Altec 1567A buzz.....ideas?

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mjrippe

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And WTF!!!!!! are there other units out there without buzz?!?!?!
Hey Doug, what level is the buzz you are seeing? You mentioned it is within Altec spec? These things have a ridiculous 97dB of gain total, so what is the buzz like with one input shorted w/150 ohms, channel and master both at 10 o'clock, eq off, meter off?
 

emrr

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Hey Doug, what level is the buzz you are seeing? You mentioned it is within Altec spec? These things have a ridiculous 97dB of gain total, so what is the buzz like with one input shorted w/150 ohms, channel and master both at 10 o'clock, eq off, meter off?

Well the altec spec is useless, it refers to master fully off. Probably for a reason!

With the mixers all off and the master at 10 o'clock, it's very audible. I stopped measuring weeks ago, it's beyond worth knowing a number.

Point is, NO INPUT STAGE REQUIRED. ONLY A SINGLE MIX RESISTOR connected and channel knob OFF.
 

mjrippe

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Damn, I can see how frustrating this must be! It is one of those "how is this happening?" sort of problems.
Just to cover all the bases here, you mention all four having the same noise - have you been doing all the testing here on just one unit? Possibly there are multiple problems causing similar symptoms and a different unit will respond differently? Also, if these all came from the same source, is there a previously done mod that could be causing a ground loop or lift?
Tried swapping V3? Replaced mix resistors with metal film? Tried shielding the resistors, pot wiring, wrapping the whole damn thing in tinfoil? o_O I am quickly running out of ideas and I'm sure you have tried all this and more.
One last crazy thought, perhaps the rivets have oxidized and made the grounding sketchy. Perhaps take a larger drill bit and use it to spin the rivets if possible to get a fresh bite on the panel?
 

Winston OBoogie

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I see you tried new heater rectifiers which didn't help.
But since you're describing the noise as a buzz and that it has harmonics up to mid band, I'm still suspicious of the power supply.

I don't know that it is diode switching noise but, for shits and giggles, have you tried any of the tricks used for snubbing diode noise?

A cap across the secondary AC windings? Both the HT and LT. 1uF or so.
5ohm or so resistors added on the HT AC line before the diode/multiplier.
0.5ohm or whatever you can spare in voltage drop on the LT line before the bridge.

On the schema you linked to, the power transformer shows a faraday sheild, your units have one?

A series R and C across diodes can tune and damp any ringing/switching but need to be properly tuned and I don't remember the exact method to go about it... Anyone?
 
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emrr

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Damn, I can see how frustrating this must be! It is one of those "how is this happening?" sort of problems.
Just to cover all the bases here, you mention all four having the same noise - have you been doing all the testing here on just one unit? Possibly there are multiple problems causing similar symptoms and a different unit will respond differently? Also, if these all came from the same source, is there a previously done mod that could be causing a ground loop or lift?
Tried swapping V3? Replaced mix resistors with metal film? Tried shielding the resistors, pot wiring, wrapping the whole damn thing in tinfoil? o_O I am quickly running out of ideas and I'm sure you have tried all this and more.
One last crazy thought, perhaps the rivets have oxidized and made the grounding sketchy. Perhaps take a larger drill bit and use it to spin the rivets if possible to get a fresh bite on the panel?

4 units. 3 previously untouched. All do it, at multiple locations. No one can SWEAR DEFINITIVELY that they get any better than this, I've never heard one any quieter, and I wouldn't use one with a microphone. But lots of people do. Is there something wrong with those people? Or these amps? Or both!

Everything from V3 on is fine.

Many new caps (all electrolytics done), many metal film resistors, 10 billion tubes auditioned, blahdy f'in blah F these things. No ground paths changed in the recap. Many ground path changes tried, all same or worse. They're all the same, regardless.

Someone needs to produce a 'quiet' unit as evidence.
 
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emrr

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I see you tried new heater rectifiers which didn't help.
But since you're describing the noise as a buzz and that it has harmonics up to mid band, I'm still suspicious of the power supply.

I don't know that it is diode switching noise but, for shits and giggles, have you tried any of the tricks used for snubbing diode noise?

A cap across the secondary AC windings? Both the HT and LT. 1uF or so.
5ohm or so resistors added on the HT AC line before the diode/multiplier.
0.5ohm or whatever you can spare in voltage drop on the LT line before the bridge.

On the schema you linked to, the power transformer shows a faraday sheild, your units have one?

A series R and C across diodes can tune and damp any ringing/switching but need to be properly tuned and I don't remember the formulas... Anyone?

I have not. Since it's all wired leads to tag strips barely squeezed into no space at all, that also sounds like no fun. I'm WAY past they point I'd have sold off this garbage and moved on if it were mine. Life's too short.
 

Winston OBoogie

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I'm WAY past they point I'd have sold off this garbage and moved on if it were mine. Life's too short.

The Norman Petty stuff sounds decent for the time period so I suppose it's "possible" to use those mixers...
But for sure, better things to be spending your time with. Is giving them back "as is" to your client an option? Without you losing out on much $. I mean.
 

5v333

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From another forum:

Altec 1567A mixer hum, help needed

Boswell Tue, 07/22/2008 - 05:04
Yes, those units suffered badly from ground-induced hum. DC heating of the input valves (tubes) was a nice idea, but it causes heavy pulse currents through the rectifiers that permeate the rest of the circuit. The optional input and output transformers were not there for fun - they were needed to isolate the mixer from what was driving it and what it was driving.
 

emrr

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From another forum:

Altec 1567A mixer hum, help needed

Boswell Tue, 07/22/2008 - 05:04
Yes, those units suffered badly from ground-induced hum. DC heating of the input valves (tubes) was a nice idea, but it causes heavy pulse currents through the rectifiers that permeate the rest of the circuit. The optional input and output transformers were not there for fun - they were needed to isolate the mixer from what was driving it and what it was driving.

Yep. Though the transformers make no difference, lots of hifi people using them w/o.
 

5v333

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Im thinking, IF the buzz is almost everywhere common mode style then maybe transfo isolated ins and outs might be the intention?
Sorry, Im just throwing guess cakes atcha.
 

emrr

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Ok...
One last guess cake.
Safety earth..?
Tried every possible way, tried every possible grounding with reference to outside equipment. That's all already at it's quietest. I frequently revisit after major changes and lift power ground to see.
 

Winston OBoogie

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Hey Doug,
here's Bill's reply in the BA-2A buzzing thread.
I think this applies to your situation too:

The noise being a buzz (rather than hum) and repetition frequency of 120 Hz is consistent with "pulsed RFI" created by the reverse recovery (rapid snapping off of conduction after voltage has reversed) of the silicon rectifiers. Most garden-variety rectifiers have no maximum listed for t sub-rr and some are micro-seconds long. It's why you'll often see 10 nF capacitors wired across each rectifier on lots of gear. Because the "snap" creates lots of ultra-sonic ringing in the power transformer winding, it can couple via unusual routes - including radiation. Just be sure the added caps have a voltage rating that's substantially higher than the peak AC voltage involved.

Whether it's worth your time investigating and modifying the Altecs further is, of course, another matter ;)
 

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