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Old 28th December 2010, 07:56 AM   #21
regal is offline regal  United States
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Many people like the sound of a tube amp where the tube rectifier signature is left in the signal path. That is personal taste, however the reputation for poor bass response is due to using PTX with too low a VA rating and rectifier tubes that can't deliver enough current for the design. Now that we have that cleared up, wouldn't one argument in favor of tube rectification be less filtering required? Some SS rectified designs have so much filtering they get sluggish by the time the switching noise is filtered out? I'm just throwing this out there, I have built/designed tube amps with either or, but do tend to prefer the tube rectifier with its slow startup which I think is easier on the amp tubes.

Last edited by regal; 28th December 2010 at 08:22 AM.
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Old 28th December 2010, 10:16 AM   #22
M Gregg is offline M Gregg  United Kingdom
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Quote:
So, let me try and understand you… you think that if a resistor sounds ‘lush’ then playing Motorhead through that resistor will make Motorhead sound lush????

Components do not have a ‘sonic signature’; they have electrical characteristics which must be understood, selected and blended to produce the desired circuit and system response.


---
If a resistor as has an impedance at AC frequency due to inductance then as you put this is an electrical characteristic.

It is the electrical characteristic that creates the signature of the component!
If you say you cannot hear the difference between polypropylene and polyester caps then yes I agree "in reverse" the circuit is a bad design!

You say that you are going to produce what is on the source CD Etc.

I say yes that is the aim, however what you get from your system and what another will get with the same amp with different components will be presented differently!

This could be detail, imaging, tighter base, improved treble or the reverse where yours system is better.

I have listened to Ongaku and money aside I was not a fan, many are! I prefered the PP circuits they had on display. Different harmonics.


Any way you are probably correct. Electrical characteristics do not effect sound only the Dielectric sorry is that a characteristic!

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M. Gregg
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Old 28th December 2010, 11:31 AM   #23
M Gregg is offline M Gregg  United Kingdom
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Sorry on the above post

To many typo's and off topic!
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Old 28th December 2010, 12:13 PM   #24
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like anything else - it depends. I've had good luck with both SS and tube rectifiers. My friends prefer tube rectification, but I personally think that is the "tube only" bias. I'm in the weird camp of thinking that SS is good for some things (MC carts and big power amplification) while tube is good for others (MM carts, linestages, low power for high efficiency speakers).

Back to rectification - for a SE (or Class A) amp, I usually stick with a tube rectifier like the 5AR4. For Class AB/high powered amplifiers - I like the "speed"/dynamics of solid-state like the lowly UF4007.
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Old 28th December 2010, 02:07 PM   #25
Structo is offline Structo  United States
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I think it comes down to R/C time constants.
If you go too big you will have problems, if you go too small you are going to have problems.

That is why power supplies are designed for the amp at hand.

Too low of filtering and the caps won't be able to keep up with the bass and sagging will occur.
Too much filtering and it can make it sound sterile and lifeless.

Get it just right and mmmmmm, that's it.

Makes me wonder about the SS amps, with their gazillion microfarads of filtering.
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Old 28th December 2010, 05:02 PM   #26
kruesi is offline kruesi  United States
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I agree with all those that have said that components do not have a sonic signature of their own, but have electrical characteristics which must be understood before a reasonable design is achieved. I would add that the farther you get from the signal (power supply, line cord, house wiring, substation, generating plant) the less impact its characteristics will have on the amplified audio.

This goes for rectifiers too- once the DC supply is quiet enough to meet the demands of the design, it makes little difference whether the design is thermionic or solid state.

I also agree with (who WAS it, now) that said it's not always about JUST the electrical design. There is a certain amount of aesthetic to it too- otherwise we wouldn't be designing with tubes at all (oops- NOW I've said it!).

All that being said, I usually prefer solid state diodes since they're far more efficient than thermionic devices- higher output voltage from a given transformer and far less heat- we usually have plenty of heat in a tubed design as it is.

And there's usually enough nice, big, warm, glowing glass bulbs visible to satisfy the desired result. -I really don't miss seeing the rectifier tube enough to justify the additional heat. I admit 5U4s are visually appealing but personally I have a hard time justifying their use.

Cathode stripping is also really not an issue below several thousand volts so there's really no need to delay B+ while the heaters get up to temperature.
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Old 28th December 2010, 05:57 PM   #27
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Originally Posted by kruesi View Post
And there's usually enough nice, big, warm, glowing glass bulbs visible to satisfy the desired result. -I really don't miss seeing the rectifier tube enough to justify the additional heat. I admit 5U4s are visually appealing but personally I have a hard time justifying their use.
I had no problem justifying the use of 5U4GBs in one project. The design nominal Vpp was 350Vdc. The power xfmr I had in the junk box was a 650Vct @ 150mA device. With SS diodes, it overvolted badly. With a 5U4GB, it produced the design nominal DC (352Vdc to be exact). Why not use the 5U4 since it's obvious this power xfmr was designed for it, and the design would either require a different power xfmr for SS diodes or some circuit to dump nearly 100V anyway. Besides, I have essssss-loads of 5U4GBs and its compactron version on hand anyway.

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Cathode stripping is also really not an issue below several thousand volts so there's really no need to delay B+ while the heaters get up to temperature.
Cathode stripping may not be an issue, but over voltage with DC coupled designs definitely is. That was another problem: the use of SS diodes to provide a negative rail. That comes up to some 450Vdc within a couple of seconds, way before cathodes can warm up. That exceeded the Vhk rating of the 6FQ7s I was using as grid drivers. Forch, I could float the entire heater supply without causing objectionable hum. Otherwise, some sort of arrangement with Zeners (and their nonlinear capacitances) would have been required to prevent poofing the HK insulation.

If you DC couple, best to preheat when using SS power supplies.

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Now that we have that cleared up, wouldn't one argument in favor of tube rectification be less filtering required? Some SS rectified designs have so much filtering they get sluggish by the time the switching noise is filtered out?
I use the same filtering regardless. Yes, you can filter more heavily with SS diodes, as these have much higher Isurge ratings. However, vintage power xfmrs weren't designed with this in mind, and may not appreciate Isurge way in excess of what designers could reasonably expect of hollow state diodes. Same goes for modern power xfmrs like those "Hammond Classic" power xfmrs which are recycled designs from the days before Si diodes were ever available.
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Old 29th December 2010, 12:38 AM   #28
kevinh is offline kevinh  United States
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Since no component is perfect, the dynamic behavior needs to be examined. The pn junction of SS rectifiers have a lot of noise and hash. The is audible. Comments from Olsen on this:

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Matt Kamna also demonstrated a technique for zooming in on the waveform on the power-transformer secondary (about 10V/div on the scope screen). The rough appearance around the zero-crossing was very obvious with solid-state diodes. HEXFRED's gave a small improvement, but conventional tube rectifiers looked much smoother, and the TV damper diodes were by far the smoothest of all. So even in low-current preamp applications, TV damper diodes give the least noise. I know from experience in the Tektronix Spectrum Analyzer division that it's much easier to eliminate noise at the source than filter it afterward. If there was an even quieter device, I'd use that, but as far as I know, TV damper diodes are the quietest from the viewpoint of switching noise. Considering that the main B+ supply is switching five hundred volts, this is not a small consideration, since switch-noise is radiated in all directions, into the B+ supply, the interior of the chassis, and back into the power cord.

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The electro-magnetic interference (EMI) noise generated by bridge-capacitor and pi-filter power supplies is responsible for a significant amount of tonal coloration as well as low-level veiling.

The first power-supply cap connected directly to the rectifier charges up very rapidly — the caps are "topped up" by a brief but very powerful spike of current. (Refer to Chapter 30 in the RCA Radiotron Designers Handbook, Fourth Edition, Fig. 30.1 for a more complete discussion of peak current flow in pi-filter supplies).

What goes unnoticed by most readers is the pulse-width of this brief current spike is in turn modulated by the moment-to-moment current draw of the amplifier. If the current demand is heavy, the spike is wider, and if the draw is low, the spike is narrower. Jumping to the frequency domain, the wider spike will have stronger low-frequency components, and the narrower spike will have more high-frequency components ... although both spikes yield a comb spectra going out to at least 100 kHz or more, depending on the residual inductance of the first power-supply capacitor. In effect the noise spectrum of the power transformer secondary/rectifier/cap antenna is modulated by the current draw on the entire amplifier.

If you recast the bridge-capacitor & pi-filter supplies as a Tesla coil (inductor, commutator, cap) the picture gets clearer. The EMI spectrum of this miniature RF transmitter is pulse-width-modulated by the inverse of the current draw.

This isn't so bad for true, all-the-time, Class A circuits, since the current demand is theoretically constant; for more common Class AB circuits, though, it is a disaster. The current demand for Class AB fluctuates a great deal, especially for transistor amplifiers that typically idle at a few watts. This means the noise spectra of the power supply (which extends into RFI frequencies) is always changing with the music.

This might be the single greatest advantage of choke-fed supplies; at least the current pulse through the rectifier is much wider and not significantly affected by current demand. It also looks like folks who are stuck with bridge-capacitor supplies (the worst kind) for heater supplies and solid-state amplifiers might be wise to slow down the bridge with modest values of resistance, rather than leave the damping to the unpredictable value of ESR in the first filter cap. This is probably the reason why adding a film bypass cap makes this type of supply sound worse; as a result of the film bypass decreasing the effective ESR, the current pulse is speeded up and EMI emission increased.

As can be imagined, this Tesla-coil-in-miniature is going to be very sensitive to physical layout, stray C's and R's in the power transformer, and stray L's and R's in the first capacitor. Slowing down the rectifier with ferrite beads might be a no-go as well, since the current pulse is very fast, very large, and can easily saturate the ferrite. Small values of resistance with inductive-type wirewounds, one on each leg of the bridge, are the best way to moderate the RF emission at the source. Not much chance of saturating something as inert as a wirewound resistor — and the residual inductance is an air-core, no saturation there either.

The distance between the power transformer, bridge, and first cap determines the loop area of the noise-source antenna, and the peak current pulse indicates the power this antenna is likely to emit. Something as simple as twisting the power supply lines with an associated ground return can reduce antenna emission by 20 dB. Noise suppression requires a combination of noise reduction at the source (any sub-circuit that switches rapidly) as well as RF-style shielding techniques.
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Old 29th December 2010, 06:58 AM   #29
Jen-B is offline Jen-B  United Kingdom
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Originally Posted by kevinh View Post
Since no component is perfect, the dynamic behavior needs to be examined.
Yes!


Quote:
Originally Posted by kevinh View Post
The pn junction of SS rectifiers have a lot of noise and hash.
Yes, but the level and it's effect surely depend upon the competence of the overall psu design.


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Originally Posted by kevinh View Post
The is audible.
Not necessarily. If the designer has dumbly just bunged in SS diodes then he / she deserves the noise. But if the designer has chosen SiC Schottky diodes, or correctly applied RC snubbers, then used an inductor, then a voltage regulator (and perhaps isolated the current loop by a shunt regulator) then the power supply is likely to be a quiet and competent foundation upon which to build the amp.


So the key to it is competent design for the specific application (in my humble opinion).


J.
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Old 29th December 2010, 09:09 AM   #30
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by Jen-B View Post

Yes, but the level and it's effect surely depend upon the competence of the overall psu design.


Not necessarily. If the designer has dumbly just bunged in SS diodes then he / she deserves the noise. But if the designer has chosen SiC Schottky diodes, or correctly applied RC snubbers, then used an inductor, then a voltage regulator (and perhaps isolated the current loop by a shunt regulator) then the power supply is likely to be a quiet and competent foundation upon which to build the amp.
So the key to it is competent design for the specific application (in my humble opinion).
J.
Jen-B,

Of course the circuit must be well designed, however that is not the be all and end all of it!

Quote:
But if the designer has chosen SiC Schottky diodes,
------
Please show us all one with 1000V 1A+ we would all like to use it!


The PSU you describe above adds components to remove problems with components, these "add" more problems even though they remove others.

How many valve amps do you see with complex power supplies?
Many "users" not builders will say they don't like the stiffled sound from regulated power supplies!
Then again some like them!

This is also true with DC or AC heaters they "sound" different. I do not mean that the music has some how changed Motor Head into a classical piece. The presentation has changed.

This is subjective to the listener. I watched a "blind guy" listening to an audio note kit amp on demo, we had accidently stood in the sound stage of the music. we were not talking, we were casualy listening. I looked up to see a frustrated person franticaly moving his head about. I walked over and apologised he said he could not "see" the music had something happened to the amp! I spent some time talking to him its interesting from a different perspective!

This can be even more of an issue with guitar amps. When the sustain and harmonic content change the overdrive sound. Yes guitarists have their idea of perfection and are just as crazy as audiofools.

You can fit components (a diode is a diode) no its not. A cap is a cap no its not. A resistor is just a resistor no its not! The design of a circuit is very very important, However a component catalogue does not say (non audiophile) this cap is smooth and lush. Then again it does not say this resistor has a high inductive value at AC frequency etc. Or the dielectric causes HF roll off at AC ?Hz. in this capacitor.

I managed to find in a TV repair shop some "Old new" paper plessey caps still in there plastic sealed bag "paper type". I substituted them in an amp, guess what they made it sound like "old valve" equipment. Sine / square wave looked exactly the same!

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