[Svart]

Ok I'm getting further with a buss channel design and decided it's time to give it a thread of it's own.

I'm sure you've read all the other threads and here will be no difference except for all of the information being in one place.

Here's the information in a nutshell:

Primarily SMD except for where I HAVE to use through hole.  sorry folks, this is what I like and it keeps the cost down through fewer drill holes and through plating.

Active impedence converter roughly based on the $$L4k and others like it.

MAT04 with footprints for the AD825/AD829/5534/DOA and servos.  

Bipolar caps, Nichicon Muse ES between stages but will have jumpers so you can leave the caps out if you are using servos.

Extra pad widths for through hole parts.

SSM2402 for switching and muting.

I'm trying to keep the devices in the audio path as few as possible while still being as fast as possible.

Any and all suggestions welcome.

I'll be getting a schemo up soon.

For those wondering about circuit flow,

input->inductor->bipolar cap->MAT04->opamp(s)->insert->cap(undecided)->opamp buffer(undecided)->fader->opamp->output

[Svart]

Topic of first discussion, DC servos.

I've been studying as many of these as possible.  $$L made an interesting servo out of a 5534 and tl052 arrangement.  the servo IC actually controls the balance pin on the 5534, not either of the input pins.  This is interesting and seems to work well.  Fred Forssell has a number of interesting servo ideas also.  With some modification it works with the FETbloak too.

The AD825 does not have a balance pin, nor a comp pin.  it is strictly a fast FET opamp that is unity stable.  fast as in 130v/us fast.

The AD829, which i heard $$L used in the 9k bussing is 230v/us!  BUT, there is a price to pay here.  It is BJT input, AND only stable over something like a gain of 20.  That means we can't use it in a unity servo'd buffer arrangement..  However upon further investigation and some Spice, I found that we could in fact use it in this arrangement with a feedback cap of ~60-70pf.  One would think this is great until you realize that the part's speed diminishes with the cap in the feedback loop.  Ultimately you are left with a 5$ opamp that is unity stable but with a speed of only 16v/us.  This leaves us with the 5534 combo with 13v/us, or just using a cap and an AD825.

Which would you go for and why?  Would a bipolar cap really change the audio *that* much?

[Gus]

Something to think about.

There was a solid state hifi preamp late 70s that was very high speed using opamps and transistors one of the things it did was split the feedback gain setting around the opamp(dc and some gain control) and also around the output cap(cap distortion reduction and some gain control)IIRC

I have not seen one but I read about in one of the mags kind of like the audio critic of that time.

Anyone remember that? I am thinking it was like a 3 letter name and something like a 1000V/us using a feedforward feedback circuit.

EDIT  AGI 511A preamp

[Svart]

I know you guys think it's overkill but how about the AD811?  Super badass fast(2500v/us) opamp that can be unity stable.  Feedback is currentmode and as far as i can see it can drive a load down to 50R.

they are only 7$ each.

[analag]

Hey Svart, you have a thread for each part of your mixer...what gives.
If the mixer and it's schemo is too big to squeeze into one thread, why dont you post each part of the schemo with each thread you create for it.
I'm sure some of us relate better to the visual implementation of what we have a hard time following literarily.
Anyway you seem to have your ideas all set, so I see this as only a debate/confirmation kind of thing. Just do it. You aint no rookie.

analag

[Svart]

Analag,

Thanks.  I keep thinking up interesting ideas as I talk about other parts of the design.  everytime I think I have something nailed down I get that *oh wow* idea.

Some of the parts are set in stone, some of them are not.  This will eventually be a complete mixer but right now i will start with the bussing.  I also want this to be a somewhat group project, with ideas from all those willing.  I could be silent and just do it but the minds of many can do more than the mind of one.

I think of myself as a rookie still as I am always learning something new and will likely be doing so until i die.  I've been in awe of engineers/techs and apparently some have been in awe with me, but that is the nature of the work, some will know more and some will know less.

I'll digress and get back to the design.  I'll edit it some more and get some changes I thought up today stuck in there and hopefully start to post something soon.

[Svart]

Did anyone ever notice that the input stage to the $$L4k summing buss is almost identical to the JH990?

Did anyone care?

[PRR]

> input stage to the $$L4k summing buss is almost identical to the JH990?

Low impedance, balanced, low-noise.... how many frikkin ways CAN you do it?

Two: transformer, and rich BJT pair.

All else is details. Critical details that make or break your sound and/or profit. But mere details.

[Svart]

true, very true, but I think what got me was that values were identical, but I guess that is to be expected when using the same active parts.

I've done a number of drawings of different setups.  I'll have to wait to post them tonight but they'll be in .SCH format(circuitmaker) for now.

[Svart]

still working on this, spare time is thin these days.

it WILL be good though, I promise.

I've made more changes and it will be totally CFB.

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