Has anyone built and heard this pcb? Wondering if its any good, reliable and worth the investment or not
180W LM3886T x6 Parallel BTL Power Amplifier w Output DC Servo CKT Huge PCB | eBay
180W LM3886T x6 Parallel BTL Power Amplifier w Output DC Servo CKT Huge PCB | eBay
This may be roughly equivalent to the National BPA300, six chipamps
three in parallel for one half and three in parallel for the other half.
It will drive an 8ohms speaker quite well.
It may be just about adequate for a 4ohms speaker, but I don't recommend that.
Don't try to drive a 3ohms speaker.
Download the National application Notes for the BPA, PA & BA series of amplifiers.
Download the 3886 datasheet. See what is involved before you buy.
three in parallel for one half and three in parallel for the other half.
It will drive an 8ohms speaker quite well.
It may be just about adequate for a 4ohms speaker, but I don't recommend that.
Don't try to drive a 3ohms speaker.
Download the National application Notes for the BPA, PA & BA series of amplifiers.
Download the 3886 datasheet. See what is involved before you buy.
AndrewT, thanks for replying to my post.
you've got me re-assessing my plans... though its been some time since ive built anything.
Ive read through the Application notes on the chip in the past, and you are correct. It should drive 8 ohms nominal, easily. Paralleling, as I understand it, permits driving lower impedance loads than the circuit would otherwise be suited to, and bridging allows one to produce greater voltage swing and boost output a couple db. (hence more "power" to control the drivers)
Ive built a few single LM3886 ChipAmps and I like their sound individually. Ive listened to these single chips bridged, which sounds ok with my Spendor BC1 speakers.
Although i've been looking at this BTL board, I have some other boards from JIMS Audio:
a pot adjusted version, which looks to be similar to Alex's BPA300 build at DIY BPA300 6x LM3886 300W audio Amplifier. Ive been planning to build this version, but many people prefer the 'better' approach of a DC servo version such as that board i referenced on ebay. This is what has lead me to ask about that pcb.
ive understood that these chips drive 8 ohm well and 4ohms ok. Ive assumed that bridged and parallel amps translate roughly similar to this:
⅛+⅛+⅛= 2.6ohm...and when bridged --> 5.2 Ohms well
and
¼+¼+¼= 1.3ohm bridged --> driving 2.7Ohms Ok.
What you've mentioned about driving 3ohm loads has brought up some questions, and if i can borrow some advice and experience, i'd like your thoughts on my intended application.
Until now, Ive assumed that the BPA300 should be able to drive loads dropping below 4 ohms relatively easily. This is a requirement i will need out of my next amplifier build.
You may be wondering, Why do I want to build a BPA type build..? Heres my plan..
My desired application is for the four Linkwitz LX521 woofers.
The Seas Woofers in this DIY speaker, have a nominal impedance of 4 ohm. The mids are 2 4Ohm with a passive x-over between hi-mid and low-mid. The tweeters are roughly 4 or 8 ohm loads, and I assume should be driven with a single chip just fine.
Each woofer driver (4 total) would have its own BPA300.
The mids and tweets i was going to power with basic single chipamps, gain matched to the BPA300.
In this proposed set up, Would a few BPA 300's and Single chip lm3887's be able to drive this DIY speaker adequately? the actual load of the Lx521's drivers may very well dip below 4 ohm, and i certainly dont want to damage anything. Looks like its on the good side of the chips abilities... but i could be mistaken and i dont want to damage anything.
Would you go ahead?
--- since i need 8 channels of amplification, are there any other AB type amplifiers that may be more suitable to the task of driving linkwitz lx521's?
Have i figured everything correctly?
id prefer to build rather than buy - better satisfaction and potentially cheaper overall.
any thoughts on thesechip based amps in this proposed application are greatly appreciated.
you've got me re-assessing my plans... though its been some time since ive built anything.
Ive read through the Application notes on the chip in the past, and you are correct. It should drive 8 ohms nominal, easily. Paralleling, as I understand it, permits driving lower impedance loads than the circuit would otherwise be suited to, and bridging allows one to produce greater voltage swing and boost output a couple db. (hence more "power" to control the drivers)
Ive built a few single LM3886 ChipAmps and I like their sound individually. Ive listened to these single chips bridged, which sounds ok with my Spendor BC1 speakers.
Although i've been looking at this BTL board, I have some other boards from JIMS Audio:
a pot adjusted version, which looks to be similar to Alex's BPA300 build at DIY BPA300 6x LM3886 300W audio Amplifier. Ive been planning to build this version, but many people prefer the 'better' approach of a DC servo version such as that board i referenced on ebay. This is what has lead me to ask about that pcb.
ive understood that these chips drive 8 ohm well and 4ohms ok. Ive assumed that bridged and parallel amps translate roughly similar to this:
⅛+⅛+⅛= 2.6ohm...and when bridged --> 5.2 Ohms well
and
¼+¼+¼= 1.3ohm bridged --> driving 2.7Ohms Ok.
What you've mentioned about driving 3ohm loads has brought up some questions, and if i can borrow some advice and experience, i'd like your thoughts on my intended application.
Until now, Ive assumed that the BPA300 should be able to drive loads dropping below 4 ohms relatively easily. This is a requirement i will need out of my next amplifier build.
You may be wondering, Why do I want to build a BPA type build..? Heres my plan..
My desired application is for the four Linkwitz LX521 woofers.
The Seas Woofers in this DIY speaker, have a nominal impedance of 4 ohm. The mids are 2 4Ohm with a passive x-over between hi-mid and low-mid. The tweeters are roughly 4 or 8 ohm loads, and I assume should be driven with a single chip just fine.
Each woofer driver (4 total) would have its own BPA300.
The mids and tweets i was going to power with basic single chipamps, gain matched to the BPA300.
In this proposed set up, Would a few BPA 300's and Single chip lm3887's be able to drive this DIY speaker adequately? the actual load of the Lx521's drivers may very well dip below 4 ohm, and i certainly dont want to damage anything. Looks like its on the good side of the chips abilities... but i could be mistaken and i dont want to damage anything.
Would you go ahead?
--- since i need 8 channels of amplification, are there any other AB type amplifiers that may be more suitable to the task of driving linkwitz lx521's?
Have i figured everything correctly?
id prefer to build rather than buy - better satisfaction and potentially cheaper overall.
any thoughts on thesechip based amps in this proposed application are greatly appreciated.
Since you're planning a full active system, I can't see a reason not to use transformers between your chipamps and drive units. This gives you the flexibility to optimize your power output and also the chance to improve the SQ of the LM3886. The transformers you need aren't anything fancy - I dismantle and re-wind smallish (20VA) mains trafos with the requisite number of turns. To drive tweeters I use ferrite cored trafos (RM14 is a good choice of core).
Thanks for your thoughts. Please Excuse my ignorance... Output transformers on the output of the amps?
certainly i've been considering input transformers to bridge line-level signal to the channel pairs... but was leaning towards a line driver for cost advantage
i'm vaguely aware of output transformers in guitar amps... and i suppose thats a similar situation (low impedance drivers)
Might you have any examples of it being done in HiFi?
(in) 2:1 (out) ratio i'm guessing, and of sufficient power handling?
Wouldn't the BPA300 be able to drive such speakers well enough as it is?
certainly i've been considering input transformers to bridge line-level signal to the channel pairs... but was leaning towards a line driver for cost advantage
i'm vaguely aware of output transformers in guitar amps... and i suppose thats a similar situation (low impedance drivers)
Might you have any examples of it being done in HiFi?
(in) 2:1 (out) ratio i'm guessing, and of sufficient power handling?
Wouldn't the BPA300 be able to drive such speakers well enough as it is?
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I use input transformers too - for going single-ended to create the two out-of-phase signals needed for bridging.
Output trafos get used on valve amps but the only transistor amps I'm aware of that use them (autoformers actually rather than isolated outputs) are McIntosh.
Using 6 bridged/paralleled LM3886s might be overkill for what you need, though I've not got the details on the power levels you need. Based on my experience you'll get better SQ (lower noise from your power supply) by raising the impedance that the 3886 sees. A pair of 3886s in bridged will easily deliver 100W but only into an impedance at least double that of your naked driver - hence the advantage of the trafo.
Output trafos get used on valve amps but the only transistor amps I'm aware of that use them (autoformers actually rather than isolated outputs) are McIntosh.
Using 6 bridged/paralleled LM3886s might be overkill for what you need, though I've not got the details on the power levels you need. Based on my experience you'll get better SQ (lower noise from your power supply) by raising the impedance that the 3886 sees. A pair of 3886s in bridged will easily deliver 100W but only into an impedance at least double that of your naked driver - hence the advantage of the trafo.
Just as a thought, I wouldn't even attempt an amplifier such as this design without using Servo amplifiers for each section.
I have seen and helped with two newbies successfully build a BPA200 with servos and it work the First time around.
I know that it has been done and made to work before.
But, I have seen many others that have attempted this design without them and struggled and just a few of them didn't end up with a pile of burn't resistors and chips.
As far as getting the two phases to drive the two halves, transformers or opamps may be used, I prefer to use opamp's for better precision, the DRV134 has typically been used in the past but there are now one or two chips available now that have even better specs, Sorry I forget the numbers at this time but you can check with LT, AD or TI and you will find them.
FWIW
jer
I have seen and helped with two newbies successfully build a BPA200 with servos and it work the First time around.
I know that it has been done and made to work before.
But, I have seen many others that have attempted this design without them and struggled and just a few of them didn't end up with a pile of burn't resistors and chips.
As far as getting the two phases to drive the two halves, transformers or opamps may be used, I prefer to use opamp's for better precision, the DRV134 has typically been used in the past but there are now one or two chips available now that have even better specs, Sorry I forget the numbers at this time but you can check with LT, AD or TI and you will find them.
FWIW
jer
That's a pretty good summary of a BPA200 build experience, actually. Getting two LM3886es to play nicely when parallel is quite a challenge. Getting three to play nicely and then having them play nicely together in a BTL amp would likely set the average DIYer up for failure...
The board doesn't appear to be anything special. It looks like your standard connect-the-dots layout. Also, the footprints for the SMD resistors are wrong. The pads are oval. They should be rectangular. This is not a huge deal for hand-assembly, but it does make me question the design decisions that went into the board.
Tom
Thanks for the advice.
As far as i can tell so far, the lm3886 in servo, is the only option.. given the risk of DC offset issues in an amp with so many LM3886's to balance, and unpredictable heat and DC drift.
I cant find a better DIY amp design with PCBs made, capable of driving large open baffle 4 ohm woofers.
Im sure there's D-Class options out there, but i've always had the preferences for A and AB class amps. Maybe i'm prejudiced against D-class by the writings of some of the great A, and AB class designers.
Modifying the load that an amp sees is not something i'm very well acquainted with. I find the impedance conversion aspects of transformers rather confusing.
As far as i can tell so far, the lm3886 in servo, is the only option.. given the risk of DC offset issues in an amp with so many LM3886's to balance, and unpredictable heat and DC drift.
I cant find a better DIY amp design with PCBs made, capable of driving large open baffle 4 ohm woofers.
Im sure there's D-Class options out there, but i've always had the preferences for A and AB class amps. Maybe i'm prejudiced against D-class by the writings of some of the great A, and AB class designers.
Modifying the load that an amp sees is not something i'm very well acquainted with. I find the impedance conversion aspects of transformers rather confusing.
So a turns ratio of roughly 1:1.4 would be ideal, if i wanted to double the impedance the amp sees.
what kind of VA ratings would be needed.. 100 for Woofers, 50 for mids and smaller for tweeters would probably be plenty. I guess frequency roll off wouldn't be a problem since each channel has only a limited bandwidth duty.
What about a chip based approach... wouldn't that be less expensive? Not sure i have a clue how to implement such a task.
what kind of VA ratings would be needed.. 100 for Woofers, 50 for mids and smaller for tweeters would probably be plenty. I guess frequency roll off wouldn't be a problem since each channel has only a limited bandwidth duty.
What about a chip based approach... wouldn't that be less expensive? Not sure i have a clue how to implement such a task.
Yes if you wanted to double the impedance, 1:sqrt(2) would be the turns ratio. As regards VA rating - I've found its more a function of what damping factor I intend to reach. Seeing as trafos add series resistance they reduce the DF. So far in my experiments I've aimed for DF >=10 and found a 10VA trafo is enough to reach this. But then my chipamps are only doing around 20-30W. So you could choose a trafo VA at a third of your max sustained power output. With normal music crest factors (>12dB) the trafo won't feel warm to the touch.
I'm unclear what you mean by 'chip based approach' ? Its a lot easier to get bridged amps to play happily than it is to get paralleled ones singing together. I also suspect a little more power may be available in bridged with a well optimized load impedance - but I've not done the math yet. The cost of a trafo here is about the same as the cost of a chipamp. There is of course the labour involved in rewinding the bobbins to achieve the desired turns ratio - but labour is what DIYA is about.
I'm unclear what you mean by 'chip based approach' ? Its a lot easier to get bridged amps to play happily than it is to get paralleled ones singing together. I also suspect a little more power may be available in bridged with a well optimized load impedance - but I've not done the math yet. The cost of a trafo here is about the same as the cost of a chipamp. There is of course the labour involved in rewinding the bobbins to achieve the desired turns ratio - but labour is what DIYA is about.
When you buy cores, they usually list VA for 60 Hz. To use them at 20 Hz for woofers, you need to degrade them by 3. In your case, you need a core of 300 VA for your 100 VA impedance matching transformer. You could buy a 300 VA power transformer to use since woofer only cares about bass, not high frequency response.
The cheapest approach is to use a chip driver and add power transistors to gain power. Most amps use this method. Only a few amp come with auto transformers.
If you want to go with transformer I can post a couple of links.
The cheapest approach is to use a chip driver and add power transistors to gain power. Most amps use this method. Only a few amp come with auto transformers.
If you want to go with transformer I can post a couple of links.
You're unlikely to need full power level at 20Hz but yep you're right the limitation is on the flux and running at 20Hz full tilt will saturate a core designed for the same voltage at 60Hz. I suspect only a subwoofer though needs full power capability at 20Hz as recorded music generally has quite a lot lower power levels at such low frequencies - organ music perhaps being the exception. I reckon subwoofers are best fed by classD amps rather than trafos.
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