No global is quite a different thing from none...there is quite a bit of discussion around here about various implementations of local. It is quite effective.
I too have a handful of 'no global nfb' amps. One runs cathode FB thanks to some Chicago BO-6 outputs, and there is a set of BO-14's waiting a home, plus a few E-Linear amps, and a 'plain' Schade-ed 26HU5 amp.
cheers,
Douglas
many thanks for the reply!
Indeed unbypassed. I agree with all of your point. My idea is to setup a test circuit and try different values for the cathode resistor, and measure actual distortion and output impedance. Was just hoping some good soul would do a simulation to compare different values for the cathode resistor.
As to the speaker, I would not dare using it with some complex loudspeaker designed to be fed by a high damping factor amp. My load does not have any xover, just plain bass loudspeaker Dipole Line array with GRS Slim 8" and Visaton WS25e
No need to, a pentode all by itself is a current source by definition.
Just don´t apply NFB.
They are pretty poor current sources, at least compared to what is a trivial task up to about 60 mA with a DN3545 and a 10M45.
If you want something that is high in output Z, just add the current FB. That can make a 2A3 look like an open loop pentode...🙂
cheers,
Douglas
It is commonplace to get a SE 2A3 to between 2 and 3 Ohms output impedance. A pentode amp( PP or SE, it makes nooooo difference), with its gain reduced with current FB can easily go to 2 orders of magnitude past that...likely further. 30 dB should be quite possible...somebody do the math...LOL
cheers,
Douglas
as Douglas pointed out, I am indeed looking for Zout >> Z load. Don't know if one can hit the 200k mark though.
Here is another thread, Smoking Amp comes with the idea of crosscoupling to raise the impedance further, but I need pictures to understand it 🙂 He also mentions the larger unbypassed cathode resistor (Current feedback).
Simplest path for a current drive amplifier.
Tomorrow I will be testing a PP stage with the 6E5P tetrode and the 6P15P pentode I am very curious how the frequency response will look like on the OPTs secondary.
Back to Reality?
A 5k output transformer might have 20 Henry primary inductance.
At 40 Hz, that primary inductive reactance is 5,027 Ohms.
You have to have some form of feedback in order to make that amplifier's output be a current source at 40 Hz.
That is true, even if the effective output tube's plate resistance, rp, is MegOhms.
Just my opinion.
A 5k output transformer might have 20 Henry primary inductance.
At 40 Hz, that primary inductive reactance is 5,027 Ohms.
You have to have some form of feedback in order to make that amplifier's output be a current source at 40 Hz.
That is true, even if the effective output tube's plate resistance, rp, is MegOhms.
Just my opinion.
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6A3sUMMER, I recall the Gordon Rankin's article (in Sound Practices?) on 10W 807 SE amp, where he ended up using 20-pound OPTs to get a decent primary L/DCR ratio, and he still got to the conclusion that the system would benefit from the additional separate LF amp.
Well...reality is sometimes over-rated... 🙂 Seems that just like voltage nfb can increase bandwidth up to the limits of the components, current FB will have limitations as well.
Like getting flat response from sugar-cube sized OPT's...response at power gets flatter and flatter, wider and wider, but power level of that response has to go down.
Good point...
cheers,
Douglas
Like getting flat response from sugar-cube sized OPT's...response at power gets flatter and flatter, wider and wider, but power level of that response has to go down.
Good point...
cheers,
Douglas
With a 5k primary and 20 Henry, and an 8 Ohm tap:
(5k:8 Ohm) = 625
20H/625 = 0.032 Henry secondary
0.032 Henry @ 40Hz = 8.04 Ohms of inductive reactance, not a very good current source.
If your turn the amplifier off, and remove the output tube(s), the loudspeaker "looking back at the output transformer' will see 8.04 Ohms of inductive reactance.
As you can see, you must have amplifier circuitry that overcomes this 8.04 Ohm secondary inductive reactance, in order to make the amplifier look like a current source.
Extremely high plate impedance rp, by itself, will not make the amplifier be a current source.
Oh . . . so to improve that, you decide to use a 5k:8 Ohm output transformer, that has increased primary inductance of 100 Henry.
Well, the secondary is going to be 5 X 8.04 Ohms inductive reactance (5 x 8.04 = 40.2 Ohms, still a very bad current source).
This post is about making a current source amplifier.
It is not about bandwidth.
Solve the current source issue first, and then figure out a way to get bandwidth.
Just my opinion.
(5k:8 Ohm) = 625
20H/625 = 0.032 Henry secondary
0.032 Henry @ 40Hz = 8.04 Ohms of inductive reactance, not a very good current source.
If your turn the amplifier off, and remove the output tube(s), the loudspeaker "looking back at the output transformer' will see 8.04 Ohms of inductive reactance.
As you can see, you must have amplifier circuitry that overcomes this 8.04 Ohm secondary inductive reactance, in order to make the amplifier look like a current source.
Extremely high plate impedance rp, by itself, will not make the amplifier be a current source.
Oh . . . so to improve that, you decide to use a 5k:8 Ohm output transformer, that has increased primary inductance of 100 Henry.
Well, the secondary is going to be 5 X 8.04 Ohms inductive reactance (5 x 8.04 = 40.2 Ohms, still a very bad current source).
This post is about making a current source amplifier.
It is not about bandwidth.
Solve the current source issue first, and then figure out a way to get bandwidth.
Just my opinion.
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thanks for the explanation. I read other threads where indeed people comment that calculating the output impedance of a pentode stage is done by dividing the Rp through the impedance ratio¨, and the secondary inductance is not mentioned. Maybe because they do not go to the dumb idea to willingly increase output impedance 🙂, and that under "normal pentode operation" with voltage feedback etc the inductances do not play a role. It is late over here... go to bed, tomorrow in the lab
6A3, so is it your point that exceeding the impedance of the secondary is not possible?
cheers,
Douglas
cheers,
Douglas
Bandersnatch,
There are many kinds of feedback topologies that can change the output impedance of an amplifier . . . regardless of the output tube's rp; and regardless of the output transformers primary inductance; and regardless of the output transformers secondary inductance.
1. Negative Feedback around the output transformer can reduce the output impedance.
The transformer primary and secondary inductances are constant, but the resultant output impedance is reduced.
2. Negative resistance output circuits . . . increases the current into the load, when the load impedance goes up.
Likewise, Negative resistance output circuits . . . decreases the current into the load, when the load impedance goes down. (The opposite of Ohms Law).
The negative resistance effect was very successfully used in the Variable Resolution Filter Amplifiers of the Tektronix 2782 and 2784 Flagship Spectrum Analyzers.
3. There are a few threads on the Tubes / Valves portions of DiyAudio, that discuss current feedback. Sounds to me, like a successful way to overcome the primary inductance, and the secondary inductance of an output transformer, and make the amplifier into a current source.
How well a design of #1, #2, or #3 is accomplished, will determine the success or failure of an amplifier that has to work into a wide range of loudspeakers that have large values of varying resistance, inductive reactance, capacitive reactance, versus frequency; as well as all the reactances of of an output transformer (such as primary and secondary inductances, distributed capacitances, and leakage inductive reactance).
#1, global negative feedback above is the form of feedback that is more widely understood and successfully used in more amplifiers.
#2, and #3 above are generally more tricky.
The failure of proper design of any of these circuits has been known to do a "Presto Chango", just like a magic wand, and instantly change a power amplifier into a power oscillator.
Good luck in producing a true current source power amplifier.
There are many kinds of feedback topologies that can change the output impedance of an amplifier . . . regardless of the output tube's rp; and regardless of the output transformers primary inductance; and regardless of the output transformers secondary inductance.
1. Negative Feedback around the output transformer can reduce the output impedance.
The transformer primary and secondary inductances are constant, but the resultant output impedance is reduced.
2. Negative resistance output circuits . . . increases the current into the load, when the load impedance goes up.
Likewise, Negative resistance output circuits . . . decreases the current into the load, when the load impedance goes down. (The opposite of Ohms Law).
The negative resistance effect was very successfully used in the Variable Resolution Filter Amplifiers of the Tektronix 2782 and 2784 Flagship Spectrum Analyzers.
3. There are a few threads on the Tubes / Valves portions of DiyAudio, that discuss current feedback. Sounds to me, like a successful way to overcome the primary inductance, and the secondary inductance of an output transformer, and make the amplifier into a current source.
How well a design of #1, #2, or #3 is accomplished, will determine the success or failure of an amplifier that has to work into a wide range of loudspeakers that have large values of varying resistance, inductive reactance, capacitive reactance, versus frequency; as well as all the reactances of of an output transformer (such as primary and secondary inductances, distributed capacitances, and leakage inductive reactance).
#1, global negative feedback above is the form of feedback that is more widely understood and successfully used in more amplifiers.
#2, and #3 above are generally more tricky.
The failure of proper design of any of these circuits has been known to do a "Presto Chango", just like a magic wand, and instantly change a power amplifier into a power oscillator.
Good luck in producing a true current source power amplifier.
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Maybe a good way to think about it is that the output transformer's magnetizing ("primary") inductance and its lumped-equivalent capacitance are in parallel with the output valve. So they limit how big the output impedance can be without feedback being taken from downstream of the transformer.
Less important if the transformer is optimized for a single speaker driver, hopefully the case because speaker-level crossovers and high impedance sources don't play well together.
All good fortune,
Chris
well then 6A3...that was not an answer. So, if you'd be so kind( I know you're tired after writing that long non-answer that is in part incorrect ), can you take a shot at answering please?
Also, global NFB can make the output impedance go up or down. It depends on how the voltage that is fed back tot he front gets derived.
cheers,
Douglas
Also, global NFB can make the output impedance go up or down. It depends on how the voltage that is fed back tot he front gets derived.
cheers,
Douglas
Gain reducing NFB around the OPT can either raise or lower output impedance.
cheers,
Douglas
Bandersnatch,
Your question:
"so is it your point that exceeding the impedance of the secondary is not possible?"
The output impedance of the secondary, entirely by and entireley of itself, can not be changed without modifying the transformer in some way or other.
But the output impedance of the secondary that is dependent on all the other factors of the amplifier (such as rp across the primary; global negative feedback: voltage dependent, current dependent; etc.), that total output impedance can be equal to, lower than, or higher than just the bare-bones output impedance of the secondary by itself.
I hope that was a more accurate answer to your how your question was stated.
Again, just my opinions.
And I am sorry that I only answered that Global Negative feedback lowers the output impedance, when it is Voltage sensed.
I incorrectly left out the fact that Global Negative feedback increases the output impedance, when it is current sensed.
Now, I hope we have brought up all the facts so that they are 'current' (pun intended).
Your question:
"so is it your point that exceeding the impedance of the secondary is not possible?"
The output impedance of the secondary, entirely by and entireley of itself, can not be changed without modifying the transformer in some way or other.
But the output impedance of the secondary that is dependent on all the other factors of the amplifier (such as rp across the primary; global negative feedback: voltage dependent, current dependent; etc.), that total output impedance can be equal to, lower than, or higher than just the bare-bones output impedance of the secondary by itself.
I hope that was a more accurate answer to your how your question was stated.
Again, just my opinions.
And I am sorry that I only answered that Global Negative feedback lowers the output impedance, when it is Voltage sensed.
I incorrectly left out the fact that Global Negative feedback increases the output impedance, when it is current sensed.
Now, I hope we have brought up all the facts so that they are 'current' (pun intended).
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