[petahpan]

Hello everyone! This is my first post in this forum, so please be gentle
I recently came across cmoy amp and lt1206-based virtual ground circuit. It caught my attention, so I built it from scratch and i don't understand why LT1206 is overheating (after several seconds of operation i can't touch it - it's too hot) - and i can hear 'clicking' sound every second or so in my headphones. It's based on popular tangentsoft website:
http://tangentsoft.net/elec/vgrounds.html
http://tangentsoft.net/audio/cmoy-tutorial/
 
I've done some measurements. Circuit is powered from two 9V batteries. Virtual ground vs. rail voltages is perfectly split, but when 1206 overheats they become unstable. Power draw total is about 90mA. The component values are exactly the same as suggested at tangensoft website.
 
Is it possible it's just DIP8 (N8) version of 1206 that causes it to overheat (instead of TO22)? Or maybe it's oscillating? How can I diagnose the problem? All voltages seem to be fine. I also double checked all components - they work fine.
 
BTW why no one actually builds cmoy with LT1206 virtual ground? I can understand that it doesn't need >250mA current, but still it's one of better VGND circuits.

 

[nikongod]

  Hello everyone! This is my first post in this forum, so please be gentle
I recently came across cmoy amp and lt1206-based virtual ground circuit. It caught my attention, so I built it from scratch and i don't understand why LT1206 is overheating (after several seconds of operation i can't touch it - it's too hot) - and i can hear 'clicking' sound every second or so in my headphones. It's based on popular tangentsoft website:
http://tangentsoft.net/elec/vgrounds.html
http://tangentsoft.net/audio/cmoy-tutorial/
 
I've done some measurements. Circuit is powered from two 9V batteries. Virtual ground vs. rail voltages is perfectly split, but when 1206 overheats they become unstable. Power draw total is about 90mA. The component values are exactly the same as suggested at tangensoft website.
 
Is it possible it's just DIP8 (N8) version of 1206 that causes it to overheat (instead of TO22)? Or maybe it's oscillating? How can I diagnose the problem? All voltages seem to be fine. I also double checked all components - they work fine.
 
BTW why no one actually builds cmoy with LT1206 virtual ground? I can understand that it doesn't need >250mA current, but still it's one of better VGND circuits.

 
Its a very fast current feedback op amp. Most people use (relatively slower) voltage feedback op amps because they are significantly easier to use. 
 
You said:
"when 1206 overheats they become unstable"
My money is on this correction being more accurate:
When op amps go unstable they overheat. 

 

[jcx]

with a dual supply active/virtual gnd is a waste
 
any active gnd doubles your power drain with output since it has to sink as much as the amp channels put out
 
 
the LT1206 draws 20 mA all the time unless you have used the shutdown pin as current programming control - @20ma, +/-9V the PDIP will get quite warm doing nothing
 
 
the LT1206 is a high speed CFA - current mode feedback amplifiers have different stability/feedback requirements limitations than the VFA "voltage feedback" op amps usually seen in cmoy, used by Tangent
 
for unity gain CFA absolutely must have a proper value feedback R for stability, not a wire like VFA unity gain circuits
 
and CFA is very sensitive to C at the inverting input - not good to prototype on solderless strip boards

 

[wakibaki]

Not really very helpful, huh?
 
The LT1206 is potentially problematic in a virtual ground, it's very sensitive to strays, and requires careful attention to layout. This can lead to a chorus of 'I told you so' type replies when problems occur. You could have an instability issue (oscillation) but that said, there's a good chance that you've got some other problem, such as a hard-to-see short (solder string). They really can be very hard to find on occasion.
 
You could buy another LT1206 and a few other bits and build this very good headphone amp:- http://www.johncon.com/john/amp/. Again, careful attention to layout will be required, THD and noise performance will probably be layout-limited, so not necessarily a good choice for a beginner.
 
Best to build a simple cmoy with undemanding opamps and undemanding specs. to start with, a Grado RA1-style clone has surprisingly good performance. See this thread for a neat implementation:- http://www.head-fi.org/t/640006/diy-wooden-amp-enclosure-and-amp-project-all-under-25-tutorial-for-diyers
 
w

 

[petahpan]

I'll try to physically shorten feedback loop and experiment with shutdown pin in LT1206. The original tangensoft circuit contains 22k resistor that bridges S/D pin with op-amp output. Most of the designs omit reducing quiescent current and directly connect S/D pin with -V.
Anyway, analyzing LT1206 documentation further, it seems DIP8 package has dramatic thermal dissipation even under moderate loads (100K/W), I could actually get CT7 (5K/W) version or try some heatsinking.

 

[petahpan]

SOLVED! 

 
The cause of overheating of LM1206 was imbalance between compensating capacitance and output capacitance, that is the capacitive load was too high. I replaced 10nF compensating capacitor with a larger one (tried both 100nF and even 680nF - they are ok) and all problems were immediately gone - feedback amp is almost cold, there are no fluctuations in voltage supply. The sound is juicy and nicely detailed from my Grados. 
I also replaced 1k feedback resistor with 500ohm, that improved virtual ground imbalance to 0,04V (relative to V+ and V-), instead of 0,1V.
 
Cheers!
 
BTW. Shutdown pin connection in tangentsoft schematic is of course absolutely right, I initially misinterpreted the circuit.

 

[wakibaki]

SOLVED! 

Cool.

w