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Old 9th August 2012, 09:44 AM   #1
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Default solid state relays for plate load switching?

Hi All,

I've been wondering about the suitability of solid state relays in high impedance/high voltage (ie tube) audio circuits. I recently built a "programmable" guitar pre-amp using around 80 mechanical DIP reed relays and it worked pretty much how I expected it to (popping and all!) . However, I have some doubts about the reliability of such a system over the long term. These relays (Hamlin HE751) should last for at least several million cycles (apparently) at 5 volts. But I have no idea how long they will last at 200+ volts! I've taken the necessary precautions and minimized any capacitive switching but I somehow don't think it will stand the test of time.

Solid State relays on the other hand have no moving parts and would seem to be more robust. The only question is are they at all suitable for audio use? I can't seem to make head or tails out the spec sheets (I'm looking at the opto-mos types), specifically in regards to output/transfer capacitance as well as "input to output capacitance". I'm sort of confused as to what these different measurements allude to (they all seem the same to me), so I can't actually choose a suitable device...

I'm using the standard scheme where the relay shorts out it's respective resistor in order to create a 'n' bit resistor, if that has any impact on choosing a device. Any comments would be helpful!
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Old 9th August 2012, 10:18 AM   #2
oshifis is offline oshifis  Hungary
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They have Triacs inside, so I believe they are intended for mains frequency only...
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Old 9th August 2012, 10:50 AM   #3
roline is offline roline  United States
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Triacs and Quadracs will turn off at 0volts across them and then trigger on to short out when there is enough charge and voltage across the device for it to latch. They require a 0 crossing to turn off again. Every turn on event will create a harsh event, dimmers create a huge amount of harmonics and noise on the main line.
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Old 9th August 2012, 11:10 AM   #4
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Quote:
Originally Posted by exclamationmark View Post
Hi All,

I've been wondering about the suitability of solid state relays in high impedance/high voltage (ie tube) audio circuits. I recently built a "programmable" guitar pre-amp using around 80 mechanical DIP reed relays and it worked pretty much how I expected it to (popping and all!) . However, I have some doubts about the reliability of such a system over the long term. These relays (Hamlin HE751) should last for at least several million cycles (apparently) at 5 volts. But I have no idea how long they will last at 200+ volts! I've taken the necessary precautions and minimized any capacitive switching but I somehow don't think it will stand the test of time.

Solid State relays on the other hand have no moving parts and would seem to be more robust. The only question is are they at all suitable for audio use? I can't seem to make head or tails out the spec sheets (I'm looking at the opto-mos types), specifically in regards to output/transfer capacitance as well as "input to output capacitance". I'm sort of confused as to what these different measurements allude to (they all seem the same to me), so I can't actually choose a suitable device...

I'm using the standard scheme where the relay shorts out it's respective resistor in order to create a 'n' bit resistor, if that has any impact on choosing a device. Any comments would be helpful!
There is none.
No matter what type you will have problems with that concept.
If you have two plates, and two decoupling caps, and two load resistors, then you can switch between two sources of audio.
Otherwise if you are trying to switch DC without decoupling and loading first, you will fry the relays or get a big pop in the audio.

Switching between two gain levels, it's better to switch the cathodes instead of the plates. (the cathode bias can be switched because it's low voltage)

Types of relays:
LDR light dependent resistor (oldest method, most proven) but not for plate switching.

PC relay w/ DC coil, OK, used in several guitar amps, but to switch audio, not DC. Very sensitive to vibrations.

Reed Relay, only made for very low level and current although it lasts longer. Faster yes, but the contacts are very delicate and can weld together very easily. The contacts are not for high voltage, that's for sure. The reed relay is used to operate the coil of another relay, or to turn on a soild state device. Reed relay is used for very small signals.

Solid state relay, depends on triac, SCR, transistor, etc... will tend to distort the audio but might work with some tweaking.

Vacuum tube relay, now you may have something there. The concept is extinct but probably holds the most potential.

Mercury wetted vacuum relay, OK that would be interesting. $$$
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Old 9th August 2012, 03:10 PM   #5
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I'm looking at opto-fet type SSR's which are essentially built like an analogue switch (ie, back to back mosfets). They don't have crossover problems like triacs but I'm struggling to understand if they are suitable due to their capacitance issues. I've hooked up a couple to a signal generator and scoped the output and can't see anything untoward. Though hearing them is another issue altogether. I might just frankenstein it and jumper some SSR's into my existing relay modules and see if I can hear a difference for myself!
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Old 9th August 2012, 04:48 PM   #6
TheGimp is offline TheGimp  United States
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If you are using them in the signal path, I expect them to contribute some noise and distortion as they were optimized for power switching. That said, they will not exhibit the bounce problems associated with relays.

You mention switching 200V+, so it is not clear what you are doing with them.

A schematic would be helpful.
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Old 9th August 2012, 06:42 PM   #7
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What about a reed relay?
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Old 10th August 2012, 08:35 AM   #8
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by exclamationmark View Post
I'm looking at opto-fet type SSR's which are essentially built like an analogue switch (ie, back to back mosfets). They don't have crossover problems like triacs but I'm struggling to understand if they are suitable due to their capacitance issues.
OptoMOSFET SSRs can indeed be used for audio switching in valve amps- I prefer them to mechanical relays in non-hifi applications.

The self capacitance of an SSR is generally around the 50pF mark. When used in series with the signal this can easily allow high frequencies to leak past, due to the high impedances normally involved in valve amps (since a CR filter is created).

However, they can be used in shunt with the signal very successfully, assuming you keep the source impedance small enough not to lose the high frequencies (RC filter) (also consider slew rate). In musical instrument applications this isn't difficult since the bandwidth requirements are pretty relaxed.

Also, when used in shunt with the signal you don't need to worry about distortion since the audio only passes when the SSR is off, and therefore benign.

As Soundguruman noted, if you try and switch DC you will always get a pop, so only switch AC if that's something you need to avoid.

Last edited by Merlinb; 10th August 2012 at 08:38 AM.
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Old 10th August 2012, 02:54 PM   #9
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Quote:
I recently built a "programmable" guitar pre-amp using around 80 mechanical DIP reed relays........for plate load switching......popping and all
I have been tinkering with a microprocessor controlled guitar preamp for a while. I have been using sub-miniature tubes and a PIC chip so that it all fits inside the guitar.

I saw the Peavey ReValver software simulator a few years ago. In the high tier version you can get the schematic of a Peavey guitar amp on the screen and tweak resistor and cap values, even change tubes, and hear how these tweaks affect the sound. I am not too sure how tonally accurate the simulation is, so I set out to make a hardware version.

Whenever you change a plate load resistor value on a tube in real time you are going to get a pop, no matter what you use to do the switching, if the resistor value changes abruptly. This is because the current through the tube, and therefore the plate voltage changes instantly. This step function transient will have audible components that ripple through the amp. The abrupt transient can push the following stage(s) into distortion momentarilly until the bias recovers.

To avoid this you need to make the change occur slow enough so that there is little energy in the audio band, and the bias isn't disturbed on the following stages. Technically, you want the rise time of your switching signal to be slow enough that it doesn't travel through the amp's coupling caps.

I use ordinary P-channel mosfets to switch additional plate load resistors in parallel with the existing resistor. I use an N-channel fet for the cathode resistor. I use the digital output from the processor rounded with a RC low pass using about a 150mS time constant to slow down the switching.

You can use your reed relays to add cathode bypass caps. Just parallel the relay contacts with a 100K resistor to keep the cap charged. This stops the DC transient that makes the pop. I haven't used reed relays, but I have used the little OMRON's. I did shake one to death with the amp head on top of the speaker cabinet, so I am also working toward a non-mechanical solution. 200 volts is probably not going to kill the relays if you are talking about the usual 12AX7 circuit with less than 1mA of current.

TOP SECRET method for making connections in the audio signal path....Silonex LDR opto couplers. These are a Light Dependent Resistor and an LED in a plastic package. The resistance is high...about 1 Megohm when the LED is off. As you apply current through the LED the resistance of the LDR drops. At maximum LED current the resistance can be about 1K ohm.

These little critters can be used to make a dynamic channel switching preamp. You can fade from the clean channel to the dirty channel and back with a foot pedal.....or an Envelope Follower......even MIDI.

I tried the LDR couplers for a dynamic plate load and they fried! they are only rated at 40 or 50 mW of power. So you can get a 1/2 watt LDR that is rated at 320 volts. Then put it in a dark plastic tube with an LED and it should work for a plate load. I have ordered the LDR's but I don't have them yet.
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Last edited by tubelab.com; 10th August 2012 at 03:00 PM.
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Old 11th August 2012, 02:36 PM   #10
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I've been using something like the attached schematic with reed relays (ignore the rest of the schematic, just look at the plate switching mechanism). It works fine as it is, but I'm assuming if I use SSR's, with the SSR OFF there would be approximately a 50pf capacitance across the resistor terminals (though I guess that would depend on the voltage across the resistor and the SSR's transfer function). If I used this scheme for a gain control (ie, constructing a potentiometer in the same way as the variable plate resistor, albeit with 2x the resistors), what issues could I run into? I'm willing to accept some dulling in tone (and the simulations I've done seem to suggest a -3b point of over 30kHz!, depending on load/source impedances), in exchange for their reliability.
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