A preamp with tone controls

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#21
Hi again Tarun,

To get the 3 band "Baxandall" equalizer to function symetrical and nice, I think you must change the mistake in the mid. section.

I have don the change in the schematic below

BTW. when the eq section are active the pre-amp are out of phase on the output, (compare with input) but when you have switch eq of/by pass you have right phase on the output.

--Bo

An externally hosted image should be here but it was not working when we last tested it.
 
#22
Bo Hansén said:
To get the 3 band "Baxandall" equalizer to function symetrical and nice, I think you must change the mistake in the mid. section.
Click to expand...
Thanks for the trouble you've taken. But in the absence of any simulations, I've decided to just build the circuit as it's given in the book. I'm ordering two PCBs (they're inexpensive in India), and will build one to just hear what the whole thing sounds like. That way, I'll see how the midband eq behaves. Once I get a clear go-no-go on this schematic, I'll decide whether to build more or make the change first. After all, I explicitly asked Randy, and he said this is no typo, it's intentional. So let me try it first.

BTW. when the eq section are active the pre-amp are out of phase on the output, (compare with input) but when you have switch eq of/by pass you have right phase on the output.
Click to expand...
I'm mystified here. I was under the impression that if I'm using an opamp in non-inv configuration, there's no phase inversion. Since both opamps in my schematic are non-inv, how come the tone control section is inverting the phase?

Tarun
 
#23
Tarun,

I'm mystified here. I was under the impression that if I'm using an opamp in non-inv configuration, there's no phase inversion. Since both opamps in my schematic are non-inv, how come the tone control section is inverting the phase?
Click to expand...

Because the first one do not have a phase reverse but the second do, and when you switch the eq section out you take the signal from the first op-amp that not reerse the phase, and when you switch the eq section in, you take the signal from the second op-amp that reverse the phase.

(The first op-amp use non inverting input and the second use the inverting input)

--Bo
 
#24
Bo Hansén said:
(The first op-amp use non inverting input and the second use the inverting input)
Click to expand...
Boy! You're right. I hadn't even looked at my own circuit carefully, and was shooting my mouth off. Really sorry. Don't know why I was carrying the impression in my head that the tone control stage was non-inv. I guess it was a blind reaction to the fact that there was a link leading to it, and it was (visually) at the top. Sorry again.

Yes, you're right about that. Don't know whether I want to change it right now... I guess changing it would mean more than minimal surgery. I guess I can keep it the way it is, because there won't be any phase inversion between channels. Either both channels are switched to tone controls or both are switched out. And I don't have much of an understanding of absolute phase anyway, so I don't care if the absolute phase reverses when I switch on the controls.

But thanks for breaking my stupid illusion about my own circuit. 🙂

Tarun
 
#26
Just got the PCBs from the fab shop

Will now assemble the preamp, on a table top, over the next few days, and will actually try to listen to it. Then will let you know whether it works at all, and if it does, then whether that controversial midband control connection works too.

SInce my PSU is not even frozen yet, let alone fabricated, I guess I'll do all my initial testing using two 9V dry cells. Should work fine, I should think.

Wish me luck. 🙂
 
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