DIY Dual Mono Power Amps

[PJPro]

Haven't posted for a while as I've been scouting around for my next project. I'm going to build a couple of mono power amps for use with my Beresford DAC.

The amps are based on the LM3886 chip amp. These chips are capable of delivering a very reasonable 68W. Each amp will have it's own dedicated power supply and be housed in a separate case.

The Chip Amp (or Gain Clone) is an extremely simple design, cheap to build and good for beginners such as me. Moreover, it's supposed to sound pretty good too.

I have already ordered the kit and it arrived today. The kit was purchased from ChipAmp.com and cost me £67. However, that only covers the cost of the PCBs and onboard components. Using the cost breakdown covered in my post on building the Novo, I expect the PCB and components to represent about half the cost of the completed project. However, as stated in that thread, the PSU supplied with the Novo was on the cheap side. So, allowing for two toroidal transformers, I expect that the total cost will probably be close to £200.

More to follow....

 

[Anonymous]

Really interesting kit, what will you be using for cases? I understand that these can be one of the surprisingly expensive items.

Good luck with it, I'm looking forward to following along with you.

 

[idc]

Excellent, another big fan of your build projects ready and waiting

 

[Dave_]

Yep,looking forward to see how the project pan's out.

 

[Andy H]

I can't wait, the last project you did has got me thinking about having a go in the near future.

 

[Craig M.]

all of the above

 

[PJPro]

zzgavin:
.....what will you be using for cases? I understand that these can be one of the surprisingly expensive items.

I've got a few ideas and have been stockpiling links to manufacturers/sources.

 

[PJPro]

Just a few pictures to whet your appetite.....

The first shows the complete kit. On the left are the two amplifier chips, then the amp PCBs and above them the other amp components. Down the middle are the two power supply kits and on the right the four enormous power supply capacitors and above them two freebie stereo PCBs.

The chip amp kit

Mono amp PCB closeup

Power supply PCB closeup

 

[PJPro]

So, here's where I am so far......

There are build instructions provided on the Chip Amp site here.

Building the Power Supplies

The first step involved identifying all the bits. Again, the resistors were a bit tricky but the kit provider had gone to some lengths to help by colouring the reel tape and refering to it in the instructions.

Power supply components and PCBs

Cracking straight on with the build, I whacked in all the resistors and a single small diode. You need to get the diode the right way round. The PCB is clearly marked to help. Two of the resistors are actually placed on the underside of the PCB.

The resistors and small diode are in.

The polypropylene capacitors and the electrolytic capacitor for the LED go in next. You need to be aware of the polarity of the electrolytic....the long lead goes into the positive pad.

The polypropylene and LED electrolytic capacitors are in.

Next up were the large MUR860 diodes. I think these are instrumental in converting the AC supply into DC. They were a bit tricky to solder in as they are a loose fit in the PCB and I wanted them all lined up and neat. However, by supporting each diode underneath by using my pinkie I got them all in pretty much straight. You do need to ensure thay are placed in the board the right way round...with the metal plate towards the bold line on the PCB.

The large diodes in.

The large diodes in.

Finally, the huge power supply capacitors go in. OMFG! These large caps are enough to, errr....never mind. Again, be aware of their polarity when placing them into the board. The terminal blocks are the final components to be soldered into place and go underneath the board (though see comment on LEDs below).

The power supply capacitors in.

Here are both of the completed boards....

Completed dual power supplies.

To fully complete the boards requires the LEDs to be soldered into place. However, I haven't finalised the case, etc so I'm going to put the LEDs in later.

These took about an hour to complete from start to finish.

 

[PJPro]

So how do these components hang together? I have provided the schematic from chipamp.com below.

Power supply configuration based off of a unregulated design by Carlos Filipe

I'll have a go at explaining the schematic, but don't take my word for it cos I'll probably get it wrong!

The power comes from the mains and through a transformer (on the left). A transformer with dual secondaries is required to power both sides (rails) of the board. The power goes through the diodes (rectifying bridge) which converts the AC to DC. The big capacitors serve to filter out the ripple introduced. The negative from the upper rail, combined with the positive from the lower produce the protected ground.

That's about as far as I can go. Grateful for any corrections.

 

[PJPro]

Incidentally, the above is for a single PSU.

 

[PJPro]

To complete the PSU I need to buy a couple of transformers. You'd think this would be easy but I've spend a few nights considering my options.

The manual which comes with the kit (link above on an earlier post) reckons that 80VAC (or volt amp current) per channel should be more than adequate. So that would suggest two 80VAC transformers then? However, when you look around poeple seem to use a lot of different sizes.....arguing that bigger is better (soundwise).

There's also another complication of which I was largely unaware....the impedence of the speakers. OK, so I knew that one had to be careful when matching speakers to amps, particularly ones which are difficult to drive, but it seems this is far more important than I imagined.

Depending on the impedence of the speakers, different transformer voltages need to be selected. So for this particular chip, for 4 ohm speakers, you don't really want above 25VDC (volts direct current), 6 ohms around 28VDC and for 8 ohms around 35VDC. It's a bit of a pain that once a transformer has been selected that it limits current and future speaker selection.

I don't have any speakers for these amps yet. I need something small so was considering the Dali Lektor 1s (8 ohms). However, the Lektor 2s don't seem much bigger so I thought about these as a possibility (6 ohms). I was also considering using a couple of my Musos (4 ohms). Damn! I finally decided to go for 8 ohm speakers (not sure which ones), though I will return to this below.

So now I knew I wanted 35VDC. However, the rectifying bridge increases the AC voltage from the transformer by around 40%. So, to get 35VDC into your amp, you need a transformer with secondaries rated at 35/1.4 = 25V. OK, so 2 x 25V secondaries. I'm narrowing it down.

Back to the VAC. The volts x the amps required = VAC. How many amps are required? Hmmm, not sure. As I said earlier, people tend to go for monster transformers which is supposed to improve the sound. I decided to go for two 160VAC transformers. This equates to 160/25 = 6.4A of available current.

So, 160VAC transformer with 25V dual secondaries, i.e 2x25V. I didn't really consider any type other than toroidal but which one to get? I wanted quality. I wanted low noise. I wanted economy. In the end I went for two of these fully encapsulted jobbies from RS Components here. Note: I believe that Nuvotem are a european version of Amveco.

Nuvotem fully encapsulated transformers.

So, back to the impedence issue. Although one could argue that an amp tuned to the attached speakers will be a better match and not suffer the consequences of commercial amps, which need to support all speaker impedences, it's a wee bit inconvenient.

Clearly, it'd be good if there was a way to accomodate any speaker. I have created a thread on diyaudio on this very topic here.

From my generally ignorant view point, voltage regulation would appear to be the way to go....so that the power getting to the amp is reduced to match the attached speakers but it's never as simple as that. Any luck, I'll get a simple solution from the posters on diyaudio.com.

 

[PJPro]

By the way, these two transformers cost me £64.73 delivered. This is only a couple of quid less than the kit at £67!

 

[PJPro]

PJPro:So how do these components hang together? I have provided the schematic from chipamp.com below.

Power supply configuration based off of a unregulated design by Carlos Filipe

I'll have a go at explaining the schematic, but don't take my word for it cos I'll probably get it wrong!

The power comes from the mains and through a transformer (on the left). A transformer with dual secondaries is required to power both sides (rails) of the board. The power goes through the diodes (rectifying bridge) which converts the AC to DC. The big capacitors serve to filter out the ripple introduced. The negative from the upper rail, combined with the positive from the lower produce the protected ground.

That's about as far as I can go. Grateful for any corrections.
Perhaps I should have mentioned that the snubberising capacitors don't appear on this schematic. Snubber caps sit around the rectifying bridge and remove high frequency noise added by the bridge.

 

[idc]

Sigh, just as I think, I can do one of these kits, you go and post the above. Maybe that Carlos Filipe does night classes.

 

[PJPro]

Sorry. To a certain extent, it's just babble. You don't need to understand how the circuit works to be able to make it. I don't!

The tricky bit is sorting out the bits that don't come with the kit. If I'm lucky I'll be able to sort it all out, providing a simple path for others to follow.

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

 

[PJPro]

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

 

[PJPro]

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

 

[Anonymous]

 

[PJPro]

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.

 

[PJPro]

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.

 

[Anonymous]

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!

 

[Dave_]

MS Mezzo 1's or 2's?

 

[PJPro]

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.

 

[Anonymous]

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.