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B2R buzz when used with SVPCL

M

mgess

Apr 8, 2008
83
9
Virginia
#1
Hey,

I am new to using separate preamps and poweramps. I have always used integrated heads. However, I recently purchased an used ampeg svpcl. I still need to save the funds to get my Crest CA9 so, in the meantime, I thought I could use my old Ampeg B2R with a poweramp in on the back panel.

I tried connecting the two using a 1/4 cable. As soon as I plug into the poweramp in on the B2R, it hums like crazy. It does this while the svpcl is not plugged in at all. So, I don't believe it is the preamp.

Is there something else that I could/should be doing? OR is the B2R the problem?

Thanks
 
#7
Possibly a bad ground on the B2R Power amp in jack?
Plug your cable in to the jack, at the other end of the cable short the tip to the sleeve with a piece of wire, the hum should stop. If it does not, the B2R is at fault. If it does stop, there's something else going on. (duh)
 
#8
<Sigh> The hum is coming from the fact that he is using an UNSHIELDED cable. That is picking up on all the hum and induced AC noise in the area.

Speaker cables are aptly named. They connect an amplifier to its SPEAKER system and are used nowhere else. That's why they are called SPEAKER cables!

Paul
 
#13
mgess said:
Hey,

It does this while the svpcl is not plugged in at all.
Click to expand...

mgess said:
I did use a "cheater" to go from 3 to 2 and it cleared up the buzz.
Click to expand...

Something doesn't add up there.:confused:


mgess said:
Is this safe to do? If not, would a power conditioner solve the problem?
Click to expand...

Safe? Not really. Done frequently? Yes.
Power conditioner help? No

I'd try using the xlr balanced out, it is transformer isolated and has a ground lift switch. Problem here is that it bypasses the master volume on the preamp.
Using the preamp out (1/4"), another way I've been able to stop ground buzzes is to attach a ground bonding wire between the two units. There should be a zinc plated (silver) phillips head bolt on the back of each unit near the AC receptacle or power cord entrance. Loosen the bolt, wrap a wire (14-16 gauge) under the head, tighten it down, repeat on the other end, buzz should be gone. Safety level high, but not very convenient if you gig with the amp. Spade lugs on the wire would be more convenient.
 
#14
JustRob said:
Quote:
Originally Posted by mgess
Hey,

It does this while the svpcl is not plugged in at all.
Quote:
Originally Posted by mgess

I did use a "cheater" to go from 3 to 2 and it cleared up the buzz.


Something doesn't add up there.:confused:
Click to expand...

that pretty much sums it up....

and as far as safety.... well, you probably wont get UL to approve it. but, the two units (which, as rackable units, *are* actually in a rack, right?) chassis' are very likely to be electrically tied together by the rails (even nix the nylon washers or add star-type lockwashers between the rail and the unit if you need to, which will dig through any isolation offered by the finish), as well as by the shield on the signal cable, which is virtually always tied to chassis ground anyway. you can check it with a voltmeter, if you like (and you should). it HAS to be, though, as otherwise there would be no ground loop. ground loops are very very very common in rack gear, and transformer isolation is often not available. if you are truly terrified of the small chance of electrocution, use a wireless, and electrically separate yourself 100%. also, the physical ground wire between the units as mentioned above would be even more assurance, and is exactly why said lugs exist. if none of this makes sense, see here:

[Invalid or Expired Link Removed])

or
wikipedia said:
Low current wiring is particularly susceptible to ground loops. If two pieces of audio equipment are plugged into different power outlets, there will often be a difference in their respective ground potentials. If a signal is passed from one to the other via an audio connection with the ground wire intact, this potential difference causes a spurious current through the cables, creating an audible buzz at the AC mains base frequency (50 or 60 Hz) and the harmonics thereof (120 Hz, 240 Hz, and so on), called mains hum. Sometimes, performers remove the grounding pin from the cord connecting an appliance to the power outlet; however, this creates an electrocution risk. The first solution is to ensure that all metal chassis are interconnected, then connected to the electrical distribution system at one point (often referred to as a "single-point ground"). The next solution is to have shielded cables for the low currents, with the shield connected only at the source end. Another solution is to use isolation transformers, opto-isolators or baluns to avoid a direct electrical connection between the different grounds. However, bandwidth of such is of consideration. The better isolation transformers have grounded shields between the two sets of windings. In circuits having high frequencies, such as computer monitors, chokes are placed at the end of the cables just before the termination to the next appliance, e.g., the computer. These chokes are most often called ferrite core devices.
Click to expand...

someone will read that as bad advice, but i read it as the truth.
 
#16
If the hum exists with the system plugged-in and all attached, but the preamp NOT ON.... then you have a "pin 1 problem".................unless the wire is really badly unshielded. Since lifting ground fixed it, shields were not the major issue, although they are good to have (!).

Most likely, there is a power transformer whch is inducing hum from its "leakage field" into the ground of the cable to the B2R. The B2R is amplifying that hum. I suspect the B2R also has some "pin 1 problem" coming in that input. It is hard to avoid with complex signal paths in amps.

Lifting ground, so long as there is no other ground, will generally fix it.

Often, moving the mains cords and signal cable around will also fix it, with no need to lift grounds.

The use of one outlet or extension cord instead of two separate ones also reduces this sort of hum.
 

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