you're not drawings nearly as much current as one would expect. The opamps account for ~14mA on their own and then you've got the 8 x EL2002's which would appear to draw 5mA each (40 total), plus the 2 x EL2001's at 1.3mA, plus a couple more mA for Class A bias, plus your LED. So that'd put my estimate somewhere in the range of 60mA.

 

[rogerlike]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif Unless I'm understanding something incorrectly you're not drawings nearly as much current as one would expect. The opamps account for ~14mA on their own and then you've got the 8 x EL2002's which would appear to draw 5mA each (40 total), plus the 2 x EL2001's at 1.3mA, plus a couple more mA for Class A bias, plus your LED. So that'd put my estimate somewhere in the range of 60mA.

Hmm well the amp itself is working in both channels, which would indicate the op-amps must be working. The LED is working. Could it mean some of the buffers in my stack (no chance of desoldering that lot :/ ) might have gone on the blink. Would that manifest as hiss?

BTW n_maher, did you ever sell your super-jung?

I've been cannibalising all sorts of things to get more resistors to change gains!

 

[n_maher]

Quote:

Originally Posted by rogerlike /img/forum/go_quote.gif Hmm well the amp itself is working in both channels, which would indicate the op-amps must be working. The LED is working. Could it mean some of the buffers in my stack (no chance of desoldering that lot :/ ) might have gone on the blink. Would that manifest as hiss?

Well, there might be something about the parallel buffer arrangement that my weak understanding of electronics doesn't "get". Best to wait until some other, more knowledgeable members can have a look and comment as well.

Quote:

BTW n_maher, did you ever sell your super-jung?

Uh, sort of.

Question: how does the amp behave with a source plugged in? Does the hiss change at all? Have you tested different headphones? I'd also suggest trying to find a cheap dual channel opamp to try. I'm hoping you socketed the adapter...

 

[rogerlike]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif Question: how does the amp behave with a source plugged in? Does the hiss change at all? Have you tested different headphones? I'd also suggest trying to find a cheap dual channel opamp to try. I'm hoping you socketed the adapter...

The hiss remains the same with source in or out. I've tested UM2 (awful hiss, completely unlistenable) ER4P (bit better but still unlistenable) ER4S (better still) HD650 (not very noticable but still definitely there, I could live with this). Makes sense the hiss gets quieter with less sensitive cans.

Yes, I socketed the adapter. I needed to in order for the BD adapter to fit above the cascode fets.

Maybe I broke something with my gas-soldering iron :/ I did wonder since the board got a bit hot soldering...

I've put the gain down to 3 now, and the hiss is OK, but it just shouldn't be there... it didn't used to be ://

Edit* Uhm, my META42 sounds different - warmer - after i set the gain down from 10 to 3. Does that make any sense? I did this by changing the value of R4 from 4.75K to 1K

 

[tangent]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif that'd put my estimate somewhere in the range of 60mA.

I buy it.

So, how confident are you in your current measurement, Roger?

Quote:

Originally Posted by rogerlike /img/forum/go_quote.gif Could it mean some of the buffers in my stack might have gone on the blink.

Chances of it happening in both left and right channels, in the same amount, is too low to believe. If it happened in the ground, it would affect both channels. So, rotate the buffers among the channels: G->R, R->L, L->G. Yes, this will be "unbalanced", but it will work well enough to do a noise listening test.

Quote:

Originally Posted by rogerlike /img/forum/go_quote.gif I've put the gain down to 3 now, and the hiss is OK, but it just shouldn't be there... it didn't used to be ://

Are you sure you're not just hearing new things because you're listening for new things?

Quote:

Uhm, my META42 sounds different - warmer - after i set the gain down from 10 to 3. Does that make any sense?

I think you just didn't notice it at first because you were focused on the noise. This is a normal result from changing from AD8620 to OPA627.

 

[rogerlike]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif I buy it. So, how confident are you in your current measurement, Roger?

Well, My Multimeter was seriously cheap (on sale, 2 for 5GBP!). So It could be pretty unreliable on current measurement ^^

Quote:

Are you sure you're not just hearing new things because you're listening for new things? I think you just didn't notice it at first because you were focused on the noise. This is a normal result from changing from AD8620 to OPA627.

I'm not sure about this. I have been listening to the new chips for a while, still with the hiss, but it still could be true. It doesn't seem to make sense to me that the feedback gain would alter the sound. The hiss, however, could easily screw with my perception.

I think I am going to leave it for the time being and pick it back up when I get hold of a cheap dual op-amp to do more tests. I might get a better meter too.

Unfortunately I haven't got the virtual ground buffers socketed ://

Thanks for taking the time to help.

 

[rogerlike]

A quick question, if I were at some point to replace the 627 with 637 op-amps, would I have a problem with a gain of 3?

 

[n_maher]

Almost certainly yes.

 

[rogerlike]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif Almost certainly yes.

Hi again, I'm not concerned by the hiss anymore. But now I have another query.

I set the gain back up from 3 to 10, and replaced my 627BP chips with a pair of 637BP chips bought 2nd hand here on head-fi. My META42 acted as if no op-amps were present with the 637BP pair fitted. I tested them again with gains between 5 and 50 all with the same results. Are the 637BP most likely dead? I replaced my 627BP pair back in, and things work just fine again. I understand the 637BP can be picky, is there something else which could cause them to act like this in the META42?

Thanks

 

[Pars]

As far as I know, the 627 and 637 are not picky opamps, other than one of them (637??) is not unity gain stable (or is that gains below 5?). I would think you have some bad ones. I'd be contacting the seller...