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Old 7th December 2009, 08:24 AM   #1
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Default Tripath amp - passing by input stage ?

Hi there.

I was trying to figure out how to pass by the opamps of my Hypex UCD to improve the SQ of the amps, when another idea hit me.

I am running a TA2021B on my main system.

As we all know , there is an inverting opamp sitting on the input of the Tripath amps.

Gain is V0/V1=-12*RF/FI

Now what would happen if I open the feedback loop and connect my input signal to the feedback pin without connecting it to the opamp input?
I guess I'd end up at an gain of 12!? Would this be working? What happens to the impedance? Any other issues I might have to look at?


Cheers
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Old 12th December 2009, 01:01 PM   #2
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Bump!

Nobody ever tried to pass by the input OpAmp?
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Old 6th April 2010, 09:59 AM   #3
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Hi folks.

I'd like to come back on that one, since I am gonna touch my amp soon.
I'd like to rephrase my question:

How about shortening RF? This should give a gain of 1 on the opamp input stage - correct? This would be basic opamp behavior.

But - according to below formula taken from the datasheet the overall gain becomes 0 if I shorten RF.

vO/vI=-12*Rf/Ri


TA2021B datasheet pdf



I'd really appreciate if somebody could give advise, what's gonna happen if I shorten Rf. Otherwise I'll potentially fry the amp.


THX a lot.
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Old 6th April 2010, 01:48 PM   #4
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soundcheck,

Give me some time and I will see if I can test this out for you.

I have an amp that uses the TC2001 that I'm not afraid to fry. I have plenty of extra IC's for it.
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Old 6th April 2010, 02:17 PM   #5
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Quote:
Originally Posted by theAnonymous1 View Post
soundcheck,

Give me some time and I will see if I can test this out for you.

I have an amp that uses the TC2001 that I'm not afraid to fry. I have plenty of extra IC's for it.
Great.


BTW: You got a small mistake in your drawing. RF is feeding the inverting input
on the 2nd opamp!
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Old 6th April 2010, 02:44 PM   #6
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Perhaps some words about the underlying story.

I am running a 1.7V DAC output right into the amp firing 98db/SPL speakers.

This forces me to apply 12-18db attenuation - digitally on the PC side. This is of course not acceptable for a 16bit signal. (I'm still running a 1543 based DAC) I would like to get the gain as much down as possible.

Perhaps a nice side effect could be that I'll improve the performance of the amp if I can get rid of RF. Perhaps a nice tweak also for others running sensitive speakers.

To go a step further the first idea was to leave out the entire input stage and to feed the signal through RI ( disconnected from the opamp input ) right into the feedback output. Perhaps it is also worth to test this approach.


Feel free to comment.

Last edited by soundcheck; 6th April 2010 at 02:46 PM.
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Old 6th April 2010, 03:00 PM   #7
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Quote:
Originally Posted by soundcheck View Post
BTW: You got a small mistake in your drawing. RF is feeding the inverting input
on the 2nd opamp!
That's straight out of the TC2001 datasheet. Tripaths fault, not mine.

When Rf is shorted, everything gets attenuated (signal AND noise). I was kind of excited until I shorted Ri as well. With Ri shorted the signal level jumps back up, but it doesn't sound right at all. All the bass is gone and the signal is a bit distorted.

Anyway, all the poking and prodding of the input pins didn't kill anything, so you should be safe to experiment on your amp.

If there is something else you want me to try while I have all this mess out let me know. I'll be putting all back away shortly.
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Old 6th April 2010, 03:11 PM   #8
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THX a lot real good news.

So. Shortening RF is working. And you even got excited. Cool.

Can you figure out what gain we're looking at?


Why did you shorten RI?

I was thinking of cutting RI off from the inverting input to feed the signal straight in the RF output ( RF stays shortened). This would keep the opamp completely out of the loop.

Cheers
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Old 6th April 2010, 03:28 PM   #9
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One more datasheet related question:

The 2020 sheet says related to gain "12*RF/RI" the 2021B says "-12*RF/RI". (notice the minus)

My guess: The 2021B is the newer datasheet and is supposed to tell us that the
signal will be inverted on the output.

Can anybody confirm this?
THX
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Old 6th April 2010, 03:37 PM   #10
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Quote:
Originally Posted by soundcheck View Post
Why did you shorten RI?
Because with just Rf shorted the output level was very low, even with an extremely high input signal.

Quote:
Originally Posted by soundcheck View Post
Can you figure out what gain we're looking at?
Ehhh..... I'm not up for math at the moment. I'll just post some voltage readings and you can glean from it what you wish.

These output readings are with a 0.3Vrms input signal (sine @200Hz)

Rf and Ri normal = 2.9Vrms
Rf shorted = 0.04Vrms
Rf and Ri shorted = 2.2Vrms

An interesting thing; the 200Hz sine output sounds like a square wave when Rf and Ri are both shorted.

I dunno soundcheck, looks like you'll have to do some tests of your own. I think I've had enough fun for now.
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