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Old 4th October 2002, 06:59 AM   #11
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Originally posted by sonnya
Another thing you can expect to work better in inverting opamp's is that if you use a feedback cap (Cf across Rf) to stabilize and to raise the phaseshift,...

Am i wrong PerAnders?
Yes, you have a point here. I wonder why inverting amps have so low reputation among audiophiles?
/Per-Anders (my first name) or P-A as my friends call me
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Old 4th October 2002, 07:29 AM   #12
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Originally posted by peranders

Yes, you have a point here. I wonder why inverting amps have so low reputation among audiophiles?
i dunno. i think it's a little more difficult in some applications (due to less flexibility with the input impedance). also, a lot of people can't keep track of absolute polarity and wouldn't know if they need to reverse their speaker cables or not. boo hoo.

anyway, i'm thinking building a little IC-based amplifier, a 47 Laboratories Gaincard clone if you will. i currently have the Blue Circle Music Pump mono amps (basically Gaincard clones in a woman's pump shoe) which i'm reviewing for a magazine, and i'm shocked at how good they sound. no not the ultimate sound, but really quite good and very musical. i'm pretty sure they use the LM1875 chip, which is so simple to implement, so i would like to build a really compact amp to use as a mini-system or for computer speakers. i am just wondering if i should try something new, and maybe run the IC in inverting configuration... wonder if that would work with what is essentially a power op-amp? any possible side effects to this in a higher-current application like this one?

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Old 4th October 2002, 07:38 AM   #13
mirlo is offline mirlo  United States
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Default alavius: You might want to add ...

... a resistor between the base and emitter of the 2n2222 --- or perhaps some other means of loading the 3904, to make the 3904 turn on hard enough to behave well.
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Old 4th October 2002, 09:19 AM   #14
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Default Noise gain

One disadvantage of the inverting config is that the noise gain is always higher than the signal gain. Sounds cryptic?

Consider that for a gain of -1 (both resistors equal) the input signal really is attenuated by the input resistor and the feedback resistor (halved in this way), so even if it looks like a gain of 1, the opamp really internally has to amplify the signal by 2. That means that all noise and distortion generated in the opamp is also amplified by 2 rather than one (hence the name 'noise gain').

If you get more fancy with feedback topologies the signal gain and the noise gain can be quite different and it will look like the feedback factor is less effective than expected.

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Old 4th October 2002, 09:33 AM   #15
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Hi folks, I won't get into the deep technical critique, cos I would have no idea what I was talking about , but here is a forum you might find interesting-


I am currently building inverting and non inverting versions using the LM3875, using the info from this forum, and hey, I think I might be learning something!
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Old 4th October 2002, 12:01 PM   #16
alvaius is offline alvaius  Canada
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Default This was one of the later schematics

I have to update this circuit, but this was one of the later configurations with some better transistors on the output stage and a better tuned feedback loop. The comment above of loading the 3906 is quite valid depending on what op-amp you are using. I actually loaded the AB output of the op-amp into the negative rail which improved the quality a bit.
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Old 4th October 2002, 07:41 PM   #17
dorkus is offline dorkus  United States
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Default inverting input and input impedance

hmm ok, so even if i need a relatively high input impedance, is inverting-input configuration still recommended? e.g. i want to have 50k input impedance with 26dB gain, this would require a 1Mohm feedback resistor - very high, would noise and distortion performance suffer? would i be better off using non-inverting to get this high an input impedance?
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Old 4th October 2002, 07:46 PM   #18
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Every time you mention inverting phase or phase
splitting to the semi technical audiophile, they go into

I have another hypothesis which occurs when the stage
is not being used for much gain:

It is my experience that at low and unity gains, a lot of
op amps are on the edge of oscillation. Over the years
I have made a number of bad sounding circuits (no, not
ones you've seen) and many of them were bad sounding
not because they were oscillating, but because they were
near the edge. If I opened up the feedback gain, they
got better as they moved farther from the edge.

As a result, whenever I use op amps in an audio circuit, which
is not often, I make sure that I either use up or throw away
about 20 dB of open loop gain. If the circuit has a gain of
10 or greater, this is automatically achieved.

If the circuit has less than 20 dB gain, I toss some away.
With an inverting amp, a resistor from the - input to ground
will do it. If not, then resistance from both + and - to
ground is used.

Works like glue....

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Old 4th October 2002, 08:26 PM   #19
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Default With an inverting amp, a resistor from the - input to ground

So the noise gain is higher than the signal gain I suppose. I notice your ZEN, Aleph X, and X amps are an inverting topology. Would there be advantages in converting our Aleph amps to inverting designs for us DYIers?

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Old 4th October 2002, 08:37 PM   #20
sonnya is offline sonnya  Denmark
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not comment on the work done by passlabs, but yes i would think that there would be a more stable amp... But the sound will change! Too the better ?? I don't know!

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