RE: Volume drop on chorus.

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Author: Kaz Kylheku
To: Darrall Dougherty
CC: ada-mp1
Subject: RE: Volume drop on chorus.

On Thu, 20 Dec 2012 15:20:27 -0600, Darrall Dougherty
<darralld@???> wrote:
> I noticed it before I started replacing any of the parts. As soon as
> I adjust the depth from 0 to 1 you hear the volume drop off. Bring it
> back to 0 and it comes back up.

I tried that and I also see the same thing basically. There is a
noticeable volume drop from 0 to 1, explained by that gain-adjusting
circuit around U15. This is probably caused by the chorus section
not being exactly unity gain; i.e. it doesn't put out enough of
a level to compensate for the drop.

The chorus section is bracketed by a compander, consisting of
the two channels of the U21 compander chip (which is a 570,
a cousin of the 572 used for the S.S. compressor).

The input signal is compressed one the way in, then the chorused
signal is expanded. The level match is probably not exact.

> Also when I adjust the depth higher
> the chorus starts to sound worse. But it only does it on a distortion
> channel. On the SS channel it sounds great.

The chorus is quite late in the chain, after the EQ and everything,
so it is quite isolated from SS versus distortion. Audio that
sounds bad is more noticeable on distortion, because of all
the high frequency content.

To me, all chorus sounds worse the more of it you add, or the
more wildly you set the parameters.

This MP-1 chorus unit has the added problem that you cannot control
the wet/dry mix! Depth only controls how widely the pitch shifting
swings (and turns it off completely on 0). When the chorus is on,
the mix level is hard-wired by the circuitry.

That chorus is not really a hi-fi circuit. I mean, the wet tone
goes through compression, expansion and a bucket brigade. How good
can it sound in the end? The compressor/expander chip has op-amps
on it. What are they? The datasheet for the NE750 says that "The
main op amp shown in the chip block diagram is equivalent to a
741 with a 1 MHz bandwidth." There is also this "The output
stage, although capable of output currents in excess of 20 mA, is
biased for a low quiescent current to conserve power. When driving
heavy loads, this leads to a small amount of crossover distortion. "
Not very inspiring. :)

> I changed U15 ( this is he only one I have replaced so far )
> with the 5532 like we discussed the other day in hopes it might fix
> it. But it didn't. I have a new 5534 opamp for the chorus I could swap
> out if you think that could be a cause.

Though the U15 is involved in the gain change when chorus is enabled,
the choice of op-amp won't affect the gain change, but the resistors
around it.

> I don't recall the unit doing
> it when I first got it. I just noticed that it didn't sound the same
> as it used to with the chorus engaged.

It's possible that all the problems you've been having are
related: the one output dropping out and these chorus issues.
I would check for signs of anything being burnt or toasted.

Since you have the 5534, might as well drop it in. But that chorus
section is sandwiched by the U21 compander. It might be worth
swapping that one. I think it's socketed, like the 572.

Anyway, I'd try to fix the nonworking B output first.

One thing at a time!