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Old 12th September 2008, 10:35 AM   #1
Elvee is offline Elvee  Belgium
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Default Improving DC-servos

Hi

I am trying to find ways of improving the performances and scope of DC servos. In general, their function is very narrow and focused, but I would like to give the servoing amplifier a bigger role.
In summary, I want to combine the accuracy, linearity and high open-loop gain of a precision amplifier with the power and bandwidth of a power amplifier.
Nothing new, and a number of solutions have already been provided.
But I want to implement it in a more general and convenient way, with some additional constraints. The picture shows a (non-working) example of what I try to achieve:

U1 symbolizes the fast, low-accuracy power amp.
U2 is the precision correction amplifier. It compares the output scaled by R4/R5, to the input signal, and corrects the input of U1 via R3.
The HF part of the signal goes directly to U1 via the feedforward capacitor C3.

Obviously, it doesn't work: C2 and C3 introduce two poles in the loop, resulting in a strong gain and phase variation in the middle of the band.

The constraints:
The summing point for LF and HF signals has to be the NI input of the power amp.
The correction amplifier has to be fully isolated from the HF signals, both on the inputs and the output: this is the reason for C2 and C4.
This is also the reason why it is not possible to introduce a lead resistor in series with C3 f.e. The role of U2 must strictly be limited to LF.

Is there a clever way of achieving these objectives, without resorting to ultra-accurate matching, or much more complex circuits?
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Old 12th September 2008, 02:27 PM   #2
syn08 is offline syn08  Canada
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It sounds like you want to build a composite amp. Here's some basics:

http://focus.ti.com/lit/an/sboa002/sboa002.pdf

I've successfully experimented something like this (not intended for high precision):

http://www.diyaudio.com/forums/showt...04#post1528904
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Old 12th September 2008, 04:32 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally posted by syn08
It sounds like you want to build a composite amp.[/url]
Sort of.
But often, like in your example, amplifiers making up the composite have similar frequency characteristics; simple summation techniques can therefore apply.
When the frequency bands have to be separated, and the summation occurs upstream of the main amplifier, things become much more complicated, as I have shown in my example.
This why I chose DC servo as a title, although it's much more radical than usual DC servos.
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Old 12th September 2008, 05:54 PM   #4
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i used to be the final test and customer service tech for a company that used a DC servo in their power amp the trick was that the servo had a -3db point of 5hz, and was isolated from the audio feedback loop. it worked extremely well (so well, in fact, that in doing the DC offset adjustment, if you forgot to disconnect the servo, adjusting the offset pot did absolutely nothing except change the mix of even harmonics in the distortion residual).

your picture is missing most vertical lines, but i think i get the idea of what you're trying to do. you have the input to the servo tapping off from part of the audio feedback, and the lack of isolation between your AC and DC feedback loops is where your problem lies. i'll upload a copy of a servo i know works later, and you'll see what i mean.
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Old 12th September 2008, 06:06 PM   #5
lineup is offline lineup  Sweden
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i do not use dc servos
and probably never will

the normal dc feedback is enough for my amplifiers
but not for some others amps, i suppose

regards
lineup
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Old 12th September 2008, 06:18 PM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally posted by lineup
i do not use dc servos
and probably never will

the normal dc feedback is enough for my amplifiers
but not for some others amps, i suppose

regards
lineup
This is not specifically intended for audio amplifiers, it is more general, applicable to instruments amplifiers f.e., but it could also benefit audio amplifiers:

If the correction amplifier is one of the new, ultra-low distortion chips, it will contribute to the linearization of the lower part of the spectrum, which can be beneficial.



Quote:
your picture is missing most vertical lines, b
Use the "maximize" tool or handle of your browser.
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Old 12th September 2008, 06:32 PM   #7
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It is easier to get high bandwidth low noise low distortion amp closing a GFB loop around stages with local feedback than the same loop around the same stages without local feedbacks. I once made an amp in class A+C with such approach, it was much better than any traditional AB class amp, both sonically and according to measurements. 3 negative feedbacks on AC, and one global on AC and DC.

Click the image to open in full size.
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Old 13th September 2008, 12:43 AM   #8
GK is offline GK  Australia
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Default Re: Improving DC-servos

Quote:
Originally posted by Elvee
Hi

I am trying to find ways of improving the performances and scope of DC servos. In general, their function is very narrow and focused, but I would like to give the servoing amplifier a bigger role.
In summary, I want to combine the accuracy, linearity and high open-loop gain of a precision amplifier with the power and bandwidth of a power amplifier.
Nothing new, and a number of solutions have already been provided.
But I want to implement it in a more general and convenient way, with some additional constraints. The picture shows a (non-working) example of what I try to achieve:

U1 symbolizes the fast, low-accuracy power amp.
U2 is the precision correction amplifier. It compares the output scaled by R4/R5, to the input signal, and corrects the input of U1 via R3.
The HF part of the signal goes directly to U1 via the feedforward capacitor C3.

Obviously, it doesn't work: C2 and C3 introduce two poles in the loop, resulting in a strong gain and phase variation in the middle of the band.

The constraints:
The summing point for LF and HF signals has to be the NI input of the power amp.
The correction amplifier has to be fully isolated from the HF signals, both on the inputs and the output: this is the reason for C2 and C4.
This is also the reason why it is not possible to introduce a lead resistor in series with C3 f.e. The role of U2 must strictly be limited to LF.

Is there a clever way of achieving these objectives, without resorting to ultra-accurate matching, or much more complex circuits?


Look up some old CRO service manuals. This is along the lines of how wide bandwidth, DC-coupled oscilloscope input amplifiers used to be made when IC opamps started to come into fashion.

The the HF response of the opamp was deliberately rolled off at a transition frequency of at some kHz. The directly coupled output of the opamp was then summed with the output of a discrete, AC coupled amplifier for the higher frequencies, coming in with a comlementary high-pass response at the transition frequency for an overall summed flat response.

I once copied the idea for a precision instrumentation thingle using an AD707 and a GHz fT discrete transistor array for the AC ampifier (can't remember the part # for the latter, but Farnell sells it).

Cheers,
Glen
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Old 13th September 2008, 07:26 AM   #9
Elvee is offline Elvee  Belgium
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Default Re: Re: Improving DC-servos

Quote:
Originally posted by G.Kleinschmidt




Look up some old CRO service manuals. This is along the lines of how wide bandwidth, DC-coupled oscilloscope input amplifiers used to be made when IC opamps started to come into fashion.

The the HF response of the opamp was deliberately rolled off at a transition frequency of at some kHz. The directly coupled output of the opamp was then summed with the output of a discrete, AC coupled amplifier for the higher frequencies, coming in with a comlementary high-pass response at the transition frequency for an overall summed flat response.

I once copied the idea for a precision instrumentation thingle using an AD707 and a GHz fT discrete transistor array for the AC ampifier (can't remember the part # for the latter, but Farnell sells it).

Cheers,
Glen
I have seen such circuits. But the LF and HF signals simply follow parallel paths, and are summed at or near the output. This eliminates the closed loop difficulties.
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Old 13th September 2008, 02:38 PM   #10
GK is offline GK  Australia
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Default Re: Re: Re: Improving DC-servos

Quote:
Originally posted by Elvee
This eliminates the closed loop difficulties.

Well yeah, that's pretty much the idea. With a little component matching it is a relatively simple way to a uV-precision DC coupled amplifier, -3dB at several hundred MHz.

Cheers,
Glen
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