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Old 21st Sep 2018, 9:25 pm   #1
PaulR
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Default Power supply voltage doublers

I have a Rogers HG88 III and a Sansui 500 and both use lower voltage on the mains transformer secondaries followed by a voltage doubler circuit. Can anyone tell me why this was used? Why not just have a higher output transformer with a conventional smoothing circuit?
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Old 21st Sep 2018, 9:29 pm   #2
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Default Re: Power supply voltage doublers

My Pioneer does the same. I have always assumed that it was done like that to reduce hum pickup, but I may be (probably am) totally incorrect.

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Old 21st Sep 2018, 9:49 pm   #3
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Default Re: Power supply voltage doublers

I can't see how a doubler would reduce hum over a traditional full wave rectifier though.
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Old 21st Sep 2018, 9:57 pm   #4
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Default Re: Power supply voltage doublers

Review by Geoffrey Horn suggests that various upgrades to a previous amplifier increased the output. One item was the use of a voltage doubler to increase the HT, to my mind this perhaps means they used the same mains transformer as the previous amplifier.

See para 3,http://www.bassboy.com.au/getreel/si...adet3/test.htm
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Old 21st Sep 2018, 10:00 pm   #5
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Default Re: Power supply voltage doublers

It may simply allow lower voltage reservoir capacitors to be used, though one will need to be insulated from chassis.

Maybe less voltage stress in the transformer, too.

Possibly bean counting may come into it too, somewhere- stocking less different values etc etc.
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Old 21st Sep 2018, 10:12 pm   #6
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Default Re: Power supply voltage doublers

The voltage doubler increases ripple currents and ripple voltages, it may need lower voltage capacitors, but it demands more farads.

It's a higher stress thing overall.

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Old 21st Sep 2018, 10:26 pm   #7
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Default Re: Power supply voltage doublers

Would increasing the value of the capacitors help with ripple or be inadvisable? I have replaced them in the amps with similar values but I have a Fisher receiver in the loft that needs recapping and this also has a doubler power supply. Maybe the increased stress on the transformer would make it unwise.
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Old 21st Sep 2018, 10:41 pm   #8
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Default Re: Power supply voltage doublers

I was surprised the HG88 had voltage doubled HT but the circuit confirms this. There may be a benefit in the AC connection to the equipment as this will limit the power that can be transferred to the HT rail without added dissipation. It will behave like a capacitor dropper.

I often use voltage doubling to get enough voltage from the heater circuit to power silicon stuff.

PS: Just seen your post and it would not be a good idea if the design purpose is to limit power transfer. The sag in HT voltage may be essential to avoid the output valves going into thermal runaway when driven hard into class B. The HG88 has RC smoothing after the doubler so you could increase this capacitor but if you are getting significant hum and the capacitors are in good condition, the fault is likely to be elsewhere.

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Old 21st Sep 2018, 10:44 pm   #9
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Default Re: Power supply voltage doublers

I think it is only the Mk III that uses it. I think the earlier versions use a traditional power supply.
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Old 21st Sep 2018, 11:41 pm   #10
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Default Re: Power supply voltage doublers

I seem to recall that a simple voltage doubler is S/C proof. That must confer advantage in some circuits.
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Old 21st Sep 2018, 11:54 pm   #11
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Default Re: Power supply voltage doublers

Quote:
Originally Posted by Herald1360 View Post
Possibly bean counting may come into it too, somewhere- stocking less different values etc etc.
That sounds the most likely! Same transformer as already used elsewhere.

As others have pointed out, stress is increased for most components.

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I seem to recall that a simple voltage doubler is S/C proof. That must confer advantage in some circuits.
Not really, although it is true that if you put a short-circuit on the output, the current is limited by the value of the capacitors (because there's never an instant when the output is getting charged without a capacitor in series, somewhere), whereas putting a short-circuit on the output of a bridge rectifier there is only the rectifier and transformer resistance to limit current. However, the values of capacitors are such that, to all intents and purposes, the limiting they provide is not enough to prevent something else dying!
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Old 22nd Sep 2018, 12:04 am   #12
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Arrow Re: Power supply voltage doublers

Reference post 1.

Reason: probably cost.
Case 1. Transformer secondary with centre tap and two rectifiers.
Case 2. Smaller secondary without centre tap and four rectifiers.
Case 1 has much more copper in the transformer than case 2 and rectifiers are much cheaper in Case 2. Extra capacitors are relatively cheaper, too.

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Old 22nd Sep 2018, 12:31 am   #13
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Default Re: Power supply voltage doublers

Perhaps by contrast, from the transformer's point of view, the traditional bi-phase half-wave arrangement so popular with thermionic double-diodes could be described as a voltage halver...

I'd noticed that quite a few later-generation valve amps showed a liking for voltage doubling- it's difficult to conclude that this was anything other than cost-driven, but the weighting applying to different factors would be interesting to know. Perhaps transformer makers preferred to do rather fewer turns of somewhat thicker wire, or maybe as electrolytic technology improved, cheap, compact high-value capacitors with good ripple rating and only requiring relatively modest voltage rating meant that the full-wave doubler's lower frequency, high amplitude ripple was less of a design objection. Semiconductor diodes also removed any heater awkwardness with doublers.
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Old 22nd Sep 2018, 7:50 am   #14
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Default Re: Power supply voltage doublers

Thicker wire costs less per kg of copper.
Fewer turns means less time on the winding machine.

SMPS often have switched doubler/bridge rectifiers and a pair of moderate voltage reservoirs, so these mass market capacitors are cheaper. Before SMPS this voltage range was common in the 120v world in TVs, so cheap back then.

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Old 22nd Sep 2018, 8:20 am   #15
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Default Re: Power supply voltage doublers

Ampex used voltage doublers for the HT in some of their tape machines, I never understood why.
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Old 22nd Sep 2018, 9:20 am   #16
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Default Re: Power supply voltage doublers

Would the bean counting matter quite so much to manufacturers like Rogers and Fisher? I think that Sansui were somewhat cost driven although I bet that the Sansui 500 was expensive in its day. It has stereo FM as standard which might be thought of as extravagant in 1962 or 63.
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Old 22nd Sep 2018, 10:58 am   #17
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Default Re: Power supply voltage doublers

Sansui probably sold more in the US market than UK/Europe- FM stereo got off to an earlier start across the Atlantic. (As an aside and reminded by another thread, I have a valve stereo decoder that was a sub-chassis module in an early 'sixties Sansui receiver, presumably indicating a degree of uncertainty over which stereo FM system would be adopted at the time of design).

I always though of Rogers stuff as a bit cheap'n'cheerful- not quite the Amstrad of its day, but pitched below the likes of Radford and Quad. Of course, the passage of time has lent a touch of mystique to any hi-fi with valves in it, and even non-descript thermionic stuff of half-a-century ago will be inoffensive to the ear, and more sturdily built than stuff churned out today. No offense to Rogers collectors!
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Old 22nd Sep 2018, 11:34 am   #18
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Default Re: Power supply voltage doublers

As a bit of an aside, the Sansui has a fiendishly complex switching arrangement which allows AM on one channel and FM on the other. I think there was a passing thought of having stereo radio using that system
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Old 22nd Sep 2018, 11:38 am   #19
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Default Re: Power supply voltage doublers

Not a Rogers collector but please don’t put them at the side of Amstrad, they were decent amplifiers, perhaps not Radford or Quad but not the same price either.
See the review in post 4 from a respected reviewer, dated 1965
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Old 22nd Sep 2018, 12:17 pm   #20
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Default Re: Power supply voltage doublers

Quote:
Originally Posted by Nuvistor View Post
Review by Geoffrey Horn suggests that various upgrades to a previous amplifier increased the output. One item was the use of a voltage doubler to increase the HT, to my mind this perhaps means they used the same mains transformer as the previous amplifier.

See para 3,http://www.bassboy.com.au/getreel/si...adet3/test.htm
In my experience, designers can win brownie points from their bean counters by specifying components that would otherwise end up as redundant stock. There's a constant tension felt by the designer between the marketing demand for improved exciting products and the financial pressure to use the existing components still filling their bins in the stores. Bulky heavy mains transformers would be a very obvious standout item when the chief accountant visited the factory stores to find out where all the cash has gone. A voltage doubler as a means of powering the next more powerful amplifier would then seem both a neat design and a politically expedient solution.

At the hobby level, I've found a voltage multiplier to be a convenient way of making use of cheap surplus transformers to produce the right DC supply for a specific job. For example, when building a mains power supply for my R1155 receiver, which needs 210VDC, I found in the junk box a nice transformer of the right power, with a 6.3V heater winding, but with HT output only 75VAC. A voltage tripler circuit soon solved the problem and has been powering the radio very well for a few years now.

Martin
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