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Thread: Channel switching in tube amp with solidstate relays

  1. #1
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    Question Channel switching in tube amp with solidstate relays

    Did anybody try it ?
    E.g. Vishay LH15xx ? They seem to be almost ideal - linear operation, maximal signal amplitude 200V and more, isolation voltage 4500V, bounce free switching, low consumption...

  2. #2
    Noodle of Reality Steve Conner's Avatar
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    I've never tried it, but there are schematics over at Mark Huss's site.
    http://mhuss.com/18watt/schematics/MosfetSwitchTrex.gif

    It's also been discussed on this forum, try the search function.

    I think the only drawback with them is the high shunt capacitance in the OFF state. It might require a series-shunt pair of switches to stop signal bleeding through.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Yes, I also noticed relatively big capacitance, maybe it's major problem..

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    I may be wrong, but I doubt very much these can replace a metallic contact relay as *audio* switches.
    I see them as power switches, turning lamps, entire preamps or small motors on, rather than feeble audio signals.
    I had a slight hope when I saw some of them labelled "Telecom switches" thinking they might be part of complex Telecom audio matrixes, but not, they handle on/off hook (switching from 24 to 48V DC) and 48V Pk dialing pulsed.
    In short, they are not 300V capable CD4016/66/53 etc.
    I see the 18W.com drawing closer to a brainstorming idea than an actual working schematic.
    Of course, it might work as we hope.
    In short, I see them closer to Triacs and such than to audio components.
    If anybody bench-tested them, welcome.
    I'll see if I can grab a pair.
    EDIT: makers post no transfer curves at all !!!!
    Besides, being Led triggered, they "eat" a lot of power for what they do.
    Last edited by J M Fahey; 02-10-2010 at 01:36 AM. Reason: Today I'm slower than usual.

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    There are the IR PVT series

    Haven't tried them, and the capacitance issue is still there, but it does look suitable for signal switching.

    Hope this helps!

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    Maybe. Hope so.
    At least they show a voltage referred to current graph, it seems to be linear and it seems to cross "0"
    That's what worried me before, because I *have* worked with solid-state relays before, and they were meant to mimic an electromechanical leaf-type relay turning motors on and off and stuff like that, but they were useless for audio, they had a voltage threshold that distorted audio signals worse than crossover distortion, were not linear, etc.
    If you turn-on, say, a garage door opener motor, whether it's 120VAC or even 12VDC, "losing" 1 volt is nothing, but on audio it's terrible crossover.
    These *seem* to be linear.
    What I'd love to have is whatever Telecom companies use on matrix switchboards.

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    Noodle of Reality Steve Conner's Avatar
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    JMF: You get two different types of solid-state relays, MOSFET and triac. You may have had triac ones. The MOSFET type are what we're discussing here, and they are theoretically linear ("ohmic"), just like JFET and analog switches, but I can't vouch for them in practice as I've never tried them.

    I've used the H11F1 JFET optocoupler in amp circuits before, as a switch and a voltage-controlled soft clipper, but it's a bit of a wuss.

    The telecom stuff is all digital now, but they used to use relays and uniselectors.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    I used some Panasonic AQW210E dual SSRs in an SPDT configuration to switch between two preamp volumes and they didn't work worth a damn.

    On paper, they looked great, high voltage, low resistance, etc.

    In practice, however, they didn't turn off properly with a high-impedance source and load (driven from a 1Meg pot into a 1Meg grid load). I had to lower the load resistance way down before the off channel wouldn't come through at any appreciable level.

    I don't know if it was capacitive bleed-through on the off switch, or just something with the way the signal modulates the MOSFET channel, or what, but I was in a hurry to get a prototype done and didn't get a chance to further investigate it, I just ripped them out and put in some real relays.

    I plan to look into them further when I get a chance. I suspect they would be great for shunt switches, but I don't know about series switches - you may have to use an L configuration with a series and shunt to ground.

    The high capacitance of some of them may also adversely affect frequency response or inadvertently pass through very high frequencies in the off state (which would add a "buzz" into your clean channel if you were switching between clean and dirty).

    Randall Aiken

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    Noodle of Reality Steve Conner's Avatar
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    Thanks for the reality check, Randall!

    I had similar weird problems when using the H11F1 as a clipper. If it was biased partway on, the signal would couple capacitively into the floating gate, and modulate it on and off, making nasty, funky sounding distortion. I never got it sounding quite right, and went back to a Marshall-style diode bridge clipper. But H11F1s worked fine for me in other applications, like changing gain by switching cathode resistors.

    Maybe the MOSFET SSRs also suffer from this kind of "Miller effect" - if that's the right name for it, when you use them at audio frequencies.

    Someday I want to try back-to-back power MOSFETs as a speaker protection "relay" in a hi-fi power amp. I wonder what their THD measures like.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Quote Originally Posted by Steve Conner View Post

    Someday I want to try back-to-back power MOSFETs as a speaker protection "relay" in a hi-fi power amp. I wonder what their THD measures like.
    I'm going to be looking into some of those myself, soon. If I come up with anything interesting, I'll let you know.

    RA

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    Hi Steve.
    Yes, you are right, the SS relays I used repairing industrial machinery *must* have been Triac based, yet I never got one open, though.
    I know these are Mosfet based, I've seen the datasheets, and precisely there lies my doubt: Mosfets are great for switching, *can* be used for audio, but unfortunately they are enhancement devices (that by itself is a small nuisance but not a fault) *BUT* they are *very* nonlinear for the first 2.5 to 3.5 volts, just look at the curves.
    I guess that must produce something that sound very close to crossover distortion, which we all know is horrible .
    What you and others add about crosstalk and feedthrough make me want to stay away for now.

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    In general it is always better to use FET switches as shunt switches. If the source resistance is large, then sometimes this is all you need.

    They can be used as series switches too, but you ideally want a low source and load impedance. Unfortunately they have higher resistance at low signal levels (tens of ohms), so you get cross over distortion. But if you load them with a high resistance then you fix the crossover distortion, but capacitative bleed-through and poor offness become a problem, so you pretty much always need a 2nd one to act as a shunt and improve the offness. Using more than one shunt in parallel improves the offness of course (6dB improvement for every doubling of the number of FETs you use).

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    Yes, that's true.
    Fact is, we have many options: Fets, CMos gates, even bipolars shunting signal to ground, if we work with low voltages, say , 15Vpp and a few handle 30Vpp, but for higher levels as usual in tube amps, we are stuck with either LDRs or relays.
    Anyway we should try everything new, someday ......

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    Just finished a 5E3 with an added stage switchable by SSR. Used two avago chips, switching between volume controls after the first stage and then directing the second stage either to the PI or into the third stage for overdrive. I think it works well, no issues to speak of. I don't think there are any audio degradation issues or incomplete shut off. I can't hear any overdrive coming through on the 5E3 channel. Perhaps it depends on where in the circuit you want to use them, but unlike Randall's post above, they worked well for my switching preamp volume controls...

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    *VERY* interesting, please post what you implemented.
    Thanks.

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    Started out as a 5E3, but changed the tone control to the Spitfire control.

    Had some initial trouble with squealing, put a snubber across the second stage plate resistor, and now it's quiet as a mouse. This is the most quiet amp i've put together, and the wiring is far from ideal. No cross talk between channels at all
    Attached Thumbnails Attached Thumbnails ampschem.png  

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    Hi Michael.
    That's GOOD and very useful
    Thanks for sharing your successful experiment.
    K1-1 isn't switching *that* strong signal, but K2-1 gets hit real hard !!! Easily a 250Vpp squarewave or worse.
    And yet you report no feedthrough, not even through wiring capacitance or ground loops.
    I would have thought that grounding V2A's grid would have been necessary.
    Congratulations.
    We need more of this high quality work posted here.
    Again.

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    About the V2a grid,

    My first scheme had V1a being switched out of the circuitry, with v1b into v2a. Input went to v1a all the time, and also to the SSR. Input to the second stage was taken from either v1a plate or the common input. This howled, and i thought it was becaue I didn't ground the plate of v1a when second stage input was switched. So i redid the circuit and switching so that V1a either went to V1b and then v2a, or straight from v1a to the PI. This scheme grounded v1b grid when not in use. Still howled so i figured out it was oscillation and put a snubber on v1b plate resistor. that cured it, and the switching was fine. But there wasn't enough gain in one stage before the PI so i switched the circuit back to having either two stages or three, as in the current schem. I was worried about having the third stage ungrounded but it works great!

    There should be a lot of voltage swing going into the SSR after the second stage but it doesn't bleed at all.

  19. #19
    Noodle of Reality Steve Conner's Avatar
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    Very nice! Can you tell us the part number of the SSRs you used? I'd like to get some and check them out.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  20. #20
    Old Timer defaced's Avatar
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    For my own education, what is the appeal of solid state channel switching elements?
    -Mike

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    These were Avago ASSR-4128. I've also used NEC PS-7141 series in my Trex, no problems there either.

    To defaced: The big attraction is that you can switch silently as sometimes mechanical relays will pop. also FETs can't handle as much voltage as SSRs. and these thins are DIP8 packaged, and also come in SMT so they can be real small. I know there are some relays like 7mm square but i think they're a lot bigger than these $2.50 chips...
    Last edited by Michael Allen; 03-05-2010 at 01:16 AM.

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    Quote Originally Posted by Michael Allen View Post
    Started out as a 5E3, but changed the tone control to the Spitfire control.

    Had some initial trouble with squealing, put a snubber across the second stage plate resistor, and now it's quiet as a mouse. This is the most quiet amp i've put together, and the wiring is far from ideal. No cross talk between channels at all
    Hey Guy's! As a newbe to this forum I thought I would let you know that Crate Guitar Amplifiers have been using the Vishay VTL5C3 opto couplers for ages and my own Crate VC-3112 Combo sounds fantastic with no problems with switching! The only problem with these amps is the solder on the valve bases becoming bad! But Hey!!
    That's an easy fix and you can pick these beautiful babys up real cheap because of this!

  23. #23
    Noodle of Reality Steve Conner's Avatar
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    Those aren't solid-state relays though, just the old-fashioned Vactrols.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Quote Originally Posted by Steve Conner View Post
    Those aren't solid-state relays though, just the old-fashioned Vactrols.
    Hi Steve! Yeah! I get what your saying but if it works why fix it!
    Do you see any advantage with using the SS type relay for the switching of valve amp circuits?
    The VTL5C3's do a very nice smooth transition from channel to channel and very reliabliy so job done as far as most musicians are concerned!
    Am I missing something in this thread? Apart from experimenting of course! Nothing wrong with that!!

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    SSR are much smaller than vactrols, and aren't a resistor so you don't need to worry about passing signal/bleed through. Maybe in some cases, but in my experimenting you don't.

    And they're getting cheaper while I bet vactrols aren't being made in large quantities anymore

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