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Old 18th December 2007, 12:47 PM   #1
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Default Using an SMPS with Amp10 Basic?

Hi all,

I'm building the Amp10 Basic kit from 41Hz - largely because it met my needs for power output, size, is bridgeable in future and was very affordable. I've built a couple of Gainclones before, as well as number Headphone amps.

I'd quite like to power this from an switchmode power supply - simply because I've used toroid xformers before and I'd like to try something different and build something a little lighter.
Looking around it seems that there is no suitable and well priced (very important!) SMPS with a dual rail supply available either prebuilt or as a kit. This said it's hard to go past £22 for a 225VA toroid from Farnell.

I have seen some people writing about using two switching supplies and chaining them to provide +V Gnd and -V, but there seems to be discussion saying that this doesn't work well or implying it's a bit of a dirty hack?

I'm also wondering if I'm missing something because I'm not searching for the right terms or haven't seen a post/website that discusses this in detail.

Any help/pointers greatly appreciated!
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Old 18th December 2007, 05:13 PM   #2
y8s is offline y8s  United States
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someone else correct me if I'm wrong but....

most SMPS have a +, -, and GND connection.

if you look on the AMP10 Basic, the AC feeds into some bridges and then to the big caps. both caps are connected to gnd (opposite polarity) and then to the output of the rectifiers.

You end up with a neg rail, a pos rail, and a common (gnd) rail.



however I'm not totally sure how the amp knows that GND is half way between the two rails. Other than the two 1W cap discharge resistors...

Matt
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Old 18th December 2007, 08:11 PM   #3
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Quote:
Originally posted by y8s
someone else correct me if I'm wrong but....

most SMPS have a +, -, and GND connection.

if you look on the AMP10 Basic, the AC feeds into some bridges and then to the big caps. both caps are connected to gnd (opposite polarity) and then to the output of the rectifiers.

You end up with a neg rail, a pos rail, and a common (gnd) rail.



however I'm not totally sure how the amp knows that GND is half way between the two rails. Other than the two 1W cap discharge resistors...

Matt
Corrected. It doesn't work that way. The secondary (-) is Zero volt reference and GND is Primary-side Earth Ground. The amplifier won't magically find a negative rail from a "typical" SMPS. And yes, I've tried.
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Old 18th December 2007, 09:39 PM   #4
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That's what I was reading.

I've done alot of googling - and it looks like the supplies from ColdAmp $$$, eBay $$, or some possibly vapourware style supplies around the web of unknown quality are the only things available.

Alas +/- 43V-60V is a little too much for the chip (and probably the components) used in Amp 10 (notes about +/- 35 V DC max?).

I'm afraid I lack the knowhow to design a Switch Mode PSU - it looks like the transformer route for me.
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Old 18th December 2007, 09:51 PM   #5
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Eek - (that is me above, really!) not sure how but I wasn't logged in as me? Something to do with BugMeNot by the looks! Apologies,
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Old 19th December 2007, 04:26 AM   #6
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Quote:
Originally posted by EnvisionAudio


Corrected. It doesn't work that way. The secondary (-) is Zero volt reference and GND is Primary-side Earth Ground. The amplifier won't magically find a negative rail from a "typical" SMPS. And yes, I've tried.
ah. i tested mine with a dual DC supply. it's been a while

but with that, the two supplies were isolated so I could run the + from one to gnd and the - from the other to gnd. supposedly you could do the same with two SMPS single supplies as long as you trimmed the output voltages fairly well and the outputs are isolated, yes?
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Old 19th December 2007, 01:30 PM   #7
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Quote:
Originally posted by y8s


ah. i tested mine with a dual DC supply. it's been a while

but with that, the two supplies were isolated so I could run the + from one to gnd and the - from the other to gnd. supposedly you could do the same with two SMPS single supplies as long as you trimmed the output voltages fairly well and the outputs are isolated, yes?
Eh...probably like this: (+)=====(-)--Zero Volt Ref--(+)=====(-)

Earth GND is chassis, not amplifier's Zero Reference. However, it is permissible to tie the two together in certain circumstances, with certain components (mainly for RF rejection).

If you were tying your amplifier's zero volt reference to Earth GND, you were no longer referencing the power supplies to their secondaries and no regulation (at least not full) could take place. It would play music, but noise measurements would be awful.
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Old 19th December 2007, 01:44 PM   #8
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Originally posted by raromachine
Eek - (that is me above, really!) not sure how but I wasn't logged in as me? Something to do with BugMeNot by the looks! Apologies,
I see nothing wrong with your appearance. Mine, on the other hand....
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Old 19th December 2007, 04:03 PM   #9
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Quote:
Originally posted by EnvisionAudio


Eh...probably like this: (+)=====(-)--Zero Volt Ref--(+)=====(-)

thats how i did it. no gnd connection to the PS for the amp.
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Old 19th December 2007, 10:37 PM   #10
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Thanks guys

Can I ask what the SMPS modules you used were? And where you picked them up?
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