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Hi,
My stereo 2a3 se amp has a 5U4- 47uf-15H-47uf power supply, putting out 320 volts of B+ . What would happen if I increase L to 25H? Would B+ voltage increase or decrease, but more importantly would it change the sonics of the amp for the better or worse? Another question, would It matter if the replacement choke was designed for a L input supply? Any insights would be appreciated.
Regards,
David
Edits: 09/22/17 09/22/17Follow Ups:
If you are looking for more "speed" I don't think that in your case increasing the inductance and DCR will deliver. Actually, it will probably slow PS recovery - something that you don't want - and your B+ would drop a little. You are running a small final capacitor so I think recovery will be important.Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 09/23/17 09/23/17 09/23/17
The question is whether the change in value will impact the way the PSU interacts with the amplifier circuitry. For that matter, how does the supply in its present state respond? I consider it essential to analyze every new supply using SPICE. That lets me see the output Z of the supply over the entire audio spectrum. Many supplies are designed apparently without this consideration, and I've modeled quite a few existing designs that perform very poorly. If you know the DCR of the choke in your supply, I wouldn't mind taking a few minutes after I get home to check this combination of values.
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Buy Chinese. Bury freedom.
Hello TK,
Thanks for the reply. My amp is running fine. There's no audible hum and it sounds smooth and pleasant. What's missing is a bit of speed and impact(slam?) that I want more of.
The choke measures 101ohms. The proposed 25H replacement is 205 ohms. So, what do you think?
Regards,
David
Bear in mind I'm only advising regarding the supply. What you're asking of the amplifier may not be related to this. Many things can influence system dynamics, including components and phenomena external to the amplifier proper.Based on a basic SPICE analysis, the supply you're using now exhibits a very smooth output impedance curve. There is only one resonance peak, and it is well below the audio band at 6 Hz. The plot is below:
I also plotted the supply with the 25H choke you're considering. That pushes the peak much lower in frequency (off the page in fact), but it does nothing to reduce impedance at 20 Hz and above. The best means to reduce the impedance in that area - and push the peak away at the same time - is to install a larger value at C2. The plot below represents impedance values using a 100uF cap:
My conclusion from these plots is that the 25H choke will not materially benefit your amplifier. I'll also say that although the additional capacitance in the second plot might appear beneficial, the reduction in impedance at low frequencies must be considered against the load presented by the output stage and the efficiency of the overall system. Because this is a SET, the output stage does not make the same dynamic current demands at low frequencies as push-pull AB1. Its loading on the supply is relatively constant regardless of output power and frequency. However, many SETs are driven into clipping at least occasionally during normal use, and a larger second cap can be helpful in dealing with that.
I have to disagree with Eli's comment regarding SS rectification in this case. Additional anode voltage might be beneficial if the amplifier is being regularly tasked beyond its limits. Otherwise, the natural characteristic of Class A to always draw the same average current renders the benefits of SS as it relates to push-pull designs irrelevant.
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Buy Chinese. Bury freedom.
Edits: 09/22/17
Thanks TK for taking the time to help me out. Based on your analysis and recommendation I will for now hold off in increasing the choke value and instead increase the final C value to 100uf.
Regards,
David
I think that is a good decision. Please, let us know how you go with that.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
"What's missing is a bit of speed and impact(slam?) that I want more of."
Switch to SS rectified B+ using "noiseless" high PIV Schottky diodes. It's most definitely not an accident that amps famous for good bass, like the McIntosh MC-275 and the Harman/Kardon Citation II, employ SS rectified B+.
Eli D.
Do you have too much B+ ripple?
Typically you want just enough filtering to make an amp acceptably quiet. If it is quiet, adding more filtering is not usually good for sonics. If you are using high quality caps and chokes, bigger ones are way more expensive.
If you have hum, most of the time it is not from the B+ rail unless you are using a poorly designed filter.
FWIW, A filter of 40uf-4H-40uf-4H-40uF @ 100mA to 200mA will be quiet and good sounding. You can get reasonably priced 300mA rated chokes that will be mechanically quiet as well. For highish current power supplies, high inductance chokes are not always better sounding.
Hi C647,
I haven't measured the ripple, but I don't have audible hum. I use Altec 604s and only a slight hum is detectable at high volume. I'm just wondering if a better made choke and/ or higher value one would provide any sonic improvements. Your thoughts?
Regards,
David
but adding more inductance than necessary in B+ filter ain't one of them. If your better made choke has lower DCR, it will usually mean that it is higher rated in current for a given inductance. Swapping a choke with lower DCR will usually increase the B+ voltage, so you have to watch out for that.
Improve the quality of your power supply capacitors, replace electrolytics with films, this may result in an improvement in sonics.
As this data sheet indicates, you could be pressing your luck with a 47 μF. part in the 1st position of the PSU filter.
IF the DCR of the 2 inductors is the same, a larger inductance yields less ripple, without changing the rail voltage.
If a "swinging" choke is on hand, I would not use it in a П section filter, as it's inductance varies with the current being passed. A "normal" choke intended for critical current duty will be fine. Inductors in choke I/P filters are subjected to much greater strain than those in cap. I/P filters and are constructed accordingly. If an inductor not specifically rated for choke I/P service is to be employed that way, substantial derating of its current handling capability is necessary.
Eli D.
Hi Eli,
Thanks for the information. To clarify, a choke designed for c I/P is also called a swinging choke?
Thanks,
David
Not all chokes used in choke I/P filters are "swinging". However, all chokes designed for critical current filter duty take the increased stresses of said service into account.
Eli D.
I can thank this forum for getting me hooked on L-critical choke input supplies. my amps are all heavier thanks to that.
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