Bridged lm3886 suggestions Please

[Scott_drake]

I have just finished my first DIY dual mono amp, is is in bridged lm3886 config. The case i made myself with 5mm thick alluminium and heatsinks as walls. The design i followed is from the national datasheet and shown below. I have changed Rf to make the amplifier gain around 30. I am getting great sound out of it, zero hum or noise that i can hear. The only complaint is that it does not seem to have very much bass. I may possibly increase the gain furthur as some sources i have to turn volume control right up to get loud volume. Previously i have just changed Rf to vary the gain, should i change Rin as well to keep things ballanced?I would like any suggestions on ways to improve my amp.



This is the power supply design i have used, I am using +-35 volt supplies, from dual 25-0-25 toroids. 10,000uF of supply capacitance per rail for each channel. do you think any performance benifets will be achieved if i add another 10,000uF to each rail. I also have 1uF accross the supply rails at the chips, should i have 0.1uf as well?


These are photos of the finished amp:

[AndrewT]

Hi,
you have different gain in each half of the bridge. 14.1 Vs 14.5
If you select each resistor @~+-0.5% from nominal you can get the gains very close.
The turn over frequency (F-3db) of the non-inverting is about 3.4Hz, but the inverting F-3db is 7.2Hz. Could this be the bass loss problem?
On it's own this is probably quite subtle.
Agh! the non-inverting channel has the NFB F-3db set higher than the input filter (F-3db~=7.2Hz). This should be set about half to one octave LOWER than the input filter.
Each channel has Zin~=4k3 this is unusually low, could the source be struggling to drive this?

But it gets worse, the two bridged amps feed ~100W into 8ohms. A normal power amp of this capability would require +-10mF to +-20mF on the rails of each channel.
But each half of the bridged amp acts as a 50W into 4ohm power amplifier. To produce good bass into that load you need +-20mF to +-40mF per amplifier. But you have two amplifiers each producing 50W into 4ohms working in unison. So your smoothing capacitance for the pair jumps to +-40mF to +-80mF per channel.
You have just +-10mF per channel.
That is one of the major difficulties/costs that is generally ignored when deciding to adopt bridged amplifiers.
This shortfall in smoothing capacitance combined with the highish F-3db could explain the bass loss.

I suggest that instead, you convert each channel to parallel operation. Each amp will try to drive 25W into 16ohms if you retain your 8ohm speakers. Change the turnover frequency to below 2Hz in the non-inverting mode and fit +-20mF to each parallel chipamp channel.
This will make a much better balanced amp capable of delivering as much current as any 8ohm speaker could ever require. It would even make a pretty good stab at running 4 to 8ohm speakers. You gain all this for a loss of 3db in maximum volume.

[Scott_drake]

Thanks for your comments

I am confused, i know The input filter is calculated from Rin and Cin, and equates to 3.38Hz.

The -3db point for the inverting opamp is calculated from Ri2 and Ci2 and equates to 7.2Hz

But i am a little confused on how you calculate the non-inverting half power point, should it not be the same as the inverting opamp but using Ri1 and Ci1 ??

If i increase Ri1 and Ri2 to a value of around 22K this will increase the input impedance substantially and also drop -3db point to around 1.5Hz.........Right?..........Of course i will have to increase Rf to keep the gain around 30.

Could you please point me in the right directions and tell me what components i need to change to get the -3db cutoff to below 2 or 3Hz.

I have no trouble in adding more PS capacitance as i had thought of doing so anyway. I will start by adding another 10,000 to each rail and see if bass improves.

[AndrewT]

Hi,
the input filters should be the restriction to the bandwidth not the NFB leg.
For the inverting using 4u7F as the DC blocking cap, the input resistor could be changed to 20k to get 90mS (f-3db=1.8Hz) RC time constant. But this needs a very high value of feedback (300k) resistor.
You could instead fit a parallel combination of 2u2F//47uF and now you can use 1k8 to give the same 90mS RC. The feedback now becomes 27k. Zin<1k8 and low noise.

But I see non-inverting as the better option if your source cannot drive 1k8 or 4k7 adequately.

For the non-inverting input, you can use 2u2 and 39k or 43k for the input filter.
the feed back then becomes 39k & 2k7 for that same 14times gain.

The NFB filter should be set at least half an octave below the 1.8Hz high pass at the input. try to get RC about 130mS to 150mS here.

Finally for a 4ohm load (that's what the bridged pair each see) the PSU RC>=1.4 *140mS = ~200mS, that amounts to +-50mF on every amplifier rail. This bridging is getting silly. Go parallel with non-inverting.
At least try it on one channel since you already have all the components and listen to the bass (if any) difference in the two channels. The ease with which the parallel drives your 8ohm speakers may sound noticeable in the midrange as well.
Remember to match the gain to about 30 in both channels if you go for this experiment.

[KSTR]

I second Andrews remarks and suggest you take a look at this application note dealing with details of bridging/paralleling LM3886's:
http://www.national.com/an/AN/AN-1192.pdf

Regards, Klaus

[Scott_drake]

Hi - why would i change parallel capacitors and change Ri2 to 1.8k, i would think i should be increasing the Inverting input impedance.

"But I see non-inverting as the better option if your source cannot drive 1k8 or 4k7 adequately."

please explain this, how is it the better option, what is the other option?

"The NFB filter should be set at least half an octave below the 1.8Hz high pass at the input. "

I though the input high pass cut off was 3.4hZ


In regards to reading the application document, i have and that is where this schematic if from. Are you saying that the national schematic, may have significant flaws in the design.

[KSTR]

Hi Scott,

The basic circuit you have built assumes a low driving impedance, otherwise the gain of inverting side will vary with source impedance. This will be significant if you have a level control pot in front of the amp (a 10k pot has 2.5k max. impedance). See circuit fig.17, there they have a buffer installed and the resistors scaled down, capacitors scaled up. I would suggest you build that circuit (omitting the paralleled sections).

OTOH, not reaching 100% perfect gain matching is not too much of a real problem, the output will just not be pefectly symmetrical. That would take away a little of max. reachable output voltage/power.

I would opt for the high-pass corner frequencies in the feedback sections to be at 3Hz, for example. Then the input RC for the non-inverting section should be at 0.75Hz (2 octaves down) so that it will not contribute much to the total roll-off. If it is at higher frequencies, the noninverting section will see additional roll-off and phase shift, both sections wouldn't be balanced any more for very low frequencies. Isn't too much of a significant problem, though.

Maybe more important -- and that is a point where the schematic fig.3 looks flawed indeed -- is the matching of the DC impedances at all op-amp inputs, to get low offsets. For DC the caps are open circuit, so the conditions are: Rin=Rf1, Rb=Rf2. A low value of Rb would be correct only if Ci2 were shorted and the source has a low impedance DC-coupled output. Actually it would be 4.3k, not 3.32k then (the 3.32k might be an empirical value for the actual circuit that was choosen to minmize offset). Again, fig.17 looks better in this regard.

Like has been said, the restrictions of the chip with 4 ohm loads would suggest you go parallel instead, like in fig.6. You will be rewarded with sound quality, the small loss in output power is only a very little price to pay for that. And in the non-inverting mode you can easily get a high input impedance (and use smaller coupling caps).

Those datasheet/appnote schematics should always be seen as starting points, not as bullet-proof and ready-to-build matured circuits.

Regards, Klaus

[AndrewT]

Those datasheet/appnote schematics should always be seen as starting points, not as bullet-proof and ready-to-build matured circuits.

I would not put it as strongly as that.

Usually the datasheets can be relied on for their accuracy.

It's just unfortunate (and unusual) that this schematic is flawed.

[AndrewT]

Hi,
decide how low you want/need the bass response to go.

Set all the input filters to that frequency.
Set all the NFB filters to half an octave below your chosen input filter frequency.
Set the PSU RC to half an octave below the NFB frequency.
Now see what components should be changed to and what extra components are needed to produce your desired roll-off frequency.

Now redo all the above for a parallel arrangement.

which is cheaper and easier to accomplish?
which is likely to play/sound better?
which is 3db down on power output?

the answer to all three should be parallel.

BTW,
I have found that changing the input filter from F-3db=10Hz to F-3db=2Hz makes a significant difference to the way small bookshelf speakers are able to play bass. Changing from 6Hz to 2Hz is also noticeable but sutble rather than significant. Using big speakers have an even greater effect.

I very much prefer setting the input filter to about 80 to 90mS (1.8Hz) and then choosing all the others appropriately.

[AKN]

Hi Scott_drake,

From what I can see from your pictures.
Mains ground is not connected to the chassis and you don't seem to have any fuses for protection in case of failure.