Distortion figures for analog muxes

[eplpwr]

I'm about to build a digitally controlled (Hi-Fi!?)
preamp, and have most of the design figured out
(thanks to a lot of good internet sites).

The critical part of the design that I'm still
confused about is how to do the input switching
in a "HiFi" way, i.e. with low distortion and
noise levels.

I have three alternatives as I see it:

1. Use gold-plated relays. (As a perfectionist I'm
a bit unhappy about the clicking sound from the
relays).
2. Use extremely expensive audio switch circuits
(i.e. SSM2402:s).
3. Use analog muxes. I have AD406 chips.

The preferred alternative would be #3, but I can't
find any information about distortion for these
devices in the datasheet. The crosstalk and
isolation figures are in the DS, but not a single
figure on distortion.

Has anyone tried analog muxes in a preamp?
Opinions? What's an acceptable "HiFi" level of
distortion?

Thanx in advance,

[peranders]

Relays have a nice sound! i mean the click

If you have your mux in a feedback chain the distortion will be very little. Think of an inverting amp and the mux signal goes through the input resistors. If you have 10 k as input to the inverting inout via the mux and you have 10k-100 k as feedback, then you will have extremly low distortion. One further advantage is that the mux have very low voltage across the switches, virtual ground you know. 4066 isn't so bad but this chip hasn't got any declicking and smooth on and off like the more expensive ones. If you have a microcontroller yoou can easily fix a small delay at on and of. The smoothness of the switching can be hard to fix though.

Consider a high performance analog switch, avoid the 4066 and similiar ones.

[eplpwr]

Hi peranders, and thanks for the input.

Regarding the mux:es that i already have, they're
not 4066:es but 16-way ADG406 from Analog Devices
(I missed the 'G' in ADG in my original post). The
quoted R-ON is quoted as 50 ohms typical, max 80
ohms. Typical +/- 4 ohms difference between
the different switches.

Since I'll have an input stage with an input
impedance of approx 47 kOhms, I don't think there's an issue with gain loss if using the device as a straightforward input switch (think
of using it as a replacement for a traditional mechanichal switch). What I am unsure of, however,
is if this kind of "simple" connection will give
me lots of distortion.

There is another chip from Analog, the AD8113,
that is a real crossbar matrix. This device is
the only analog mux circuit (apart from SSM2402
switches), that I have found any distortion figures for. The AD8113 is, however, rather expensive and also has a high-density pinout that
I will be having major problems with when/if I
try to make a suitable PCB.

The ADG406 runs on +/- 15V and has a R-ON of 50
ohms, but will it give me acceptable sound if I
use it in an "straight-through" connection, or do
I need some kind of feedback circuit?

[eplpwr]

BTW, I forgot to mention that I'' be using PGA2310:s for the volume controls, and don't
think there'll be any problems muting strange sounds during source switching.

[mlloyd1]

I'll probably get drawn and quatered for this , but my "interim" preamp uses a scheme like this one from the Hafler 915 preamp.
It's not bad, sound-wise, and you can throw it together VERY quick. I use Maxim analog switches- higher power rails, MUCH better specs all the way around . I use 2SK389 as the JFETs (I like seeing the zero for DC offset).

Here is an old netnews posting of some related discussion. Enjoy and good luck.
mlloyd1
++++++++++++++++++++++++++++++++++++++++++
Newsgroups: sci.electronics.design
Subject: Re: 4053 THD results
Date: Thu, 03 May 2001 17:28:10 GMT
On Thu, 03 May 2001 13:10:22 +0100, Lamebert <lambert@newsguy.com>
scribbled:

>wavefront@bigfoot.com (Martin Griffith) wrote:
>
>>
>>Just done a quick test of a 4053 analogue mux when used as an audio
>>switch in a virtual earth config
>>
>>opamp Philips 5532
>>mux Nat semi CD4053bcn, about 10 years old
>>i/p R 10K
>>f/bR 10K
>>opamp output load impedance >10K
>>
>>input level +18dBu @1KHz
>>THD on o/p was better than .005%
>>
>>input level 0dBu @1KHz
>>THD on o/p was better than .002%, probably lash up limited!
>
>Interesting, I wonder what the original poster was hoping for.
>
>What supply was the 4053 on?
>
>I suspect an HC4053 would be better, from memory they have substantially
>lower on resistance than 4000 series.
>
Oops!, sorry for the omission
5532 was on +-15, with 100nF to gnd from each rail
The 4053 was on+-5v derived 3 1K resistors in series between the 5532
supplies.
The opamp needed 47pF across the feedback R to calm it down, haven't
done noise or F response, but should be adequate

I'll play around a bit, and put a couple of the gates in parallel, and
see how it improves.

Martin

[phase_accurate]

This thread might be interseting for you:
A good digitally controlled resistor pot

NP's opinion looks quite promising !

BTW: it wouldn't be that difficult to implement declicking measures for 4066 and the like - even without a microcontroller.

Regards

Charles

[peranders]

Is it possible to make slow on and offs with a 4066? I don't think so. Isn't the inputs buffered (= high gain)?

It's peculiar that AD not have mentioned any distortion figures at all....

[jackinnj]

don't bother with the CD4051.

[phase_accurate]

peranders wrote

Is it possible to make slow on and offs with a 4066? I don't think so.

No, it is of course not possible - but it isn't necessary either.
If you switch at a low (or better NO) signal voltage then there won't be a click. So you have to build a zero crossing detector that triggers a latch which controls the 4066s.

Regards

Charles

[peranders]

Originally posted by phase_accurate
If you switch at a low (or better NO) signal voltage then there won't be a click. So you have to build a zero crossing detector that triggers a latch which controls the 4066s.

You can clicks anyway due to DC offsets. It's also hard to design a fast zero crossing detector capable of a few micro volts or at least under 1 mV.

It's also possible to use discrete JFET's as analog switches. Then it's very easy to get them slow. National have application notes about this somewhere....