Need experts for vexing hum problem

Hi RL,

Some good news!

I posted your message in the diyaudio.com thread. But, by the time I got there to post it, I found the message below, from the preamp's owner/builder

- Tom

------- QUOTE:

Great News...

It would seem the star grounding has worked; the 120 Hz hum is now absent from the preamp out and amp out when preamp is off. There is still 100mV 60Hz hum when they are on but I see this as an important victory and feel very happy. It was previously theorized that this hum was the sum of several issues possibly compounding each other.

Here is a pic of the current state. Once again, thanks for the constant support and advice:

Here is a close up of the starground:

Please let me know if this can be improved to make it final.

Here is where you guys are going to get annoyed with me...I made the grounding changes prescribed most recently by megajocke. I tested the system without a successful outcome and proceeded to install the 4 (replacement) output caps and scrub all the pcb connections with alcohol and toothbrush. Then I realized that the rca output cable was still plugged into the old, now floating, output jack. So I can't conclude which one of the three changes caused the improvement. I'm assuming it's the ground scheme. Mark, I'm sorry about this. I will gladly replace the old caps if you are interested in a conclusive answer. Let me know.

Divide and conquer. Tom, I'm going to order resistors to step down the scope probe signal to my computer. What is the next step?

You guys are the best. Talk to you on Monday.

gary

ps, I cleaned up the picture site:

I will open up the tranny box and take pictures, I get the feeling there are some improvements I can make with the fuse and switch wiring.

------ END QUOTE

The PC scope will be the largest source of 60Hz leakage current in an ungrounded system. It's supply, being a major potential source of EMI, has a built-in line filter that connects the chassis to the AC line through typically 6800pf of safety capacitors. This is roughly ten times what you'd find in audio or high quality low noise test equipment.

With no ground wire on his wall socket, this current will travel wherever it has to, to get back to ground or neutral.

RL

Good news, it sounds like, anyways. Not quite as foolproof as a Dynaco was, despite the simplicity of the new kit circuitry. Wonder what happened to the guts of the PAT4 that's being canibalized. I hope he's not using the same neon on-off switch.

I've heard no reference to what's connected to the preamp input or output sockets, when powered-on testing is done.

What's the power amp input load look like?

What's the source - a shorting plug?

RL

Hi RL,

Some good news!

I posted your message in the diyaudio.com thread. But, by the time I got there to post it, I found the message below, from the preamp's owner/builder

- Tom

------- QUOTE:

Great News...

It would seem the star grounding has worked; the 120 Hz hum is now absent from the preamp out and amp out when preamp is off. There is still 100mV 60Hz hum when they are on but I see this as an important victory and feel very happy. It was previously theorized that this hum was the sum of several issues possibly compounding each other.

Here is a pic of the current state. Once again, thanks for the constant support and advice:

Here is a close up of the starground:

Please let me know if this can be improved to make it final.

Here is where you guys are going to get annoyed with me...I made the grounding changes prescribed most recently by megajocke. I tested the system without a successful outcome and proceeded to install the 4 (replacement) output caps and scrub all the pcb connections with alcohol and toothbrush. Then I realized that the rca output cable was still plugged into the old, now floating, output jack. So I can't conclude which one of the three changes caused the improvement. I'm assuming it's the ground scheme. Mark, I'm sorry about this. I will gladly replace the old caps if you are interested in a conclusive answer. Let me know.

Divide and conquer. Tom, I'm going to order resistors to step down the scope probe signal to my computer. What is the next step?

You guys are the best. Talk to you on Monday.

gary

ps, I cleaned up the picture site:

I will open up the tranny box and take pictures, I get the feeling there are some improvements I can make with the fuse and switch wiring.

------ END QUOTE

I have a few observations and questions about the design and the layout. I don't claim to be an audio expert, and I gave up using tubes about 40 years ago, so there may be some element of "why" in the following:

1. The AC supply appears to be fed by a two-prong plug. There needs to be an earth ground somewhere, and it should be at the same point for all equipment enclosures. The shields for the signals should not be used for this purpose. A separate ground wire should be used to connect the preamp to the main amp.

1. The filaments of the tubes are fed by DC, which is a good idea to reduce hum. But the filament supply ground seems to be connected to the same ground as the signal reference, so the capacitor current will create some

1. I don't understand the purpose of V301A in the schematic2 for the preamp. It seems like it is used as a constant current load resistance for V300A, which actually provides the amplification. It is only dropping about

1. What sort of output voltage is needed on R312? The +15 dB figure with no reference indicates a power ratio of 34 times (6 mV/500 ohms), or about 2.5 mW. If it is in dBm, then it is 34 mW. The circuit can provide probably 50 VRMS at 3 mA, which is 150 mW into a 17 k ohm load.

2. What do you mean by ordering resistors to step down the scope probe signal to the computer? You will be looking for signals in the microvolt and millivolt range that are causing the hum, and stepping them down won't help. And you can't just isolate the scope input with resistors. You really need a differential input, with high impedance probes that can be placed at nodes in the circuit to measure potentials across components to track down the source of the noise. Otherwise, the capacitance of the scope ground will induce noise into the circuit (if it is floating), or else it will create a ground path. Either way, it won't help you.

1. It seems to me that this whole circuit could be made with an IC audio amplifier and it could be powered by a 9 volt battery or maybe a wall wart supply. But, then, I don't understand why anyone would design something like this using tubes. Maybe there is some rationale behind using them for high power output stages, where a "warmer" sound (IOW, pleasant distortion) may be produced, but it seems that it is asking for trouble to use high voltage components to amplify microvolts to millivolts or a few volts.

Good luck,

Paul

That is good news.

RL

Yours may be poser, the real moose is more careful.

I hope you are ready for this. I just noticed that the shielded cables have the shield connected at both ends. Ground loops. Select the ends to cut and ground only from one end.

I think that's something that only works on differential (two wire) sheided loops. These are single ended signals, using the sheild as a signal return path.

RL

Well, it works in TV and radio broadcast control rooms. if you don't, you have an AC current flowing through the shield.

It is fine for the shield to act as the signal return path, and as such needs to be connected to the signal ground at both the preamp and the amp. But one problem I see is that there is no separate earth ground connection to the chassis, and this should be done by means of a separate wire from the preamp to the amp, with a single earth connection there. But it might be OK if both units had 3-wire cords plugged into the same set of outlets.

Paul

I'll have to try it on single ended audio myself sometime, but my experience with duff patch cables suggests it's a misapplication.

I can see where it could work at HF.

RL

I was talking about audio equipment, nothing HF. The 60 HZ component on a heavy braid on the video cable doesn't cause many problems. As far as single ended, it depends on the power source for each piece of equipment. OTOH, an AM radio transmitter too close to the control room can cause fits, trying to filter the AM modulation out of the baseband video. We had to add more 80 foot ground rods to lower the ground resistance to an acceptable level.

Hum can be a pain in an older building, or where the equipment is powered by different phases. If you want to run audio a long distance, it should always be balanced, or converted to FO.

Yes, good grounding is the first step in hum elimination. In the areas i was talking about, all the equipment is bolted to grounded steel racks.

Hi everyone,

my name is Gary, I am the electronically impaired owner of the above preamp. First let me thank Tom for going out of his way on my behalf. We have been talking about this preamp on diyAudio.com. If it is not mentioned above here is a link to some pictures and drawings:

It should become clear quickly that I am new to this but thanks to the help I have been getting I am making progress.

The current issue is this: I have fixed the 120Hz ground related problem. I have an FTT and oscilloscope application on my computer (I notice has generated some concern) that tells me that I have a very orderly looking 60Hz sine wave (100mV) measured at the amp's speaker out. It is not subtle. I do not get this from either preamp or amp when they are not connected to each other. The preamp has a 3-prong outlet with earth ground connected to the chassis. The amp does not, it has a 2 prong plug, it's tranny has a CT connected to the chassis. One of the steps I took a while ago was to remove the preamp tranny and place it in a box separated by a 20" braided umbillical. There is a picture on the above page of it open, the black rectangle is the back of a 3Amp fuse holder.

All help is greatly appreciated, please forgive tardy responses for now I am juggling 3 forums.

thanks,

gary

With 120Hz defeated, you'd go through the same series of iterations to tackle the 60Hz problem, starting simply with a minimum of external components or connections.

If you don't trust your ears, a battery-operated voltmeter permanently across the speaker leads can give unambiguous AC output readings, while individual items (such as your PC scope probe ground lead or component line cords / line power ) are being alternately connected or disconnected.

The input to the preamp should be shorted with a characteristic impedance, when the source is disconnected.

You might occasionally venture to put the lid on the preamp to see if there's any effect on a specific iteration.

RL

I strongly recommend converting the main amp to a three prong plug. It is also a safety issue.

In an earlier post, the OP indicated that his house wiring is two-wire.

RL

For safety and for optimal audio performance, I would recommend retrofitting with 3-wire outlets, or at least get a good ground connection to a water pipe or a ground rod for earthing the enclosures.

Paul

Allow me to clarify. Most of my apartment is 2-prong wiring, I suspect knob and tube over 50 years old. Renovations long before my arrival here required a grounded plug in the kitchen. I am using this outlet, with nothing else plugged in, to power an isolation transformer off of which I run my home stereo. I work on and test the preamp using a 2- prong power source (desk/work bench location,) but check all my changes by bringing the preamp back to the grounded outlet, (listening room.) I have also brought the preamp, amp and a test speaker to a brand new house and powered them up right off the pole, feet from the earth ground stake with no improvement.

It was suggested on another forum that if the preamp manifests hum immediately when switched on with all the tubes removed, the transformer might be the problem. Any thoughts on this?

Thanks,

gary