Pros & Cons of tube rectification in a preamp?
- Thread starter MadFloyd
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Hey MadFloyd,
Look forward to hearing from the designers if possible, as they will have excellent technical knowledge of the implications of this kind of PSU on sound. All I know is that the Zanden tube preamp is considered one of the all-time greats and has tube rectification which Yamada-San uses quite a lot. Big fan of his work.
Look forward to hearing from the designers if possible, as they will have excellent technical knowledge of the implications of this kind of PSU on sound. All I know is that the Zanden tube preamp is considered one of the all-time greats and has tube rectification which Yamada-San uses quite a lot. Big fan of his work.
Handwaving, simplified, and not my area of expertise (just played around with them) so take with block of salt.
Tube rectifiers provide "automatic" soft-start since they are thermionic devices that must heat up before they operate. That is probably their most oft-cited advantage. Their "knee" is much softer than typical SS rectifiers, so they tend to radiate less HF noise, though the filtering after the rectifier should kill all of that anyway. Tube rectifiers have voltage drop that is ~10's of volts instead of <1 V for SS diode, a significant drawback, but given the way tube circuits operate not a major one. That also means their regulation is much worse -- the output voltage varies much more with load current than a SS rectifier. They need high-voltage, natch, and in general cannot provide the same amount of current that SS diodes can support. They are also much larger and hotter, and may require a filament supply so are more complex to use. The current-limiting can be a plus, of course, by providing some current limiting, but OTOH they can arc and provide wideband nasty pulses onto the filter caps. Their lifetime is generally less than that of SS diodes. But they are easier to find for HV applications like tube preamps and amps -- HV SS devices are harder to find (not impossible, just less common). That said tube audio circuits tend to have B+ well below 1 kV.
As far as the audibility, that is essentially up to what comes after the rectifier, i.e. the amount of filtering and regulation in the rest of the power supply.
There are tube experts here who will have better and more complete answers. Mine is based upon work done years ago. I did use a tube "zener" in one preamp after I got tired of the zener diode string blowing up because the manufacturer ran them way too hot.
FWIWFM - Don
Tube rectifiers provide "automatic" soft-start since they are thermionic devices that must heat up before they operate. That is probably their most oft-cited advantage. Their "knee" is much softer than typical SS rectifiers, so they tend to radiate less HF noise, though the filtering after the rectifier should kill all of that anyway. Tube rectifiers have voltage drop that is ~10's of volts instead of <1 V for SS diode, a significant drawback, but given the way tube circuits operate not a major one. That also means their regulation is much worse -- the output voltage varies much more with load current than a SS rectifier. They need high-voltage, natch, and in general cannot provide the same amount of current that SS diodes can support. They are also much larger and hotter, and may require a filament supply so are more complex to use. The current-limiting can be a plus, of course, by providing some current limiting, but OTOH they can arc and provide wideband nasty pulses onto the filter caps. Their lifetime is generally less than that of SS diodes. But they are easier to find for HV applications like tube preamps and amps -- HV SS devices are harder to find (not impossible, just less common). That said tube audio circuits tend to have B+ well below 1 kV.
As far as the audibility, that is essentially up to what comes after the rectifier, i.e. the amount of filtering and regulation in the rest of the power supply.
There are tube experts here who will have better and more complete answers. Mine is based upon work done years ago. I did use a tube "zener" in one preamp after I got tired of the zener diode string blowing up because the manufacturer ran them way too hot.
FWIWFM - Don
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Hello MadFloyd and good evening to you sir. Of all of the combinations I have experienced through the years, I prefer tube. Reason being is that I can change it on a whim. Personally, I think this is an underrated aspect of a systems performance.
Tom
Tom
Cannot disagree more strongly. No amount of filtering and regulation can make a solid state diode sound like a tube.
There are simple reasons why SS and tube rectification are not easy to compare under equal settings. Most tube rectifiers tolerate a very modest amount of filtering capacitance connected directly. If a large capacitance bank is necessary there are two choices: it can be separated through either resistors or chokes. The resistor approach brings other complications such as a long time constant and voltage loss. The choke approach works perfectly, but the added cost and weight of a nice choke discourages most designers.
The very best sounding glass diodes use mercury vapour but unlike luminescent lights these have been condemned by the environmental lobby. Unlike many vacuum rectifiers these have a low and constant voltage drop of about 5v.
The future most probably belongs to synchronous rectification, which thanks to advances in automotive charging systems is becoming more widely available. Most of my SS equipment already made this transition and soon hopefully the tube equipment will follow. It is only a question of time high end manufacturers wake up to this approach.
I have both, but would go with tube rectification in all future projects.
Tube rectification produces less noise, which can be an issue not only for the component, but for the whole AC supply. Also, with the 5AR4 I use it prevents B+ from being applied to a cold tube, with SS rectification I have one component with 2 switches so B+ can be applied after the tubes warm up, in another I have a timed relay for the B+ voltage.
The way different tube rectifiers sound is likely due to differences in sag, the voltage provided by a tube isn't totally constant, there is a bit of voltage drop in between cycles called sag.
The disadvantages are mainly reliability, rectifiers can fail, they do so much more often vs SS, especially new production 5AR4. I've had a few fail and one took out a resistor, another melted-down the power transformer. I've yet to have SS diodes fail on me.
Tube rectification produces less noise, which can be an issue not only for the component, but for the whole AC supply. Also, with the 5AR4 I use it prevents B+ from being applied to a cold tube, with SS rectification I have one component with 2 switches so B+ can be applied after the tubes warm up, in another I have a timed relay for the B+ voltage.
The way different tube rectifiers sound is likely due to differences in sag, the voltage provided by a tube isn't totally constant, there is a bit of voltage drop in between cycles called sag.
The disadvantages are mainly reliability, rectifiers can fail, they do so much more often vs SS, especially new production 5AR4. I've had a few fail and one took out a resistor, another melted-down the power transformer. I've yet to have SS diodes fail on me.
I had in mind the other way around; adding a regulator after the tube rectifier and initial decoupling capacitors, which will "stiffen" the supply and is thus closer to a SS design. I did that to an old preamp with mixed results. Measurements and perceived bass were a bit better, mainly due to tweaking coupling time constants possible after stabilizing the supply and biasing of the signal path devices, but it introduced additional HF noise (hiss) that took a while to tame. You can also use chokes after SS rectifiers, of course, and resistors, if you want to more closely emulate a tube power supply. I built a capacitive multiplier using a tube at one point but in the end decided it wasn't worth the effort. Large capacitor banks have been used in some power amps but usually (always IME) require some sort of soft-start circuit when powered on to give the bank time to charge from the tube rectifiers.
Can you still get the old Hg tubes?
How does the dead zone compare between tube and SS synchronous rectifier circuits? I know the SS side but that is something I have not done with tubes, although decades ago my dad's CB radio actually used a circuit like that in its power supply.
Regulation can work well with pres but not amps... They are different enough I have a different PS for the voltage gain and current gain stages of my amp, the voltage gain is regulated and the power tube's PS is tube rectified with CLC filter stages. Both use very low internal resistance film caps., and massive capacity is not required for excellent results. Using a CRC type PS filter w/ a big carbon resistor and electrolytic caps is cheap but sounds bad, makes for a lot of the "tubey" sound people associate with old tube amps. A CLC filter with an inductor and film caps is a massive step up.
This option has never ever crossed my mind. I must be brainwashed from reading too many orthodox texts.
. In my defense such a scheme will not work for me as all my power supplies are choke input.You can most certainly still get Hg rectifiers, but the NOS are getting scarce and expensive. Attached is one of my favourite with a 5687 for scale.
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20200717_203024.jpg
You can get better performance from solid state supplies. But like all things they have to be properly implemented and that is not a trivial matter.
If I were to point to the main issue that I think people hear, it has to be 'diode noise', and to this end, often expressed as an intermodulation, making the output harsher. Tube rectifiers really have almost no noise. The problem as best I can make out is the issue of snubbing rectifiers is poorly understood, starting with the simple fact that you have to snub the power transformer not the rectifier! This is because the rectifier has a capacitance and the transformer an inductance; this can set up a high frequency swept resonance which is responsible for the diode noise. This is why some rectifiers will work with some transformers and and be low noise while they will not with other transformers, and vice versa.
But once the snubber issue is properly dealt with, then the issue of regulation and the like has to be addressed. If you really want to be serious about your preamp, regulation is pretty important. Again, you can face noise issues if the regulation is poorly implemented. This is because whatever regulator you have is essentially a small power amplifier; it must have good linearity and low noise. To this end it also has to be properly bypassed, as it has bandwidth too. This is the case whether tube or solid state! Once it rolls off, the bypass must be of the right value such that the output impedance, expressed as a curve against frequency, is as flat as possible! This is tricky if you run single-ended tube circuits, as they are so much more sensitive to power supply issues.
If the regulation is properly implemented, the issue of tube or solid state rectification is often rendered moot, unless the solid state rectification is poorly implemented and able to induce noise downstream.
This is all tricky stuff; as a result many will prefer to implement tube rectification as it is so much easier, and without much effort can get better performance than poorly executed solid state. If you want to see an example of how it was done right, take a look at the Citation 1 preamp power supply. Properly refurbished this is one of the better vintage tube preamps money can buy and is no slouch against modern preamps (keeping in mind low output cartridges did not exist at the time).
Of course, regulation of the filament circuit is nearly as important as the high voltage, and here is becomes obvious that solid state is the only way to go. If you want an all tube phono section that can work with a low output cartridge, you'll need to regulate.
If I were to point to the main issue that I think people hear, it has to be 'diode noise', and to this end, often expressed as an intermodulation, making the output harsher. Tube rectifiers really have almost no noise. The problem as best I can make out is the issue of snubbing rectifiers is poorly understood, starting with the simple fact that you have to snub the power transformer not the rectifier! This is because the rectifier has a capacitance and the transformer an inductance; this can set up a high frequency swept resonance which is responsible for the diode noise. This is why some rectifiers will work with some transformers and and be low noise while they will not with other transformers, and vice versa.
But once the snubber issue is properly dealt with, then the issue of regulation and the like has to be addressed. If you really want to be serious about your preamp, regulation is pretty important. Again, you can face noise issues if the regulation is poorly implemented. This is because whatever regulator you have is essentially a small power amplifier; it must have good linearity and low noise. To this end it also has to be properly bypassed, as it has bandwidth too. This is the case whether tube or solid state! Once it rolls off, the bypass must be of the right value such that the output impedance, expressed as a curve against frequency, is as flat as possible! This is tricky if you run single-ended tube circuits, as they are so much more sensitive to power supply issues.
If the regulation is properly implemented, the issue of tube or solid state rectification is often rendered moot, unless the solid state rectification is poorly implemented and able to induce noise downstream.
This is all tricky stuff; as a result many will prefer to implement tube rectification as it is so much easier, and without much effort can get better performance than poorly executed solid state. If you want to see an example of how it was done right, take a look at the Citation 1 preamp power supply. Properly refurbished this is one of the better vintage tube preamps money can buy and is no slouch against modern preamps (keeping in mind low output cartridges did not exist at the time).
Of course, regulation of the filament circuit is nearly as important as the high voltage, and here is becomes obvious that solid state is the only way to go. If you want an all tube phono section that can work with a low output cartridge, you'll need to regulate.
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