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Soulution preamp DC-offset protection

Good morning,
I've just bought a used but mint Soulution 721 preamp. It has replaced a Spectral DMC30SS Series 2. Apart from the sonic differences between the two, I'd like to know more about the DC-offset protection that the Soulution preamps have. Soulution claims that the input signal is permanently monitored for DC-components and at a level > 15 mV a high-class coupling capacitor is switched into the signal path. If DC-offset has decreased, the coupling capacitor is removed automatically after 15 seconds. During the first listening hours, I noticed that this capacitor switched on at least 5/6 times (a capacitor symbol appears in the display and after 15 seconds, when it's removed you hear a mechanical "click" sound). I'm wondering if this is normal or maybe it's a symptom that requires further investigation. I read that the DC-offset could came from the recordings itself and, actually, if I play the same CD and the same song, the DC-offset protection circuit of the Soulution switches on every time in the same point. Maybe it just something I did not realize before because the Spectral preamp showed me nothing of all this. It could be something like "if you don't see... you don't worry". Can someone help me to understand if I have to be worried or not? Thanks a lot!!
katfnc
20 responses Add your response
atmasphere's avatar
atmasphere
11,793 posts
 
First, the DC can't come from the recordings!!
We make a preamp with a DC coupled output. Such a circuit needs something called a 'servo' to correct the DC voltage so that the DC output of the preamp is always at zero volts.
By any chance does the capacitor light only come on at higher volumes?
imhififan's avatar
imhififan
2,180 posts
 
What is the CD player/DAC?
katfnc OP
14 posts
 
@atmasphere: No, it doesn’t seems to be related to high volume. And the strange thing is that it occurs always in certain points of certain CD/songs... For example near the end of Chris Jones “Long after you’ve gone”... 

@imhififan: Soulution 540 Sacd/Cd player... Cardas Golden Reference RCA signal cables. 
atmasphere's avatar
atmasphere
11,793 posts
 
@katfnc I would contact the manufacturer. I can't imagine why the circuit would need something like that, but that's me.
katfnc OP
14 posts
 
My dilemma is: when I had the Spectral preamp I was simply unaware of this thing just because the Spectral doesn’t have this kind of protection (or perhaps it doesn’t show anything to the user)? Because I don’t hear anything wrong with the music... I think I will try to use a balanced connection between the CD player and the preamp, just to see if the same thing occurs when using another input and with another type of connection...
atmasphere's avatar
atmasphere
11,793 posts
 
when I had the Spectral preamp I was simply unaware of this thing just because the Spectral doesn’t have this kind of protection
Probably because it didn't need it. We don't have a protection circuit on our direct-coupled output either; in 29 years of production the preamp has not seemed to need it.
katfnc OP
14 posts
 
@atmasphere: I didn’t meant to imply any kind of criticism towards Spectral. I had Spectral equipment for some years (after a long time spent with Levinson Reference) and I liked them a lot. I was simply try to understand what happens now because nothing has changed apart from the preamplifier and the strange things is that the DC-offset capacitor switches on always in the same points of the same CDs/songs (I’ve just tried again 10 minutes ago) so it seems that in those points something arrives at the input of the preamplifier and makes it react by switching on this DC-offset capacitor. Anyway I’m gonna change the input and type of connection and play the same song to see if anything changes. And I have already written also to the manufacturer. I hope they will answer me...
gs5556
1,234 posts
 
I would check the CD processor to see if that preamp indicator light is functioning properly. Download a 0 dBFS test signal at 50 or 50 hertz and burn it to a CD. Play the disc and measure for DC at the outputs with a DMM. If it is zero (or below 3 mV) then I would test the CD at that point where the cap light comes on.
katfnc OP
14 posts
 
In the meanwhile I’ve swapped the cables between CD player/preamp and preamp/power amp. Now I have a balanced connection between CD player and preamp. Symmetrical input # 1 instead of asymmetrical input # 2. Things got worse. With this connection the cap light comes on much more than before, practically it lights on, remains on for 15 seconds (it’s the minimum time of activity that the manufacturer indicates in the manual) then goes off for a couple of seconds then it turns on again and so on... I have used the same song that with the unbalanced connection made the cap light comes on one time only at 5’17”... Pretty annoying. 
stereo5's avatar
stereo5
6,384 posts
 
If it were I, I would definitely contact Solution.  Sounds like a problem with the unit.
erik_squires
16,049 posts
 
Gang,

This is a unique, but by no means the only, DC offset solution.

Many solid state electronic makers employ some sort of servo circuit with very long time constants to actively correct incoming or outgoing DC bias.

I say SS makers, because 99% of tube preamps have a DC blocking cap at the output, and a lot of inexpensive gear has DC blocking caps at the input.

In addition to DC blocking, discrete analog circuits may have some sort of auto-bias sensing circuit. While playing music I could imagine this starts to look like DC on the outputs during some circumstances. Is the CD player using discrete output stages? If so, I’d look there. :)

Best,

E
imhififan's avatar
imhififan
2,180 posts
 
While waiting for the manufacturer's reply, maybe try the player and preamp with factory default setting.
atmasphere's avatar
atmasphere
11,793 posts
 
@katfnc  I am now suspicious that this unit may be direct-coupled from input to output.


If that is so, it could be that the source is the problem and not the preamp. Can you try a different source (cell phone for example) and see if it still misbehaves?
katfnc OP
14 posts
 
Thanks a lot for all the suggestions guys.  The next test I’ll do will be with a different audio source, with the same CDs/songs where I saw the DC-offset protection switching on...
dionakra
13 posts
 

the problem with Soulutions protections is that they are not trustworthy, sorry

I know it very well on my skin and i paid a lot cause of that

katfnc OP
14 posts
 
@dionakra Could you explain better what you mean?

I wrote to Soulution and received a kind answer from Cyrill Hammer.

He told me that:

“Yes, this is normal behavior, but please let me explain.

The 721 is DC coupled design. This means that DC which shows up at the inputs of the 721 could pass through preamplifier and would present at its output. This would be no problem for the preamplifier at all, but it could harm components downstream, especially loudspeakers. Therefore, we have added a protection system to the 721 which prevents your system from critical DC offset levels. The 721 does permanently monitor the DC-offset at its inputs and as soon as there is more than 10mV DC offset the DC-protection goes on. There are some tracks on some CDs which in fact produce DC offset. This will trigger the protection. There is nothing to worry about your unit and your system is perfectly OK.

Your previous preamp the Spectral DMC30SS Series 2 is most likely an AC coupled design and does not need kind of additional protection.”


It makes perfect sense to me, because I noticed that behavior only in some points of some songs. These days I’m listening to several CDs and I’ve never seen the DC-offset protection switched on again, therefore this behavior is limited to some rare cases and with specific songs/CDs.

Speaking of balanced connection between CD player and preamp and the fact that the protection switches on more often, Cyrill told me that:


“The balanced connection will present twice as DC offset to the input stage than the unbalanced connection. Therefore, the DC-protection gets activated more often. The CD-player does of course produce a small amount of DC-offset by itself. However, as the DC-protection does not get activated when there is no music playing, the CD-player’s own DC-offset level is for sure below the threshold level for the protection circuit in the 721.”


I still use unbalanced connection and the activation of the protection is very limited. This, in conjunction with Cyrill Hammer’s explanations and reassurances, makes me feel peaceful. In any case next week I’ll make a new test with a different CD player, just to see if (using the same CDs/songs that caused the activation of the 721 protection) I’ll notice an identical behavior.

dionakra
13 posts
 
Yes Cyrill is kind, but after i sold my cd player (flawlees with every other preamp) the protections went on seeing DC somewhere else till the preamp burned
And no warranty at all
i moved to another Dc coupled preamp and i live happy now, with no Dc detected
obviously i wish you’ll not have such a bad experience
katfnc OP
14 posts
 
@dionakra I decided to return the preamp to the store (I didn’t feel good about this thing). They have been absolutely kind and understanding and since only few days had passed they gave back to me my Spectral and money. 
dionakra
13 posts
 
I moved to the new Boulder 1110 and i’m glad
Glad for your choice as well
katfnc OP
14 posts
 
@dionakra Congratulations for your Boulder preamp!
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