Phono preamp buzz

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cfishbowl

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Joined 2015
#1
Hi all. I own a hagerman bugle 2 phono preamp that I built and like very much. Schematic here last 2 pages of pdf. Gain on the bugle is set to 40db

http://www.hagtech.com/pdf/bugle2.pdf

I recently finished some monoblock amps and some 96db/w/m kit tower speakers. What I notice is that with my turn table plugged in (rega rp1 with stock carbon cartridge, 2.5mv) is that when I get up past 12 o clock on the volume pot, I start to get a buzz. Buzz is same in both channels and increases as volume pot is turned up.

I know this is not intrinsic to the amp because my other source, a schiit audio Dac, does not do this and the speakers are dead quiet with volume pot maxed.

With the bugle inputs shorted to ground, the noise drops quite a bit to a level I consider acceptable.

Hoping for help troubleshooting. I don't own an oscilloscope so hopefully someone has some insight not requiring one. Thanks!


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#4
Sy, the preamp is on a pcb inside a plastic case. There is a ground lug which is not used as rega turntables do not utilize a grounding wire. There is a ground plane within the phono stage pcb but otherwise there is no way to ground the pcb i.e nothing there marked "ground." The hagerman kits are sold as complete with parts and plastic chassis. I assembled as per instructions.

Jon, see above. The rega Tt doesn't have any external ground, nor does the phono stage.

I guess I should say i don't know the difference (to my ear) between hum and buzz. All I know at this point is that the noise floor drops to an acceptable level with the inputs on the phono preamp shorted to ground. As a next step should i try another table with it if I can find one? Or should i try another phono stage?

Thanks for the replies!

Corey


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#7
Hamish119 said:
Visit this!

Change the frequency to 60Hz, 120Hz, Sine (hum) and Square (buzz) waves.

Which sounds closest to the problem you're hearing?



Good Luck!
Click to expand...


Alright Hamish, Christmas shopping delayed me a bit but it is definitely predominantly a 60hz square wave. There is some "dirtiness" to it, in that the tone generator 60hz square wave was definitely more pure than what's coming out of my speakers but the pitch is the same. I've been doing some reading... Could it be my turn table? I think by shorting the phono stage inputs to ground and having the noise go away means that the phono stage maybe isn't the culprit?


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#9
cfishbowl said:
With the bugle inputs shorted to ground, the noise drops quite a bit...
Click to expand...
cfishbowl said:
...but it is definitely predominantly a 60hz square wave. There is some "dirtiness" to it, in that the tone generator 60hz square wave was definitely more pure than what's coming out of my speakers but the pitch is the same.
Click to expand...
As other contributors have suggested, it may be a PSU related issue.

You can troubleshoot the issue by disconnecting the external 24V DC switched mode power supply and temporarily powering the Bugle2 from 3 x 9V batteries connected in series.
If the interference is still present, you may have to look elsewhere.

Good Luck!

How to connect batteries in series 'Ghetto' or 'Redneck' style;
Link 1
Link 2

PSU test idea 1
PSU test idea 2

REGA tonearm wiring suggestion
 
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