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

[agedhorse]

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.

Yes, roughly.

The point of my comment is that a number/spec that may look spectacular compared with another number/spec until the equivalency calculations are made.

 

[CatOnTheBass]

Yes, roughly.

The point of my comment is that a number/spec that may look spectacular compared with another number/spec until the equivalency calculations are made.

Ah gotcha. What I meant to say was, "here is a suggestion OP could try, because it has worked well for me in the past," not "here is a suggestion OP could try, because the specs look amazing on paper." In other words, my recommendation was based on personal experience with both products (not on the specs).

I would imagine your Mesa DI's are way quieter than the Ampeg's. 60dB is not very good.

 

[Old Garage-Bander]

[QUOTE="CatOnTheBass, post: 27242967, member: 364833]60dB is not very good.[/QUOTE]

60db is not a stand-a-lone measurement. For it to be so, would require one or more additional letters after "db" to give a reference. When you say xdb it must be relative to something else. 60db below (-60db) nominal operating level would convey something meaningful as it has a reference, the level measured as nominal operating.

Interestingly, and just to put your 60 db noise figure in perspective, -60db from nominal operating level was the minimum required standard applied to the noise levels for stereo FM stations in the U.S. Stations were required to meet this during their annual Proof of Performance.

This noise level was measured from one of the microphone inputs on the station console, (mic disconnected) through all the equipment at the studio, the link between the studio and transmitter snd the transmitted signal as picked up on a receiver. The station consoles were, some still may be, the noisiest part of the chain. So it was tough to achieve required noise levels. Many stations up until the eighties relied on telephone lines as the studio-transmitter link (STL). Those often barely made spec. So your favorite FM station was essentially operating at 60db below program level. I used to laugh when some stereo salesman tried to steer someone to a receiver that had a noise spec of -80db vs one that was -70, and for a lot more money.

These measurements are no longer required by the FCC, so there may be stations that have let this slip a bit. So if you had good FM reception, you were pretty much experiencing music, who's noise level just barely made the -60db standard. And in some cases, due to how some interpreted the rules, you were really only listening to noise just 50 db below program level.

For every 20db drop in noise level below reference program level, the noise is 1/10 of program. 60db down is 1/10 of 1/10, of 1/10 of or 1/1000 the level of the programming. Noise at -60 db below reference is considered too low to be heard. So while -60 is "bad" by today's standards, there are diminishing returns on getting much lower. Comparing it to a noise spec of -128db isn't even worth the bother. You just don't hear the difference while listening to the audio.

 

[Zatenm]

@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).

Thanks CatOnTheBass, to answer your questions:
1) the buzzing sound does increase with the volume
2) Yes, I kind of have: I removed the Scarlett altogether (good point about the signal to noise ratio!) and sent the DI out from the PF-20T to the FX Loop Return signal of my other tube amp, an Ashdown little Bastard 2.N which happen to have an onboard power soak + cab sim from Two Notes. The amp + cab sim colored the sound as you would expect but again, the buzz was there! As annoying, artificial and intrusive sounding as it was through the Scarlett, that's why I'm really doubting the PF-20T.

(Note: the ashdown amp sounds very consistent through it headphones out or the Scarlet)

Yes to your point, the Scarlett has a very good tone on its own (very clean) but I'm partial to the Ampeg sound and would like that tube sound through my headphones (the Ashdown sounds great through headphones pre and post the cab sim!).

Good to know about the EQ placement in the signal chain too, makes a lot of sense, thanks!

 

[Zatenm]

Thanks for all the helpful replies guys!
The replacement amp should in here in a few days. I'll let you know if the outcome is any different (crossing my finger and toes!)

Side note: I enjoyed reading the technical pieces you shared Not always accessible info to a noob like me but always good to stretch oneself and learn from such an impressively informed group!

 

[CatOnTheBass]

Good luck @Zatenm! I hope the replacement unit is quieter. It was mentioned in the 'humbuster' thread, if you buy from Sweetwater, they know about the problem, and you can ask them to test your unit before they ship it to you.

@jnewmark I want to thank you for sharing that link. Around page 16 is where things get interesting for PF20T owners. It's not a super high priority for me, but at some point, I am definitely going to try that mod! 20dB is a big difference.

 

[Zatenm]

Hello again! As promised, the update on the replacement amp:
So I received it yesterday (3 business days shipping from Cosmo Music, always very happy with their service) and proceeded to testing it out as soon as I got back from work.
1st thing I notice, the sound is noticeably cleaner and clearer through the speaker cab - very encouraging and quite unexpected (On the subject, always uber-impressed by the wonderful presence, volume and articulation of these little amps!).
Unfortunately, I had to return my Scarlett interface for a refund while I still could (found out I'm not that interested in recording after all) so I could not try the new amp through it. That said, the buzz experienced in the previous amp was as present and noticeable through the Ashdown LB 2.N FX loop, so I tried that.
First seconds were buzz-free but when I turned in the volume a bit more, I could hear the buzz coming back but ONLY on higher notes (D & G strings). The buzz on the previous amp was more pronounced and across all notes of the fret board.

I don't know what exactly what to make of it but here are my preliminary thoughts:
1- Looks like there was indeed something wrong with the prior amp seeing the improvement on the replacement unit.
2- The Transformer DI does sound 'cleaner' on the new unit but the issue is not effectively resolved (the buzz is still there but weaker and more limited to high notes)
3- I LOVE the tone through the cab (now more than ever) and I'm not as interested in recording anymore. I won't be gigging / jamming in general and this likely going to remain a practice / home amp for the forseable future.

As a result, I'm tentatively thinking of keeping the amp figuring that I love its speaker tone and I can always use the preamp DI if ever needed. On the downside, I'll be missing out on the Tranny DI which seems highly appreciated by other owners and will be living with the notion that my amp is potentially not a perfect unit.

Much interested in your thoughts as usual!

 

[grouse789]

Hello again! As promised, the update on the replacement amp:
So I received it yesterday (3 business days shipping from Cosmo Music, always very happy with their service) and proceeded to testing it out as soon as I got back from work.
1st thing I notice, the sound is noticeably cleaner and clearer through the speaker cab - very encouraging and quite unexpected (On the subject, always uber-impressed by the wonderful presence, volume and articulation of these little amps!).
Unfortunately, I had to return my Scarlett interface for a refund while I still could (found out I'm not that interested in recording after all) so I could not try the new amp through it. That said, the buzz experienced in the previous amp was as present and noticeable through the Ashdown LB 2.N FX loop, so I tried that.
First seconds were buzz-free but when I turned in the volume a bit more, I could hear the buzz coming back but ONLY on higher notes (D & G strings). The buzz on the previous amp was more pronounced and across all notes of the fret board.

I don't know what exactly what to make of it but here are my preliminary thoughts:
1- Looks like there was indeed something wrong with the prior amp seeing the improvement on the replacement unit.
2- The Transformer DI does sound 'cleaner' on the new unit but the issue is not effectively resolved (the buzz is still there but weaker and more limited to high notes)
3- I LOVE the tone through the cab (now more than ever) and I'm not as interested in recording anymore. I won't be gigging / jamming in general and this likely going to remain a practice / home amp for the forseable future.

As a result, I'm tentatively thinking of keeping the amp figuring that I love its speaker tone and I can always use the preamp DI if ever needed. On the downside, I'll be missing out on the Tranny DI which seems highly appreciated by other owners and will be living with the notion that my amp is potentially not a perfect unit.

Much interested in your thoughts as usual!

I’m glad you received a “better” amp.
I’m curious as I did not see it asked. what were the amp settings you used? What are the settings on the bass you played?
Does the pf20t have a hi input button?
Thanks

 

[agedhorse]

Was the noise present when the bass cable was unplugged at the amp end?

 

[Zatenm]

I’m glad you received a “better” amp.
I’m curious as I did not see it asked. what were the amp settings you used? What are the settings on the bass you played?
Does the pf20t have a hi input button?
Thanks

Thanks @grouse789 !
I played the amp flat, then boosted the bass and highs a bit and reduced the mids (my usual setting at home). I played with the gain and master, boosting 1 and cutting the other and vice-versa. Played it through a Fender Classic 50's P-bass (2017 production) with a set of well broken-in Labella flats. Started with the bass tone dial wide open to all the way to off.
The PF-20T only has 1 input, no pads or sensitivity switches. It does have a ground lift which improved the noise level out if the DI but the buzz is still there even when engaged.

 

[Zatenm]

Was the noise present when the bass cable was unplugged at the amp end?

It only comes out when I play the notes with the bass plugged into the Transformer DI. Relatively quiet otherwise.

 

[agedhorse]

You are plugging the bass INTO the transformer DI?

 

[Zatenm]

You are plugging the bass INTO the transformer DI?

No, I typed too quickly: I meant 'out of' the transformer DI (as in bass plugged in the front of the amp and coming out from the DI). Sorry for the confusion!

 

[agedhorse]

If your bass in plugged in, the bass's volume turned down but you are not playing, is it quiet?\

To clarify, it there a noise or is there a distortion of additional sound that's only present when you are actually playing?

 

[Zatenm]

If your bass in plugged in, the bass's volume turned down but you are not playing, is it quiet?\

To clarify, it there a noise or is there a distortion of additional sound that's only present when you are actually playing?

Thanks for the clarification. Yes to both questions.. Quiet when volume down / not playing and only buzzing when I do (out of the DI)

 

[agedhorse]

When you turn the bass's volume up but don't play, is there any noise?

Do you hear the noise through the speaker also, or is it just the DI?

(there are 2 very different possibilities, this is the only way to rule in or out one or the other.

 

[Zatenm]

When you turn the bass's volume up but don't play, is there any noise?

Do you hear the noise through the speaker also, or is it just the DI?

(there are 2 very different possibilities, this is the only way to rule in or out one or the other.

@agedhorse Confirming no noise when volume is up and i’m not playing, just tried it to be sure. Also it appears to be only from the DI (I’m listening to it through headphones and I’m assuming if the same buzz was reproduced through speakers it’d be loud enough for me to hear it)

Intrigued!

 

[agedhorse]

Ok, so this is completely different than the often cited hum problem.

What is the DI driving, and how do you have the gain on that device set? I believe the output is line level and may overdrive a mic level input if the device's input gain is not set correctly.

Are all of the LED bias indicators operating correctly per the owner's manual?

 

[Zatenm]

Ok, so this is completely different than the often cited hum problem.

What is the DI driving, and how do you have the gain on that device set? I believe the output is line level and may overdrive a mic level input if the device's input gain is not set correctly.

Are all of the LED bias indicators operating correctly per the owner's manual?

Yes I too have ruled out the infamous hum problem from what I read about it (someone here has gracefully shared the related thread earlier).

LED bias indicators turn and stay green a few seconds after turning on the amp, so no issues there.

I tested the DI through 3 different components to compare results:
1-A Scarlett 2i2 3rd gen Interface (connected to laptop with headphones out)
2-the FX loop of my Ashdown Little Bastard 2.N with has a built-in cad simulator & headphones out
3-My ART Headtap (passive headphone monitoring pedal)

The buzz is as present and clearly heard through the Scarlett and the Ashdown amp (although as you'd expect the Ashdown does color the tone).
The ART headtap produces very low volume out of the DI (meant to go through the speaker out) but if you pluck the string hard enough and listen carefully you're still able to discern the buzz.
I also tested 2 different cables and two separate rooms/wall plugs with the same results.

This is the second amp that shows that Transformer DI buzzing issue, although the first one was more even pronounced, so I'm genuinely puzzled...

I may choose to live without the Transformer DI but what I'm most worried about is the eventuality that the speaker out is also not clean as it should be (i.e buzzing) but I don't have the hearing sensitivity/experience to detect it through the speakers (headphones are always more HIFI). Do you think that is likely/possible?

 

[beans-on-toast]

The Focusrite indicates if you are overloading the input, keep the level set below seeing any red. If you have a DAW meter plugin you could verify how hot the signal is coming out of the transformer output. Too hot and the op-amp driving the line out could be overloading.

You mentioned that the balanced line out is fine. This implies that the source is in the power amp. Try swapping the fourth 12AX7 with one of the other 12AX7’s in the pre.

You could listen for the high frequency noise in both the 4 and 8 ohm settings, with and without a speaker connected. The resistor load shown in the diagram below is incorrect. It isn’t just when set to 4 ohms.