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Just how bad is it? Audio mixer with headphone amp.
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BrianHG:

--- Quote from: Audioguru on February 25, 2018, 04:58:11 pm ---Why do you have so many coupling capacitors? Most of your opamps have a gain of only 1 so their output will have the tiny maximum DC output offset of 0.004V without a coupling capacitor and resistor load to ground. Who cares about an offset of only 0.004V max? The offset voltage might even be nothing.

Why do your coupling capacitors have the huge value of 100uF? Feeding 22k ohms then they pass earthquake frequencies down to 0.07Hz! That is one cycle each 14 seconds. Why not use 1uF then the audio -3dB cutoff frequency is 7Hz then will be flat down to the 20Hz lowest frequency that we can hear. The formula to calculate a coupling capacitor value is very important and is simple.

Can you hear a difference between true phase and reversed phase? Does every radio in your house produce true phase? Do you want to add many other mixers to this mixer?

An audiophool would use 14 NE5532 opamps in parallel to drive headphones through solid gold Monster cables. An audiophile uses only one fairly high power NE5532 opamp through ordinary copper cables to drive 600 ohm Sennheiser headphones.

--- End quote ---

Now I know I like low frequencies, but I must agree with you Audioguru.  Paulca has gone off the rails in all respects when his first schematic would have worked fine beyond his ability to hear have any better.

However, Paulca may be doing this just for creating an over electronically glorified mixer, not that all of this may actually improve the audio.  In fact, it will most likely lower the sound quality just having the audio passing through so many transistors in so many op-amps with so many feed-backs and gain stages.  (I can almost guarantee a proper laid out board of the earlier design will have a better sound especially in the high frequency range...)

As for driving headphones, I stand by my earlier suggestion of using 1 op-amp, driving a 2N3904/6 emitter-follower setup.  Too many op-amps will kill sound fidelity.
paulca:
I'm borrowing from the design of pro-audio mixing desks.  I found it a little odd that I was hearing two different things from two different places.

Yes the original might have worked fine and no I probably wouldn't notice the difference, but I have at least a small part of my background and family working in pro-audio.  Things are done differently.

An example was pre-amp gain.  Everything audio I know prior to electronics told me to add gain at the input.  But all the amatuer audio stuff in EE forums use pots on the input to attenuate then boost which is a pointless noise generator circuit.

A the same time I still need you guys to help me as borrowing blocks from a book can lead to trouble until I learn the details of how to calculate my way out of a pickle.

Out of interest, if you put all the blocks together from the book and built a 24 channel desk with full inserts, parametric EQ, sends, returns, channel groups, PFLs, AFLs etc. etc. it would probably have in the order of 500-1000 op amps.  Do your CDs sound distorted or noisy?
BrianHG:

--- Quote from: paulca on February 25, 2018, 05:12:10 pm ---On the gain.  The Baxandall stages have a gain of around -70db...

--- End quote ---

With headphones especially, you wont be able to mute the volume with only -70db.  Better add a mute button to each stage.

Be careful when signal mixing at lower impedance with higher uF caps.  These caps begin to exhibit some funny responses with their internal ESR and using cheap electrolytics or ceramics here may do funny thing to your signals below 500hz.  In really high quality audio, at the line level, we tend to go for 4.7uF or 10uF film type cap, with only a 50k or 100k load to avoid this issue.  On rare occasion, I've seen 22uF used, but, it was in a 24k/12k ohm XLR balanced load.
paulca:

--- Quote from: BrianHG on February 25, 2018, 05:13:46 pm ---As for driving headphones, I stand by my earlier suggestion of using 1 op-amp, driving a 2N3904/6 emitter-follower setup.  Too many op-amps will kill sound fidelity.

--- End quote ---

This is the books suggestion for a cheap, simple headphone amp:

paulca:

--- Quote from: paulca on February 25, 2018, 05:24:28 pm ---As someone on here said, if an audiophool spends £400 on a set of speaker cables it WILL sound better to him.

--- End quote ---

Just to clarify.  I do not mean I want to throw money at the thing and hope that it sounds better because of it.  I believe what I am doing is bypassing the marketing and mark up and building something that would cost 10 times what I could afford to buy already built.

Audio gear falls into three categories. 

* Domestic stuff which will have a quoted figure of 1000 when it only delivers 10.
* Audiophool stuff which quotes 1000, delivers 2000, but only 5 was needed at 100th the price.
* Pro-audio stuff which quotes 100, provides 100 and 50 was needed, but you pay a shed load more.

It's emulating the later I am aiming for.

Of course, most of my gains may be lost in my poor board design, crap cheap components and poor source cabling etc. etc. etc.
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