By curiosity I have put a 100nF (instead of 470nf), with same R=150k and I have not found any minimum change, not in up nor in down, in the measured frequency response (both at -0.7dB @ 20Hz at maximum GNFB).
Hence it is the model which is not correct.
Hence it is the model which is not correct.
These amps were designed in the days of three music sources, radio, records and magnetic tape. The original music storage medium in all of these was usually the vinyl record. The vinyl recording / playback system evolved over the years, as did the equalization curves. In most cases frequencies below 50 Hz were attenuated in the recording and record cutting process to avoid over excursion in the record cutting head.
Playback systems usually didn't extend far below 50 Hz since there was limited music information below 50 Hz. Many "high end" systems included a switchable "rumble" filter to further cut the low end to eliminate low frequency sounds generated in the turntable itself.
Today we have the ability to record a 10 Hz or lower frequency, put it onto a CD or other digital storage system and play it back, all with a simple PC.
The "basser" kids that lived across the street from me in Florida had a "test CD" with test tones down into the 10 to 20 Hz region. One of them had a "system" in his car so powerful that a cranked test tone in the low bass region would kill his engine. 80's Ford products had an inertia switch in the trunk to kill the fuel pump in a crash event. Where do kids put their subwoofers?
Do we need, or want these subaudible frequencies in our modern design tube amps? In most cases, no especially if the OPT's are not designed to pass those frequencies at full power. Attempting to do so drives the tube current into the red(plate) zone and blows stuff up.
Now when you have OPT's (marked 400W @ 20 Hz) and tubes (36LW6...6 per CH) like these.........
I'll settle for 500 WPC at 30 Hz. There are two RC poles in the audio path. Each is set at 10 Hz (.1uF - 1 megohm) for now. Breadboard level testing is underway.
Attachments
"Each is set at 10 Hz (.1uF - 1 megohm)"
It is 1.5Hz, why 10Hz?
By the way mine is now 10Hz (150K and .1uF)
It is 1.5Hz, why 10Hz?
By the way mine is now 10Hz (150K and .1uF)
1.59 Hz to be more precise.....why, because I was trying to think first thing in the morning before I am awake.
It should have become obvious when my own measurements show the lower 3 dB point of the entire amp including a tiny Edcor matching transformer used as a phase inverter to be 11 Hz at 100 watts output.
I took a prototype SE amplifier that uses BIG TV sweep (line output) tubes and drove the two channels out of phase using the little Edcor transformer as a PI. I wired the big OPT across the two plates and connected it all to a BIG power supply. Over 300 watts occurred before a resistor in the screen grid regulator went open.
The test is seen in posts #21 and #25 here:
If UNSET and the RCA50W Had a Baby
don't know if its the right place to ask..
By mistake for my mod for my OTL HP Amp, I bought some 0.22uf Miflex KPAL-01 instead of *KPCU-01
now I can't return em cause I already soldered them without checking.
But they perform pretty well, but I heard good stuff about *KPCU-01 only!
Now I'm thinking what if I add some *KPCU-01 say about 0.022/0.047 in parallel with KPAL-01 to get the copper foil oil flavour?
Is this is a good/bad idea?
By mistake for my mod for my OTL HP Amp, I bought some 0.22uf Miflex KPAL-01 instead of *KPCU-01
now I can't return em cause I already soldered them without checking.
But they perform pretty well, but I heard good stuff about *KPCU-01 only!
Now I'm thinking what if I add some *KPCU-01 say about 0.022/0.047 in parallel with KPAL-01 to get the copper foil oil flavour?
Is this is a good/bad idea?
It seems that feedback theory is beyond most people on DIYA? I wouldn't call myself an expert, but I can see that rayma is the only one in this thread that understands this problem. For a directly coupled amp, we normally don't have to worry about phase margin at low frequencies, but most tube amps require a transformer which will be the dominant pole at low frequencies so adding a similar pole with a small coupling cap makes the amp unstable and/or causes peeking. A larger coupling capacitor does not cause much phase shift so that the feedback remains negative. A smaller cap shifts the feedback phase towards positive feedback territory.
This 1958 Radio Electronics essay by Norman Crowhurst may help to explain further.
Especially see figure 3, and the related text.
Especially see figure 3, and the related text.
Back to the OP's original question: Yes, under some circumstances, decreasing a coupling capacitor value can increase the bass response at the extreme low end of the frequency range and vice versa. This is counterintuitive, but the effect is real and measurable.
Shannon Parks, the designer of the Poseidon driver for the Dynaco Mark III amplifier, addressed a question from one of his customers on coupling capacitor sizing in a discussion on the diytube forum; see Shannon's last comment in the thread. Note that too large a coupling capacitor can cause low frequency instability.
http://www.diytube.com/phpBB2/viewtopic.php?f=2&t=5255
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In my MA-1 design, I found a LF instability that caused the woofers to breathe as George describes... In my case, I solved it by making a 220nF/510k into 1µF/510k which was also counter intuitive given the LF.
A smaller cap doesn't always make for more stability and a larger cap doesn't always improve things. It's really the entire design that will determine what caps are needed for coupling.
A smaller cap doesn't always make for more stability and a larger cap doesn't always improve things. It's really the entire design that will determine what caps are needed for coupling.
In respect to the side issue of desired frequency response I think it is good to keep in mind that for a power amp the output transformer will have a limit to the low frequencies it can pass effectively and there is no point in exciting it with frequencies it can't handle. In fact to keep the rest of the spectrum clean it is a really good idea to avoid passing those frequencies. So even though in my world 15Hz response matters it is unlikely that any of us will have trannies that can do that. Even a really good tranny is going to run out of steam below 30Hz and a lot below 40Hz.
I feel like it is better to not stress the amp and fill in the last 8va or 8va and a half with a sub. JMHO.
I feel like it is better to not stress the amp and fill in the last 8va or 8va and a half with a sub. JMHO.
That's why when I use a turntable, no matter how good they are, rumble-wise, or even simple record changers, I add in Rod Elliot's sensational Sallen-Key fiter right after the RIAA preamp.
It's amazing how it literally removes any rumble, and even the built-in subsonic noise floor of the vinyl.
It even eliminates any speaker pumping and feedback to the turntable at high volumes.
And all without any noticeable effects to what you hear from the record.
In fact, the elimination of amp-robbing power for such things as warped records, speaker pumping, allows more power to be used for music, and cooler-running amps.
I build my S-K filters to start cutting off around 25Hz (38dB/octave), because anything below that is mainly noise anyhow.
Rod's chart gives additional cut-off points.
Broskie shows how this can work in this blog post a little more than half way down:
"On the other hand, we can create a bass boost by selecting the right capacitor values, as shown below." "These smaller than expected capacitor values result in a bass boost with low impedance headphones, but no boost with high impedance drivers."
https://www.tubecad.com/2019/03/blog0458.htm
"On the other hand, we can create a bass boost by selecting the right capacitor values, as shown below." "These smaller than expected capacitor values result in a bass boost with low impedance headphones, but no boost with high impedance drivers."
https://www.tubecad.com/2019/03/blog0458.htm
Morgan Jones had a whole chapter or so in one of his books devoted to "socking the dominant pole" or something like that. I haven't read it in years, but it is valuable information for designing an amp that uses GNFB that includes the OPT. Basically the OPT is a LF pole and one must avoid another pole in the design that is at a similar frequency. Here the phase shift will be large enough to make some of the feedback positive in the low frequency region. This can cause a bass boost in some situations, and total instability in others.
The common FB Pair is almost bullet proof. I've run more than 20 fb NFB with no problems.
Can the OP pls post the circuit. lets have a look. 🙂
Can the OP pls post the circuit. lets have a look. 🙂