Channel switching relays.

[Colonel_Sanders]

Hi again,

Just wondering if these relays are suitable for channel switching in a tube preamp(NA-12W-K @ Mouser)? http://www.fcai.fujitsu.com/pdf/na.pdf . Seem good to me, but the coil resistance is higher than the other relays I've been scoping out. More resistance = less current draw?

Any other recommendtions are welcome!

Thanks,
C_S

[txstrat]

Hi,

I built several amps with a switchable preamp by relays. Only thing, I got little hum while the relay was "on" (in my amps the gain channel and therefore not too bad).
In another thread of this forum, I recently learned not to share the ground of the relay coils with the signal ground to eliminate the hum. I haven't modified my circuits yet, but I surely will. At least I believe it wouldn't do any harm.
You can find the relais I'm using here:
http://catalog.tycoelectronics.com/TE/bin/TE.Connect?C=1&M=BYPN&TCPN=1462000-7&RQPN=C93402
According to Ohms law you should have less current while having a higher resistance in the coil.

Matt

[Bruce / Mission Amps]

Hi again,

Just wondering if these relays are suitable for channel switching in a tube preamp(NA-12W-K @ Mouser)? http://www.fcai.fujitsu.com/pdf/na.pdf . Seem good to me, but the coil resistance is higher than the other relays I've been scoping out. More resistance = less current draw?

Any other recommendtions are welcome!

Thanks,
C_SI think Weber VST still sells a few mechanical relay board system kits.
One of these might work fine although they are a little clunky.

https://taweber.powweb.com/store/chansword.htm

[cbarrow7625]

I have a 5 channel amp that uses a whole slew of those relays (or one's prettymuch identical to them - from Mouser). I chose them because the coil resistance was high (yes, = less current draw). Running a bunch of them, I needed the lowest current draw possible.

Here's atrick to keep the relay hum from getting into your signal - Wire the relays so that the realy switches off for the active channel instead of switching on. That way the signal & the relay coil never pass the same place in the circuit. Simple, huh?

I did this on my 5 channel amp, it has everything from Fender & Bassman clean up to modded DR full on screaming gain & it is dead quiet.

Good Luck.

Chris

[Colonel_Sanders]

5 channel! Thats massive man. How many relays did you use? Keen on sharing the schematic? No worries if not, but I'm trying to make a 3 channel amp and its hard enough.

TX, I was looking at them as well, they look pretty similar.

Thanks for the tips guys, I'll go with the Fujitsu's.

Cheers,
C_S

[txstrat]

Here's atrick to keep the relay hum from getting into your signal - Wire the relays so that the realy switches off for the active channel instead of switching on. That way the signal & the relay coil never pass the same place in the circuit. Simple, huh?Chris

Chris
I did that too, and I also switch a LED to indicate a channel is ON.
Tried that with a switchable master volume control too, but don't really got a clue how to do it.
Do you have a trick handy how to switch the first master volume OFF and at the same time make the LED light up to indicate the second master volume is ON? Maybe some sort of schematic?

[cbarrow7625]

I'd love to post a schematic (and someday will) but two small children & a busy job make it really hard to find the time (not to mention the endless tweaks that always change it anyway). The switching isn't anyhwere near perfect either. I don't make any claims that it is ready for prime time, there are definitely some small switching pops but for my uses it is perfect.

[cbarrow7625]

What I did is effective, although not terrible elegant.

I have what I call a "reversing relay" for each channel. The "reversing relay" is turned "on" and it, in turn, switches "off" the realys on the channel I am using. Any LED's are run off of the coil on the "reversing relay". That will free up both halves of your DPDT relay to switch a master volume in & out.

I have all of my LED's in my footswitch & they work perfectly. I actually just have the LED's in series with the voltage going to each coil (with a series protection diode as well to protect the LED's from reverse bias). Quick, dirty, effective. You just need enough voltage headroom in the circuit to make this work.

Something equally effective & more elegant could be done with transistors or digital logic but my whole amp is built on copper ground plane boards so there are no PC Boards at all for fancy stuff.

[txstrat]

The way you use the reversing relay sounds pretty effective. If it's not elegant, what the heck?
I use the relay (DPDT) to switch the not needed channel to ground right after the first volume pot (over a 47K to ground) and the second half of the relay just to leave the signal path open right before the reverb recovery stage. That works great for the channel switching.
I thought of transistors for channel switching too but I'm not into electronics that much and would have to read and learn a lot about transistors to make things work. And as you stated: two kids and a busy job... me too.
I opened another thread for the master volume switching problem (also added a schematic how I think it could work) but nobody could help me along so far.

[cbarrow7625]

Post it again here. I'll see if I can't offer up something useful.

[txstrat]

Well, my idea is to have two independent master volume controls switched by a relay. I'd like to find a way to switch the two pots in the same manner like the channels. That would mean the relay shunts one of the volume pots to ground while the other pot lets the signal pass. I added a schematic of the way I think about it. Would this work? And what should the values of R1 to R4 be?

[cbarrow7625]

That would work but it is not the best solution. The values of the resistors is where you will run into trouble. To have good isolation & keep the circuit from loading down the tone pots you want R1-R4 to be fairly large (like 470k - 1M). That is good for isolation & loading purposes but is not real good for your tone. Most times, the signal will start to sound pretty "flat" and spongy after running through too much resistance.

If you want to try it, I would start with 470k in each position. I don't think it will sound particularly good but you never know. It's all about the compromises that you can bear to make to get functionality vs. "perfect" tone.

Assuming all you have to work with are SPDT relays, I would make one small wiring change. Instead of having "Relay A" shunt the signal to ground, I would have it jumper around R1 or R2 to short that resistor out of the circuit on the channel that is active. Just move the wire that is currently going to ground over to the junction of R1/R2. You will likely have to reverse your NO / NC (Normally open / Normally closed) contacts as well so that when energized the relay shunts around the resistor for the MV that you want to be active.

With that arrangement, you can make R1 & R2 as large as you want (2.2M? 3.3M?) and pay no "tone penalty" becasue you are no longer passing active signal through them. The signal totally bypasses those resistors & goes through the relay instead. Leave R3 & R4 as 470k, they should work OK for this application (maybe even 220K if you prefer the tone you get).

Let me know how that works for you. I don't have a real quick program to work up schematics so I have to try to decribe them verbally. If I have to I can do something in AutoCAD but it is not exactly "schematic ready". If this isn't to your liking I'll see if I can draw up something using two DPDT relays instead, that will give you a lot more flexibility.

Good luck.

Chris

[txstrat]

Chris
thank you for your detailed instruction. I know exactly what you mean (describing instead of drawing is pretty OK). I'm currently using one DPDT relay (no more space for two of them) but I unfortunately labelled each half relayA and relayB.
Your described way is a third version I haven't thought of yet. Up to now I switched the IN and OUT wipers of the different pots open (out of the circuit - see schematic) with either half of the relay (just a 1M resistor to calm the pop). But that makes the signal go THROUGH the relay and it might catch up some hum. Thats why I thought about switching either master pot to ground.
Do you see any sense in that?

Matt

[Colonel_Sanders]

Man, great thread, this is exactly what I'm trying to do as well...

[cbarrow7625]

I think you may be better off (from a switching noise perspective) if you change 'Relay B' back tothe way you had it before. Your new drawing definitely provides the best isolation but it is probably also the noisiest. It will probably 'pop' when switched.

If you want to keep this arrangement then I would go ahead and add 4 x 1M (or 2.2M) resistors around the relays. On each relay connect one end of 2 x 1M resistors to the relay center contact & the other ends to the NO/NC contacts of the relay, respectively. If you do this you can also get rid of the 1M relay to ground (the one before the first relay) because you will never lose continuity to ground if you are using the bypass resistors around the relay contacts. This setup should also help tame down the switching noise a little (or at least soften it). Changing the bypass resistors to 2.2M will give you better isolation but it may not be as quite as 1M. Try both and see what works best.

If you do what I described above to 'Relay A' and put 'Relay B' back like you had it before, you will probably have about the best comprimise between isolation & switching noise.

Let me know what you find.

[Colonel_Sanders]

I have a question to add. Would the circuit you're recommending work coming off the plates of V1 to switch the 'Volume' pots pre coupling cap in a 5F6a circuit? I don't see why not but just want to confirm...

Cheers
C_S

[cbarrow7625]

No. You always need to have a coupling cap before going to any pot. High voltage from the plate going to something you will touch with your hand? You do the math....

This would work fine afterthe coupling cap and before the grid.

[Colonel_Sanders]

Cheers for the reply. Lack of description on my part. The cap would come before the pot, but the relay will come before the cap. I suppose it would be better to put the relay after the cap though eh?

C_S

[txstrat]

Chris
Good proposal to use the compromise. I added a schematic like I would assemble it.
Do you think there would another pair of 1M resistors around relay1A be necessary to calm the pop?
And what value should the resistors R1 and R2 have? I believe they don't have to be that big (around 100 K?).
This is similar to the master volume section in the Peavey 5150 amp, except they don't switch the ground of the volume pots with a relay. I think there must be some influence in the range of either pot caused by the other pot in case they are not switched "out" of the schematic completely. Right?

Matt

[cbarrow7625]

Yeah. You want to avaoid switching DC if at all possible. It would be very noisy swithcing at best.

[txstrat]

Chris
Thats right. I want to switch the ground, cause I use a footswitch too (stereo phone plug for channel and master volume switching) and I don't wanna send the DC over the footswitch. Ground feels better to me to be switched. Do you think it'll work the way I drew it?

C_S
Have you done that before, or are you looking for a way like this for the first time?
I think it would be best to have the complete switching section in between two caps (one cap from the preamp - switching relays and master volume controls - cap to the PI).
You might leave out the cap to the PI, cause I don't think it would do any harm. I used both kinda ways in two different amps and either works fine. Never leave out a cap before a pot... you know the rest. HV over the pot = no good

[cbarrow7625]

It's OK to send low voltage DC down to the footswitch and use the footswitch to route the DC somewhere. That's no problem at all and it is down all the time.

I was referring to the Colonels question about putting the relays in the circuit between the plate & the coupling cap. Not a great idea.

[txstrat]

I don't think so either, even over a good relay I wouldn't feel good to send the HV.

Do you think it would be less noisy to switch the DC instead of the ground? What about the current draw? I sometimes use a footswitch cable a long as about 20 feet.
Not that I'm used to the big stages all the time. But sometimes lay the cable around the drumset and other stuff ;-)

[cbarrow7625]

That would work but I was thinking something more like the attached sketch. I've never used the "attach" feature here so I hope it works.

Sorry for the hand sketch but it was the quickest way for me.

[cbarrow7625]

It really shouldn't be a problem. Relays are very forgiving. A 12V relay usually has a minimum "on" voltage somewhere around 7-8 Volts.

I also have a very long footswitch cable (switching the LV DC) and a very low DC supply for my relays but even that works.

[txstrat]

Well this is a fourth way to switch.
I believe this is it. As far as I'm concerned I think it will reduce the pop to a minimum and ALSO have the not used pot switched to ground.
Absolutely great. The hand sketch is pretty OK. Thank you.

[txstrat]

I could use a different IC instead of the IC7812. Is there a IC 7814. I read in the manual of the relays they have max voltage of around 30 volts. 14 or 15 volts shouldn't be a problem. Is this really the less noisy way to switch?

[Colonel_Sanders]

Thanks Chris, legend.

TX, Its my first time using relays, and switching pots. I'll go after the coupling cap, but I'll leave the PI cap off for now.

Cheers,
C_S