DIY Amplifier Suggestions - on a tight budget

[Taligent]

An amp built around a LM3886 or LM3875 is a good example of the ease with which costs for a DIY project can spiral if you allow it - the amp itself is a single chip, but the components around it can be as expensive as your imagination will permit.

If you're new to electronics, you may want to put together a LM3886 amp with good quality components and later experiment with components like PTFE caps or such if you wish.

The main expenses are in the power supply transformers, power supply capacitors, and heatsink - but all of these can be easily found inexpensively at local hamfests, swapmeets, surplus stores, old equipment, or eBay - without sacrificing performance in the least.

As an example, some of the ordinary expenses ($USD) I've come across when looking for parts to build audio systems...

• Transformers - $10 for a well built 330VA EI core, apexjr.com (if there are a few Signal BL1733A's still available, go for it - perfect for LM3886 chips), free from older equipment also in fine condition.
  
  The LM3886 can average 40-60% efficiency - a transformer should be rated for around 120-150VA or so per channel at the desired supply voltage, at minimum. 300VA is a good size, due to the versatility with other projects and with running two chips from one transformer if need be (surround channels, active crossovers, etc). Also note that the LM3886 is capable of 80w or so into 8ohms with higher supply voltages, without increasing distortion. The datasheet show it as possible, as well as the bridge-parallel version of the amp noted through National - to be tested at +/- 42v and beyond in a car amp soon to see what it takes to destroy a chip under load.
  
• Heatsink - $5, 12"x8"x1.5" beautiful black anodized aluminum, hamfest
  
• Capacitors - $0.50 per 10,000uF/50v Philips capacitors, lot of 50 on eBay
  
• Good quality passive components - Sprague 716P/Panasonic P-series polypropylene capacitors, Panasonic FC series electrolytic capacitors, metal-film resistors - are easily available through Digi-key, Mouser, and other online distributors.

Built yourself a case (shield with metal if using wood, acrylic, etc for the enclosure), and the total can be as low as $50USD/channel, fully individual, depending on your willingness to look for components.

The natural and immediate urge is to throw in the most expensive components available because of the extreme ease of substitution (with strong encouragement of such by vendors) and low absolute cost at the part level, but there are usually better ways to spend those funds. For example, the difference in perceived sound could be potentially far higher by designing an active crossover for your speakers and using an amp per driver, rather than placing expensive components in an amp driving a passive crossover. It's a choice between throwing money at a problem versus learning about where the majority performance losses are in an audio system and addressing those issues.

The LM3886 is well designed and very easy to use - put a case around it with components you can easily afford and listen to the result. Even if you find yourself unable to sleep at night with speculations of other components, it will serve as a control for later experimentation. Also, be sure to read through application notes available from National, TI, Analog, Maxim, etc to get some ideas on general design of the power supply and other supporting components. It's fairly straightforward, really...

Have fun!

-Nikhil

Edit: Added series type for capacitors...

[pinkmouse]

All my many GCs are built using good quality off the shelf parts, Evox rifa for the small caps, Rubicon ZL for the electrolytics, and standard metal film resistors. This was because I was more interested in comparing the different chips, and if I had spent £40 on parts for each one then I would be even more broke than I am currently!

The finished results compare favorably with commercial £1000 amps, even tho I prefer OPA chips to the LMs. If you want to know why, read Peter Daniel's comments in the NJAGC thread, his experiences are similar to mine, we just come to opposite conclusions from a taste point of view

[mrfeedback]

Hi all GC owners.
I am moving house this easter, and i have dug out a few old chassis that are not worth restoring.
The TINAGC thread is too long to read through, so my \question is - What is the concensus on supply voltages for LM3865 and LM3886 chips, and what is the concensus on sonic differences for these two chips ?.

Thanks, Eric.

[joensd]

Hi all GC owners.
I am moving house this easter, and i have dug out a few old chassis that are not worth restoring.
The TINAGC thread is too long to read through, so my \question is - What is the concensus on supply voltages for LM3865 and LM3886 chips, and what is the concensus on sonic differences for these two chips ?.

Thanks, Eric.

Looking at the datasheet I´d say go with 18V secondaries if you´re using 4Ohm speakers.
This will get the most out of 4Ohms and the least power dissipation with those.
For 8Ohm´s I´d use 24V.
I´m using 18V myself and it´s enough for me with any speaker.

Soundwise I didn´t have the chance to compare.
Others might help here.

If you want to p2p-wire it, use the LM3875 then you don´t have to connect 8 pins (LM3886) but just 5.

Regards
Jens

[elizard]

ok, so more questions!
thanks so far .. helped me lots

1) If apexjr has the signal BL1733A transformers, should i get one or two? (if i'm building monoblocks)

2) If above is not available, would it be wise i order a torroid from plitron (they're in canada, preferably order as much as possible from canada)

3) I know the spirit of gainclone is as simple as possible, therefore no pre-amp. But I need one, switching inputs is simply too much of a hassle for me (have 4 inputs, listen to at least 2-3 different ones all the time). I just built an active preamp, should I use that, or build a passive one (like rarkov)? As suggested before, just a 50k resistor connected to the IN jack and to one end of the 4.7uF cap should do the trick?

I found the schematics for both the gainclone and its PS that were posted by algar_emi earlier in the gainclone thread, This is not just another gainclone
and his price list too. That's something a newbie like me needs!!!

The only thing I'm not too clear on is how I'll connect the rectifier diodes, but I'll worry about that later. Other than that, its all good!!

Oh yeah
4) What sort of wire should I use? Something that I can get for maybe $5/foot (for internal wiring) or under. Or should I start off with just normal (what gauge?) copper wire, and work from there?

Thank You

[Philo]

Elizard,
You don't need to swap inputs if you can get away from monoblocks. My amp is a integrated version that is fully enclosed and sound excellent. No hum, crosstalk or any other spurious problems. I also cut down on my machining time and material costs. It easier to move and has four inputs. Not as "hifi looking " as monoblocks but I don't care as this amp has spanked every other power unit of friends and neighbors in shootouts. It hasn't gone up against anything more expensive than a couple of Crown A/V Monoblocks ($2k apiece) but did humble one Creek 5350SE owner last week who thought the GC had a richer, warmer and more involving sound. Take a look at the picks and decide for yourself

[elizard]

damn .. well what the hell did i build my preamp for

ok .. how about this .. i can always re-build the things, right?
so start off w/ using my preamp to cut down the costs .. then i can move to integrated as you have

i was debating of putting the monoblocks into one chassis anyway though ..

[Philo]

I have changed a few things since these picks, like using MSR860 diodes and done some dampening of the transformer mountings. I will be changing out the binding posts soon but that will require some additional drilling of the back plate and downtime I don't want to have.

Here a pick of my rectifiers. Four of these are going into an intergrated AlephX/BOSOZ/APOX-1 unit I am thinking about... ok, ok dreaming about. Look at the datafile for the diodes you have for the orientation and build it from that.

Edit: You built the preamp because all this stuff we do is FUN! . Don't you need another stereo later in another room? Anyway, try the monoblock version to go with your preamp. You might need to work out some ground loop hum problems that I encountered when using my GC with my Parasound pre. I ended up loaning my pre to a friend who had his system stolen in burglery, so I built an integrated version. Which, will end up going to my daughter as soon as I can part with it.

[elizard]

Allright, awesome
Well, I guess you gotta try stuff, eh ..

Still my question about the transformer (my main worry) goes unanswered.

If I were to get the Signal BL1733A from www.apexjr.com/Clearance.htm should I get 2 of them? They're $10/piece (if they still have em) so that's VERY cheap for me .. Would I also need any modifications? Or are they good to go as they are?

[elizard]

Or how about 2 torroidals from plitron, $55/each
I can get them either in 18,20,or 22VAC .. and they're rated at 160VA?
part# 057015201 for the 22VAC version
Would 2 of those be good?
Still, I'd rather get the 2 from apexjr if there isn't anything major for me to do, or a lot of drawbacks from them ..

plitron i'd prefer because they're in canada - cheaper shipping/no customs