Help with LM1877 power amp design

[bancika]

I want to build something like this to replace crappy amp in crappy genius speakers. They have decent box and power transformer I plan to use, but will swap the amp and most likely the drivers. It's way too bassy now and there aren't any mid/highs or highs to speak of, so when I use them for monitoring guitar they sound awful.
Given the current capabilities of the transformer (9V 1A), I think that 2x2W should be decent. LM1877 seems to fit the bill and I can source it locally. Schematic from the data sheet is below. Now the questions:

* how much voltage gain do I need for line level signal coming from my audio interface? Their example has Av=200 which seems high. I'm thinking about reducing gain to approx 40 and replacing 10uF caps with 1uF poly film caps, since I don't need freqs under 60Hz anyways. So the feedback network would be 100K+2.7K+1uF. Should 40 be enough?
* is 2.7ohm + 0.1uF combo Zobel network? Can I swap places of 500uF cap and that network to be able to mount it on the speaker directly and save some board space?



Thanks 

[teemuk]

* how much voltage gain do I need for line level signal coming from my audio interface?

It' basically up to you to decide. What's the signal level of this "line level" source to begin with? Is it really line level (there are actually several standards for such) or lower, like "instrument level". With 9VDC power supply headroom limit per each half wave will be less than 4.5V (peak) so your input level and this headroom define the minimum gain you need to reach max. output power. On the other hand, you might want to leave some margin so that you can reach the rated power also with signals that have lower amplitudes. Sometimes even considerably lower... Is the design goal headroom or power amp overdrive? Are there volume pots before the power amp? Are they linear or log? Do you want to reach full output power only at "10" of the volume dial or before that? If earlier, how early?

The answer to your question really depends on many different variables.

* is 2.7ohm + 0.1uF combo Zobel network? Can I swap places of 500uF cap and that network to be able to mount it on the speaker directly and save some board space?

Yes and yes.

[bancika]

I wanted to avoid volume controls because I have a huge knob on the interface that I would use, it's more convenient for my setup.

Since it would be a clean power amp for monitoring, I don't want it to go into overdrive, so ideally, it should be able to reach maximum power when my input is maxed.

Here's what specs for the interface say, have no clue what it means

Line Outputs (D/A)           
Full Scale Output Level @ Max. Volume   +11 dBu       
Dynamic Range @ Max. Volume   102 dB (A)       
THD+N   1/2: 0.01%, 3/4: 0.02%       
Frequency Response (96 kHz), -1 dB   10 Hz - 45 kHz

Thanks!

[teemuk]

Full Scale Output Level @ Max. Volume   +11 dBu        

Ok then...

"Full scale" is simply a term used in digital systems (your source has digital/analog converters in output) to indicate that signal has reached its maximum or minimum representable value. Beyond that point signal's amplitude cannot possibly increase, the signal will only clip.

11 dBu is equivalent to about 2.8VRMS, so expect peak voltages of about 3.9V from that source.

Basically you would thus need only 1.1x of gain to reach the theoretical clipping point of 4.5 volts, defined by the power supply voltage. In practice the amp will likely clip somewhat earlier than that (consult the chip's datasheet). Probably even gain of 1 would therefore be too much.

---

Although I still believe you will eventually find out that designing that amp to reach its maximum output power only during highest input signal peaks and with volume dial on "10" isn't such an ideal setup to begin with. It will not sound very loud (human hearing is very insensitive to peaks and loudness perception is more related to average signal levels) and it will not compensate lower input signal amplitudes.  

[bancika]

Thanks!

I could make Av = 10 or maybe 20 to make it friendly to other inputs and still be able to have usable control over volume. Sounds better? Or I need more?

[bancika]

Quick update, I finished the amp, gutted out the stock amp and replaced it. It worked from the first try and it sounds pretty good.
Starts clipping with volume about half on the interface, so there's enough controllable range. It's plenty loud for my bedroom.

Couple of pix





Thanks for help, teemuk!

[Seljer]

Is there a notable improvement over the stock amp in the speakers?

[bancika]

yes, much more neutral frequency response, now I can actually hear the guitar. It brought bass to normal levels, so it's not boomy anymore. I'll think about replacing drivers, may need a bit more highs, these drivers don't seem to have enough highs, but it's definitely usable now.

[Seljer]

So the original was was doing some kind of built in smiley face EQ or a bass boost for 'better' sound?
I had a $50 2.1 computer speaker system that I replaced with some bookshelf monitors and noticed a huge improvement in midrange audibility but I figured it was the old speakers' drivers (two 7cm speakers + a 12cm 'subwoofer') instead of their amp. I've since been using the monitors and hooked the PC speakers up to the TV.

[bancika]

yes, they advertise as warm woody-sounding speakers, so they just slapped a huge bass boost. No mids or highs (the latter is probably due to speakers). I was surprised how much bass it had, to be honest, but it's no good when everything else sounds dull. I thought about modding the stock amp to somehow turn off bass boost, but they installed huge heat sink above the whole board, and I didn't want to bother taking it off, so I just gutted it out