Using multiple (like 40) TL074s... - Page 4 - diyAudio
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Old 28th April 2004, 06:07 PM   #31
hermanv is offline hermanv  United States
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You're determined to make me get out the calculator (and maybe even read the data sheet ). But I'll stick to guessing.

Each op-amp is good for about, what, 5 milliamps? So a 1 Ohm resistor would cause (current) saturation if one op-amp wanted it's output at +5 millivolts and some other op amps thought that -5 millvolts was dandy.

Even with 10 Ohms the offset will limit the total voltage swing on some of the op amps pretty quickly. If the output current is substantially higher than 5 milliamps then you are right, my guesses of resistor values are high.

Depending on power supply voltage there is an exact answer, but it is most likely higher than 10 Ohms.
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Old 28th April 2004, 06:55 PM   #32
Stocker is offline Stocker  United States
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CALCULATOR CALCULATOR CALCULATOR CALCULATOR CALCULATOR

Break out the datasheet!
From the horses' mouths:
http://focus.ti.com/lit/ds/symlink/tl074.pdf
And don't forget the bass drivers!
http://www.national.com/ds/LM/LM324.pdf

Product folders...
http://focus.ti.com/docs/prod/produc...rtNumber=TL074
http://www.national.com/pf/LM/LM324.html
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Old 28th April 2004, 07:31 PM   #33
Stocker is offline Stocker  United States
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It just occurred to me:

Could the resistors be largely eliminated or reduced in value if each amp had its own feedback network instead of one global network?

BTW the TL074 datasheet says about a hair over 200mW Pdiss each.
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Old 28th April 2004, 10:14 PM   #34
hermanv is offline hermanv  United States
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There was a whole flurry of Op-amp evaluations for audio some years back. All of the TL0.. series were considered pretty much OK but (and memory grows dim) I seem to recall the 324 was thought to be a true stinker.

The headphone idea would allow evaluatiion with considerably less time investment. Headphones, headphones, anyone? Anyone at all?
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Old 29th April 2004, 04:00 AM   #35
jcx is offline jcx  United States
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a 44 op amp/channel power amp?

http://headwize2.powerpill.org/proje...meier3_prj.htm
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Old 29th April 2004, 03:07 PM   #36
Stocker is offline Stocker  United States
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Headphones? you want to use 300 opamps to power *headphones*?!

If I am going to do this, it better be able to push amps through low-single-digit-ohms voice coils, be they headphones or no.

The 44 opamp amp is cool. It is also in the 6th post in this thread.
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Old 6th May 2004, 01:47 PM   #37
Stocker is offline Stocker  United States
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I was talking to an engineer friend of mine after church last night and he surprised me by not thinking this was all that crazy an idea. I proposed using the LM324s as drivers and TL074s as active feedback chips, and he said that I'd have to be careful. Since the 324s are bipolar and 074s are CMOS the 074s might pick up switching noise from the power supply rails from the 324s loading it... "Careful bypassing will be required".

That stumped me until I remembered my pile of voltage regulators...Put them on different power supplies altogether. Figure 14 LM324s and maybe 50 TL074s per 7815/7915 combination. and 0.1uF caps like candy all over the place...

For ease of signal wiring I was thinking of alternating 324s and 074s, driver, feedback, driver, feedback etc. all the way across...
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Old 8th June 2004, 06:58 PM   #38
Stocker is offline Stocker  United States
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Default Still noodling this...

The positive rail of the TL07x is included in the common mode range, so separate power supplies are out, but careful (read: lots of caps) bypassing should take care of it. The bigger problem may be ground loops with such long rails, but a fat ground rail or a ground plane may cure that.

I have heard that the TL07x has a 15mA wire-bond limit and I have also heard 18mA. Seems better to shoot for 12mA to be on the safe side.

I never knew large sheets of proto board were so expensive!

I generated a spreadsheet based on some simple algebra and it's pretty useful if you want to parallel any number of amplifiers. Let's see if anyone has some comments on it.
Attached Files
File Type: zip ampcalx.zip (4.2 KB, 29 views)
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Old 18th December 2004, 05:41 AM   #39
Stocker is offline Stocker  United States
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I was looking for something else and found this thread. What a gas!

Anyhow, all this fuss was about DC offsets and one time an engineer told me something like...if you aren't amplifying DC, why not just use a cap? duh.

Good thing he didn't tell me that when I was thinking of doing this mess!

also, the 15mA wire-bond current-limit was a misheard 50mA which seems more like a thermal dissipation limit with any sort of moderate output voltage!

Anyways, please join me in rolling your eyes (at me)
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Old 19th December 2004, 02:50 PM   #40
Mr Evil is offline Mr Evil  United Kingdom
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I for one applaud your adventurousness! So many naysayers... ignore them.

Using a capacitor to block DC offset per op-amp is a good idea, but you would still need a resistor to account for differences in gain between op-amps. Using matched resistor arrays would alleviate that problem a bit.

Hmmm... perhaps you could try making a 40-phase class D amplifier instead. There's a thought...
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