Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: terminalgs on August 17, 2010, 01:21:22 pm
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While looking at some old Acoustic schematics, I noticed that Acoustic used LDRs in series with the signal to turn off and on channels. I think GT did this as well on their Dual-75 or maybe on their Soul-o amps.
anyhow.. anyone here try it? experience/advice?
I have an 5V winding on a SE amp I'm not using. it'd be easy to use a footswitch
to control a couple fast acting LDRs.
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When channel switching there are a few things to consider:
- switch noise
- high off resistance, very low on resistance
- can handle 50V voltage swings
- low control logic part count
- leakage
- total cost
There are relays, mosfets, LDRs, JFets and other transistors.
LDRs are pretty good in the above categories, but a little pricey.
I really like JFets, but the one JFet I like a lot is no longer available.
Relays are inexpensive and do the job very well. Just watch out to make sure you dont get any switching noise. Relays have the bonus of DPDT.
LDRS and transistors dont realy require a little board for mounting, but it sure makes it a lot easier to work with.
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yeah, I figure with an LDR used like pictured in the attachment would have little if any switching noise...
It seems like it could be used in either of these ways: LED#2 mutes the signal, LED#1 could be a single cut (or boost, if thought of in 'reverse'). This would be with an LDR that has a dark R value of 20M or so,, and light R value of less than 1k.
the circuit to power the LEDs could be anything lightweight,, like 5V or 6V rectified with a filter cap (1000uf - 2000uf), a couple resistors to get the current right for the LEDs and 2 switches.
the cost of the LDRs and low voltage power supply,, would be what $15-20 ???
what am I missing...?
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I always try to switch "shunt to ground" rather than "series". Reduces switch noise and keeps leakage to a minimum although with an LDR of your specs no biggy. Other than that it'll work fine. Which LDR are you using? Vactrol's work well VTL5C1 is common. I have tried others without much luck - too sensitive to soldering.
When I speak of cost I think of making many amps not just one. For a one off amp, the expense LDR is minimal.
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I've used many of those $10 Weber relays over the last few years. I've never had any switching noise, pops, clicks, etc. The only drawbacks are having to isolate the jack from the chassis (I use stereo jacks and plugs to get around it), and it does use some real estate, altrhough not much, only 2" x 2 1/2".
Dave
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Which LDR are you using? Vactrol's work well VTL5C1 is common. I have tried others without much luck - too sensitive to soldering.
I've used Vactrols and CdS's, I haven't run into soldering issues with either one.
When I speak of cost I think of making many amps not just one. For a one off amp, the expense LDR is minimal.
I do 5 or 6 devices (preamps, or amps) a year, most are one offs, so ultimate cost of materials is not paramount, neither is speed of assembly.
thanks for your input ,, I'll give it a shot.