Simple hybrid amplifier (35W/8Ohm) - Page 2

12E1 · 19th January 2013, 10:57 AM

Nice concept IMO, using a power op-amp in this way for the buffer function. If I had the time (I don't) I'd be tempted to build one myself just for fun. What's the purpose of the LM317 configured as a current sink on the output? I note that you added it in the second version, but I could not find a reason for it when I scanned the OPA458 data sheet.

kacernator · 19th January 2013, 11:07 AM

LM317 constant current source makes OPA548 to stay in sinlge ended class A for first cca 0.5W. Simply it sounds better. It moves crossover to higher power levels.

Look at the bottom of the page at last schematics:
"http://www.tubecad.com/2004/blog0013.htm"

12E1 · 19th January 2013, 11:17 AM

Cool. Thanks for that. It makes sense. I see that JB's schematic shows the old LM12 - I remember those from many years back - not the first high power op-amp, but one of the first that was really usable for audio.

kacernator · 20th March 2013, 08:48 AM

Some tweaks there and there...

FOXYE · 20th March 2013, 01:12 PM

Very interesting amplifier, just need to find a way to get more power.

kacernator · 20th March 2013, 01:39 PM

I am thinking two options:

1) make LM3886 or TDA7294 unity-gain stable
2) OPA549 unity-gain in bridge mode configuration

Second should be doable, hopefully I can use CCS on output too woth bridge mode. BUT OPA549 is quite expensive, cca 25$. It is a lot money just for output stage. It should do about 150W into 8 Ohms. Nice thing about OPA549 is that you can current limit them with resistor, which should be usefull with bridge mode especially.

Junm · 21st March 2013, 02:07 AM

I wish you can do Lm3886 since I also have 6 pcs of it to use..also LM3875 and Lm1876. If these chips can be stable as unity gain buffers..My plan before is to have a gain of 5 and the rest is from the tube pre...but not done yet hope you will...

kacernator · 21st March 2013, 07:36 AM

Quote:

Originally Posted by Junm

I wish you can do Lm3886 since I also have 6 pcs of it to use..also LM3875 and Lm1876. If these chips can be stable as unity gain buffers..My plan before is to have a gain of 5 and the rest is from the tube pre...but not done yet hope you will...

I am trying LM3886 non-inverting mode at unity at this thread, but I need help with it:

LM3886 unity gain

jcx · 08:00 PM

I suspect that with the shown Hybrid Amp schematic's series isolating 1 k and 220 pf to gnd the source impdance may be controlled enough to try the noise gain hack

the "sportiest" I would want to try with LM3886 would be to aim for noise gain of ~20 with asymptote/pole at 200kHz, which puts the low frequency zero at 10 kHz, I'd try noise gain R ~ 1kOhm

heyyou · 6th April 2013, 12:03 AM

I like your idea of making cooling holes using a shotgun.

20th March 2013, 01:39 PM	#16
kacernator diyAudio Member Join Date: Jan 2008	More power I am thinking two options: 1) make LM3886 or TDA7294 unity-gain stable 2) OPA549 unity-gain in bridge mode configuration Second should be doable, hopefully I can use CCS on output too woth bridge mode. BUT OPA549 is quite expensive, cca 25$. It is a lot money just for output stage. It should do about 150W into 8 Ohms. Nice thing about OPA549 is that you can current limit them with resistor, which should be usefull with bridge mode especially.

27th March 2013, 07:56 PM	#19
jcx diyAudio Member Join Date: Feb 2003 Location: ..	I suspect that with the shown Hybrid Amp schematic's series isolating 1 k and 220 pf to gnd the source impdance may be controlled enough to try the noise gain hack the "sportiest" I would want to try with LM3886 would be to aim for noise gain of ~20 with asymptote/pole at 200kHz, which puts the low frequency zero at 10 kHz, I'd try noise gain R ~ 1kOhm Last edited by jcx; 27th March 2013 at 08:00 PM.

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19th January 2013, 10:57 AM	#11
12E1 diyAudio Member Join Date: May 2012 Location: Near Huntingdon, Cambridgeshire, UK	Nice concept IMO, using a power op-amp in this way for the buffer function. If I had the time (I don't) I'd be tempted to build one myself just for fun. What's the purpose of the LM317 configured as a current sink on the output? I note that you added it in the second version, but I could not find a reason for it when I scanned the OPA458 data sheet.

19th January 2013, 11:07 AM	#12
kacernator diyAudio Member Join Date: Jan 2008	LM317 constant current source makes OPA548 to stay in sinlge ended class A for first cca 0.5W. Simply it sounds better. It moves crossover to higher power levels. Look at the bottom of the page at last schematics: "http://www.tubecad.com/2004/blog0013.htm"

19th January 2013, 11:17 AM	#13
12E1 diyAudio Member Join Date: May 2012 Location: Near Huntingdon, Cambridgeshire, UK	Cool. Thanks for that. It makes sense. I see that JB's schematic shows the old LM12 - I remember those from many years back - not the first high power op-amp, but one of the first that was really usable for audio.

20th March 2013, 01:12 PM	#15
FOXYE diyAudio Member Join Date: Feb 2009	Very interesting amplifier, just need to find a way to get more power.

21st March 2013, 02:07 AM	#17
Junm diyAudio Member Join Date: Dec 2008 Location: Dau	I wish you can do Lm3886 since I also have 6 pcs of it to use..also LM3875 and Lm1876. If these chips can be stable as unity gain buffers..My plan before is to have a gain of 5 and the rest is from the tube pre...but not done yet hope you will...

6th April 2013, 12:03 AM	#20
heyyou diyAudio Member Join Date: Feb 2013	I like your idea of making cooling holes using a shotgun.