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PostPosted: 11 Feb 2019, 15:47 
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Joined: 19 Oct 2018, 15:30
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Location: Montréal, Québec
Hi guys. I’m in the process of designing the cheapest power supply possible using a triad n-68x isolation transformer and a voltage doubler rectification. I used psu designer to do the basics and see if it can be done, but i’m stuck in the filtering and smoothing part of the psu. I’ve read the article on power supply design, but I still have questions.

My goal is to design a small power amp for the beginner who doesn’t want to spend 100$ or more for a power transformer, and at least 100-120$ for each opt. I know that some compromises will need to be made in order to acheive this goal, so I want to stick to common, cheap and easily available parts. Hence the use of a 22$ isolation transformer as a power transformer. Btw, if you have some experience with building power amps on a budget, I welcome all suggestions.

The problem : How do I choose the right reservoir and smoothing capacitors size, and resistor size ?

I would like to Have the least voltage drop possible, so chokes are out of the equation (they also cost a lot and they aren’t that easy to find)

I think i’ll buy two 470uf 250v electrolytics for the doubler. Psu designer « suggested » 220uf for the reservoir cap and filter cap.

I will add more information as my research goes on. Thanks to everyone !


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PostPosted: 11 Feb 2019, 17:23 
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What tubes? What power? What configuration?

The transformer you choose (any kind) must be designed to transform current and voltage to the amp circuit and making sure that the amps and volts are enough - power that is. So is an isolation transformer cheaper than a "normal" mains transformer? I mean, that whatever transformer you pick, it still must be able to supply the voltage/current/power needed and maybe the difference in price is very little.

Check EdcorUSA, their transformers have a nice price/quality ratio.

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PostPosted: 11 Feb 2019, 18:12 
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Soundbrigade wrote:
What tubes? What power? What configuration?

The transformer you choose (any kind) must be designed to transform current and voltage to the amp circuit and making sure that the amps and volts are enough - power that is. So is an isolation transformer cheaper than a "normal" mains transformer? I mean, that whatever transformer you pick, it still must be able to supply the voltage/current/power needed and maybe the difference in price is very little.

Check EdcorUSA, their transformers have a nice price/quality ratio.


I am not exactly shure which tube i'll use. The idea behind this project is to find a way to build a small amplifier with cheaper parts (mostly iron) than what Edcor or hammond cost. I also tried to find parts that are more accessible than true audio transformers. I am not seeking a perfect audiophile too ! I just want to build something that is fun to build, cheap, and that works too.

I was thinking push pull because it seems that push pull OPT are easier to find at cheap prices, but it requires double the amount of output tubes, wich raises the cost again. Single ended circuits are simpler, but the transformers are harder to find and not as cheap. Since I found new 10w 70V line matching transformers for 5$, I decided it was hard to beat.

This kind of answers the power question : below 10W.

As for tubes, I think that the el84 would fit this kind of project. not shure of the exact topology yet. If you have any suggestion of a small power tube that can be run at about 300V plate voltage, let me know !


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PostPosted: 11 Feb 2019, 18:22 
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Joined: 19 May 2011, 05:38
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Triad n-68x isolation transformer is a 50 va transformer with a dual 115 vac primary and a single 115 vac secondary available for about $16.00 us. voltage doubler will yield 300 to 330 volts dependling on load. Install it backward using the secondary for primary will yield about 250 ish volts. Very versatile transformer for the money.


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PostPosted: 11 Feb 2019, 19:36 
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Cheap transfomeres from aliexpress is always consideration, you typically get scammed fake transitors but not fake caps or fake trasnfomers and resitors.


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PostPosted: 11 Feb 2019, 20:16 
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Location: US Pacific Northwest
Gab wrote:
The problem : How do I choose the right reservoir and smoothing capacitors size, and resistor size ?
You start by assessing your chosen power transformer. You are using a line isolation transformer. This type of transformer is built and rated for 360º secondary conduction. When you run any type of rectification bridge from these, conduction angle is a major concern. You will have to derate the current available roughly proportional to conduction angle to avoid overheating the transformer core and windings. So first, always use a full wave bridge or doubler, and second, choose the smallest first capacitor you can (conversant with the desired ripple) to maximize conduction angle in the secondary. Then worry about ripple reduction afterward.

Gab wrote:
I would like to Have the least voltage drop possible, so chokes are out of the equation (they also cost a lot and they aren’t that easy to find)
This makes no sense to me. Chokes typically give the lowest voltage drop verses filtering and are relatively cheap. For example, the Triad C-14X (6H, 200mA, 150Ω) is available for $15.48 from Mouser and the C-24X (1H, 240mA, 50Ω) is only $9.78. I would hardly call these options "expensive". Especially given the level of filtering provided when coupled with a reasonably size capacitor.

Gab wrote:
I think i’ll buy two 470uf 250v electrolytics for the doubler. Psu designer « suggested » 220uf for the reservoir cap and filter cap.
Again, be very careful of the conduction angle. Isolation transformers are not rated for pulsed loads. Too large a first capacitor and you will burn up your transformer even at nominal loads.

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PostPosted: 11 Feb 2019, 20:33 
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Joined: 19 Oct 2018, 15:30
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Location: Montréal, Québec
I wasn’t aware about conduction angle. That might explain why i’ve read people who said that the current ratings dropped by about 80%.

I’ll recondsider chokes. I might not have catch everything about them. That is exactly the kind of answers I wanted !


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PostPosted: 12 Feb 2019, 03:03 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
When ordering from China you must take in consideration that freight may add a lot to the costs.

Building a "cheap" amplifier can be done if you manage to find some old stuff that can be butchered, like old tubular tape recorders, radios or similar stuff.

Also the use of mains transformers has been seen and here's a fun use of such transformers. It's a way of turning a SE-amp almost behaving as a PP-amp. Try to find more info about this odd design, which is called Matrix. Here, two 2x12V mains transformers are used as OP-trannies.

Attachment:
matrix_darling.gif


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PostPosted: 12 Feb 2019, 08:21 
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Joined: 19 Oct 2018, 15:30
Posts: 101
Location: Montréal, Québec
Soundbrigade wrote:
When ordering from China you must take in consideration that freight may add a lot to the costs.

Building a "cheap" amplifier can be done if you manage to find some old stuff that can be butchered, like old tubular tape recorders, radios or similar stuff.

Also the use of mains transformers has been seen and here's a fun use of such transformers. It's a way of turning a SE-amp almost behaving as a PP-amp. Try to find more info about this odd design, which is called Matrix. Here, two 2x12V mains transformers are used as OP-trannies.

Attachment:
matrix_darling.gif


I’ve never seen this design before. Does anyone know how it sounds ? It is weird that they used two separate transformers to create some kind of center tap between them. There is no phase splitter stage too. If I understand correctly, the upper transformer pair takes care of the positive phase of both left and right channels, and the lower pair reverses the positive signal to create the negative phase of both channels. Kind of like phase splitters that use interstage transformers.

I frequently look up for old tube stuff for sell, but there’s not much to find in my area. I frequently see old organs that people are willing to give, or old radio consoles. I could use the parts inside those old hammond organs and build an amp with their transformers, but I think it’s bad to destroy a music instrument that’s perfectly good.

And for Chinese transformers, their prices are very appealing, but most of the time shipping costs more than the transformer’s price. I don’t use aliexpress, but there might be some interesting deals with free shipping. I’ll have a look at this.


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PostPosted: 12 Feb 2019, 08:54 
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Joined: 19 Oct 2018, 15:30
Posts: 101
Location: Montréal, Québec
Triad n-68x 120v : 22$
Triad f-16x 6.3v : 19$
Caps for the doubler : 5$ each (approximately)
Already 50$, and I didn’t count the cost of the added parts needed to smooth out that rectified dc.

A hammond 269ex power transformer costs 54$ excluding shipping, so there is absolutely no need for me to go through all the trouble of creating a power supply with parts that were not meant for this purpose.

« I haven’t checked aliexpress yet, but I doubt that i’ll find something that is of the same quality at this price point. » - output transformers are available at much lower prices, but most of the power transformers offered have a 220v primary wich makes them useless in North America.


Last edited by Gab on 12 Feb 2019, 12:51, edited 1 time in total.

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