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Old 15th March 2014, 12:07 PM   #11
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I asked 30 Euro for 7 pieces (2 x 2402, 5 x 2404). Will have to check where they are.
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Old 15th March 2014, 03:21 PM   #12
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Back to that ADG1612 Analog Device chip I mentioned, they also have a related intem, the ADG1613 (ADG1612 datasheet and product info | 1 ? Typical On Resistance, 5 V, +12 V, +5 V, and +3.3 V Quad SPST Switches | Analog Switches | Analog Devices). The 1613 is a 2 gate "make before break". Mouser has the whole family (adg1611,12, and 13) and they are reasonably priced. Certainly better priced than any resistive opto isolators I've seen.

But the point of my thread is to hear opinions as to whether or not digial analog switch chips (any kind) would work as well as a resistive opto isolator, and maybe what drawbacks and advantages each has. because when it comes to audio, I can think of many of occasions where even after looking at all the numerical specs, the part that performed best and worst were a total surprise.
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Old 15th March 2014, 06:42 PM   #13
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Organs of a certain era used a lot of CD4016, CD4066, and CD4052 analog switches to do the keying of the notes. The actual keys switched DC voltage, which controlled the analog switch. The elimination of AC keying occured at the same time as the elimination of palladium/rhodium plating on the key contacts in the popularly priced organs.
Traditional organ sounds stop at about 7000 hz so they were able to use heavy low pass filtering to eliminate switch pops. As these weren't zero crossing switches, the pops were pretty radical if the note switched on at a waveform peak. IMHO the whole analog switch product line variations are variations on these pioneering products. The part number prefix now is MC14xxx or others, RCA the originator has been shut down.
The slowness of the CDS cell to turn on and off seems to be a tremendous advantage in the strictly analog world. At $5 each they had better be superior, and I can't say much for the PC board layout compatibility of the packaging of the Vactrol either .
As far as the human interface of analog switches, the actual copper switches we humans use to control them have "contact" bounce as a common feature, which goes on for some milliseconds, which is very audible. To keep this "keyclick" from being audible, either heavy low cut filtering has to be used, or some sort of debounce circuit has to be used that switches only ones. Once debounced, the analog switch will only switch once, which gets the pop up into the megahertz realm. With a switch going back and forth several times over 10 millisecons, the pop would be down in the thousand hertz range which is right in the middle of the music you are trying to pass.
One form of debounce is the flip flop, that switches once, instead of going back and forth several times. These work best with a double throw copper switch.
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Old 16th March 2014, 02:31 AM   #14
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I have tinkered with both. The Vactrol type devices function as a variable resistor. Some can be found with ratings high enough so they can be directly connected into tube electronics and used to vary the parameters of the tube. Try that with a silicon chip.

Most of the analog switches, and e-pots on the market today restrict the signal voltage swing to between ground and Vcc which can be very restrictive in some situations...especially in tube circuits. If you use enough series resistance the chip won't blow up when overdriven, but the distortion produced doesn't sound good, and there can be a noticeable recovery delay.

Both have their place in an amplifier depending on what you are doing, and how high the maximum signal level is.

Want to play with Vactrols cheap, try these:

Electronic Goldmine - SALE! (Pkg of 50) Audio Opto Coupler
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Old 17th March 2014, 02:44 AM   #15
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One problem I'm aware of with CMOS switches is that driving any of the analog pins beyond the rails causes leakage and/or noise to all the analog channels. I don't know if the ADG parts suffer from this problem.
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Old 19th March 2014, 03:44 AM   #16
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All analog switch products suffer problems up to and including blowing up when you take input signal beyond supply rails. They can also latch ON.
The Analog Devices "High Voltage" parts will pass/switch about 35 V pk-pk signals maximum.

The old H11F "Photo Fet" Optocoupler can handle about about 60V pk-pk.

The IRF Solid State relays can handle 200V pk-pk.
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If using JFET switches, Analog Switches (CMOS or JFET) or "PhotFET Opto Couplers then you have to be very careful about where in the circuit you use them to ensure you do not exceed those signal swing limits.

Sometimes you just have to use a garden variety mechanical type relay.

Nothing worse than a 2 channel amp using JFET switches to channels select, than when you start getting weird distorted breakthrough from the supposedly OFF Channel. You only ever design one like that and I've done mine.

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Old 20th March 2014, 12:40 AM   #17
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What do you think of this analog switch circuit?
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Old 20th March 2014, 12:43 AM   #18
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Voltwide,
That is the absolute best way to go about using jfet or cmos analog switches. That is, use them at a virtual earth point where there is effectively no signal voltage just signal current.
It doesn't help when overdriving results in a gain of zero and the vitual earth point "disappears". The voltages will be clipped by previous SS stage to be within the jfets voltage range so I don't think your back to back diodes do anything except form a low pass filter from that 100K + the diode capacitance.

This scheme is not that suitable for tube amps. First we don't like feedback circuits within the amp, they tend to react badly to overdrive. The '170 is a 40V rated part limiting its use to where you can gurantee that the signal voltage is less than about 36V pk-pk (Actually 40V - the Vgs turn off Voltage).

Cheers,
Ian

Last edited by gingertube; 20th March 2014 at 12:46 AM.
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Old 20th March 2014, 01:18 AM   #19
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Quote:
Originally Posted by gingertube View Post
It doesn't help when overdriving results in a gain of zero and the vitual earth point "disappears". The voltages will be clipped by previous SS stage to be within the jfets voltage range so I don't think your back to back diodes do anything except form a low pass filter from that 100K + the diode capacitance.

Ian
to be honest, I do not have a clue what you are talking about...

From my point of view during the off-state, the diodes clamp the signal to +-0.7V and the fet disconnects this from the summing input.
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Old 20th March 2014, 01:36 AM   #20
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Sorry I was'nt clear.

If using an OPAMP then the circuit is fine, because there is no signal voltage at that summing point (summing point = virtual earth).

You do need to watch the gain of the opamp which you have set to 1/10th. 10K/100K = 1/10.
If gain is sufficiently high that the output of the opamp saturates (hits the rail voltage) then the summing point stops working like a summing point (because the incremental gain has gone to 0, that is, more input results in 0 more output).

With +/- 15V rails on the opamp then this circuit should be able to cope with +/- 150V input. The ONLY negative? is that gain of 1/10th.

The same thing can happen if using a tube stage as the active gain element rather than an opamp.
You must set the gain such that the stage never saturates.

I was wrong about the diodes - because I was thinking of the JFEt as a shunt switch to 0V rather than a series switch. Sorry.

Hope this make it clearer.

Cheers,
Ian

Last edited by gingertube; 20th March 2014 at 01:49 AM.
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