DC Offset Issue in Don Sachs Preamp

Is this a new behavior?

With a capacitor coupled tube preamp, the output capacitor will develop a fairly large DC voltage across the cap. When the preamp is turned off, the voltage on the input of the cap goes to zero, but the DC voltage across the cap doesn't just disappear immediately. The larger the cap value, the more energy will be stored and the longer it will take to dissipate. If the power supply voltage drops faster that the coupling caps can drain, this will result in a DC offset for a short time which will cause a pop or thump if you still have the amp powered up. 

Amp is used (3-4 years old) but new to me.  

Today, I swapped out the CV181Zs for classic 6sn7s, and things may be better. Would that make sense? Doesn't the CV181Z have higher heater current value than 6SN7s?  

The only cv181 tubes that have a higher heater current are the nos tubes, the newer production cv181 tubes are identical to the 6sn7.

Don is still with us, contact him by email. My guess is the load resistor on the output of the cap has gone open, but I wouldn’t expect both channels to do that at the same time. Maybe something has gone wrong with the regulator. It would help to know what power amp you are using and how efficient your speakers are. (Khorns? Magnepans?)

The only way to be sure is look at the output with an oscilloscope and see how big that pulse is. You can get handheld Chinese models for about $200.

Thank you! 
 

I’m running 100dB sensitive horns with larger woofers.  I’m currently playing the Sachs into a Marantz Ruby set to Power Amp mode.  This hybrid setup sounds sublime. 

Definitely call Don. But before you do, disconnect all inputs into the preamp and cycle it on and off to rule out the signal source as the problem. Without a schematic it is difficult to guess what the problem is. 

The final stage of the preamp is a cathode follower with an active pull-down, which is cap-coupled to the RCA output. The output is shunted by a large-value resistor to discharge the output cap. Don’t remember if there is a muting relay to mute the output during power-up or power-down. In the meantime:

1) ALWAYS power on the DS Preamp before activating your power amp. I’d give it a full minute to glow and stabilize operation before powering on the power amp. If any of the tubes do not light, do not use it. If the tubes make weird scratching or whirring noises, they need replacing.

2) When powering down, turn off the power amp FIRST, then power off the preamp about 10 seconds to a minute later. (You want the power amp fully discharged and providing no gain. No clicks should audible at all.)

I would definitely NOT power on or power off the full system on a common power outlet. If you are using a 12V relay system, you need to add logic to add the delays shown above.

This recommended power-on/power-off sequence applies to all mixed tube/transistor systems (with a few exceptions, noted below). The vast majority of tube preamps use cathode followers with the cathodes (the output node) at 100 to 150 volts DC. The coupling cap blocks the DC, but there can be transients when power goes on or off, as the cap charges or discharges.

The Raven preamp Don and I designed is an exception, because it is transformer coupled, not cap coupled, so on/off transients are far smaller (the transformer does not have a DC charge on it, and the preamp is internally balanced). The DS preamp you have is a superb preamp, but it is a more traditional design with cap coupling in the output section (there are no caps anywhere else, though).

All-transistor or all-tube systems can be switched on a common power switch, but it’s not good practice, since small DC transients can make it through the power amp, get amplified many times, and harm the loudspeaker (and startle the listener). The safest and least annoying approach is following the above sequence.

(Some manufacturers mitigate the problem with muting relays, but it is difficult to find relays that can switch 150 volts without developing contact deposits over time. A low voltage "dry circuit" which does not arc (typical input selecting relays or volume controls) is not a problem, but a high voltage relay that can tolerate a 150 volt arc is extremely difficult to work around. It makes no difference if a 120/240 volt light switch develops a little distortion, but the output of a preamp is one of the most critical nodes of the entire hifi system.)

lynn nailed it

It’s the rectifier tube. Don will tell you tot always power amp down first than preamp. 
I know because I had the same thing happen on his preamp and he told me it’s completely normal. Pre on first, wait 1 minute, then amp on. Amp off first, then pre. 
This is normal, no worries. 

Yes. Amps on/off first per Don. (For the Raven/Blackbirds) My one word review of the Raven/Blackbirds would be. Get. 

Thank you everyone!  Lynn, you are the man! 
 

don’t tempt me on those new models! 

Don is awesome people and one day I aspire to have enough saved up to buy his great products (And Lynn Olsen too for that matter).

Here’s proof that Don is "Alive & Kicking". Another awesome fellow Canuck (for a West Coaster anyways)

https://youtu.be/iUk0vc9FyXY?si=KtsNDt2KD1ze8jlS

Actually, posting these little "how-tos" on the forum is good for me, since the content can be added to the product manuals. I’m the one that writes them for Spatial.

Plenty of experience writing these things ... nine years at Tektronix in Oregon, followed by several years of editing Glass Audio and Vacuum Tube Valley. So it’s just keeping in practice.

Many of my designs are "thought experiments" to see if an idea works or not. In audio, you need to physically build something to hear what it actually sounds like. Before it’s built, it’s a sound you hear in your head, but it’s always a surprise to hear it for the first time. That’s been my experience since the Shadow Vector intro at the 1975 Chicago CES to hearing the Raven intro at the Seattle show in 2023.

(By the way, I am the owner of a DS preamp. Great design, and yes, I know what's inside.)

@jbhiller If this problem continues, one of the more common reasons is failure of the bypass capacitor on the power switch. This can result in a loud pop at times, depending on the Voltage of the incoming AC line when you shut it off. So it can seem to come and go.

Thank you Ralph! 

The unit had some super microphonic tubes (CV181s) that sounded sublime but were so noisy that after several hours of low volume play, the tubes could pick up nearly any activity in the room and subtlety amplify it, which I've never experienced (outside of guitars, only 10 year of tube experience here). 

I've got a pair of run-of-the mill reissue Tung Sol and EH in there now and things are quiet. 

I'll open the unit up shortly to see if there's a bypass cap on the power switch.  I could always replace it for like uF/voltage to see if that helps.