SPEAKER HUM


I have been playing with a new system. I was driving my speakers with a MingDa MC 34 AB tube amp (75 wt/ch ultralinear 40wt/ch triode) which is powered with 8 EL 34 tubes. It can be used as an integrated amp, and when I used it as such, it was DEAD quiet.
I decided to make a good thing better, and just got the matching MingDa MC 2A3X preamp to go with it. The speaker hum when both are powered up is awful...not that very low level stuff you sometimes get with an amp/preamp set up...but loud enough to hear during quiet passages of musical pieces.
I have been tormented in the past with trying to eliminate speaker hum...any help out there for this system??
Thanks so much
rsasso
i do not envy you, it is a pain. my last bout was resolved by shorting the unused xlr inputs on my second amp. you will just need to go through and isolate by disconnecting pieces until you locate the source...god speed sir...
Have you ruled out bad tube on the preamp (or simply a defective unit)? Also, are the amp and preamp both plugged in the same power conditioner or separate outlets?
Although it is a bit technical, the following paper provides some good background on system-level causes of hum (as opposed to hum that is introduced within a particular component, which is definitely also a possibility here, as Millicurie suggested):

http://www.jensen-transformers.com/an/an004.pdf

Regards,
-- Al
To Millicurie

amp and preamp are plugged into separate outlets
To Millicurie

I substituted a small Tono preamp for the big Ming Da...result was LESS hum (and higher frequency)...but still some hum. I am not sure what that tells us.
IIRC, plugging the amp and pre to separate lines is a very good (BAD) way to induce a ground loop. However, since my electrical skills/knowledge are slim, I could easily be wrong. However, its an easy experiment to try them on the same duplex.
At the risk of being too obvious - have you tried a cheater plug yet at the amp/current source? If not you should. I would guess that it will reveal that your problem is an old fashion ground loop. Once you know its a ground loop you can take further steps to try to eliminate it, or just leave the cheater plug in place (that is what most folks do provided the pre-amp is grounded).
To Millicurie

OK: I substituted the interconnects between amp and preamp with heavier shielded cables and that had no effect. The hum is NOT there if the amp only is turned on.

The hum is there when the preamp is on, even with no components connected to it, as soon as you turn on the amp.

If you TAP the preamp when the amp/preamp is humming...there is a ringing sound out of the speakers. If you even gently tap the ceramic bases of those 6SN7 tubes, there is a louder ringing through the speakers...now THAT can't be right.
Does that speak to defective tubes, or, more likely, a defective preamp????
To Newbee

Someone stole my cheater plug. I will get one at the hardware store and give that a try. Thanks
If you TAP the preamp when the amp/preamp is humming...there is a ringing sound out of the speakers. If you even gently tap the ceramic bases of those 6SN7 tubes, there is a louder ringing through the speakers...
Rsasso (Threads)

That's typical of a microphonic tube (or tubes). Hum can also be caused by bad tubes.

Have you tried installing a set of new tubes (or systematically rolling a new tube through the different positions)?
I would try another preamp, if you can get your hands on 1. Maybe borrow one from a friend. Sounds like there are several issues with the preamp that are being amplified in your circuit. Your comment that you have been tormented by speaker hum "in the past" may indicate the presence of yet another issue.

Another thought might be to connect the amp/preamp using balanced ICs if currently using single-ended, or vice versa. That cured a speaker hum issue I had in the past (which is documented somewhere on this site). Good luck.
To Fplanner2000

I have to share my ignorance. Don't you need XLR input/outputs to connect the preamp and amp with balanced IC's? These components have RCA connectors.
To Tvad,

I dont have any of these 6SN7's to rotate right now. The microphonic tube you mentioned as common; is that normal as well? The microphonic effect of tapping the tube doesnt mean the tube is damaged, correct?
The microphonic tube you mentioned as common; is that normal as well? The microphonic effect of tapping the tube doesnt mean the tube is damaged, correct?
Rsasso (Threads)

That's a good question. Many of the most sought after and expensive tubes sound the way they do because of a certain amount of microphony: Siemens CCa, Amperex 6922 Pinch Waist, Tung Sol 6SN7GT Round Plate, for example. All these tubes create a very large image, due in large part to their microphonics.

Usually, tubes known to be microphonic will sell at a discount to the same tubes that are not microphonic.

So, does microphony mean a tube is damaged? Not necessarily.

However, you might try moving the microphonic tube to another position to listen for any change in the sound of your system.

Also, it'd be a good idea to have a set of new, non-microphonic tubes standing by that will allow you to troubleshoot situations like this. You don't have to spend a lot of money. Just buy some good quality, new production tubes.
As Swampwalker mentioned, amp and preamp in two different outlets can cause serious hum. Since you tried another preamp and it still gives you hum (albeit less), maybe it is indeed the ground loop introduced by different components in different power lines.

Can you borrow a power conditioner/surge protector and plug everything into one shared line?
To Tvad,

Thanks again for your very knowledgable input on my setup. It is appreciated, and I will get some 6SN7's and try out your suggestions.
To Millicurie999

Since the amp is about 10ft away from the preamp,it would take an extension cord, but yes, I can get everyting (sources included) into the same surge protector.

Out of curiosity though, if that is the problem, would a cheater plug on the amp (plugged into its own outlet as it is now) diagnose the problem...would I expect the hum to disappear if the separate outlet situation is causing the hum?
Out of curiosity though, if that is the problem, would a cheater plug on the amp (plugged into its own outlet as it is now) diagnose the problem...would I expect the hum to disappear if the separate outlet situation is causing the hum?

Yes, it might. With components that are connected via single-ended (unbalanced) interconnects, the interconnect shields serve as the return path for signal currents flowing between the two components. However, the component signal grounds, the component chassis, and ac safety ground (which is the ground prong on the three-prong power plug, assuming the component has a three-prong plug) are all connected together in the component (the chassis being connected to ac safety ground so that the circuit breaker will trip if an internal short makes the chassis electrically "hot").

Therefore any voltage offset between the chassis of the two components (which can result from their being plugged into different ac outlets or different ac runs or different surge protectors or line conditioners) will cause extraneous currents to flow in the shields of the interconnects between the components. The receiving component cannot distinguish between those currents and signal currents, and hum can therefore result.

Isolating the safety ground on one of the two components, using a cheater plug, will eliminate that voltage offset between the two components, since the only connection between them will be via the interconnect shields, and not via the ac safety ground wiring.

An argument could be made that using a cheater plug defeats the safety purpose of the safety ground (and in fact the author of the paper I linked to in my preceding post, who is a distinguished authority, makes that argument), but as long as one of the interconnected components is connected to ac safety ground, the chances of a problem arising as a result are remote.

When you try the cheater plug, if necessary also try both possible connection polarities of the ac plug.

Regards,
-- Al
One other potential cause of hum would be a cable TV that shares a common outlet. I had this problem until I disconnect the cable from the TV and bought an isolation transformer.
Another thing to check is whether the tubes are propertly seated in their holders.