I'm looking to buy an LM3886 kit as my first DIY amp and was wondering if there is much difference between the kit on the chipamp website:
Non-Inverting LM3886 Stereo Kit | Chipamp Electronics
and this much cheaper kit on ebay:
audio power amplifier LM3886TF AMP board KIT+Rectifier filter power DIY KIT | eBay
It looks as if the circuits are different designs as on the Chinese version there are more resistors on the amp boards, and more capacitors in the power supply.
Does anyone have any experience with these Chinese boards that could offer some advice as to whether they are decent quality?
Any help would be much appreciated.
Non-Inverting LM3886 Stereo Kit | Chipamp Electronics
and this much cheaper kit on ebay:
audio power amplifier LM3886TF AMP board KIT+Rectifier filter power DIY KIT | eBay
It looks as if the circuits are different designs as on the Chinese version there are more resistors on the amp boards, and more capacitors in the power supply.
Does anyone have any experience with these Chinese boards that could offer some advice as to whether they are decent quality?
Any help would be much appreciated.
The Chinese kit is missing a couple of components that reduce the high frequency oscillations and stabilize the output stage. You can add these components (2.7 ohm resistor and 0.1uF capacitor) if you're familiar with the datasheet, otherwise, just get the chipamp version. I have the Chinese kit, and it will play for a short period of time, then shut down if these components are not in place. Hope this helps.
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Thanks for all the quick replies!
I'm going to go for the Chipamp version as it seems very good quality and has so much documentation. I'll move onto more adventurous projects once I've got started.
Does anyone have any advice with regards to a soft-start circuit? I've been told a soft start /power delay kit will help the life of the fuse?
And does anyone think speaker protection kits are necessary?
Thank you for the recognition.
The chipamp boards are not bad. I do see room for improvement in the supply and ground routing. I would also include a Thiele network (L||R in series with the output) for stability with capacitive load (think long speaker cables). That said, I should note that I'm a tough grader and aim to eek every possible bit of performance out of the chips.
I have yet to be impressed by an eBay layout. Most of the kits are connect-the-dots layouts offering poor performance and are missing components that are critical for stability. They are incredibly cheap, though.
Tom
Just use a larger fuse. Aim for a fuse with a rating of 3*VA/Vmains, where VA is the VA rating of the power transformer and Vmains is the RMS mains voltage. This is a rather conservative rating. 2*VA/Vrms is probably sufficient in most cases. Recall, the fuse is there to prevent your house from burning down in the event of catastrophic amp damage. The fuse does not protect the amp.
With the smallish transformers needed for a typical LM3886 amp (I recommend a 200 VA transformer for a stereo amp), there is normally no need for a soft start.
I personally don't use one. The LM3886 is a rugged chip. If treated well (i.e. operated below the ABS MAX ratings in the data sheet, mounted on a large heat sink, etc.) it will work well for many years. That said, it can fail and if it does fail, it can take your speaker out with it. Whether you think that's worth the risk is up to you.
I perform all testing on a resistive load first. Once I'm confident in the design and assembly, I will move on to real speakers.
Tom
Hi Swazbuckle,
Can you tell more about your source and your speakers (impedance) ?
Waiting, have a look :
http://www.chipamp.com/docs/User Guide - LM3886 Kit.pdf
Phil.
Can you tell more about your source and your speakers (impedance) ?
Waiting, have a look :
http://www.chipamp.com/docs/User Guide - LM3886 Kit.pdf
Phil.
I'll be using a sabre DAC connected to a raspberry pi or my CD player as source material. I've ordered the dual mono + the ALPS pot for volume control. I'll probably build a preamp at some point in the future for use with a record player perhaps.
This is my DAC: HiFimeDIY Sabre USB DAC ES9023 96Khz/24bit
My speakers are 4ohm at the moment, I'll be looking up the transformer values a bit later on. Will I need smaller transformers for a dual mono set up?
Thanks Tomchr for all your help. You mentioned there could be some upgrades made to the circuitry, how would I calculate the values for a Thiele network? If I'm running a dual mono setup I don't think the wire length would be too long but it might be useful if I change the setup. What sort of resistive load do you use for testing?
Thanks again.
For dual mono and 4 Ohm speakers, I use 120/160 va with 20/0/20 volts, like this :
http://www.diyaudio.com/forums/chip-amps/265827-lm3886-fullrange.html
Phil.
http://www.diyaudio.com/forums/chip-amps/265827-lm3886-fullrange.html
Phil.
You can use as low as 80VA to 100VA for a small chipamp and it will work.
But the big downside to small transformer is the very high regulation that is inherent to them. The voltage on no load rises a lot. And you need to select the transformer voltage such that worst case supply voltage does not blow up the smoothing capacitors and does not blow up the amplifier.
For that reason I recommend a minimum of 160VA for a power amplifier and roughly:
VA = 1times to 2times (the total maximum power of the amplifiers).
Thus a 50W in to 4ohms amplifier could work from 50VA to 100VA transformer.
A stereo amplifier would need around 100VA to 200VA.
In all of the above my 160VA minimum applies and would work well with a mono bloc, or with a stereo assembly.
But the big downside to small transformer is the very high regulation that is inherent to them. The voltage on no load rises a lot. And you need to select the transformer voltage such that worst case supply voltage does not blow up the smoothing capacitors and does not blow up the amplifier.
For that reason I recommend a minimum of 160VA for a power amplifier and roughly:
VA = 1times to 2times (the total maximum power of the amplifiers).
Thus a 50W in to 4ohms amplifier could work from 50VA to 100VA transformer.
A stereo amplifier would need around 100VA to 200VA.
In all of the above my 160VA minimum applies and would work well with a mono bloc, or with a stereo assembly.
Thanks for your help with choosing a transformer. I've been reading the guide on decibel dungeon to building a gainclone power supply (Which it seems you helped create, I love how many experts there are on this forum) and have been looking at transformers such as this:
0160P1-2-018 | 2 Output Toroidal Transformer, 160VA, 18 V ac | Nuvotem Talema
Would these sort of specs be correct?
Many Thanks.
That'll work. That transformer will provide about +/-24 V rectified DC under load. You can use a 2x22 V AC transformer if you want a bit higher voltage and higher output power.
Just build it the way you want it from the start. +/-28 V DC is suitable for 4 and 8 ohm loads, provided that the LM3886 is fitted with a large heat sink (1-2 K/W). To get to +/-28 V, use a 2x22 V RMS transformer, rectifier, and 2x22000 uF reservoir caps. You may have nearly +/-30 V at idle. That's fine. The voltage will droop a bit under peak power loads.
I use simulation to determine the values of the Thiele network. 1.5-2 uH || 1.5 ohm works well. 2 uH is about 15 turns of AWG 18 (1 mm dia) wire, 15 mm long wound on an AA battery. Use enameled wire (transformer/magnet wire).
My dummy load is ten 0.8 ohm Vishay-Dale RN-25 resistors in series on a forced air cooled heat sink. I picked up the resistors and heat sink up on eBay years ago. The heat sink is from a cell phone base station. I connected the resistors with a tap in the middle for 4 ohm loads. Connect the two halves in parallel for a 2 ohm load. I can dissipate over 200 W into that thing without issues.
Tom
Hi,
When you buy heatsinks, don't forget thermal paste!
First time I built amp, I must wait a week for listening cause I have not..
If you tighten too much the transformers, they can become noisy.
What kind of music files you play, I think you play MP3 or compressed (with poor dynamic range) sometimes ?
@ Tom
Do you think it's better to sold the Thiele network at output on binding posts ?
Phil.
When you buy heatsinks, don't forget thermal paste!
First time I built amp, I must wait a week for listening cause I have not..
If you tighten too much the transformers, they can become noisy.
What kind of music files you play, I think you play MP3 or compressed (with poor dynamic range) sometimes ?
@ Tom
Do you think it's better to sold the Thiele network at output on binding posts ?
Phil.
If you use the isolated LM3886TF, get the paste. If you use the non-isolated LM3886T, get a thermal pad such as Bergquist SilPad 1500ST.
If you forget to throw the thermal goop in with your component order, you can typically find it at computer component stores and computer repair places.
As my undergraduate advisor put it, "the electrons don't know which end of the wire they're in". It doesn't matter greatly whether you have the Thiele network at the amp output or at the speaker binding post inside the amp. The idea of the Thiele network is to provide some isolation between a capacitive load and the amp so the amp remains stable.
Moving the Thiele network to the speaker defeats the purpose as the cable capacitance will now load the amplifier, thus, increasing the possibility of instability.
Andrew's concern is that the Thiele inductors of a stereo amp will couple, resulting in crosstalk. In theory, that's true, but the coupling coefficient is really, really low and any inductive coupling will be terminated by the amplifier's output impedance (low impedance) and speaker impedance (also low). The crosstalk in a stereo amp is most likely the result of coupling through the power supply and ground nets.
Common sense applies. Don't place the Thiele inductors right up against each other in a stereo amp. If the inductors are, say, 15 mm long, I'd guess a spacing of 75-150 mm would be more than enough to prevent any coupling from rising above the noise floor of the amp.
Tom
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