Hello can anyone explain what the feedback exactly does on a push pull amp and where to connect it on the secondary and why
Thanks
Thanks
It is explained here.
https://www.google.com/search?q=fee...wY1LjEzLjGgB77IAQ&sclient=mobile-gws-wiz-serp
https://www.google.com/search?q=fee...wY1LjEzLjGgB77IAQ&sclient=mobile-gws-wiz-serp
Dr. Williamson, pretty much considered the inventer of the first wideband, low-distortion feedback amplifier, summed it up thusly:
The functions of negative feedback are:
a) To improve the linearity of the amplifier and the output transformer.
b) To improve the frequency response of the amplifier and the output transformer.
c) To reduce the phase shifts in the amplifier and the output transformer within the audible range.
d) To improve the low frequency characteristics of the output transformer, particularly defects due to the non-linear relation between flux and magnetizing force.
e) To reduce the output resistance of the amplifier.
f) To reduce the effect of random changes of the parameters of the amplifier and supply voltage changes, and of any spurious defects.
Williamson's original articles make good reading about the benefits of feedback in a tube amplifier:
https://dalmura.com.au/static/Williamson 1952 The Williamson Amplifier.pdf
Regarding where to connect the feedback, it depends on the output transformer. Generally it is taken from the 16 ohm tap since this will bring the entire secondary into play. But you can also take it from any of the secondaries, especially if you are only going to employ, say, the 8 or 4 ohm tap. But you have to experiment with a 'scope to determine which tap will give you the smoothest response.
Here are two threads from Audiokarma which are excellent primers in the art of optimizing a feedback amplifier. They are well worth reading for any DIYer who wants to build or improve a classic feedback amp:
http://www.audiokarma.org/forums/index.php?threads/regilding-the-gilded-lily-heaths-w-2m.767851/
http://www.audiokarma.org/forums/index.php?threads/heathkits-w-3am-a-new-beginning.772780/
The functions of negative feedback are:
a) To improve the linearity of the amplifier and the output transformer.
b) To improve the frequency response of the amplifier and the output transformer.
c) To reduce the phase shifts in the amplifier and the output transformer within the audible range.
d) To improve the low frequency characteristics of the output transformer, particularly defects due to the non-linear relation between flux and magnetizing force.
e) To reduce the output resistance of the amplifier.
f) To reduce the effect of random changes of the parameters of the amplifier and supply voltage changes, and of any spurious defects.
Williamson's original articles make good reading about the benefits of feedback in a tube amplifier:
https://dalmura.com.au/static/Williamson 1952 The Williamson Amplifier.pdf
Regarding where to connect the feedback, it depends on the output transformer. Generally it is taken from the 16 ohm tap since this will bring the entire secondary into play. But you can also take it from any of the secondaries, especially if you are only going to employ, say, the 8 or 4 ohm tap. But you have to experiment with a 'scope to determine which tap will give you the smoothest response.
Here are two threads from Audiokarma which are excellent primers in the art of optimizing a feedback amplifier. They are well worth reading for any DIYer who wants to build or improve a classic feedback amp:
http://www.audiokarma.org/forums/index.php?threads/regilding-the-gilded-lily-heaths-w-2m.767851/
http://www.audiokarma.org/forums/index.php?threads/heathkits-w-3am-a-new-beginning.772780/
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Also it's important to observe the polarity of your output transformers when wiring up so you get Negative fb instead of Positive fb. The primary side has 3 wires, one on each end and one in the middle. The secondary side has wires corresponding to the speaker impedances offered (0-4-8-16) with the understanding that you'll connect 0 to the (-) terminal of the speaker and Ground, and that the other end of the secondary is your Feedback connection point.
If you get a high-pitched whine when you hook up your amp to a speaker, your OPT may be wired backwards. Easier fix may be swapping the end wires on the Primary. Ymmv.
w
If you get a high-pitched whine when you hook up your amp to a speaker, your OPT may be wired backwards. Easier fix may be swapping the end wires on the Primary. Ymmv.
w
I built a PP 45 amp with 0, 6, 12 and 18dB of feedback. 0 and 18 dB did not sound quite as 6 and 12dB that made no difference in sound.
IMO, PP triode amps are better off without feedback, or with a very small amount to smooth things out a bit. Your output impedance is already pretty low and adding a lot feedback kind of defeats the point of directly-heated triodes. It also depends on the output transformer. I recently experimented with a very expensive Japanese output transformer that already exhibited excellent bandwidth from 20-20K. It did not take kindly at all to a lot of feedback, exhibiting enormous ultrasonic peaks that were impossible to tame.
We're in the Tubes/Valves forum, so I assume you're asking about tube amps. Solid state amps follow the same rules, but there is no output transformer to complicate things. Almost all tube amps will have an output transformer, which you'll need to take into account.
There are basically two ways negative feedback is employed in push pull tube amps.
- Global negative feedback from the output transformer (OPT) secondary to the input stage of the amplifier
- Local shunt (parallel) negative feedback around the output stage, from the output tube's plate (or screen grid) to its control grid.
By far the most popular way to apply negative feedback in PP tube amps is from the OPT secondary to the amp input stage.
Here is a simple PP amp with a global NFB loop (the loop is highlighted in orange):
The output signal from the OPT secondary is 'fed back' inverted (in 'negative' polarity) to the cathode (a non-inverting input) of the input stage. That feedback signal goes through a voltage divider made of two resistors (3,3k ohms and the first stage's 100 ohm cathode load resistor) so that only a portion of the output signal is fed to the first stage cathode. As was pointed out in grovergardner's post, if you feed back the right proportion of signal back to the input, there are many benefits. This all happens at something like the speed of light, so it is instantaneous for all intents and purposes.
If you want to read up on how and why this happens, here are a few places to look:
On the innerwebs:
https://www.aikenamps.com/index.php/what-is-negative-feedback
https://www.aikenamps.com/index.php/designing-for-global-negative-feedback
https://www.valvewizard.co.uk/localfeedback.html
Books:
Designing Tube Preamps for Guitar and Bass by Merlin Blencowe
Valve Amplifiers 4th Edition by Morgan Jones
Radiotron Designers Handbook 4th Edition by Langford-Smith
The details of how feedback works in an amplifier and how exactly to apply it for best results is a complicated subject, and beyond my understanding. I just figured I'd provide some basic materials to get started.
Unless I miss-read this thread, nobody has proposed the following:
1. Push Pull and global negative feedback:
Output Transformer secondary connections:
Ground the 4 Ohm tap.
Use Balanced negative feedback feedback from the Common Tap and the 16 Ohm Tap.
Bring the negative feedback to the push pull output cathodes, or to the push pull driver cathodes.
Pay attention to the proper connections, or you will have positive feedback instead of negative feedback.
Push Pull output stage
Push Pull negative feedback
If you want to use global negative feedback on a push pull amplifier, that seems reasonable to me.
As always, if your output transformer is saturating on high power low frequency signals . . . Global negative feedback makes the saturation Worse!
2. A different design approach:
I do not use global negative feedback at all.
My latest amplifiers use XLR balanced input, balanced volume control, balanced input tubes, and balanced output tubes.
The output tubes are Beam Power tubes that are Triode Wired.
I have a thread in diyAudio Tubes/Valves that tells about my [low power] amplifier.
. . . Next, is to try the same circuit, except to connect the screens from a series resistor to the Ultra Linear Taps (Balanced Ultra Linear Amplifier).
The power will increase, the distortion will increase, and the damping factor will be lower.
Tradeoffs. Oh my!
1. Push Pull and global negative feedback:
Output Transformer secondary connections:
Ground the 4 Ohm tap.
Use Balanced negative feedback feedback from the Common Tap and the 16 Ohm Tap.
Bring the negative feedback to the push pull output cathodes, or to the push pull driver cathodes.
Pay attention to the proper connections, or you will have positive feedback instead of negative feedback.
Push Pull output stage
Push Pull negative feedback
If you want to use global negative feedback on a push pull amplifier, that seems reasonable to me.
As always, if your output transformer is saturating on high power low frequency signals . . . Global negative feedback makes the saturation Worse!
2. A different design approach:
I do not use global negative feedback at all.
My latest amplifiers use XLR balanced input, balanced volume control, balanced input tubes, and balanced output tubes.
The output tubes are Beam Power tubes that are Triode Wired.
I have a thread in diyAudio Tubes/Valves that tells about my [low power] amplifier.
. . . Next, is to try the same circuit, except to connect the screens from a series resistor to the Ultra Linear Taps (Balanced Ultra Linear Amplifier).
The power will increase, the distortion will increase, and the damping factor will be lower.
Tradeoffs. Oh my!
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