CurtZHP

Re: Interstage curious
« Reply #120 on: January 12, 2020, 10:21:25 AM »
That's PSU and chassis related. The lid is redirecting the PT flux. 

What else changed in the last 24?  Besides moving spaces?   Distrust the outlet you are using now.  Or a light dimmer.  Etc. 

Take the PT outside and at a distance, run the AC to the PSU through 3 feet of wire and see what happens.  Or skip that and use an outboard PSU.   

What's the chassis?  Should you trust it?  I've seen chassis that proved to be the problem, no good continuity between any panels.


One site is my workbench at home.  Been building stuff like this for 15 years on that bench.  The most obvious sources of noise are generally acoustic in nature, but just for troubleshooting I've started unplugging things whether they could be the cause or not.  No dimmers to be found anywhere in the house.

The other site is my workbench at work.  Again, been working there for years.  Never had this much trouble.  On days when I know it'll be slow, I bring this stuff with me.

As for any changes, I had reoriented the power transformers.  They were originally laying flat on the bottom of the chassis.  I moved them to being vertically attached to one side, opposite from the rest of the circuit, in an attempt to reduce noise.  Fat lot of good that did!

The chassis is a 2RU aluminum body with a steel front panel.  It used to be some sort of SCA monitoring equipment that a radio station had tossed.  I stripped the guts out and replaced the front panel with a steel blank.  Your comment about it brought to my recall an interesting situation.  One of the "features" I was hoping to incorporate into this beast was an instrument input, using a switching 1/4" jack just south of the input transformer.  I had done this exact thing in my last preamp with positive results.  Of course, it hummed like crazy (worse than it does now).  In the process of troubleshooting that, I discovered that, as long as the jack was NOT touching the front panel, the hum was significantly reduced.  Sometimes, just turning the jack or bumping it a certain way got rid of the hum.  Another clue?

Anyway, I decided I wasn't married to that feature and did away with it.  Again, didn't gain much for my trouble.

I suspect that part of the continuing problem is the introduction of the "series/parallel" switch.  That was another very recent addition.  I should eliminate that entirely to see if that eliminates the problem.

Oh, by the way....
This thing's phantom supply appears to eat toggle switches.  I'm trying to put a toggle switch on the phantom supply at the output of the actual supply rail, for obvious reasons.  I've gone through two switches already.  They only lasted for a short while before the voltage passes unabated regardless of switch position.  Granted they were used switches, but both were rated for plenty more than this rail should be putting out.  And it does put out the correct voltage.  I guess I'll try again with a really beefy switch.  You know the old saying:  "Once is happenstance, twice is coincidence, three times is enemy action!"

Electrons don't read schematics.


CurtZHP

Re: Interstage curious
« Reply #121 on: January 12, 2020, 10:24:14 AM »
What frequency is the hum?  This gives a clue.

If it's 60Hz then it could be a hum loop.

If it's  180Hz then you have radiation from your transformers.

Think guitar pick-ups.............coils/transformers  pick up whatever is available, this is why good input transformers have mumetal cases.   Try fitting a temporary earthed steel screen in various places to see what changes.

Gain: do you have more than you need?

Is there a faulty electrolytic cap anywhere?   When we make stuff like this, we often use old parts just to try the circuit out.

Don't give up, come back to it later with a fresh mind!

Post some pics of your layout.

DaveP


Getting ready to try to look at this grunge on the scope.

As for gain, I definitely have more than I need.

As for parts, most of the audio circuit is used parts, but the power supply was built with all new components.  And obviously, the audio transformers are antiques.

I'll try to post some pictures soon.
Electrons don't read schematics.

DaveP

Re: Interstage curious
« Reply #122 on: January 12, 2020, 10:53:01 AM »
Quote
I stripped the guts out and replaced the front panel with a steel blank.
Steel could conduct magnetic flux from the PS to the input end.

Quote
In the process of troubleshooting that, I discovered that, as long as the jack was NOT touching the front panel, the hum was significantly reduced.  Sometimes, just turning the jack or bumping it a certain way got rid of the hum.  Another clue?

Those good quality jack sockets with the chrome bezels are insulated so that they don't make contact with the chassis, this is probably the reason why.

DaveP

Soundcloud: Delayed Action.

CurtZHP

Re: Interstage curious
« Reply #123 on: January 13, 2020, 09:05:21 AM »
Made a couple changes....
Completely reoriented all the transformers.  Put the power transformers back on the bottom of the chassis, since putting them against the side gained me nothing, and likely introduced more problems.  Took all three audio transformers and located them against the opposite side from the power transformers, thus getting them as far away from the supply as possible while keeping everything in the same chassis.  Cleaned up a little more wiring as a result of all this relocation.

Still the same noise.  Hum and hiss.

Looked again at the output on the scope.  The 20kHz oscillation is back.  I thought I had licked it earlier when I added 68K resistors in series with the grids to the push/pull stage.  I had, but here's where I apparently went wrong.  I had initially installed those resistors right on the IS transformer secondary pins, because that was easy to get to.  After seeing that it had solved the problem, I thought I should locate them closer to the tube socket, so I moved them to the perf board and attached them directly to the socket pins.  That was apparently the wrong thing to do.  So I put them back on the transformer pins, and that got rid of the oscillation.

Didn't do a thing about the hum or hiss.

Shorting out the primary of the IS transformer eliminates both.  Shorting out the either winding of the input transformer does neither.  Having the gain all the way off (essentially shorting out the pot) gets rid of the hum, but not the hiss.

At this point, I'm at a loss for what's causing the hiss.  As for the hum, I'm starting to think there's a ground loop or two somewhere in the first two stages.  The perf board layout is my prime suspect.  I think the next course of action is to take that all apart and either come up with a completely different layout or just go point-to-point for now.
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #124 on: January 13, 2020, 11:10:17 AM »
By the way, I think I tracked down the hum, and it's definitely 60Hz, so ground loop.

Wondering if the hiss is just a result of the boatloads of gain bringing up the noise floor.
Electrons don't read schematics.

Spencerleehorton

Re: Interstage curious
« Reply #125 on: January 13, 2020, 11:15:29 AM »
Or could be impedance missmatch?
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!
Or a snake!!!

EmRR

Re: Interstage curious
« Reply #126 on: January 13, 2020, 11:28:22 AM »
I don't remember if there was a definitive update on total gain. 

Still the 12/9/19 schematic?  Minus the DI input?  C8/9 not really needed, but may not hurt either.

Is C1 a 1 mfd or 0.1mfd?   1 mfd would be asking for motor boating.   0.1 is plenty, or go 0.22. 

You might consider another 4K7-10K between R7/8 with another filter cap. 

You might want to put 200Ω in series with the phantom after the switch. 

The grid stoppers don't make sense.  It would seem a symptom of something else, just getting a band-aid.   It's a single stage low gain portion.   Given it's a transformer coupled PP input, there should be no need for grid stoppers.  Does it oscillate with T2 primary shorted?

I'm more strongly suspecting the power ground itself, I would definitively eliminate that possibility. 

What do you get with V grid grounded?  Shorting T1 secondary as you've done should be doing that, but...

Hiss may really be a noisy tube.  Hiss practically always comes from the first stage....it's amplified the most.  Output stages should always sound quiet...there's very little gain there.   Listen to a half dozen or more tubes.  I went through a lot of (30) 12AY7's looking at noise after a one week burn-in when restoring the BC-2B console, and selected the two lowest noise for the two high gain amps.  It makes a huge difference there, much less in the lower gain amps. 





Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

CurtZHP

Re: Interstage curious
« Reply #127 on: January 13, 2020, 11:51:50 AM »
I don't remember if there was a definitive update on total gain. 

When last I calculated that, it was around 80dB all told.


Still the 12/9/19 schematic?  Minus the DI input?  C8/9 not really needed, but may not hurt either.

Is C1 a 1 mfd or 0.1mfd?   1 mfd would be asking for motor boating.   0.1 is plenty, or go 0.22. 

Yes, still about 90% that schematic.  Ignore C8,9.  They're not in at this point.  C1 and C2 are both 0.1uF.  Not sure why it says 1uF.  (I'll repost...)


You might consider another 4K7-10K between R7/8 with another filter cap. 

You might want to put 200Ω in series with the phantom after the switch. 

I'll have to scrounge the parts.  Sounds sensible, decoupling each stage individually.


The grid stoppers don't make sense.  It would seem a symptom of something else, just getting a band-aid.   It's a single stage low gain portion.   Given it's a transformer coupled PP input, there should be no need for grid stoppers.  Does it oscillate with T2 primary shorted?

Yanked them and tested.  Oscillation actually got slightly louder!


I'm more strongly suspecting the power ground itself, I would definitively eliminate that possibility. 

Will take a much closer look at that.


What do you get with V grid grounded?  Shorting T1 secondary as you've done should be doing that, but...

Hiss may really be a noisy tube.  Hiss practically always comes from the first stage....it's amplified the most.  Output stages should always sound quiet...there's very little gain there.   Listen to a half dozen or more tubes.  I went through a lot of (30) 12AY7's looking at noise after a one week burn-in when restoring the BC-2B console, and selected the two lowest noise for the two high gain amps.  It makes a huge difference there, much less in the lower gain amps.

Grounding the grid of V1 (I assume that's what you meant...) didn't change anything.  Would that put the problem in the second stage?

The tubes are almost brand new and were pulled from devices that have worked reliably from the get-go, for what that's worth.
Could it be the type of tube I'm using for the first stage?  Would something else be advisable?
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #128 on: January 13, 2020, 01:03:52 PM »
Reposting an updated schematic.

Made some corrections and removed what is not currently there to reflect what we are truly dealing with.

Electrons don't read schematics.

EmRR

Re: Interstage curious
« Reply #129 on: January 13, 2020, 01:13:57 PM »
OK, 0.1 is not big enough for C2.  1mfd makes sense there, at least 0.5mfd. 

Hiss with gain pot down (grounding of V2 G), takes roughly half the gain out of the picture.  Probably shouldn't be significant hiss.   

Oscillation tested how?  The full amp?  Gain up or down makes no difference?  Does it oscillate with T2 primary shorted?    Again, this oscillation seems a symptom of some other problem; grounding, proximity, layout, etc.   You should be able to eliminate that without use of a grid stopper.  It also points to layout or interference if the grid stoppers at the transformer instead of the socket position is a yes/no proposition. 

Put a 12AX7 or 12AY7 in there just to see what difference you notice.   Measure gain with those to quantify differences in noise level.  Switch to a different type if you like.   I can't say one type over another is going to be noisier.  More gain reveals more noise.  Many new tubes are noisy.  Noise is not a function of new versus used, until used is near death.   80 dB is going to show you all sorts of noise sources.   The venerable RCA OP-6 is not a quiet amp, primarily for the reason it is extremely high gain, secondarily because it's power supply is on the same chassis in close proximity. 

The noise floor difference in those 30 tubes was as much as 20 dB. 

Quote
If I remove the mic and place a 100R resistor across pins 2 and 3 of the input, the hum goes away it seems

Is that still true? 

« Last Edit: January 13, 2020, 01:18:18 PM by EmRR »
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


CurtZHP

Re: Interstage curious
« Reply #130 on: January 13, 2020, 01:40:31 PM »
Oscillation was tested connecting the scope to the output.

I rechecked this:  I connected the scope to the primary of T2, and it turns out the oscillation is there too.  I just didn't see it before because it's at a much lower level.  I don't see it across VR1. 

The puzzling thing is it still oscillates with T2 primary shorted.  As mentioned earlier, it in fact gets a little louder.
If, as it seems, the oscillation is coming from the second stage, I would think that shorting the primary of T2 would kill it, not make it louder.


As for your question about the 100R resistor across input pins 2 and 3 killing the hum....
I'm not entirely certain that was the case, as it still hums with that resistor in there.  It may lessen considerably, but never goes away.


I think I just need to take the perf-boarded section apart and start over.  Somehow, something is too close to something else, or there's a ground loop in there.
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #131 on: January 14, 2020, 12:44:57 PM »
This morning I went completely "scorched earth" on this thing and disassembled the audio circuit.

Last night I thought about my last build, and something occurred to me.  The schematic for that device and the schematic for the first two stages of this device are nearly identical as far as topology is concerned.  In fact, I could just remove the IS transformer and PP stage, connect the output transformer to the second tube stage, and I'd have almost exactly what I built last time.

It just so happens that I have a "copy" of the audio circuit board I used for that last device.  So, I figured I'll slap together another one and connect it to this power supply (which is identical to the last one) and see what floats.  If it works, then I swap the tube for a 12AU7, change one or two other components, and I have the first two stages of the new device.  Then I can introduce the IS and PP hardware and see where things fall apart.

I'll skip to the end.  It didn't work.

Now I'll back up a little....
The good news is that this particular incarnation didn't hum at all!  So, obviously, the perf board I took apart was certainly suffering from a ground loop.  The problem is that the hiss I was hearing before is now much louder.  And it's not just a clean hiss.  It's a wash of white noise.  It still passes audio with a mic plugged in; just imagine singing in a hurricane!  Placing a resistor in place of the mic, the white noise remains.

Tried a different tube, no difference.  Shorting the grid of the first stage gets rid of it (along with the signal...).  I removed the input transformer and subbed an instrument input.  Placed a resistor across that.  Still noisy.  Double and triple checked all connections.

I'm starting to get the feeling that the problem is a bad cap or two in the power supply.  They're all brand new, but maybe one of them took a hit when I was experimenting with this thing early on.  Or it's just a lemon.  I suppose I could pull them and test them.

I had already swapped the regulator for the plate supply, because I thought that was bad.  There's not much else to that supply except for a few fat resistors and some protection diodes across the regulator.  Filament supply isn't much different.
Electrons don't read schematics.

EmRR

Re: Interstage curious
« Reply #132 on: January 14, 2020, 03:43:42 PM »
Said another way, it’s noisy with the input transformer or instrument input, and quiet with V1 grid grounded?
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

DaveP

Re: Interstage curious
« Reply #133 on: January 14, 2020, 03:44:36 PM »
You have 80dB of gain right?  That's probably 40dB more than you need.

Measure the output of hiss in dB then take off 80dB and work out your E.I.N.

This will tell you if something is amiss or not.

DaveP
Soundcloud: Delayed Action.

CurtZHP

Re: Interstage curious
« Reply #134 on: January 15, 2020, 08:57:10 AM »
Said another way, it’s noisy with the input transformer or instrument input, and quiet with V1 grid grounded?


Keep in mind, at this point, the only two stages in the circuit are the first two.  The IS and PP stages have been removed.  Second stage goes right to the output transformer.

When I ground the V1 grid, the white noise doesn't disappear entirely, but is drastically attenuated.  The noise gets louder or softer depending on where the gain pot is set.  With the gain all the way down (essentially shorting the grid to V2) it disappears.
So that would mean the noise is being generated in the first stage, right?

Here's another observation....
I looked at the supply rails on the scope.  There appeared to be a nasty ripple on the B+, even with all the decoupling I have in place.  (I'll have to test again to confirm the numbers...)  When I removed the audio circuit, the supply rails were just about ruler flat.

What's odd about this is that I'm now essentially using an audio circuit that I built before and have had great success with.



Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #135 on: January 15, 2020, 08:58:05 AM »
You have 80dB of gain right?  That's probably 40dB more than you need.

Measure the output of hiss in dB then take off 80dB and work out your E.I.N.

This will tell you if something is amiss or not.

DaveP

Right now, I've stripped out a good chunk of the circuit to narrow down a few things, so we're now dealing with less than 80dB total gain.
Electrons don't read schematics.

CurtZHP

Re: Interstage curious
« Reply #136 on: January 15, 2020, 11:01:23 AM »
I don't see how this could be anything other than a power supply problem.  I've tried everything, from additional decoupling, to moving things around, to replacing components.  I've tried different tubes, most from known working devices.  I've cannibalized three other perfectly reliable pieces of gear to find other parts to try.  I've checked and rechecked wiring.  I even cleaned the gain pot (even though it wasn't scratchy...).

Of course, the supply by itself acts like nothing is wrong with it.  But, like an errant teenager, you put it to work, and all it does is piss and moan and do a half-a$$ed job!

I'm going to have to order some parts.  (Yeah, let's throw $$$ at this boondoggle!)  Needed another break from this thing anyway.

Electrons don't read schematics.

EmRR

Re: Interstage curious
« Reply #137 on: January 15, 2020, 11:07:33 AM »
Sounds like it's first stage noise....from....something.....

Pulling another thread, if this is indeed pretty much the same first two stages as the other preamp, why not lift the output cap from that preamp and feed it to the interstage transformer of the new one?  Connecting ground and power to the new parts of course.....

....or lift and swap the power supply sections between the two, see if the problem moves.   
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

CurtZHP

Re: Interstage curious
« Reply #138 on: January 15, 2020, 11:29:04 AM »
Sounds like it's first stage noise....from....something.....

Pulling another thread, if this is indeed pretty much the same first two stages as the other preamp, why not lift the output cap from that preamp and feed it to the interstage transformer of the new one?  Connecting ground and power to the new parts of course.....

....or lift and swap the power supply sections between the two, see if the problem moves.

I have entertained the thought of borrowing the power supply from the other preamp, since it's identical to this one.  I'm hesitant because that preamp actually WORKS, and I have visions of putting it back together after all this foolishness and having it NOT work.  Unless I want to whip out the credit card, I'll have to get over that fear.

What are the chances that it's something wrong with the input transformer?
Electrons don't read schematics.

EmRR

Re: Interstage curious
« Reply #139 on: January 15, 2020, 11:35:40 AM »
I have entertained the thought of borrowing the power supply from the other preamp, since it's identical to this one.  I'm hesitant because that preamp actually WORKS, and I have visions of putting it back together after all this foolishness and having it NOT work.  Unless I want to whip out the credit card, I'll have to get over that fear.

What are the chances that it's something wrong with the input transformer?

Well, you've got too much noise without the input transformer too, right?  Swap the input transformers?

The transplants I propose can just be disconnections and wire moves, you don't have to move the whole PSU/etc. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


 

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