Slight hum with PAB relay ...

[Tonegeek]

I have an amp that hums w/PAB engaged. It is dead quiet in all other settings. No problem with the OD relay. The PAB is wired like you would expect and I am using the 2 22m resistors across the contact points. I am running 12v relays using a voltage doubler off the filament supply and I thought this might be part of the problem so I tried powering it with a 9v battery and it still hummed. Something is radiating off the relay into the preamp circuit, I believe . I tried shielding to no avail. To explain this next part U have to understand how my PAB relay works. On my amp applying power turns on PAB (breaking the bass/bass wiper/treble pot conncetions). No power and PAB is off. Unplugging the relay is the same as turning PAB on. Got it? Now here's where it gets strange: If I unplug the relay, PAB comes on, but THE HUM IS GONE!. If I plug the relay in and switch it on with the footswitch, I get some hum! Even if I disconnect the power to the relays from the doubler (so there is no chance of any AC near the relays), and power it with a battery, it still hums! The only explanation I can come up with is that the relay coil is somehow acting like an transmitter. I think the relay coil needs a ground reference and you can't do that with the doubler method. (well, i havent figured out how to...) Anyone else noticed this problem? I will say the hum is not that bad, and if you also switch a cap across the treble pot ccw lug and the slope resistor like I do with PAB then it is not bad at all. Still, I want to eliminate this bug, if possible. I never noticed anything like this on my other amps where I use a separate relay tranny. I always wondered why HAD went to the trouble to have a separate relay transformer and this might explain it.

[Structo]

That is strange.
Do you have the diode across the relay coil taps?

I think before I tore too much real estate up in the amp I would try a different relay, or have you done that?

Below is how I have mine wired.

[David Root]

It does sound like the problem is in the relay. Is your OD relay wired the same way as the PAB relay?

The only one I have done so far (two more planned!) I used a PT with two 6V3 windings and used the lower rated winding to power the relays using HAD's exact doubler circuit, and no hum when relay is powered.

I have read that there can be trouble running the relay power off the heater winding, and that is why HAD used a separate transformer, but I don't remember anything about relays humming when powered.

In my case it's a separate winding, which would no doubt explain why I didn't have any problems. And BTW I didn't put in the diode across the relay coil either.

[Tonegeek]

I do use a diode, but I believe the diode just protects the coil from back EMF so probably would not cause or eliminate hum. PAB is wired the same as OD in the sense that they both have voltage on the + side of the coil and I complete the circuit from the - side to latch the relay. The OD relay does not cause hum. I did swap my relays and the hum stayed with the PAB circuit. I have not tried a different brand of relay though. I think there is something to the fact that I am using the heater tap for power. If i had an isolated 6v tap, I could use that and then tie one end to the bus bar, but I can't do that with the heater supply. I really would like to understand exactly why this happens because it just seems like it should not be an issue. The coil in the relay is isolated from the contacts and is fed with well filtered DC, so it just makes no sense why that would cause a hum. One funny thing though. If I put one probe of my meter on the + terminal of the relay coil and the other probe on either 6v heater tap, I can measure a DC and an AC voltage. If you look at the schematic of the voltage doubler, you can sort of see why this might happen. I think that potential AC there is inducing just enough hum to be noticed. Its a wild guess really, but all I have at the moment...

[markr14850]

If you have a scope, see if the hum is visible coming out of the first stage of V1. If it is, then probably your rectifier is causing extra ripple in your heater supply, which then couples to the cathode.

[odourboy]

I have an amp that hums w/PAB engaged. It is dead quiet in all other settings. No problem with the OD relay. The PAB is wired like you would expect and I am using the 2 22m resistors across the contact points. I am running 12v relays using a voltage doubler off the filament supply and I thought this might be part of the problem so I tried powering it with a 9v battery and it still hummed. Something is radiating off the relay into the preamp circuit, I believe . I tried shielding to no avail. To explain this next part U have to understand how my PAB relay works. On my amp applying power turns on PAB (breaking the bass/bass wiper/treble pot conncetions). No power and PAB is off. Unplugging the relay is the same as turning PAB on. Got it? Now here's where it gets strange: If I unplug the relay, PAB comes on, but THE HUM IS GONE!. If I plug the relay in and switch it on with the footswitch, I get some hum! Even if I disconnect the power to the relays from the doubler (so there is no chance of any AC near the relays), and power it with a battery, it still hums! The only explanation I can come up with is that the relay coil is somehow acting like an transmitter. I think the relay coil needs a ground reference and you can't do that with the doubler method. (well, i havent figured out how to...) Anyone else noticed this problem? I will say the hum is not that bad, and if you also switch a cap across the treble pot ccw lug and the slope resistor like I do with PAB then it is not bad at all. Still, I want to eliminate this bug, if possible. I never noticed anything like this on my other amps where I use a separate relay tranny. I always wondered why HAD went to the trouble to have a separate relay transformer and this might explain it.

This is mysterious indeed!

So there's one thing that I want to ask. You mention that due to the way you've wired the ground lift, unplugging the relay is the same as not powering the relay. However, the relay does provide a connection between the treble pot and the phat PAB tweak that would not be there if it were removed from the socket. Is that correct?

So assuming that your relay is not acting as a weird receiver, I think you have to look at the connection made by the 'Normally Closed' relay contacts versus unplugging the relay. I'd try unwiring the Phat PAB tweak. If that works, maybe you need some sort of a ground reference around your 680p cap?

[Structo]

I guess I am lucky, my PT has a 5v winding.
I made an 8v DC regulated power supply.

It has 5 470uf 25v caps and a .1uf film cap.

Deric made the pcb boards for me.

[badtweed]

I have an amp that hums w/PAB engaged. It is dead quiet in all other settings. No problem with the OD relay. The PAB is wired like you would expect and I am using the 2 22m resistors across the contact points. I am running 12v relays using a voltage doubler off the filament supply and I thought this might be part of the problem so I tried powering it with a 9v battery and it still hummed. Something is radiating off the relay into the preamp circuit, I believe . I tried shielding to no avail. To explain this next part U have to understand how my PAB relay works. On my amp applying power turns on PAB (breaking the bass/bass wiper/treble pot conncetions). No power and PAB is off. Unplugging the relay is the same as turning PAB on. Got it? Now here's where it gets strange: If I unplug the relay, PAB comes on, but THE HUM IS GONE!. If I plug the relay in and switch it on with the footswitch, I get some hum! Even if I disconnect the power to the relays from the doubler (so there is no chance of any AC near the relays), and power it with a battery, it still hums! The only explanation I can come up with is that the relay coil is somehow acting like an transmitter. I think the relay coil needs a ground reference and you can't do that with the doubler method. (well, i havent figured out how to...) Anyone else noticed this problem? I will say the hum is not that bad, and if you also switch a cap across the treble pot ccw lug and the slope resistor like I do with PAB then it is not bad at all. Still, I want to eliminate this bug, if possible. I never noticed anything like this on my other amps where I use a separate relay tranny. I always wondered why HAD went to the trouble to have a separate relay transformer and this might explain it.

I just completed my first builds (2 heads) of the 100 watt Overdrive and have encountered the same problem. I'm using a separate tyranny with a regulated dc supply and am also puzzled but the slight amount of hum when the PAB is engaged. Noise and hum levels without the PAB engaged are quite low. As I'm not wild about the PAB feature I will likely come up with another boost of my own design to replace it with anyhow but I am pretty curious to discover where the hum is getting introduced and how to get rid of it.

[Tonegeek]

I have an amp that hums w/PAB engaged. It is dead quiet in all other settings. No problem with the OD relay. The PAB is wired like you would expect and I am using the 2 22m resistors across the contact points. I am running 12v relays using a voltage doubler off the filament supply and I thought this might be part of the problem so I tried powering it with a 9v battery and it still hummed. Something is radiating off the relay into the preamp circuit, I believe . I tried shielding to no avail. To explain this next part U have to understand how my PAB relay works. On my amp applying power turns on PAB (breaking the bass/bass wiper/treble pot conncetions). No power and PAB is off. Unplugging the relay is the same as turning PAB on. Got it? Now here's where it gets strange: If I unplug the relay, PAB comes on, but THE HUM IS GONE!. If I plug the relay in and switch it on with the footswitch, I get some hum! Even if I disconnect the power to the relays from the doubler (so there is no chance of any AC near the relays), and power it with a battery, it still hums! The only explanation I can come up with is that the relay coil is somehow acting like an transmitter. I think the relay coil needs a ground reference and you can't do that with the doubler method. (well, i havent figured out how to...) Anyone else noticed this problem? I will say the hum is not that bad, and if you also switch a cap across the treble pot ccw lug and the slope resistor like I do with PAB then it is not bad at all. Still, I want to eliminate this bug, if possible. I never noticed anything like this on my other amps where I use a separate relay tranny. I always wondered why HAD went to the trouble to have a separate relay transformer and this might explain it.

This is mysterious indeed!

So there's one thing that I want to ask. You mention that due to the way you've wired the ground lift, unplugging the relay is the same as not powering the relay. However, the relay does provide a connection between the treble pot and the phat PAB tweak that would not be there if it were removed from the socket. Is that correct?

Not quite. unplugging the relay is the same as powering the relay. when the relay coil is powered it turns on PAB which is exactly the same condition as unplugging the relay. About the phat PAB tweak, that came later. It was humming worse before I added that tweak. With the tweak, it is true that unplugging the relay would not close that contact. I mentioned the tweak because with it, the hum is a lot less than it was without it.

So assuming that your relay is not acting as a weird receiver, I think you have to look at the connection made by the 'Normally Closed' relay contacts versus unplugging the relay. I'd try unwiring the Phat PAB tweak. If that works, maybe you need some sort of a ground reference around your 680p cap?

I tried unwiring everything I could think of and nothing really makes sense. with PAB on, If I remove the wire from the relay that comes from the ccw lug on the treble pot, the hum goes away. This wire is only 2 inches long. I just can't fathom how that 2 inch wire hums when attached to an open relay contact and it doesn't when not. There can't be more than one inch of wire from the solder point to the contact! I even tried removing the 22m silencer across the contacts. no difference. that is why I suspect the coil. Maybe the coil is just too close to the signal wiring. It is a really tiny relay. I mentioned earlier that there is only DC flowing through the coil, but there is AC and DC potential between the coil and the 6v heater supply feeding the voltage doubler. I should like to try a larger relay sometime.

the other thing I havent tried is wiring the relay backwards so that when the relay is unpowered, then PAB is on and adding power turns it off. I am curious as to how this might behave.

[Tonegeek]

I have an amp that hums w/PAB engaged. It is dead quiet in all other settings. No problem with the OD relay. The PAB is wired like you would expect and I am using the 2 22m resistors across the contact points. I am running 12v relays using a voltage doubler off the filament supply and I thought this might be part of the problem so I tried powering it with a 9v battery and it still hummed. Something is radiating off the relay into the preamp circuit, I believe . I tried shielding to no avail. To explain this next part U have to understand how my PAB relay works. On my amp applying power turns on PAB (breaking the bass/bass wiper/treble pot conncetions). No power and PAB is off. Unplugging the relay is the same as turning PAB on. Got it? Now here's where it gets strange: If I unplug the relay, PAB comes on, but THE HUM IS GONE!. If I plug the relay in and switch it on with the footswitch, I get some hum! Even if I disconnect the power to the relays from the doubler (so there is no chance of any AC near the relays), and power it with a battery, it still hums! The only explanation I can come up with is that the relay coil is somehow acting like an transmitter. I think the relay coil needs a ground reference and you can't do that with the doubler method. (well, i havent figured out how to...) Anyone else noticed this problem? I will say the hum is not that bad, and if you also switch a cap across the treble pot ccw lug and the slope resistor like I do with PAB then it is not bad at all. Still, I want to eliminate this bug, if possible. I never noticed anything like this on my other amps where I use a separate relay tranny. I always wondered why HAD went to the trouble to have a separate relay transformer and this might explain it.

I just completed my first builds (2 heads) of the 100 watt Overdrive and have encountered the same problem. I'm using a separate tyranny with a regulated dc supply and a also puzzled but the slight amount of hum when the PAB is engaged. Noise and hum levels without the PAB engaged are quite low. As I'm not wild about the PAB feature I will likely come up with another boost of my own design to replace it with anyhow but I am pretty curious to discover where the hum is getting introduced and how to get rid of it.

In your case you might try connecting the minus side of your relay power supply to your ground bus for the amp. I have to do this on my amps that are using separate trannies for the relay power, or I get some hum. Before you do this, look your schematic over to makes sure nothing will be shorted, or just measure the 2 connections with a voltmeter. Should be close to zero volts.

[David Root]

Badtweed, I had thought of putting a 470K trimpot in the circuit of each of the bass, mids and treble with a 3PDT relay to actuate them and a panel switch for each one so you can preset one, two or all three tone circuits.

I think this is difficult to do with the standard Skyline circuit, but I Dumbleized an existing design that uses Steve Bench's LC tonestack from Duncan Amps website. This design floats all three tone pots on a single resistor but I can't see why that couldn't have the three trimpots inserted in it as I have described.

Why not take a look at that and see if it would work? The Tonestack Calculator is a useful tool. I guess this LC stack could be remodeled as an RC stack too.

[Tonegeek]

If you have a scope, see if the hum is visible coming out of the first stage of V1. If it is, then probably your rectifier is causing extra ripple in your heater supply, which then couples to the cathode.

I wrote the text of the original post a while back and should have revised it to say that I don't actually have access to this amp now to try different things. The PAB tweak solved the issue with that amp, but I want to understand this going forward in case i build one without that tweak and it hums, I will know how to fix it. I think it is something from the heaters getting into the signal, but it just baffles me that it goes away when I pull the relay. There is still relay power present for the other relay and if the rectifier cause more ripple at the heaters, seems like it would still get in the cathode regardless. I wondered if the extra draw on the PS with both relays were on was causing some ripple but I think i got hum even when OD was off and PAB was on. i also tried adding extra caps in the relay supply, but this made no difference.

[badtweed]

If you have a scope, see if the hum is visible coming out of the first stage of V1. If it is, then probably your rectifier is causing extra ripple in your heater supply, which then couples to the cathode.

I wrote the text of the original post a while back and should have revised it to say that I don't actually have access to this amp now to try different things. The PAB tweak solved the issue with that amp, but I want to understand this going forward in case i build one without that tweak and it hums, I will know how to fix it. I think it is something from the heaters getting into the signal, but it just baffles me that it goes away when I pull the relay. There is still relay power present for the other relay and if the rectifier cause more ripple at the heaters, seems like it would still get in the cathode regardless. I wondered if the extra draw on the PS with both relays were on was causing some ripple but I think i got hum even when OD was off and PAB was on. i also tried adding extra caps in the relay supply, but this made no difference.

I failed to add that in my case that when you toggle the DEEP front panel toggle switch on, the hum goes away when the PAB boost is engaged. Seems that the hum is 120 hz as opposed to 60 hz hum. I'll try relocating the separate regulated dc power supply circuitry's grounding as you proposed and see if that helps.

To David Root; Thanks for those suggestions, appreciate you taking the time to respond with that advice.

[ChrisM]

If the regulators ground tab is in contact with the chassis.....did you also ground the middle lug of the regulator? If so, you might have a ground loop.

[Structo]

On mine I have a heatsink on the 8v regulator. So it is vertical.

I didn't want to end up chasing down issues with it bolted to the chassis.