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Old 12th March 2003, 03:45 PM   #1
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Default GainClone with OPA548 - impressions


After my (failed) attempts to get the GC with a LM3876 dead quiet, I have just ordered LM3875, and decided to fiddle around with the OPA548 I have lying around...

So I took apart the (up to now unused) second channel of my LM3876 GC, and put in the OPA548 instead. I used the same setup like the actual P. Daniel GC. One problem is, that my supply is a little bit out of the specification - but TI itself dosn't seem so sure about what to use.
The original datasheet says that up to +-30 V can be used, but in application note found under
they say that a OPA548 can handle up to +-40V...
So I just connected the supply build with a 25 - 0 - 25 toroid, the 221k feedback resi (on the backside of the IC, nice position, just enough space for a resi), the 1200 uF Panasonic FC to pins 5 and 4 and to ground, the non-inverting input to ground, no input cap, 10k in series with signal and 47k from signal to ground...

Fired it up, and hummmmmm
So I inserted a 10k resistor, bypassed with an Epcos 100nF MKP from non-inverting input to ground, and hummmmm is gone...

But, oh horror, the same small hissing noise like with the LM3876... but since that doesn't impact the sound when music is playing:

now to listening experiences:
For highs:
I am right now listening to the killer instrumental guitar track "Voice Of The Soul" from Death's "The Sound Of Perseverance", and I can tell you, that high pitched guitar sound at the beginning really hurts... but not because it sounds bad, it is just the way it is intended to sound I have the impression that Chuck himself is standing directly in front of me with a studio monitor pointing at me, playing those sounds....

I then took Heather Nova, "Island". Her siren voice in the beginning sounds cleaner than with my Marantz amp (a STK080G based amp), if a little bit too direct - but could be my speakers...

Mid-Bass - mids:
Not clearly to define, but fine tuning my Xover to between 60 Hz and 250 Hz I can tell you that my mid-range speakers put out some quite dry kick-bass, combined with my sub it is quite impressive.

Tuned my xover to let the main speakers handle all the bass for themselves (without additional sub) - then listened to "The Art Of Coffins" by Bohren & Der Club Of Gore (german jazz combo specializing in what they call "horror jazz", this track lets everything vibrate in my flat with sub enabled...) and I was quite impressed by the bass coming out of my small 13.5 cm Visaton bass-mid woofers (Ok, at highest levels they lose control, but that's what I have my sub for).

Overall experience:
They sound real well. I don't know if I like the overall sound more than that of my (broken) Marantz '78 amp. The GC build with OPA548 reproduces every little detail of the music, sometimes maybe a little bit too accurate. But of course that also depends on the rest of the whole setup.
Until my LM3875 (and parts for P3A) arrive, I will definitely use the OPA548, since it needs less parts than LM3876 (it is totally comparable to LM3875 for connections, you do not have to use the current limiting pin and E/S pin...), is smaller, and also doesn't get very hot...

I would advise all GCers out there to give this one a try (remember TI's excellent sample program...?), maybe especially P. Daniel should try them...

Until then,

A Sacrifice For Freedom
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Old 12th March 2003, 05:04 PM   #2
tiroth is offline tiroth  United States
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Location: Pittsburgh, PA, USA
Cradle22, your hissing problem must be something to do with your implementation. I have had no problem with a 3876 in a gainclone-like configuration. I suggest you solve this problem with your 3876 or OPA chips before going on to duplicate it with your new 3875s.
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Old 12th March 2003, 05:04 PM   #3
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How about this one?

As soon as I close the lid of the case, the amp begins heavily to oscillate... ICs get very hot, distortions in sound...

Even when I open the lid again, I have to switch the amp on and off several times before those oscillations disappear...

I must be one of the few to actually find building discrete amps easier than IC-amps...

OK, so I will listen to an open GC...

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Old 12th March 2003, 05:04 PM   #4
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In a near future I'll be checking both TDA7294 and OPA549 chips.
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
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