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Old 2nd October 2007, 06:21 PM   #1
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Default increased choke inductance - recommended?

I just can't stop tweaking my PSU for my 6SN7 Aikido. At present, the PSU is:

125-0-125 into a diode bridge using 1N4007
0.68uF "tuning" cap
10H 102R choke
65uF ASC cap
10H 102R choke
65uF ASC cap

the load is between 18 and 23mA total.

Without the tuning cap, PSUD estimates B+ of around 258V. However, I actually get more like 370V. Hence the tuning cap to make the PSU more like a cap input filter rather than choke input suffering from ringing.

ANyway, with the 0.68uF cap, PSUD estimates around 312V with 1.73mV of ripple. I measure around 312V - so PSUD is close on this one.

I have thought about changning out to a tube recitifed PSU, but the fundamental problem of choke ringing would presumably still apply.

Anyway, I ran some simulations with 20H 181R chokes and by simply changing these out, I get the same voltage in PSUD (312V), but ripple drops to 375 uV!

Furthermore, the 20H 181R chokes are not all that expensive, and they're smaller and lighter than the 10H 102R chokes that I am using at present.

Anyway, I expect that increasing inductance will be a good thing, IS IT? While there are other methods at improving the PSU, part of me is thinking that if I get the fundamentals correct, then my hum will almost completely removed, meaning that further, more exotic tweaks may become unecessary. Having said that, the present level of hum is only audible with my ear close to the speaker. BUT I am seeking audio nirvana!



Charlie
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Old 2nd October 2007, 06:36 PM   #2
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Default Re: increased choke inductance - recommended?

Quote:
Originally posted by cbutterworth

Anyway, I expect that increasing inductance will be a good thing, IS IT? While there are other methods at improving the PSU, part of me is thinking that if I get the fundamentals correct, then my hum will almost completely removed, meaning that further, more exotic tweaks may become unecessary. Having said that, the present level of hum is only audible with my ear close to the speaker. BUT I am seeking audio nirvana!


Charlie

If your idea of audio nirvana is hum below the measurement threshold then go with the largest possible chokes. IME, chokes become more audible in a PS with increasing inductance. Don't know if it's the added kilometres of wire or what. I seldom use more than 1.5-2H and find the subjective dynamics and "speed" much better.
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Old 2nd October 2007, 07:46 PM   #3
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OK, is there a noticeable sonic difference depending upon choke size?

What about going for a purely CRCRC type filter? What problem are associated with these types of PSU?

Becase one of my projects for this winter is re-housing my Aikido, I have the ability to look at a new PSU, and maybe chokeless would be better.



Charlie
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Old 2nd October 2007, 10:37 PM   #4
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Hi Charlie

Just noticed this was a preamp. Apparently my suggestion for using 1.5H is not very appropriate.

You can achieve very low ripple using a CRCRC filter, especially if you have enough extra volts in the PS which you can burn accross the Rs. This supply will sound quite different to a choke supply, at least it does to my ears.

After building amplifiers with RC - type supplies for many ears i didn't like much my first attempt of using chokes. Took me a while to realise that choke brands, similarly to capacitors vary a lot in subjective sound. And smaller chokes usually sound better. YMMV.
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Old 2nd October 2007, 11:35 PM   #5
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I can certainly experiment. If I remember, I do not have much extra voltage. I have a new 275-0-275, but may be able to get the B+ up to just over 300V. I'll see what PSUDII gives.

Of course, my present configuration has the ironware mounted on a steel plate, with the big PSU caps glued on the underside, beneath the chokes. I am wondering if this is not a big source of hum right there! I mean electrolytic caps separated from 10H chokes by 20 gauge steel. The more I think about it, the more I wonder if this is not the source of hum.


Charlie
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Old 10th October 2007, 11:45 PM   #6
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Charlie,

Just to ask .... that 1,7mV of ripple is already quite low. It will depend on the pre-amp circuit, but if you still get audible hum, then I suspect the "hum rejection" qualities of the amplifier more than the quality of the power supply. You are not using this as a millivolt output pre-amp?

You could experiment with effect of component proximity by the simple (well) procedure of removing the choke/power transformer temporarily and connect it through wires. Moving it around in spce by hand (watch out for touching h.t!) will reveal whether there is any effect.
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Old 11th October 2007, 05:31 AM   #7
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Johan,

OK, so I guess that my PSU has sufficient hardware to make ripple issues negligible in my circumstances. One thing that I should try is to increase the first cap from 0.68uF to 1.5uF. This would increase B+ voltage to something like 360V, which would result in around 180V per plate. If I have read the datasheet correctly, then the max for each plate is 450V at 20mA. I'll be nowhere near this as the total curent draw (for four 6SN7GT) is around 22mA.

My thinking is also that if I were using a true choke-input filter, with 10H chokes, 312V B+ and 22mA, I am in the region where the current is insufficient for the chokes to function properly. The first cap (0.68uF) makes the filter a cap-input, but to what degree? Does even the mere hint of capacitance ensure that the chokes would not ring?

So, if I change the 0.68uF to 1.5uF, then maybe I get closer to a true cap-input filter?

I also noticed that when I removed the 20K bleeder resistor from the final cap, I got more B+ voltage, but at the cost of lower current (14mA less) and a little more hum.

So, maybe 1.5uF cap with 20K bleeder I would get 16mA from the 20K plus the 20mA from the 6SN7s giving around 36mA and a filter that is , or closer to, a true cap input filter. This may reduce hum?

Other things to consider:

1. Groung Loop. I am working on a schematic of my Aikido, which may allow me and others to find possible ground-loop sources.

2. Maybe simply removing the signal GND from the output may break a possible ground loop with my main amp, which is also fully grounded. Doesn't this mean that signal ground is earthed twice?

3. Layout - Your layouts are immaculate (at least from the pictures I have seen). Do you have a website with photos?

Thanks,
Charlie
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Old 15th October 2007, 10:59 PM   #8
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Quote:
Originally posted by analog_sa
{...} Took me a while to realise that choke brands, similarly to capacitors vary a lot in subjective sound. And smaller chokes usually sound better. YMMV.
ooh, care to share some of your findings/favorites here?
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Old 16th October 2007, 05:18 PM   #9
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Quote:
Originally posted by Majestic
ooh, care to share some of your findings/favorites here?
I second this... Also, what are some of the parameter involving "speed" in a PS? Is this like the term "low impedance" PS? How does one go about building a low impedance PS?
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