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What's the Buzz?


Markyboard
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Leslie buzz that is. I've been troubleshooting a nasty loud buzz coming from my Speakeasy preamp over the last month. Special thanks to Mate Stubb and WesG for their inputs and allowing me to bounce ideas off of them. I'm now looking for a bit of help from other Speakeasy Vintage Tube preamp owners - maybe OB Dave , Aussie Chicago (or anyone else with a SE preamp that might know or can look)

 

 

1)What is the audio output transformer make/part#? The label on both channel transformers is removed (maybe intentionally)

 

2) When running your Speakeasy with Leslie amp full volume do you get any buzz happening (let's assume no alcohol involved :laugh:)? I suspect that this problem has always been there on mine. I've always contained it by keeping my Leslie amp volume very low and boosting the the SE volume and signal feeding the SE.

 

 

For the gory details you can read the following:

 

This is truly buzz, not ground-loop induced 60 Hz Hum. I've done a lot of study on ground loops and along with a whole lot of troubleshooting this is not a ground loop issue - although it may be ground related. The Speakeasy is dual channel and both channels exhibit the same buzz - makes troubleshooting somewhat easy. Normally only channel 2 is wired to the 122 and 147 Leslie connectors but at one point I rewired these connectors for channel 1 and same problem. The buzz is present whether using either of 2 147 s (normal configuration) or through a 122 amp (borrowed for troubleshooting). I've torn into this preamp pretty good which is a very simple circuit. I don't have a totally accurate schematic but point to point wiring makes it easy to see what's going on.

 

To cut to the chase (finally) the buzz is eliminated by using a shielded cable on the audio signal. I'm fairly certain it's the AC coupling onto the audio signal line - but WHY? I have 2 types of very old Hammond/Leslie cables; the stiff brown type where 2 conductors are unshielded twisted pair (intended for 122 balanced signals) and the other 4 conductors just straight wires - no shielding. The other type (black cable) has 1 conductor shielded and 4 other straight conductors non shielded. Now the shielded conductor and it shield were connected to the relay switching signals (pin 2,5). I rewired this cable such that the audio signal would be shielded (pin 1, pin6). Now - NO MORE BUZZ using just this cable. Any unshielded extensions will increase the buzz level based on length of the extension.

 

I should not need shielded cable and 6 conductor cable made today has no shielding. This is a problem. My M3 works fine when connected directly to these same Leslies (no Speakeasy) with unshielded cable ( and longer distance). As a work-around I disconnected the audio signal wire of the preamp going to the 147 connector and now I'm running just the audio signal (and ground) through a mono cable externally to each 147 amp from the Speakeasy 1/4" outputs. The AC and relay controls and ground are still going through the Leslie cable.

 

Btw I actually like the idea of running the audio externally. This allows independent dual channels from the Speakeasy as opposed to only single channel through the Leslie cable. Still these type problems drive me nuts not knowing the big WHY :mad:.

 

 

 

Here's the simple mod for each Leslie 147 Amp:

 

Added aluminum mounting plate to existing brake cutout

http://i1062.photobucket.com/albums/t490/Markyboard/IMG_5461_zpso9geltdg.jpg

 

http://i1062.photobucket.com/albums/t490/Markyboard/IMG_5462_zps64mszdix.jpg

 

 

Wired in swtching/shorting jack to select between external input (Speakeasy plug) and (with no plug) the Leslie connector audio signal ( for M3 direct with no Speakeasy)

http://i1062.photobucket.com/albums/t490/Markyboard/IMG_5463_zpsplshv8av.jpg

 

 

Thanks for reading.

 

 

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