My DIY Stereo Amp Thread

[Crafty]

Ok from time to time, I will come back to this thread to post questions regarding the Stereo Amp I am building, rather than creating a new thread for every issue I encounter.

1st things first. I've shelved the amp kit, and started with the power supply.
the toroids will be linked to give v1 0 v+ output. After rectification, comes the caps. What is best practice here?
Some diagrams show 2 caps per side. Others say one big one.
Best I've seen so far is 2 per side, and then it's these puppies:
http://shop.rabtron.co.za/catalog/elect-stud-15000mfd-p-6402.html
Do I really need 4 of these for a 2 x 50w amp?
The rule of thumb is 1000uf for every ampere drawn.
So how many amps can I expect from a 2 x 50w amp?
Will 10K uf be sufficient?
Where Is the best place to buy caps from?

[tangmonster]

Hi crafty

caps in parralel is almost allways better than a single bigger one.

that 15000uf is a bit on the expensive side.

You can go very boutique with caps ,but for 50W x 2 into 8 ohm loads i would say 10000uf per rail is nice.

This all depends on your toroidals.

"the toroids will be linked to give v1 0 v+ output"  i am not sure i understand?

What size are the toroidals?

Be snazzy and use 2 x 4700uf in parallel per voltage rail. something like this:

http://www.mantech.co.za/ProductInfo.aspx?Item=14M7965

[Crafty]

Sorry I typed the output wrong

it should have been v+  0  v-

Size indication of the toroidals:
They are standing on a small form factor IBM desktop PC.

[fdlsys]

Size indication of the toroidals:
They are standing on a small form factor IBM desktop PC.

!!! Consider the weight once everything is in the box!

Caps: As Tang said, those 15000uF are WAY overpriced. Check Mantech & RS, much  better pricing.

Crafty, the main reason why multiple caps per-bank in the PSU is lowering of the ESR.
For example, it's better to have two 10000uF at 0.03ohm ESR (total in parallel 0.0015), then a single 20000uF at 0.02ohm.
But, you will have to find the balance of all important parameters - ESR, life expectancy, temperature.

The other equally important reason for multiple parallel banks of caps is the ripple-rejection PSU filter configuration itself - with a single pair of large caps you will have the (R)C filter only, while with two sets (banks) of half-size caps you will have the (R)CRC which is perfectly fine for most amps. First (R) is the resistance/inductance of the transformer secondary, the second R are the power resistors inserted into the + and - rail between two banks of the caps; usually between .1-.2ohm, 10-20w (depends on the current draw).

IMHO, your best bet are the "computer-grade" electrolytes (eg. Sprague Powerlytics) because they are (allegedly) subjected to quite rigorous testing and are being over-designed to begin with.

[Crafty]

Thanks for the brilliant replies!
Are "Electrolytic Can" capacitors better in any way, that the normal Electrolytic radial surface mount caps?

[tangmonster]

You must compare the series resistance and ripple current to compare capacitors.
I would guess they have slighly better heatsinking?

But you must also remember for a 50W stereo amp i doubt ripple current or heat is really going to be an issue.

You can sepnd hours trying to figure out which cap is the best.

You will be using a transformer per amplifier.

Which amplifier kit will you be using?

For 8ohm loads at 50W with a dual mono setup i would say personally between 4700uf per rail per transformer will be just fine.

For possible 4 ohm  loads ( can your amplifier kit handle 4 ohm loads even? ) I would say 2 x 4700uf per rail par toroidal ( 8 in total)

Both of these are far above that commercial (low to midrange cost amplifier) standard believe me.

check for example this marantz 5 channel amplifier.

http://www.novomusica.com/sites/default/files/imagecache/ampliacion/Marantz_sr6005_inside.jpg

It is rated for 110W x 7 and it has a single 15000uf capacitor per channel. 



ps.

not dissing marantz , just goes to show that a a normal good amplifier PSSR is great and you won't really hear a 0.5 db drop if the average rail voltages drops a bit under high load. But in DIY you can go crazy with the capacitors and sleep easy that your power supply is rock solid!

[Crafty]

I bought the Velleman K4004B
Ratings:

2 x 50W /4ohm; 2 x 40W/ 8 ohm
1 x 100W Bridged 8 Ohm
Harmonic Distortion: 0.01% At 1kHz
Stereo Channel Separation: 85dB
Consumption (2 X 8 Ohm): 2.5A Max.

PSU: Power Supply: + 28VDC And - 28VDC Max.

I can just go and buy the velleman power supply kit, but that will strip away some of the fun. I like the idea of designing my own PSU, and on some components I already have.

Full Details on the Kit:
http://octopart.com/k4004b-velleman-5349264

[tangmonster]

For that kit you can only use one of the 2 toroidals. I would not feel safe to parallel them.

2 x 4700uf per rail will be more than enough for 40W 8ohm x 2

[tangmonster]

Also

what is the transformer AC voltage?

they must not be higher than  20V AC to get around +- 28V DC

[Crafty]

Well each of the Toroidals give 25v if measured separately.  Then they only have 2 output wires. (Not talking about secondary windings)
So How will I then go about wiring to get the v+ 0 v- type output?
After the 2 bridges you bring them together to form that output.
If I use only the one, then I will be slightly under powered?
Heinrich had a look at them yesterday, and perhaps he can shed some light as to what type they are?
He drew a little diagram that clearly illustrated the concept, but cannot recall exactly.

[tangmonster]

Do NOT parralel the dc after the bridge. You will have smoke and fire probably. it might work , but NO it won't.
Some toroidals are specced to be able to parralel their windings  but don't do it to unknown ones.

Each of the toroidals have 2 windings. So EACH toroidal is then 25 0 0 25 ( 4 wires in the picture per toroidal)

look at these diagrams to see how you connect the 25 0 0 25 to get +-35 DC

http://www.tnt-audio.com/clinica/ssps1_e.html


You cannot use these toroidals for the velleman since the velleman is specced for +-28V dc and you are planning to supply it with +-35V DC.

+-35V is perfect for a lm3886 chipamp. You can have a nice dual mono 50W into 8ohm x 2 using 2 lm3886 chips. with those two toroidals.

ask rudi for lm3886 pcb's he has designed some. I have only ever built them using strip board.

[Heinrich]

it is 2 toroids with only one output winding each , so he will have to use one transformer for the + rail and the other for the - rail.

i suggested similar as the last diagram in the link above, with the exception of to individual transformers with one individual output each instead of one transformer with 2 output windings.
i am however not sure what the rectified voltage would be exactly if the AC input is 25V into each bridge?

[tangmonster]

i am however not sure what the rectified voltage would be exactly if the AC input is 25V into each bridge?

picture shows 2 red wires per toroidal and 5 wires on the one and 4 wires on the other? My guess is the two red's are 230V AC input.
Not sure about the single secondary winding each though? Or do i have the secondary and primary confused.


roughly multiply by 1.414 so it will be +-35V DC after rectification.

[Crafty]

Look, what I can do, is build the psu, using both toroidals. If it is well above 30v each, I will save that power supply for a next project.
If push comes to shove, then I buy the psu kit for R190-00, and get a small toroid transformer with a center tap (Hope I have the right term there)
Is it possible to find one mother of a resistor and bring the output down a couple of volts?

tangmonster: The red wires is the input, correct. Then each toroidal has 2 thick output wires that went into the rectifier of the previous amp it came from. The other wires are a lot thinner, and went into the front panel for the lights and buttons, and from the second one went to the pre-amp on the mother board.

[tangmonster]

"If it is well above 30v each"

It will be 35V.


"The red wires is the input, correct. Then each toroidal has 2 thick output wires that went into the rectifier of the previous amp it came from. The other wires are a lot thinner, and went into the front panel for the lights and buttons, and from the second one went to the pre-amp on the mother board."

AH! copy that!