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A Tube amp without coupling capacitors? Possible?

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This is my project in which actually I'm playing on.
 

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A simple no coupling cap design: Tim Mellow OTL www.mellowacoustics.com/articles/Tim_Mellow_25w_OTL_Tube_amplifier.pdf Take out the 0.1uF coupling C3 and C4 between 1st and 2nd stage Ground C1 and feed the input directly into R2 thereby eliminating C1 and C2 Increases Output impedance and reduces nfb and I am not shure whether I want to feed the audio signal through a 4.7MOhm resistor but nonetheless maybe a starting point ... And it comes with a free DC sevo.
 
Lower bandwidth than a capacitor.
Nope. Why do you always comment on transformers if your experience and knowledge on this subject are quite limited at very least?
There are (commercial) interstage transformers that can reach 1 MHz bandwidth.

I can accept you don't like transformers, that's your problem, but please stop sharing wrong information.
 
Wavebourn stated the true problem. If the coupling cap sees a non constant time varying load when driven with real music, it can and will misbehave. This is why different caps will react differently.

In the TSE-II and the UNSET designs the coupling caps see a constant load of 1 MEG OHM in parallel with a few pF of gate capacitance. The fets that follow the coupling cap are chosen and biased such that that capacitance remains constant through the range of signal seen in the amplifier's use.

Frequency response of either design stretches from about 10 Hz to hundreds of KHz except for the OPT which is by far the most dominant pole in the amp, and there is no feedback applied from the OPT's output.

The TSE-II, like the original TSE is a no-feedback design while the UNSET uses local feedback loops around each pentode to emulate triodes. No global feedback is applied.

Enclosing a capacitor inside a feedback loop is another way to bring out it's worst qualities, since correction signals through the cap can be nonlinear.

"Every stick has 2 ends". Yes, feedback around a coupling capacitor increases sharpness of dynamic distortions, but decreases their impact, making recovery faster and percentage of distortions lower. The essence of design of electronics equipment is optimization. There are no "absolutely bad" or "absolutely good" solutions, everything has to be carefully weighted, except of course obvious errors.

And one of ways to kill several ducks by a single shot is to enclose a coupling cap before a follower, instead of before an output tube's grid that guarantees to work with non-linear grid current, especially if it is a triode.

Here is what I use to drive grids of tubes in my high power amps:

A2 driving boards (for tube amps)
 
H

200KHz is the bandwidth of most of the BEST fully DC coupled solid state amps...

My Pass F5 (+ 2SK175/2SJ55 ) goes to at least 5 Megaherz. My Hiraga Le Monstre goes north of 500 kHz. I had a design by Kaneda that open loop went beyond 200 kHz.
But there are good reasons to limit the BW intentionally, specifically when in commercial equipment - such as preventing interference and following FCC requirements. :p
 
But there are good reasons to limit the BW intentionally, specifically when in commercial equipment - such as preventing interference and following FCC requirements. :p

Indeed. Another reason: In the late 1940's and early 1950's, when very good recordings (by the standards of the day) became available, it was noticed that amplifiers with bandwidths well over 20 kHz - 30kHz or so often sounded better than amplifiers that had bandwidths less than 20 kHz, even though it was well established that almost all adults cannot hear past about 15 kHz. Even older adults whose ears couldn't go beyond 8 or 9 kHz noticed the difference.

It was one of the origins for the urban myth that measurements don't count for much. Measurements do count if you do the right measurements.

What was not appreciated back then was that typical amplifiers of the day had a rise in intermodulation distortion somewhat before the bandwidth limit. So, if in the music you had 2 tones, say 19 kHz and 20 kHz, both of which you can't hear, but the amplifier will beat them together and produce the sum and difference - and 1 kHz you certainly can hear - so you had distortion of a more nasty kind. This is related to the problem known in solid state circles as slew rate distortion.

By extending the amplifier bandwidth, the onset of noticeable intermodulation was also pushed up - past the point where any frequencies in the music source existed.

The best way to stop this kind of distortion is to put a passive 20 kHz low pass filter in the input to the amplifier - which fixes RFI problems as well.

The vinyl record recording and reproduction process produces quite a bit of 2nd harmonic off the higher audio frequencies. Say the recording (intentionally) has 2 frequencies 18 kHz and 19 KHz. The recording & replay process will generate 36 and 38 kHz as well at the input to the amplifier. You certainly cannot hear that, and in a very wide bandwidth amplifier all is well. But if the amp is struggling to reach about 40 kHz, it will produce the difference - 2 kHz. Again, you can certainly hear that - as distortion. But any way of limiting the bandwidth, preferably before the power amp, and there's no problem.

To guarantee stability with a wide range of loudspeakers, it is common to put an LR filter in the amplifier output. Since this is passive and outside the feedback loop it is easy to design it so it doesn't introduce distortion - but is does intentionally limit the bandwidth.
 
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" "Originally Posted by DF96: Lower bandwidth than a capacitor."
Nope. Why do you always comment on transformers if your experience and knowledge on this subject are quite limited at very least? There are (commercial) interstage transformers that can reach 1 MHz bandwidth."

You may have a point that interstage transformers can have quite a high bandwidth, but the figure you quoted does not invalidate DF96's statement. The capacitor assuredly does have a greater bandwidth than a mere 1 MHz.
 
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My Pass F5 (+ 2SK175/2SJ55 ) goes to at least 5 Megaherz. My Hiraga Le Monstre goes north of 500 kHz. I had a design by Kaneda that open loop went beyond 200 kHz.
But there are good reasons to limit the BW intentionally, specifically when in commercial equipment - such as preventing interference and following FCC requirements. :p

Yes there are good reasons to limit the bandwidth. In a transformer coupled amplifier the interstage transformer is the one you don't want to set limits of any kind. My point was simply that good transformers do not set that limit by default. It was true in the remote past but not anymore since quite a few years. Because many people don't know this and/or don't use them they still believe it's like at the old times. That is like metropolitan legend now....

The capacitor assuredly does have a greater bandwidth than a mere 1 MHz.

Which is pretty useless point.
 
45 said:
I can accept you don't like transformers, that's your problem, but please stop sharing wrong information.
Audio transformers reaching 1MHz bandwidth will be rare. Coupling capacitors failing to reach 1MHz bandwidth would be much much rarer. So what is "wrong information" about saying that transformers have narrower bandwidth than coupling caps?

I realise that some people are transformer fans, and I realise that most people use transformers where a transformer is the best component for the job. I just don't want newbies picking up the "wrong information" that transformers are somehow inherently better than coupling caps.
 
Audio transformers reaching 1MHz bandwidth will be rare. Coupling capacitors failing to reach 1MHz bandwidth would be much much rarer.
You are wrong again because many DIYers now know how to make them! And they are cheaper to make than high quality capacitors.....
So what is "wrong information" about saying that transformers have narrower bandwidth than coupling caps?
It is wrong because cheap or poor transformers are like poor or cheap capacitors. Not all capacitors are good. Actually most are crap. So your point doesn't stand because you want to pass it as a general rule which is not the case really.

I realise that some people are transformer fans, and I realise that most people use transformers where a transformer is the best component for the job. I just don't want newbies picking up the "wrong information" that transformers are somehow inherently better than coupling caps.

I am not a transformer fan. I just use the best solution for what I am doing. For power triodes capacitor coupling is the worse no matter how ideal is the capacitor. Suggestions like driving a 300B with RC coupling are not good and far from ideal. Telling that it is the best because the capacitors are more ideal is wrong information. The 300B is certainly not a rare device.....plenty of DIY stuff and COMMERCIAL products. If you can't afford it or don't like it doesn't matter. This is not a place where people have to follow the mass and often the economical factor is of secondary importance....if I want to do that I just go to the shop and look at what's on the shelves.
 
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@45, mind showing plans of how to build your 1mhz audio transformers? this is iy after all and members share....

I would like to see the plans too.

I worked several years for a transformer manufacturer. I was their chief engineer. Now something a budding engineer learns in the first few weeks of his training, or sooner, is that no matter how much money or expertise you throw at it, the best possible transformer is always extremely poor compared to any 10 cent capacitor.

Sure, I and the people I worked for knew how to make a transformer that could go past 1 MHz. But not down to bass frequencies in the same transformer. Or not something the size of a shoebox and with a price to match. Or one that is considerably lossy and with considerable (and I do mean considerable) harmonic and intermod distortion.

It's pretty much a case of pick any two that you want (high frequencies, low frequencies, low size and cost, and low loss & distortion) and forgo the other two.

Interstage transformers were common when tubes cost a month's wages each and had very little voltage gain. And back then nobody cared much about rotten frequency response and really bad distortion - because then electronics was new, novel, and they had nothing to compare it with. Just that it worked was pretty good! But they did have tubes with low anode resistance and high grid resistance, so you could use a step-up transformer and get the gain that way. As soon as tubes with decent gain and a decent price became available, capacitive coupling was used and transformers dropped like hot coals.

Coupling transformers persisted for a while in high power professional amplifiers dues to the use of Class B output stages, which draw grid current. Suitable driver tubes where not available, so here a step-down transformer was used to get a low driving impedance. But Class B is not high quality, unless the output is kilowatts, spending a man's month's wages on a transformer could be justified, and the bias is checked ever day, as was done in AM broadcast transmitter modulators.

Best YOU stick to what you know, 45.
 
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Transformers load like a choke the tube which is optimal to maximize power and decouple the tube. So this is the advantage of IT and there is nothing wrong with them.

There are professional amplifier manuals of 600 pages with 3 or 6 stages with IT and it works fine.

What I research about OT is very different, the last thing that you would want is a bi-filar. I think you are picking up rumors and then you want it.
 
45 said:
And they are cheaper to make than high quality capacitors.....
High quality capacitors are very cheap. It is expensive capacitors which are not cheap, but in some cases they are lower quality than cheap capacitors.

The 300B is certainly not a rare device.
Yes, I realise that there are 300B fans. So much so, that one manufacturer produced an amp with 300B in the name but no 300B in the circuit!
 
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