Ampeg PF-20T - buzzing noise from the Transformer DI out (pls help!)

Discussion in 'Amps and Cabs [BG]' started by Zatenm, Feb 13, 2023.

  1. Zatenm


    Feb 7, 2023

    This is my 1st post, although, like many before me, I admit to lurking on this superb resource of a forum for several years now :)

    Kindly seeking advice re my recently acquired brand new Ampeg PF-20T head.
    After 2 decades of SS, this is my 1st ever tube amp and I'm in love with the presence, fullness and 3D effect of the tube sound as you may have heard countless times before from 1st time tube owners.
    Through the speaker cab (SVT 210AV), it sounded fantastic to my untrained ears.
    The problem arose when I decided to record with it.
    I connected the Transformer DI out to my Scarlett 2i2 3rd gen interface and was very disappointed with the sound that came through my headphones: over and above the regular tone of the amp, there was a weird high-frequency buzzing sound on the top end. It kinda sounded as if the tiny speakers inside my headphones were rattling/vibrating with every note I played. I still heard the note itself but that annoying buzzing was on top of it. It was more pronounced on the higher register but still very present on the lower notes and I could not dial it out with EQ even when I completely kill the treble.
    The preamp DI out was clean and fine though.

    I was puzzled as I read great things about the Transformer DI out of this amp and did not expect this one bit.
    I verified my XLR to 1/4 jack cable, headphones and Scarlett unit by trying the exact same setup on my other newly acquired tube amp (an Ashdown Little Bastard 2.N) and it was clear and clean as it was meant to be, so I knew these components were not at fault.
    Next I removed the PF-20T tube cage to make sure the 2 6V6 tubes were properly seated. One of them had an hair of space underneath it on 1 side it so I pushed down on it to completely make it flush with the amp's surface. Turned on the amp again and let it warm up for 20 min before testing but still the same problem.

    Any other owner experience a similar issue with the PF-20T? Do I have a faulty amp?

    Appreciate all the knowlgeable feedback I can get!
  2. jnewmark

    jnewmark Just wanna play the groove. Supporting Member

    Aug 31, 2006
    NW Ohio
    Third St. Cigar Records staff musician.
    CatOnTheBass likes this.
  3. Why not try XLR to XLR?
    grouse789 and CatOnTheBass like this.
  4. beans-on-toast


    Aug 7, 2008
    Either it’s you amp, the cable, or the interface. You have to determine which. If you have a mixer, connect the pre-amp xlr out to it, use a good quality xlr-to-xlr. To connect to the interface use an xlr to xlr cable as seamonkey said. The cable should be just long enough for your needs. Use a shielded xlr mic or patch cable with neutrik connectors.

    The xlr to tip-sleeve ¼” cable is not balanced could be picking up noise and causing your problem.
    seamonkey likes this.
  5. Zatenm


    Feb 7, 2023
    Thanks for the suggestion but I didn't think it was the cable, it works very well on my amp with the exact same setup. Am I oversimplifying it?
    Last edited: Feb 13, 2023
  6. Zatenm


    Feb 7, 2023
    Thanks. The thing is I used the same cable, interface and headphones with my other amp and there was no problem at all, so I'm guessing it is my PF-20T by elimination.
    Still worth trying (and buying) an XLR to XLR cable in your opinion?
    beans-on-toast likes this.
  7. Wasnex


    Dec 25, 2011
    You should still try an XLR to XLR connection, if you can. Every piece of equipment in your collection may be fine, but you can still get a ground loop. Use the same equipment on a different AC plug, and you may not have a problem.

    XLR is balanced, 1/4" TS is not. The best way to convert from unbalanced to balanced is to use some sort of 1:1 isolation transformer. An isolation transformer preserves the full signal level and also reduces the chances of getting hum.


    1/4" TS to XLR adapters can be wired different ways. The best choice can vary with the equipment you are using.

    Here is the recommended configuration:
    But there can be situations where a different configuration would work better.

    Ref: RANE Commercial - Knowledge Base
  8. beans-on-toast


    Aug 7, 2008
    The signal level of your other amp might be higher and not a problem.

    I would use a balanced cable. An unbalanced cable is prone to picking up noise. Putting it close to a monitor, computer, router, or other source of noise can cause interference problems.

    I’m not saying that it can’t be your amp. Check out the simple things first.
    Old Garage-Bander, Zatenm and Wasnex like this.
  9. Zatenm


    Feb 7, 2023
    Thanks Wasnex, beans-on-toast and seamonkey for the very helpful advice. I agree it's worth a shot to test a more appropriate (balanced) cable. Will try a XLR to XLR cable and report back. Cheers!
  10. Don't automatically blame any one piece of equipment without understanding the concepts if ground loops and balanced to unbalanced interfacing.
    In most cases, it is the way the two pieces of equipment are connected together that is causing the problem. Not the equipment itself.
    I did this kind of work for nearly fifty years. There are some basics, well covered by @Wasnex. However, these subjects can also lead you on a much deeper dive. Fortunately, most of us can solve these issues without barely getting our ankles wet. It just takes a little bit of understanding what happens and how you can avoid, or at least minimize related problems.
  11. Zatenm


    Feb 7, 2023
    Alright, so I bought a new XLR to XLR cable and was hoping that would do it. Unfortunately the buzzing sound is still there, as is :( So I think it's now safe to assume it's not the cable.
    I even moved the whole setup to another room to try it through another wall plug to no avail.

    I called Cosmo music for help (bought it from them) and spoke to their amp specialist. He agreed that the problem seem to be from the amp itself. I'm going to send it for a replacement.

    Thanks all for the kind advice and support.
    Zbysek, Wasnex and beans-on-toast like this.
  12. Nope. Not quite safe to assume anything just yet.
    Since I wasn't in on that discussion, I won't comment on the "amp specialist's" over the phone diagnosis.
    An amp specialist may know amps, but you got to ask yourself, is he also a specialist in interfacing audio gear. Do you feel lucky? :smug:

    Until you connect the amp to another piece of gear, and be sure you have ground loop issues resolved, the balanced to unbalanced (if any) is proper, and you are matching levels (line vs mic) at both ends, it is my over the internet opinion that it is too soon to offer a substantive opinion.

    There are certain troubleshooting steps, that if not followed, can take you down the wrong path. Plenty of people here willing to help. If you are willing to follow the experts, or even me in a pinch, then fine. If not, that's fine too, but please let us know.

    Question 1) Is there a Ground Lift switch on either of the pieces of equipment you are connecting with the XLR?
    Question 2) Are there mic/line switches on the equipment at either end of the XLR cable. Or alternately, is there an input gain control on the gear you are connecting to your smp?

    And yes, XLR is the preferred cable, so don't write that off. You just have another issue somewhere.
    Zatenm and CatOnTheBass like this.
  13. Hi again...
    Just re-read your initial posting. (Maybe I should have done thst earlier, sorry.)
    I'm confused.

    "The preamp DI output was clean and fine though."
    Is this a separate, stand-alone DI? And the trouble is when using the Amp's DI OUT?
    Just needed some clarification.

    Also, some amps take a feed from the output transformer that also feeds the speakers. This may or may not be a balanced signal depending on how it was done. "Transformer output" does not necessarily mean transformer balanced. Apologies if this point was previously covered.
    Zatenm likes this.
  14. I think I can answer this: The PF20t (and PF50t) have two XLR outputs: Preamp Out (which is nice, clean and loud) and Transformer Balanced Line Out (which is the gnarly and overdriven signal coming off the output transformer). The Transformer Out is definitely more of a "rock & roll" tone.

    Both sound good, but I've always preferred the Preamp Out, personally. It is cool that Ampeg gave us both options. :)

    If the signal is noisy, try the Ground Lift switch (that's what it's there for).
    Last edited: Feb 16, 2023
    Zatenm likes this.
  15. Sorry to call you out but you are spreading F.U.D.

    Ampeg PF20T block diagram
    Old Garage-Bander likes this.
  16. @Zatenm a couple of things:
    1. Does the "weird high-frequency buzzing sound" depend on the level of the input? In other words, if you turn down the Volume knob on your bass (so your bass isn't hitting the tubes as hard) does it change the weird buzzing sound? Or is the weird buzzing sound always constant, no matter how quiet or loud you play?
    2. Have you tried using a "cab sim" effect? Very few bassists like the sound of tube overdrive directly into headphones. Most bassists who use tube overdrive as part of their sound seem to end up using a cab sim. (Or--shocking suggestion!--mic the cabinet.)
    Finally, an observation based on reading the manuals of your two devices. Your Ampeg has a 60dB signal to noise ratio, whereas your Focusrite boasts an astounding -128dB noise EIN! Your Focusrite is much more "hi fi" than your Ampeg, which means there will be many situations where the Focusrite is "exposing" the old-school gnarliness of the Ampeg. If you want a totally clean and hi fi signal, my advice is to just plug your bass into the Focusrite (bypassing the Ampeg entirely). I personally never ever use my Ampeg for recording (only for jamming with my friends) because the specs are so much better on my audio interface.

    PS The reason why you can't "dial it out with EQ even when I completely kill the treble" is because the tube overdrive is happening after the treble control, in the amp's power stage. Try adjusting the treble EQ in your DAW, I think you will hear, it makes a huge difference where you put the EQ in the signal chain (before, or after, the overdrive).
    Last edited: Feb 16, 2023
  17. OK. Yup! You are correct.
    It appears from the block diagram that both XLR outs are electronically balanced.
    Thanks @CatOnTheBass for calling me out on on that. I had it coming.

    And this also helped with the question of Ground Lift switches.

    Not as any kind of excuse, but as way of info for others (hopefully F.U.D. free...) Outputs are either electronically balanced or transformer balanced. It is also possible to to have unbalanced outputs, either electronic or transformer.
    This Ampeg uses an unbalanced transformer tap and then balances it electronically. Technically not a Transformer output. More of a transformer sourced output. But the output is electronically balanced, which may be helpful to know depending on what you are connecting to this output.

    "Tater - Toter?" I think we're all on the same page. Thanks for getting me to the right chapter. :thumbsup:
    CatOnTheBass likes this.
  18. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Note that EIN is not the same as S/N. As an approximation, subtract the circuit gain from the EIN to get the S/N ratio. For example, if the EIN is -128dB, and the circuit gain is 50dB, the approximate S/N will be 78db.

    The EIN will be specified at a specific gain, and the output will be specified at a particular level.
    Peter Torning and CatOnTheBass like this.
  19. Thanks for the info @agedhorse! If my maths are correct, that means the noise floor on the Ampeg is roughly 12dB higher than the Focusrite. That seems consistent with my results in my totally amateur home studio.
    grouse789 likes this.