Seeing -40db on My Hafler P3100 With no Playback.

cortes

New member
This is my first post here. It's a generic audio question, but since it involves recording gear this seemed a good place to ask.

I had been routing my computer audio output vis USB to a Radial USB Pro to my Hafler P3100. With no playback, the lights on the P3100 indicated idle. I also have a Focusrite Scarlett 2i4 that I use for audio input. I recently realized it will do output from USB as well. Looking to simplify my hardware setup, I ran my computer audio output vis USB to the 4i4 to my P3100. With this setup the P3100 lights shows -40db output (two lights on). I can't hear any hiss on the speakers unless If I max the 2i4, it goes to -30db, but I still don't hear any hiss. Is this normal? Am I worrying about nothing?

The only input to the USB Pro is USB from the computer while the 2i4 has analog audio inputs as well (a Roland RD 2000 in my case) so the analog audio might have something to do with it. I'm only guessing. I can always go back to the USB Pro. I'm mainly asking just to better understand what to expect in the way of sound levels.

P.S. When I turn off the Roland RD 2000, I get a noticeable hiss on the on the speakers, but that's a separate issue.
 

Folkcafe

Active member
I have Hafler amps including trans novas and trans anna. A couple interesting things about these amp in their design is you feed DC in, you'll get DC out. If your device output has a bit of DC it translates over to the output of the amp. I found this out with a Yorkville sub I had that had just a bit of DC showing on the XLR out. Odd there are no blocking caps on the input but one of the reasons they sound good.

If not DC offset from your interface, this class A/B amp has a couple adjustments including offset. They are also prone to some of the caps drying out. I've got one now that I need to get on the bench as it has a more than slight DC offset. Turn on the amp and you can see the woofer on the monitor push out (not a good thing) with no signal. So first thing, take a meter and measure DC offset. Setting it via the service manual is typically done with an amp meter across test pins and is set to some small mAmp. Good news is there are service manuals online for most of the models and the procedure is pretty clearly detailed. Love the sound of these but as they've gotten older, I keep a close eye on them. Probably just re-cap them at some point.
 
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gecko zzed

Grumpy Mod
What you may be seeing indicated on the Haffler is a signal coming through from the input side of the 4i4, specially if you have some input monitoring going on.
 

cortes

New member
@Folkcafe, the amp is only a couple of years old so hopefully they are no dry caps. I didn't know caps could even dry out. I do have a multi-meter. What pins would I test on an XLR cable or does it make a difference
@gecko zzed, what you say makes sense. There is a headphone input monitoring section.

I was able to get rid of the hiss increase when I turned off the input (Roland RD 2000) into the 4i4. I had previously been using it for a microphone and had left the 48v phantom power on.
 

Folkcafe

Active member
@Folkcafe, the amp is only a couple of years old so hopefully they are no dry caps. I didn't know caps could even dry out. I do have a multi-meter. What pins would I test on an XLR cable or does it make a difference
@gecko zzed, what you say makes sense. There is a headphone input monitoring section.

I was able to get rid of the hiss increase when I turned off the input (Roland RD 2000) into the 4i4. I had previously been using it for a microphone and had left the 48v phantom power on.
That's right, they are part of Radial now so what you have is the reissue if you will. Was mixing it up with the P3000 which like my amps are a bit ancient (like going on 30 years). Don't know if they updated the input and what I could read is the output is a similar fet design but with toroidal transformers and who knows what else might have been updated.

So pretty much disregard what I said about the bias adjustments as it is not in the manual I found online. Service information isn't in manuals like in the old days.

As to DC offset, I've had this issue a couple times with my old Hafler amps because there are no DC blocking caps in the circuitry. In addition to the Yorkville sub I mentioned, the bus powered interface I am currently using freaks my amp out on computer boot up. I get negative DC output on my amp and my woofers retract into the cab for a second or two. I always have to remember to turn the computer on first, then power the amp up. Trust me you'd be able to tell if there was offset on the amp output as the woofer would either be pushed out or in after turning on the amp. Test procedure is to put a meter set to DC and measure across + and - on the speaker output. As to an XLR output, Pin 1 is is shield and 2 is hot with 3 minus. Test between both shield and 2&3 along with between 2&3 if you are paranoid.

The Yorkville sub was a real piece of crap that started falling apart the day I got it so I wasn't surprised. Power LED stopped working the first hour. Next day the port tube fell out. I patched it up and gave it to my wife for her Karaoke set up.
 
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