Help with 60Hz hum in Foreplay I in custom chassis

[atalcott]

A little while back I picked up a Foreplay I kit that had been sitting in someone's closet for the last 30 years. I'm just getting starting with tube electronics, and putting it together was a great learning experience. It came with the Anticipation and Sweetest Whispers upgrades, though I only installed the former. Once I had it running in the original configuration I started thinking how I could make it better for my tastes and purposes. I built a new case for it out of thinner walnut with a lower profile, moved the PT underneath the chassis inside the case, moved the input and output jacks to the back of the case, replaced the dual volume pots with an Alps Blue Velvet stereo unit, added a choke in place of the second resistor in the CRCRC network, replaced the coupling caps with Solens, added 100k resistors at the outputs to attenuate the output level a bit (without them the gain was just ridiculous when matched with my Lafayette KT550 power amp) and got a matched pair of Tung Sol reissue 12AU7s.

The thing sounds great, but it's got a pretty pronounced 60Hz hum. I tried to replicate the grounding scheme as much as possible when moving to the new chassis, but with the move of the i/o jacks I did have to make some changes. Like I said, I'm more or less a complete beginner, so I'm not sure where to start with tracking down the hum. It’s in both channels, is unaffected by input source or volume control, though there is an additional, slightly louder 60hz buzz over the top of the hum when I touch the volume pot. I’ve also tried moving things around (various wires, the Anticipation circuit boards) with a chopstick and no movement seems to make any difference in the volume of the hum.

Any suggestions for things to look at? Any obvious mistakes I've made with the grounding scheme? Pics attached. Thanks very much for any input.

[Paul Joppa]

One widely used mod from back in the day was to tie the signal ground to the chassis ground. If I recall correctly, that connected terminal 13 to 14 - but it's been at least a couple decades so I might have mis-remembered. It solved most hum issues.

[atalcott]

Thanks Paul. I just tried that out but no luck I’m afraid.

In the original setup signal ground goes to the grounds for the volume pots, the CF/VA boards, the cathodes of the first section of each tube, and the heater center tap of socket B (pin 9), then to terminal 13, and finally a line ran from there to terminal 1. I put a jumper from 1 to 3, connecting the circuit to chassis ground at the first filter cap ground, and there only. With that configuration, I didn’t get any hum.

Now the signal ground goes to a jumped dual terminal strip near the volume pot. That strip is connected to the grounds for the volume pot, CF/VA boards, the cathodes of the first sections of each tube, and the heater center tap of socket B. Finally, a line runs from that jumped terminal back to terminal 1.

To my thinking this should be effectively the same as the original, but maybe I’m missing something. The only effective difference I can see is that now the output signal ground comes after the input signal grounds.

[Paul Joppa]

Well, it was worth a try  :^)  Hopefully PB will join in; he has the most experience fixing these things. Meanwhile:

* If you haven't already checked the resistances and voltages, it's always a good thing to do first to be sure it's working properly.

* The second thing is to re-flow every solder joint, making sure the solder flows and wets both wires and terminal lug. It takes only a little time, and solves a great majority of issues without wasting a bunch of time figuring out where the problem is.

(N.B. If you are using the original single-crystal copper ground wire, it had an amazingly strong enamel insulation that makes soldering difficult worth checking each joint. Usually just re-soldering does the trick, but this wire sometimes needs serious scraping.)

* The third thing is to isolate the preamp from other components and cables as much as possible - shorting plug at the input are most helpful, but turning the volume all the way down is often nearly as good.

* Finally, posting some photos of the wiring is a big help.

I don't understand your 100K output resistor - can you clarify?

[Paul Birkeland]

With that configuration, I didn’t get any hum.

So...problem solved?

[Karl5150]

Very nice and very original take on the vintage BH kit.
Karl

[Paul Joppa]

I looked at the photos again more closely, and I see the 100K resistors, along with some resistors to ground whose color code I can't quite read - are they the original 470K? If so, the combination increases the output impedance from 1K to 82K - way too high to drive cable capacitance, and can much more easily easily pick up hum by capacitive coupling. Also, the resulting attenuation is very small. You could change the 470K to 10K, which will reduce the output impedance to a hair less than 10K, and will reduce the output by 20dB - including any hum.

The wiring layout of the original was not that great, and yours is somewhat worse, primarily because the path is longer. Ideally, the signal and ground wires should be bundled together all the way to the tube and then to the output jacks - the idea being to minimize the loop area enclosed between them. (I see that you have done this with the Anticipation boards.) That will be a big job, so try the 10K first!

[atalcott]

So...problem solved?

Sorry, that was unclear I guess. What I meant was that I didn't have hum in the original configuration, the first one I described. I do have hum in the second configuration though.

I looked at the photos again more closely, and I see the 100K resistors, along with some resistors to ground whose color code I can't quite read - are they the original 470K? If so, the combination increases the output impedance from 1K to 82K - way too high to drive cable capacitance, and can much more easily easily pick up hum by capacitive coupling. Also, the resulting attenuation is very small. You could change the 470K to 10K, which will reduce the output impedance to a hair less than 10K, and will reduce the output by 20dB - including any hum.

Yep, those are the original 470k resistors. I saw a couple other posts here on the forum about attenuating the output like that, but I didn't see anything there about adjusting the output-to-ground resistors, so thanks for the suggestion! Sadly, I replaced the latter with 10ks and though that does seem to have improved the bass response  a bit it had no effect on the hum. The gain is still a bit much for the power amp--at 1/4 volume it's already way louder than I'm ever going to use--but it's workable. Really, the Foreplay probably isn't the best match for the fairly sensitive KT-550. But even with the hum and some imbalance in the voltages at the #1 sockets (I had to replace a couple of the LEDs and I think the forward voltage on them doesn't match), the Foreplay sounds way better than the $100 SS headphone amp + pre-out I've been using as a volume control so far.

The wiring layout of the original was not that great, and yours is somewhat worse, primarily because the path is longer. Ideally, the signal and ground wires should be bundled together all the way to the tube and then to the output jacks - the idea being to minimize the loop area enclosed between them. (I see that you have done this with the Anticipation boards.) That will be a big job, so try the 10K first!

I re-checked the resistances and voltages and all is well there (except for the imbalance I mentioned above, but I don't think that would be causing hum?). I reflowed all the solder joints, and also installed a 3-prong plug with the ground connected to terminal #3. Alas, no joy with any of it.

I'm not opposed to tearing down and revising the wiring layout. This is all part my process of learning about tube circuits and aside from just getting rid of the hum I'd like it to be right. Can you expand on the "big job" changes I might make in the wiring layout, given the custom chassis? What would be the ideal layout?

Should I be thinking about faulty components yet? I know these were good tubes since they came from a very reputable source, but who knows. I also wonder about the buzz over the top of the hum when I touch the volume pot, which does vary in volume with the pot position. What could that indicate?

Thanks again for all the help. Despite the frustrations, it's all a blast.

[Paul Birkeland]

I also wonder about the buzz over the top of the hum when I touch the volume pot, which does vary in volume with the pot position. What could that indicate?

That tends to indicate that the metal body of the pot ins't earthed, which will manifest as that kind of issue. 

What power amp are you using and what speakers?

[atalcott]

I see, that makes sense. Because the pot is mounted right in the walnut its body is indeed not grounded. That’s easily fixed at least.

The power amp is a 50wpc PP 7027A Lafayette KT-550 and the speakers are ADS L520s.

[Paul Joppa]

Yep, those are the original 470k resistors. I saw a couple other posts here on the forum about attenuating the output like that, but I didn't see anything there about adjusting the output-to-ground resistors, so thanks for the suggestion! Sadly, I replaced the latter with 10ks and though that does seem to have improved the bass response  a bit it had no effect on the hum.

OK, it looks like we are still at square zero. I would expect the 10K to affect the treble slightly, but not the bass. And it would certainly make a substantial reduction in hum that's generated upstream of the output jacks.  Does that mean it's generated in the power amp? ... there's no certain way to know without gathering more information and thinking logically about it.

At a guess, there is something wrong with the ground connection - could be cables, could be RCA jacks, could be internal wiring, could be something I didn't think of. I suppose a simple test is to see whether the hum is present with the Foreplay turned off, or with the tubes pulled out, or both. For each case, check the hum at full volume and with the control all the way down. If your meter has an AC voltage range, try to measure the hum voltages as you do these checks. That might guide you in the process of checking the wiring. There's no point in fixing small things that might be an issue until we fix the big thing(s) that are certainly an issue.

This may well be a long and frustrating project, but I assure you that the longer and more frustrating it is, the more you will have learned by the end!

[atalcott]

Well, I'm pretty used to troubleshooting and frustrations from my work as a software engineer, so bring it on!

I could be wrong about the effect on the bass. I had the impression it was stronger, but it sure wasn't a proper test and that impression could have been due to where I was sitting, the song I was listening to, etc. Anyway...

BTW, grounding the body of the volume pot did get rid of the buzz. Is what's happening there that a small signal from my hand is conducting into the body of the pot, and since there's no path to ground it induces that signal in the signal path?

I think you may be right about the grounding and  I've played around a bit with it this morning and here's what I've observed.

• There is no hum in the speakers when I connect this other preamp I have to the power amp using the same interconnect cable.
• With the Foreplay connected to the power amp and the Foreplay powered down there is also no hum.
• With the Foreplay connected, when I power it on the hum is completely absent at first, then gradually comes up to volume as the tubes start conducting. When I power off the Foreplay the hum disappears immediately.
• The volume control has zero effect on the hum. It's exactly the same at zero volume and at full.
• The hum is exactly the same in both channels.
• I'm reading zero AC voltage at the output jacks of the Foreplay when it's plugged in.
• With the interconnect disconnected from the Foreplay, I can easily induce very similar hum by holding the jacks close to an AC source like a power cord or my DAC.
• Plugging the interconnect into the Foreplay (or the other preamp for that matter) while the power amp is on creates a lot of loud noise--hum and pops--as the connection is made

I have a sense from these latter two items that the issue might actually be with the grounding of the RCA inputs on the power amp?

[Paul Birkeland]

I would put a 3 wire power cord on your Foreplay.  Earth the chassis plate with the power cord.

[atalcott]

I would put a 3 wire power cord on your Foreplay.  Earth the chassis plate with the power cord.

Thanks Paul. I have already installed a 3-prong cord, with the ground going to the mount of the first terminal strip (Terminal 3). It didn’t make a difference I’m afraid.

[Paul Birkeland]

You have the chassis earthed back by the power supply, I would move the ground connections for the power supply caps off the chassis terminal and to the power supply negative terminal instead, then break the connection there between the PSU negative and chassis earth.

Move the ground wire that you have to the other side of the chassis so it runs up front around the two blue power supply caps.

Earth the signal ground up at the front of the chassis, probably best done on that 3 lug strip you have up front.

I can't see the second power supply filtering resistor in your build...