DIY Dual Mono Power Amps

PJPro:

How many amps (current) are required? Hmmm, not sure.

PJPro:

At the moment, I'm concerned that I've gone for too lower powered transformers. Oh well, they only cost £65 (sob).

I'm not entirely happy with the above statements and would like to address the current required. So, I've had a look at the datasheet for the LM3886 here.

If the chip is supplied +/- 35VDC it will deliver a average output power of 50W when driving a 8 ohm load. I believe W = AV, so A = W/V. Therefore, the transformer must be capable of supplying 50/35 = 1.42A.

However, this isn't the whole story. We need to calculate the peak current required. Thankfully, the datasheet helps.

    Peak Current = sqrt( ( 2 x average ouput power ) / load )

So, with a average power output of 50W and a load of 8 ohms....

    Peak Current = sqrt ( ( 2 x 50 ) / 8 ) = sqrt ( 12.5 ) = 3.5A

Therefore, the supply needs to be able to deliver: +/- 35VDC @ 3.5A.

My transformers each produce 2 x 25VAC @ 6.4A. However, the rectifying bridge has the effect of increasing the voltage seen on the power rails by a factor of 1.4. So, 25 x 1.4 = +/- 35VDC supplied to the amp.

Assuming I haven't made a silly error and that I roughly understand what I'm talking about (dangerous assumption?) the tranformers I bought should be OK. What I don't understand is why people buy transformers supplying 12A?

As always, grateful for any corrections/comments/clarifications

Perhaps I'm getting a little bogged down in the detail here, so pressing on with the build....

I firstly identified and then placed in all of the resistors. When placing in R3, I went for the option which utilised the feedback capacitor which produces a lower DC offset.


Resistors in

I next put in the capacitors, being careful to observe the polarity with the electrolytics. With these, the long lead has to go into the positive pad.


Capacitors in

Finally, I put in the LM3886 integrated chip. I soldered in one lead first and then used that to hold it in place while I moved it into the precise position I wanted and then soldered the rest. Easy.


LM3886 in

I then completed the other board.


Both completed amp boards

Here's a couple of photos showing both amp boards and their respective power supplies


Dual mono amps and power supplies


Dual mono amps and power supplies

A poster called Redshift187 (diyaudio.com) has alerted me to a resource provided by National for the calculations I was trying to perform. It's called Overture Design Guide and is available here as a spreadsheet. The accompanying pdf guide is available here.

Well, my transformers are here. They arrived towards the beginning of the week. I need to get on with ordering the remaining components and case. But for some reason I'm dithering. I've spent nights looking at cases without making much headway.

Anyone care to recommend some small, wall mountable speakers? 8ohm impedence only. Ideally biwirable. The Dali Lektor 1s spring to mind. DB1s would be my ideal but rather too much money.

Great post, enjoying seeing the pics and what you're doing. Fancy it myself but I'm not sure I've got the patience or skill with a soldering iron!

How about a nice pair of AVI Neutrons? Doesn't need to be the latest Vs. Amazing little boxes.

EDIT - there's a pair on hififorsale.com, piano black finish, £350, Neutron V (latest). Excellent!

MS Mezzo 1's or 2's?

igglebert:

Great post, enjoying seeing the pics and what you're doing. Fancy it myself but I'm not sure I've got the patience or skill with a soldering iron!

How about a nice pair of AVI Neutrons? Doesn't need to be the latest Vs. Amazing little boxes.

EDIT - there's a pair on hififorsale.com, piano black finish, £350, Neutron V (latest). Excellent!

Ooooh. They look gooood. Can you confirm 8 ohm impedence? I've had look on the web but, strangely, the impedence isn't listed.

Yeah, I noticed that too. I had a look around and a couple of places stated them at 8 ohms nominal. I guess it would be more valuable to know a bit more about their overall impedance, i.e. a curve. I'll have a dig tonight and see what I can find. I reckon they'd be a good long term purchase though.

Shame to hear about the delay for the Starving Student amp build. Since you are asking about upgrades, how about the AKG K702 over the Grados?

I think I'm narrowing down the cases. Take a gander at these. I think I'm going to go for the 330mm by 170mm (same version but different dimensions than the one shown below).


Galaxy Maggiorato with beefy front panel

These are much bigger than I thought I would choose but I don't want things to be too cramped inside the box. The main thing is that they are shallow (170mm). I've wanted an amp to fit on a 200mm wide shelf in the kitchen above the telly for some time now.....but for some reason all commercially available amps are around 330mm deep or more. So, the size of this case meets my design requirement.

Infact, I've also had a look around at Tube PreAmp kits as well, to ensure that I can find something that will fit in the same size case. If I make a Tube PreAmp, it'll be based on the Aikido. This is a classic and supposedly sounds excellent. Trouble is, this will need two cases....one for the preamp and another for the power supplies. So that'll be four boxes in total.

Have I mentioned how much these cases cost? Errrr, ?26.80. The extra thick 10mm front (in black) costs an extra ?16.00. So a single case is ?42.80 and for two it's ?85.60. Postage is ?17.00. Then there's 20% VAT on top. So, ?123.12 or about £105 just for the power amps.

Hmmmm. Dual mono kit £67. Two transformers £65. Two cases £105. Other stuff? Errrr, £70? So total cost of about £300. They better be good!

In fact, with the Aikido preamp, I reckon the total cost will be close to £600. That's serious money.

The trouble is that this DIY stuff is hugely additive and immensely satisfying (so far). Until now, it's been cheap too. Anyone seen the b24?

Well, I've taken the plunge and bought the cases. I went for the 330mm x 230mm in the end. With the thick front panels and 8 feet, the total cost, including VAT and delivery was £130.94. Gulp!

My wife isn't going to be pleased. We're supposed to be cutting back!

igglebert:

Yeah, I noticed that too. I had a look around and a couple of places stated them at 8 ohms nominal. I guess it would be more valuable to know a bit more about their overall impedance, i.e. a curve. I'll have a dig tonight and see what I can find. I reckon they'd be a good long term purchase though.

I was looking at AVIs on ebay. They look quite deep. I need something shallow really. In fact, the shallower the better. Given the eventual cost of the power amps and ultimatley the pre-amp, the DB1s would seem a better purchase. I'm just having trouble justifying it to myself.

Anyway, I'd be grateful for any info you can find.

idc:

Shame to hear about the delay for the Starving Student amp build. Since you are asking about upgrades, how about the AKG K702 over the Grados?

I have been wondering if I should upgrade my headphones. I'd like to but.....