1) Provide a load on the rectifier such that it switches on and current flows, in one direction, out of the transformer and into the filter.
2) Store up the charge that comes out of the transformer and deliver it to the load even if the rectifier is not conducting.
3) Provide a low source impedance to the active circuitry at all frequencies from DC to the ultrasonic.
4) Reject hum and noise and keep it from getting through to the power supply output.
https://www.aikenamps.com/index.php/chokes-explained How to select a choke:
Chokes are typically rated in terms of max DC current, DC resistance, inductance, and a voltage rating, which is the max safe voltage that can be applied between the coil and the frame (which is usually grounded).
<hidetext Читать дальше> DC current
If you are using a choke-input filter (not likely, unless you are trying to convert a class AB amp to true class A and need the lower voltage, or if you are designing an amp from scratch and want better supply regulation), the choke must be capable of handling the entire current of the output tubes as well as the preamp section. Note that this doesn't mean just the bias current of the output tubes, but the peak current at full output. This usually requires a choke about the size of a standard 30W-50W output transformer, since the choke must have an air gap (just like a single-ended OT) to avoid core saturation due to the offset DC current flowing through it, and the choke also must have a low DC resistance, to avoid dropping too much voltage across it, which will lower the output voltage and worsen the load regulation. This combination of low DCR, air gap, and high inductance (more on that later…) usually results in a substantial sized choke. To calculate the required current rating, add up the full power output tube plate currents, screen currents, and the preamp supply currents, and add in a factor for margin. For a 50W amp, this may be 250mA or so. If, on the other hand, you are selecting a choke for a capacitor input supply (such as the typical Marshall or Fender design), then the requirements are relaxed quite a bit. The purpose of the choke in these type supplies is not for filtering and voltage regulation, but just for filtering the DC supply to the screen grids of the output tubes and the preamp section. The screens typically take around 5-10mA each, and the preamp tubes draw about 1-2mA or so (for the typical 12AX7; 12AT7's are usually biased for around ten times that). This means that you can get by with a much smaller choke, and, in addition, the preamp supply current doesn't vary that much, so you can get by with a higher DC resistance, which means smaller wire can be used to wind the choke, which means higher inductance for a given size core. Just add up the current requirements of the screens and preamp tubes, and add a bit more for margin. For a 50W amp, a typical value might be 50-60mA.
For a typical choke input supply, you need a choke with no more than 100-200 ohms or so DCR. A capacitor input supply typically might use a choke with a 250 ohm - 1K DCR. The higher the resistance, the more voltage drop and the poorer the regulation, but the cost will be lower.
As for the inductance value, this depends on how much filtering you want. The inductance, in conjunction with the filter capacitance, forms a lowpass filter. The larger the inductor, the lower the cutoff frequency of the filter, and the better the rejection of the 120Hz (if full wave rectified) or 60Hz (if half wave rectified) AC component of the rectified DC. In general, the larger the better, within reason (larger inductances at low DC resistances mean larger chokes, which cost more money). Typically, 5-20 Henries is a good choice with the standard 32-50uF electrolytic capacitors. The inductance and capacitance values also determine the transient response of the supply, which means the tendency for the supply to overshoot or «ring» with damped oscillations whenever a current transient is applied (such as at startup or on a heavy current surge, such as a hard «E» chord at full power!).
The voltage rating must be higher than the supply voltage, or the insulation on the wire may break down, shorting the supply to the frame.
I highly suggest going to Duncan Munro's website (http://www.duncanamps.com/ ) to download his power supply calculator program. It will allow you to experiment with different inductance and capacitance values and see the resulting residual AC ripple and transient response of the supply filter. Both capacitor input and inductor input filters can be simulated. It is a great educational tool.
http://education.lenardaudio.com/en/14_valve_amps_6.html Choke input filter
The primary limitations of early valve rectifier power supplies was the high forward resistance in the rectifier valves and the small storage capacity of the Electro's (approx 20uF). 470uF Electro's are often used in high quality valve amps today.
Most valve amps above 40 Watt are configured in Class AB (Negative bias). The quiescent current through each output valve is approx 50mA. When the amp is driven to full power the current can increase to approx 150mA. The B+ Voltage from a traditional Capacitor input supply will readily drop to a lower level. The B+ Voltage will be modulated by the varying current through the output valves. As the B+ Voltage reduces, so does the gain of the output valves. A modulating B+ Voltage causes massive inter-modulation distortion.
Choke input filter power supplies, also referred to as Swinging choke power supplies, are often seen in industrial electronics where a large amount of DC current is required with high regulation. Choke input filter supplies were often used in early valve rectifier power supplies for 100+ Watt valve amps. The Swinging choke is nearly as large as the power tranny. Two extra large valve rectifiers were often used. Each valve rectifier has the Anodes in parallel to reduce its internal resistance, enabling greater current.
Swinging choke power suppy
The pulsating DC from the rectifier valve goes through a large Choke (non saturable Inductor). The Swinging choke converts the pulsating DC to its RMS value without loosing energy. The internal losses in the 2 valve rectifiers is minimal. AC to DC conversion is close to the academic formula.
AC Volts x 0.9 = DC Volts
640V AC x 0.9 = 576V DC
I went to a reliable source (Morgan Jone's book Valve Amplifiers) and looked up some choke-input supply info. He notes that in the «old days» the solution was to use a swinging choke. You have a very tough time finding those these days, so that's probably out of the question.
He gives a detailed formula for the minimum current draw needed, but then goes on to note an approximation for 50/60Hz power supplies. It's Current min. (in milliamps) = VIn (RMS) / L (in henries). So for your 500vac transformer (which is RMS volts), you'd divide that 500v by the inductane rating of the choke you intend to use to see what the minimum current draw must be. As an example, for a 20H choke, 500v/20H = 25 milliamps of minimum current draw. The implication is you want a very high inductance rating for your choke.
He also notes that it would seem the choke only needs to be rated for the maximum d.c. load current, but that the choke really needs to be rated in excess of this. Why? The choke generates a magnetic flux in the core proportionate to the size of the current passing through it, and if that flux is too great, it saturates the core and the inductance of the choke falls to zero. Bam! You're back at 700v.
The current that the choke will support is the max d.c. load current and the instantaneous a.c. charging current (meaning the current used to charge the filter caps). So that's
IDC + IAC = Itotal peak current.
He derives a formula for the a.c. peak current. For a 60Hz wall outlet, it's
IAC (positive peak) = VIn (RMS) / [1386 * L)
Vin is the RMS transformer voltage again, and L is choke inductance in Henries.
С сайта Вестерн Электрик
It is evident from the regulation characteristics of Figures 4 and 5 that for a given output current and voltage, the choke-input filter requires a somewhat higher alternating voltage applied to the plates of the tube than the capacitor-input filter. With the capacitor input filter, however, the normal charging and discharging of the capacitor each half cycle requires the tube to supply relatively large peaks of current during each charging period. The peak current increases in value as the capacitance of the capacitor is increased and may be much larger than the average rectified output current, though its duration in such cases is only a short fraction of a cycle. Since for good tube performance, the anode current must be considerably less at every part of the cycle than the total emission current from the filament, the maximum permissible output current must be limited to such a value that this condition is satisfied. The permissible output current may be larger, therefore, for a choke-input filter, in which the peak anode current is only slightly larger than the output current. The choke-input filter also gives much better regulation than the capacitor-input circuit. The choke-input filter, therefore, should always be selected in preference to the capacitor-input filter wherever possible. With a capacitor-input filter, the capacitance of the input capacitor should preferably not exceed 4 microfarads.
Опубликовано: 4 мая 2008
Ден 123 писал: Привет. У меня SE 6С4С. 4 отдельных выпрямителя, 2 обмотки на каждый канал своя. Ставил Г фильтры на драйверы особой разницы не заметил. С Г фильтром на выходных лампах драйва добавилось, усилитель стал более "ритмичным", детальность возросла, но в целом ощущение, что картинка "развалилась". Вернул СЛС обратно. Причем с расстройства воткнул Мбгч 20 мкф вместо электролитов. Стало заметно "свежее", немного посравнивал с электролитами 100х450. Оставил МБГЧ. Отсюда у меня вопрос не в тему ветки ко всем: может быстрые конденсаторы надо ставить сразу за выпрямителем, а не только шунтировать ими выходные кондесаторы фильтра? Доброго времни суток, Уважаемые)
В моём случае с Г-фильтром в преде стало ЗНАЧИТЕЛЬНО лучше, при том же др. и тех же фильтрующих емкостях.
«Быстрые конденсаторы» НАДО ставить сразу за выпрямителем (0.1-0.8uF), можно и без них, но оч точно придётся подгонять L, у Сергея Рубцова это наглядно описано.
Сам за несколько ночей перепробовал кучку разных типов и номиналов. Тут безусловно всё индивидуально, но на звук оказывают влияние весьма и весьма существенное, полностью с Сергеем согласен.
Александр, к Вам вопрос как к автору генеального нововведения в аудио БП ака «синхронный выпрямитель» отчего же ему Г-фильтр контрадишн? Мои скромные эксперименты показывают что скорей Г-фильтр есть отличное дополнение и как раз 3его зайца-то и убиваем.
Сейчас так и слушаю в преде, звучит просто потрясающе)) В каждом канале ДППВ на ульрафастах + синхронник на двух 19ах, накопительные по 8uF полипропилен в масле + 0.1uF ССГ-2 дальше ФТ-3 0.47uF + 0.1uF ССГ-2 - 7.5Hn - 6600uF Rifa + шунты-шмунты.
И это притом что дроссели мягко говоря не то - 150 Ohm ! Буду менять на 30-40 Ohm. И ещё не пробовал ставить/подбирать R-слива.