Amp Bridging

[glens]

Yes, NCore.

 

I don't need any amps at the moment, but the next one(s) will be Hypex.  I seriously doubt there will be any negatives regarding bridging NCores.  Hurry up, put it together, and give us your positive report!

[glens]

On 6/13/2019 at 8:15 PM, babadono said:

The topology In the Crown amp that I am testing on my bench just uses the second channels error amp as a unity gain inverter to make what i will call the negative side driver just an inverting slave of the high side drivers error amp. So it should behave as one composite amp.

 

I looked over the schematic and service manuals and don't quite see what you stated.  Both amp channels still run with their own error corrections.  I don't know the merit in doing it the way they did over the Hypex implementation but I'm sure they had a reason.  Hypex's method doesn't require an explanation for me.

 

Along with the load appearing to the output as half impedance, I'm pretty sure there's a halving of damping factor as well, though with the astronomical level of that in the Hypex it won't be a factor even if I've got that right.

 

Which boards are you going to use?

[babadono]

On 6/14/2019 at 7:08 PM, glens said:

 

I looked over the schematic and service manuals and don't quite see what you stated.

http://sound.whsites.net/project20.htm

 This is how the Crown D75 amp that I am bench testing does it. See the first portion of linked page and Fig. 1. What schematic are you referencing?

This is the D75:

 

D75_J0116-4_original.pdf

[babadono]

I am going to use the NC400. As far as I know it is the only one available for DIY.

[CECAA850]

2 minutes ago, babadono said:

I am going to use the NC400.

You done yet?

[babadono]

This will prolly be next winter's project.

[Pete H]

5 minutes ago, babadono said:

This will prolly be next winter's project.

You all booked up until then?  What does your list look like?  Must be pretty big, or you've set the goal bar pretty low like someone else we know that is building the "garaj mahal"

 

[glens]

1 hour ago, babadono said:

On 6/14/2019 at 10:08 PM, glens said:

 

I looked over the schematic and service manuals and don't quite see what you stated.

http://sound.whsites.net/project20.htm

 This is how the Crown D75 amp that I am bench testing does it. See the first portion of linked page and Fig. 1. What schematic are you referencing?

This is the D75:

 

The very one I looked at.

 

I just reviewed the situation and see that I was referring to what you'd said after the snippet I'd quoted.  Somehow I'd gotten the notion that you'd gotten the notion that the inverted channel wouldn't be using its own negative feedback in the Crown.  I don't see that suggestion today.  It was a bad inference on my part.

 

Of the two methods, I'd prefer to feed the second channel with an inverted version of the original, the way it's done on the NCore boards.  Distortion is going to be minimal, but not non-existent, through the first amplifier channel, and feeding the inverting channel with the result of that doesn't seem as elegant to me.  Assuming both channels were perfectly matched that means the inverted portion of the waveform would contain twice the distortion using the feedback-derived method.

 

Granted that the two input buffers on an NCore board aren't going to be exactly equal, thus the chance for dissimilar "treatment" between the two halves of the output waveform, but at least there won't be a double "stepping on" for the one channel.

 

This entire post merely serves to illustrate my ability to (overly?) consider minutiae...

[babadono]

3 hours ago, Pete H said:

You all booked up until then?  What does your list look like?  Must be pretty big, or you've set the goal bar pretty low like someone else we know that is building the "garaj mahal"

 

Outdoor landscaping projects keep me pretty busy all through the summer and fall. Then there's that darn pilgrimage thingy that I like to attend

[babadono]

5 hours ago, glens said:

This entire post merely serves to illustrate my ability to (overly?) consider minutiae...

OK so you now see how the second error amp is a unity gain inverter? In the Crown D75 I mean.

[glens]

I saw that right off, but had mistakenly thought you'd suggested that in so doing, the second channel would be giving up its own use of its error amp.

 

Do you agree that the method would tend to impart a double dose of "house sound" to only the inverting channel? 

[babadono]

No it still uses it but as a unity gain inverter. I don't really get the question, sorry.

[glens]

It also uses it as a unity gain inverter.  It still gets used in its normal capacity for error correction of its amp channel.

 

Instead of simply inverting the low-level input signal and driving the second channel with that, the method used takes the already-amplified signal, drops it down, and injects it (paralleled) into the negative feedback loop of the second channel.  So the second channel starts with all the distortions / colorations (house sound) of the circuitry.  The distortions et. al. inherent in the amplifier appear once in the directly-driven channel, then the second (inverted) channel adds them again for the inverted signal.   Do you agree that this is what happens with this topology?

[babadono]

Huh? You are saying that it is using the second channels feedback resistor as another Rin to the summing junction of the second error amp instead of as the feedback resistor? You're blowing my mind. I did not think inverting amplifiers worked that way.

[glens]

I think you're blowing my mind.  Not quite sure what you said (I work nights and just awoke).  

 

Channel 1 output goes via R108 to its negative feedback section as well via R3 to channel 2 nfb section, which still is getting fed channel 2 output via R208.

 

Are we on the same page?  In terms of negative feedback both channels still operate 100% normally, it's just that channel two is primed with the inverse of the channel 1 output instead of its own discrete signal.

 

Maybe we're saying the same thing in that regard, just differently?

[babadono]

Yes in bridge mode R3 becomes the Rin for the unity gain inverter U200. The summing junction of the second error amp U200 becomes a virtual ground. For AC signals by virtue of R201 being set fully CCW (per manual) and through R204 and R203 for DC. R208 becomes the Rf of the classic unity gain inverter. In this configuration the signal being acted upon by the second error amp is the same signal being acted upon by Channel 1s error amp. Only channel 2s error amp is inverting it. That's how I see it.

[glens]

No disagreement on that much here.  But are you suggesting that channel 2 does no error correction of its own any longer?  I'm kind of back to getting the impression that's what you're suggesting.  (Channel 2's error amp inverting input is fed equal parts of both channel outputs.)

 

And you've not yet answered my question regarding the double dose of distortion at the channel 2 output relative to that of channel 1's output.  Do you see or not see that occuring? 

[babadono]

I do not. The feedback resistor is not another input resistor per se. Sure it has to be there, it closes the feedback loop but i just do not think of it as another input resistor. It's "input" is the output, how can it be an input resistor?

[glens]

It depends from which direction you're viewing it whether it's an input or output element.

 

Look at it this way: without error correction the second channel would be able to "run amuck" with the signal and the situation would not only be non-optimal, but actually (quite?) undesirable.  Certainly, the channel 1 error amp cannot do all the correction for both channels.

 

In any event I much prefer the method employed by the Hypex boards.