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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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4th Feb 2016, 6:46 am | #1 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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HT regulation
I've been exploring regulating HT for an amp I'm building and thought I'd ask your advice. To get 300v @ around 500mA peak x 2 how would you do this? Valves or mosfets? Two 509's or similar in parallel with a couple of OA2's seems to be standard for valved based regulation but the physical size is a problem.
Big mosfets (QFET) like the FQA24N60 - 600v 23.5A 310W diss are cheap, but I don't have much experience using them in anything this big. I'm unsure the gate side of the circuit after doing a bit of reading up on fets. I'd like to build some protection in as well obviously. Any ideas/comments most welcome, Andy.
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4th Feb 2016, 10:05 am | #2 |
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Re: HT regulation
The question is why?
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4th Feb 2016, 10:52 am | #3 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,996
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Re: HT regulation
At that sort of power level - if you're thinking of using power FETs why not build a switched-mode PSU which will - by its very nature - regulate itself?
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4th Feb 2016, 11:05 am | #4 |
Octode
Join Date: Jan 2003
Location: Ware, Herts. UK.
Posts: 1,082
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Re: HT regulation
Andy
I don't think there is any point in using a regulated HT supply for a valve power amplifier. Push-pull amplifiers are inherently good at rejecting power supply ripple, and high voltage electrolytic capacitors of large capacitance are easily available and relatively cheap, so supply ripple needn't be a problem. A linear regulated supply capable of delivering 300V at 500mA will be large, potentially hot and definitely dangerous. Using a regulated supply for a valve preamp with a magnetic cartridge input makes some sense, but in this case, the current required will be much lower, so design would be much more straightforward. John |
4th Feb 2016, 1:38 pm | #5 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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Re: HT regulation
That sort of supply would traditionally have used a choke input filter but would not have been regulated otherwise. This type of filter requires higher voltage ac to the rectifier but otherwise gives the transformer an easier time of it (lower VA rating required) than a capacitor input type.
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4th Feb 2016, 2:14 pm | #6 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,996
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Re: HT regulation
If the amp is a pure Class-A or AB1 then the current drawn should be pretty much constant and the need for stabilisation of the HT is essentially nonexistent.
Only when you start looking at class-B does 'stiffness' of the HT supply start to become relevant - choke-input filters [often using a 'swinging choke' whose effective impedance varied in response to the DC flowing through it] were the traditional way to handle the variations in load current while at the same time limiting the peak current through the [valve, often mercury-vapour] rectifiers; these days we'd use rectifiers that don't care about such peaks, coupled with lots and lots of capacitance (which is now cheaper and easier to get than a swinging choke, as well as being a lot lighter!] |
4th Feb 2016, 2:50 pm | #7 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
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Re: HT regulation
Might it be worth considering an alternative approach, design the power supply with good inherent regulation by use of an oversized transformer and generous smoothing circuitry.
To protect against mains voltage variations use a ready made AC voltage regulator for the mains input to the equipment. |
4th Feb 2016, 8:20 pm | #8 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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Re: HT regulation
Dr. Wobble: Andy - you have a P.M.
Al. |
4th Feb 2016, 9:54 pm | #9 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,394
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Re: HT regulation
I'd go along with G6Tanuki and broadgage's points- things that are expected to have highly consistent and stable performance with low noise, like low-level pre-amps and test-gear might justify stabilised PSUs but with a significant size power amp, the PSU output volts and amps represent a significant investment and you don't want too many of either frittered away in regulation to insignificant effect. A decent (or even half-decent) power amp design with a good dose of feedback will be fine with olde-worlde combinations of chokes and capacitors- and high-voltage microfarads have probably never been cheaper or better- and will be thus able to make full use of the available PSU rail without the inevitable headroom loss incurred by regulation. Additionally, a regulator circuit represents a power amp in its own right with all the bandwidth/step response/phase at frequency extremes considerations implicit in that to add to that of the actual audio power amp- keeping them from compromising each other under all possible circumstances might be a headache. I assume you're not pursuing the high-power single-ended triode output with zero feedback and 100dB sensitivity horns approach!
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4th Feb 2016, 11:25 pm | #10 |
Banned
Join Date: Jun 2015
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Re: HT regulation
I assume you're not pursuing the high-power single-ended triode output with zero feedback and 100dB sensitivity horns approach!
Maybe 50 12AX7's as I have seen somewhere!!! whichever way, regulation isnt necessary Joe |
5th Feb 2016, 6:55 am | #11 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Re: HT regulation
Ta for your advice. I asked because I was thinking of going down the AB2 route but won't bother now. Just sounding out my options. As you all say, it's not needed for AB1 and it's easier/cheaper to do with alternative means.
Cheers, Andy.
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5th Feb 2016, 3:53 pm | #12 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
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Re: HT regulation
I had a similar discussion with a co-worker, many years ago. We ended up concluding that as long as there is sufficient headroom available, a well-designed (meaning in this context, that the output voltage should not depend on the supply voltage) output stage will effectively be acting as its own voltage regulator.
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