COLIN WONFOR: LM3886 Gainclone

Colin Wonfor

Hello Colin. Can you enlighten me please? I'm building a LM3886 Gainclone design for a second system, and have a few questions on the best types of passive components to choose. As it’s a second system, not looking for any exotic/expensive components, just want to make best choice of sensibly priced options, while trying to achieve as good a sound quality as reasonably possible. My questions are around the passive components choices as follows:

This device would not be my first choice. All Class D amps have a problem I hate. When playing deep bass the PSU will pump positive or negative and can cause over-voltage. I suspect you are using this on a 28V twin rail at 8 ohms so for example you will get let’s say 38W as in the spec sheet by Ti, now note 1) Low frequency it says is 20Hz and is the -3dB? Next it is not quoted as RMS so … never trust data sheets. Then it says Class D – but it also says Class AB so, ok which is it? Now if it is Class D why is there no output filter to remove the carrier? So why not build yourself a nice AB with discrete component see photo it is simple it performs well and it is cheap.

  1. Low wattage (1/8 or 1/4 W) resistors for most locations in the circuit - are metal film the best choice? These seem to offer much better tolerance (1%) than carbon. Do resistors of different type really show much noticeable difference?

Never use Carbon in audio hiss off and uses Metal Film 1% is fine but try and afford 0.1% and the biggest wattage that fit.

  1. Same question, but for the slightly higher wattage resistors for Zobel & Thiel networks which will carry larger currents?

Zobel use Non Inductive wire wound. Start with 3W. Now the only reference I recall about Thiel Network is in politics so please enlighten me.

  1. Using a non-inverting configuration, I was intending to include an (electrolytic) capacitor of ~22uf in series with the resistor from the inverting input of the LM3886 to ground. Since this will have no permanent DC bias, does it have to be a non-polarising electrolytic, or (since its carrying only a small-signal current) will any type be OK?

NB If leave this out, based on datasheets, I think the maximum DC offset across the loudspeaker terminals might be 20mv (typ) or 200mv (worst-case). Is that likely to be acceptably low?

Try two polycarbonate caps in parallel 10uF at min of 35V each

  1. I'm using capacitance-multiplier type regulators for the supplies to the LM3886, and looking to use ~ 10uf 40v types or similar here. Any recommendations for Capacitor type? (In previous posts I've seen people advocating wet tantalums, but looking at the Farnell catalogue these are very expensive, e.g. ~£40 or more each. Does it really make much difference? It doesn't seem at all obvious to me that capacitor type would have any impact at all especially if I parallel this electrolytic with a smaller value ceramic type) Any guidance appreciated. Many thanks.

Firstly why? You can build cheap regulated PSU at 10A with just 5mV of ripple and noise from a few cheap bits. As to Tants I would never use them in audio; they are also poor diodes and cause distortion. I had a bad experience with them when some caught fire in a Harrier Jump Jet. Never use them. And for £40.00 build a proper amp please.

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