Autoformer volume control problem

merlin el mago · Post by **merlin el mago** » Mon Mar 30, 2015 1:40 pm

I have a really strange problem going passive or active, when I play DSD sounds bad, isn't the system because when used a stepped attenuator goind passive or active the DSD plays superb.

Post by **dave slagle** » Mon Mar 30, 2015 3:10 pm

Can you provide more information on the source?

dave

merlin el mago · Post by **merlin el mago** » Mon Mar 30, 2015 3:29 pm

Thanks for support Dave, source is Twisted Pear Buffalo III + Legato I/V.

TIA
Felipe

merlin el mago · Post by **merlin el mago** » Mon Mar 30, 2015 6:45 pm

Attached Legato manual

Post by **dave slagle** » Tue Mar 31, 2015 2:10 pm

could there be DC on the output? I see the Legato doesn't have output blocking capacitors and relies on the user to null the DC offset.

dave

merlin el mago · Post by **merlin el mago** » Tue Mar 31, 2015 2:12 pm

I measured Legato and haven't DC offset

merlin el mago · Post by **merlin el mago** » Wed Apr 01, 2015 10:37 pm

If was DC the problem I will have the same problem listening PCM files.

merlin el mago · Post by **merlin el mago** » Sat Apr 04, 2015 1:01 pm

I use the AVC after the #26 tube preamp so the source for the AVC is the OPT LL1660, you think there is DC?

merlin el mago · Post by **merlin el mago** » Sat Apr 04, 2015 1:17 pm

Playing vinyl also sounds fine.

Post by **dave slagle** » Mon Apr 06, 2015 2:56 pm

Hey,

sorry for the late reply, I was out of town.

If the AVC is after a #26 and IT then there can be no DC. The only possible issue is the parallel combination of the AVC inductance and the IT inductance might be too low for the 26 causing a loss of low frequency info. Since Vinyl sounds good, I don't suspect this is the issue.

Is it possible that the grid of the #26 is causing issues for the Dac?

dave

merlin el mago · Post by **merlin el mago** » Mon Apr 06, 2015 3:25 pm

I Dave,

Using input coupling cap just to block DC before the AVC works used as passive preamp & now sounds OK, I don't understand why DSD files generates DC offset.

I use grid leak resistor & grid stopper in the #26 preamp.

Felipe

crazy bill the eel killer · Mon Apr 06, 2015 8:02 pm

Hey guys,
I don't have enough on the ball to figure this out, but I can't stop thinking that the quantisation noise inherent to DSD has something to do with this. I'll let you guys with enough on the ball actually figure it out.

Felipe,
Is there any way you can play a DSD track and see if there's any DC offset, as you stated you measured none with no signal present ?

Cheers, Crazy Bill

merlin el mago · Post by **merlin el mago** » Wed Apr 08, 2015 4:36 pm

Measurements playing DSD files:

Left channel -1.6VDC Right channel -0.8VDC

merlin el mago · Post by **merlin el mago** » Wed Apr 08, 2015 4:42 pm

DC offset when NOT playing any kind of file it's the same.

Left channel -1.6VDC Right channel -0.8VDC

merlin el mago · Post by **merlin el mago** » Wed Apr 08, 2015 4:46 pm

I have disconnected physically the RCA between Legato and the Intact Audio AVC (autoformer volume control) and there is no DC output at the Legato output

merlin el mago · Post by **merlin el mago** » Thu Apr 09, 2015 10:51 am

I used the AVC as passive and 300 Ohm headphone to avoid grid & OPT tx and playing DSD files I still have Left channel -1.6VDC Right channel -0.8VDC, another curiosity is when I stop playing DSD files still remain the DC offset Left channel -1.6VDC Right channel -0.8VDC, only goes to zero when I play PCM files, another test done is changing the volume setting don't change the amount of DC offset so DC offset isn't related to volume setting.

merlin el mago · Post by **merlin el mago** » Thu Apr 09, 2015 3:24 pm

I used a new DVM because don't trust the other DVM, now with DSD files measures both channels -6VDC of offset, of course PCM files measures near 0VDC both channels.

Post by **dave slagle** » Thu Apr 09, 2015 4:22 pm

-6Vdc at the output will not make the autoformer or the grid of a 26 very happy.

dave

merlin el mago · Post by **merlin el mago** » Thu Apr 09, 2015 5:09 pm

I never used the AVC between the DAC and the grid of 26, I used the AVC between the Lundhal LL1660 OPT and the SS power amp (100K input impedance) or between the DAC and the headphone Sennheiser HD600 (300 ohms nominal impedance).

I know AVC doesn't works ok with DC offset, also I know using a cap input can fix the issue.

What the question is: why if I use a cheap ladder resistor DSD files don't produces any kind of DC offset at the cheap ladder resistor input?

merlin el mago · Post by **merlin el mago** » Thu Apr 09, 2015 5:41 pm

From the explaination in wikipeadia DSD files the appears to be much more more modulation of the signal and may be this is being rectified ?

merlin el mago · Post by **merlin el mago** » Fri Apr 10, 2015 12:48 pm

Situation without nothing connected after the AVC isn't changedr: left channel -6VDC right channel -5VDC

merlin el mago · Post by **merlin el mago** » Mon Apr 13, 2015 3:26 pm

@Dave
moglia suggested me the dc current coming out from the DAC playing DSD files is saturating the AVC core, do you know how many mA can handle the AVC without saturating the AVC core?

Post by **dave slagle** » Mon Apr 13, 2015 4:39 pm

1ma will take it down. 6V across the 60ish ohm DCR will allow for 100ma but I'm sure there is no way that amount of current can be delivered.

the effect of the saturated core should just be loss of bass.

dave

merlin el mago · Post by **merlin el mago** » Tue Apr 14, 2015 11:13 am

I have the effect of the saturated core lossing of bass.

Post by **dave slagle** » Tue Apr 14, 2015 2:20 pm

do you also get a "pop" or "snap" when you adjust levels?

dave

merlin el mago · Post by **merlin el mago** » Tue Apr 14, 2015 3:17 pm

I get "pops" when I move up or down the volume pot.

Post by **dave slagle** » Tue Apr 14, 2015 7:34 pm

that means you have DC on the autoformer.

dave

merlin el mago · Post by **merlin el mago** » Thu Apr 16, 2015 4:56 pm

No DC offset playing DSD files but I still have the effect of the saturated core lossing of bass.

Post by **dave slagle** » Thu Apr 16, 2015 10:31 pm

what is the output impedance driving it?

you can create a file with some LF sinewaves to see if you are really rolling off.

dave

merlin el mago · Post by **merlin el mago** » Fri Apr 17, 2015 10:53 am

I'm using single ended output and it's practically zero to 100khz.

Felipe