From tomhiwat–(at)–ol.com Wed Jan 8 15:17:08 CST 1997
Article: 33663 of alt.guitar.amps
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From: tomhiwat–(at)–ol.com (Tom HIWATT)
Newsgroups: alt.guitar.amps
Subject: Re: Channel Switching?
Date: 8 Jan 1997 20:04:01 GMT
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>BlueStrat wrote:
>>
>> What do channel switching amps use, or what is the preferred device for
>> channel switching? I’m guessing relays.
>>
>> Any circuit suggestions would be most welcome.
>>
> Mike
>
>Most of them use VACTEC VTL5C4/1 opto-couplers (available from MOJO,
>Allied, and several other places…) Relays can make an audible click
>or pop when switching if not careful…
>
>Randall Aiken
>raike–(at)–ellsouth.net

Exactly right. The VTL5C1 is my choice, the 5C4 has too low an off
resistance. Use two per channel, one in series, one to ground. To turn
the channel off, open the one in series and ground the output. To turn
the channel on, reverse. do the opposite action for your other channel
and you have channel switching! They use 5 volts and you set the current
with a resistor in series with the led. 35ma is good. You can use a hex
invertor to give you your opposite actions.

As for relays, they can be used but you need to test several to find a
quiet one. I have found using a film cap across the relay windings helps
a lot. Try Magnecraft W76EURCPCX-61(available from Mouser) with a .1 film
cap. It’s dpdt, giving you your alternating action without any chips!

Vactec VTL5C1s are light dependent resistors, not transistors

From raike–(at)–ellsouth.net Wed Jan 8 15:17:32 CST 1997
Article: 33668 of alt.guitar.amps
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From: Randall Aiken
Newsgroups: alt.guitar.amps
Subject: Re: Channel Switching?
Date: Wed, 08 Jan 1997 15:45:22 -0500
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Tom HIWATT wrote:
>
> >BlueStrat wrote:
> >>
> >> What do channel switching amps use, or what is the preferred device > >> for channel switching?

> >
> >Most of them use VACTEC VTL5C4/1
^^^

> Exactly right. The VTL5C1 is my choice, the 5C4 has too low an off
> resistance. Use two per channel, one in series, one to ground.

You are absolutely right…typo on my part…the VTL5C1 is the unit of
choice for the high on/off resistance range needed in audio channel
switching. It is used in most of the amps on the market today. I use
the VLT5C4/2 in linear-servoed circuits for video and chroma gain
controls with a +/- 6 dB range…but that’s another story…:)

Randall Aiken
raike–(at)–ellsouth.net

From mgarvi–(at)–anix.com Wed Jan 8 17:00:59 CST 1997
Article: 33678 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Channel Switching?
Date: 8 Jan 1997 17:09:23 -0500
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>BlueStrat wrote:
>> What do channel switching amps use, or what is the preferred device for
>> channel switching? I’m guessing relays.

In <32D3F5B8.611--(at)--ellsouth.net> Randall Aiken writes:
>Most of them use VACTEC VTL5C4/1 opto-couplers (available from MOJO,
>Allied, and several other places…) Relays can make an audible click
>or pop when switching if not careful…

Hi Randy (and Mike, etc),

Correct that relays can cause pops, but that’s easy enough to avoid,
right? (Make sure DC levels are matched) LDR’s tend to slew into
place, but you could still get some popping if dc levels are wrong.

I like LDR’s, as you know (Randy and I have emailed about using them
in amp and fx circuitry). The problem with LDR’s for channel
switching is that it’s difficult to avoid some signal bleed.
Especially a problem if one of the channels is overdriven. I’ve heard
several LDR-switched amps that have a slight buzz in the background
due to crosstalk from the OD channel.

The problem above can be solved with multiple LDR’s, but I still like
relays for pure switching apps. Some may also argue for FETs, but
there are different problems there.

The problem that I encounter with relays is that it’s a bit tricky to
power the coils without going to an external supply. I’ve used the
filament supply to do this, as the power drain is usually not severe,
but the fil supply is not hooked to ground on one end (it pivots via
two 100 ohm resistors or center-tapped xfmr winding). Also, the coils
on the usual dip relays are 5v DC, which means you’ll have to work up
a small rect, regulator, etc. Some regulators have fairly high
drop-out voltages, so ‘LDO’ (low dropout) regulators are recommended.

The fil supply would still work fine if used end-to-end with no ground
ref. The reason I mention ground referencing is that it is usually
required for the switching circuitry. For switching, I often use
high gain PNP’s on the front end so the circuit can be config’d as
‘active low’: pulling the footswitch lead to ground turns on the
transistor. That elim’s having to run a + voltage thru the footswitch.

I’ve also used CMOS logic for more complex switching arrangements,
but usually with aforementioned PNP in front to avoid possible static
problems, etc. and to allow the ‘active-low’ switching arrangement.
Darlingtons will work very well (very high gain), but they have a
high ‘Vce Sat’ which uses up a precious volt from the supply. So
there’s a problem driving the coil directly. I used PNP darlingtons->
feeding digital circuitry-> feeding TO-220 bipolar drivers in a
recent circuit. Worked very well. The digital circuitry in that
case allowed me to use a front panel switch in addition to the foot-
switch without one switch overriding the other. (hint: EXOR gate)

Some of the same problems may be encountered when lighting LED’s
for the opto assembly, of course, except the voltages would not
have to be as tightly controlled.

Note that Radio Shack stocks LDR’s. Not sure of quality or turn-on/
turn-off time, but in a pinch you could build your own LED/LDR pairs.

By the way, remember to put a reversed diode across the relay coils
to avoid spiking the driver transistor. Some relays have ’em built
in…some don’t.

MGarvin

PS: Randy already knows this stuff. Posted for the group and
possible discussion.

 

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