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Topic: Diagnosing Preamp Turn-on Thump (Read 8019 times) previous topic - next topic
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Diagnosing Preamp Turn-on Thump

My system has a noticeable turn on thump and I have always powered things up (and down) manually and in order.  I now want to be able to power everything up at once and so I examined my system component by component.  I found that the preamp is causing the turn on and off thump.  (Without the preamp, there is a very small thump that I can live with.)  The culprit is my Sumo Athena preamp, which sounds great and seems to be working well except for this problem.  So, I would like some help in diagnosing and repairing  this problem.  Can anyone direct me to posts or articles on this subject?

BTW, I don't have schematics for this piece.  Apparently, Sumo never released them and they are very hard to come by.

Re: Diagnosing Preamp Turn-on Thump

Reply #1
I would guess it's just a design weakness, not a failure, so there's probably nothing to "diagnose" or "repair".   

Re: Diagnosing Preamp Turn-on Thump

Reply #2
Check out this article

https://www.stereophile.com/content/sumo-athena-preamplifier

It states there is no switch on (or off) thump, so I assume there is a problem with your amp if it is indeed responsible for the thump

"I generally leave preamplifiers on all the time, and did so with the Athena as well. But for those who don't, the Sumo preamp has no turn-on or turn-off pulse; the output is muted for 15 seconds at turn-on to allow for stabilization of the circuitry—a welcome feature."

Re: Diagnosing Preamp Turn-on Thump

Reply #3
15 seconds sounds like an extremely long time though. My amp only takes ~3 seconds to click on after I press the power button. Maybe it's a typo and should read 1.5 seconds?

Re: Diagnosing Preamp Turn-on Thump

Reply #4
My system has a noticeable turn on thump and I have always powered things up (and down) manually and in order.  I now want to be able to power everything up at once and so I examined my system component by component.  I found that the preamp is causing the turn on and off thump.  (Without the preamp, there is a very small thump that I can live with.)  The culprit is my Sumo Athena preamp, which sounds great and seems to be working well except for this problem.  So, I would like some help in diagnosing and repairing  this problem.  Can anyone direct me to posts or articles on this subject?

BTW, I don't have schematics for this piece.  Apparently, Sumo never released them and they are very hard to come by.

One common strategy for avoiding preamp turn on spikes is to turn the power amp on after the preamp has stabilized.  There are power management devices that do that.  This device looks about right: https://www.monoprice.com/Product?p_id=604830&gclid=Cj0KCQjwz_TMBRD0ARIsADfk7hQVzk9BXUMipdl4W8TT7_yzgaBuFQ7OL0K45baDlBMrU2-1rvCcIVcaAmepEALw_wcB

Another strategy is to ensure that the power supply of the preamp stabilizes very rapidly or very slowly. Either strategy can work, or not. The theory of the fast stabilizing strategy is to let the preamp get stabilized before the power amp starts passing signals. The theory of slow stabilizing is to try to reduce the rate of stabilizing until it is so slow that  any thumps that are created are at such a low frequency that they are below the bandpass of the power amp and get lost in its low frequency roll-off. Neither approach has to necessarily work, but either often may work.

Common means to accomplish these strategies is to make the power supply and coupling caps in the preamp either as large or small as possible consistent with general goals for adequate frequency response.

One common set of circumstances is that the electrolytic caps in the preamp degrade over time as they are naturally prone to do, and this essentially changes the design of the preamp. This is one way that re-capping or capacitor upgrades can yield real improvements. The sound is really no different, but the thumps may be reduced or eliminated.

Other strategies include simply throwing this legacy crap away and getting good modern gear that works right. A lot of modern gear does this by design. For example my fairly modern AVR is so digital that it has to boot which takes time, and it has a relay that doesn't hook up the speakers until the AVR finishes booting. This takes about 3-5 seconds.  The other components in  my system could be thumping away to high heaven, and by the time the speakers get hooked up, the excitement is all over.

Re: Diagnosing Preamp Turn-on Thump

Reply #5
OP here.  Thanks for the help.  After looking at my original - and rather sparse - owners manual, I can confirm that Sumo states that there is a muting circuit that delays output for up to 15 seconds.  However, the thump I am hearing happens about one to two seconds second after the switch is pressed, so clearly something is wrong somewhere.  It happens at any volume setting, including zero, and is present with all inputs selected.  I will start by inspecting the power supply and replacing the filter caps; that's undoubtedly a good idea anyway.  However, without any schematics, it's going to be tough sledding after that.  Does anyone know where I can get the the schematics or service manual?  Or if there are any qualified service providers for this preamp out there?

Re using a power sequencer, it may come to that, but I'd rather avoid it and besides, it appears there is something wrong with the preamp and I'd like to address that first and/or instead.  And for what it's worth, I had already researched the power sequencers and found two that were a little pricier but seemed to be well made: the Furman M-8S and the Juice Goose CQ1520, both in the $280 to $350 range.

Re leaving the preamp on all the time, I'd rather clean up whatever is wrong.  It may come to this, though.  Either that or the power sequencer.

Finally, regarding replacing the "legacy crap" Athena with a modern AVR, I think I'll pass on that. 

Re: Diagnosing Preamp Turn-on Thump

Reply #6
Finally, regarding replacing the "legacy crap" Athena with a modern AVR, I think I'll pass on that. 

yeah, but of course replacing old and possibly not so well performing tech with cutting edge, feature rich and inexpensive modern hardware is just unthinkable!

Re: Diagnosing Preamp Turn-on Thump

Reply #7
It appears the power on mute delay has failed. Are you capable of repairing it or will you need to 'take it in'? Depending on the design and age of the unit, it could be a trivial as a bad capacitor.


Re: Diagnosing Preamp Turn-on Thump

Reply #8
I  imagine I could fix it myself but not without a wiring diagram or service manual.

Re: Diagnosing Preamp Turn-on Thump

Reply #9
I  imagine I could fix it myself but not without a wiring diagram or service manual.

So far I've found pictures of  3 variations of the Athena, 2 of which are shown as attachments. They are probably electrically similar in most areas of the circuit.  The third is has added XLR connectors. That means that there probably is a fourth variation of the second type, also with added XLRs.  Confusing enough? ;-) 

There also seems to be a variation with IC amplifiers replacing the discrete op amps shown here. It may have a different name because the photos I found on the web don't have clear markings. So that would be 5 variations, and there is probably a sixth with XLRs.

The anti-thump circuit is probably the few parts clustered by the associated sealed box (probably reed type) relay(s) in the lower left corner of the circuit board. The most likely failing part would be the smaller electrolytic capacitor than the larger power supply capacitors that are mounted in pairs near by.

The circuit board appears to use the older through-hole component technology that is easier to solder and unsolder parts from.

Re: Diagnosing Preamp Turn-on Thump

Reply #10
I think those PC boards are the same. The higher res picture is 7-430 REV.G  The lower res is ?-430 REV G.  I worked in small electronics manufacturing in California and we were pretty religious about revision numbers in boards. They might get stuffed differently but the art etched on the board was identical for a given revision number. I think the box on the left is mechanical protection over the mains wiring, power transformer, switch, fuse - the usual stuff. I don't see any integrated amplifiers on the high res picture  and lots of discrete transistors.

I believe the power enters the board at the lower left of the main PCB, uses a discrete diode bridge rectifier, 'lytics,, regulators and more 'lytics. My guess is K1 in the upper left is the mute relay. Relays are preferred as they achieve milli-ohms of resistance on the contacts. I would design it as relay NC (normally closed) contacts shorting the output to ground and activating the relay to lift the short to run but there is nothing right or wrong about the choice to activate vs de-activate for mute.

In switch mode power supplies (and this is NOT one of those) the small value (<470 uF) lytics are the most common failure. I would not 'shotgun' all the caps as they appear to be decent grade and since it's not a switching power supply, the ESR (equivalent series resistance) of the caps is not critical to the performance of the unit.

It does not seem to be 'service friendly' as getting at the bottom of the PC board looks like a chore but worth the trouble. If it was mine I'd repair it even without the manual though I always prefer to have a manual.


Re: Diagnosing Preamp Turn-on Thump

Reply #11
I think those PC boards are the same. The higher res picture is 7-430 REV.G  The lower res is ?-430 REV G.  I worked in small electronics manufacturing in California and we were pretty religious about revision numbers in boards. They might get stuffed differently but the art etched on the board was identical for a given revision number. I think the box on the left is mechanical protection over the mains wiring, power transformer, switch, fuse - the usual stuff. I don't see any integrated amplifiers on the high res picture  and lots of discrete transistors.

Excellent points, and on review I have to agree with them. Thanks.

Just for grins and giggles, I have attached a picture of what I previously called the IC version, which also has balanced outputs. Sorry about the lo rez, but that's as good as I can find.

Quote
I believe the power enters the board at the lower left of the main PCB, uses a discrete diode bridge rectifier, 'lytics,, regulators and more 'lytics. My guess is K1 in the upper left is the mute relay. Relays are preferred as they achieve milli-ohms of resistance on the contacts. I would design it as relay NC (normally closed) contacts shorting the output to ground and activating the relay to lift the short to run but there is nothing right or wrong about the choice to activate vs de-activate for mute.

Again, seems right. The relays used for this function in audio are typically reed relays for line levels signals, and conventional relays for loudspeaker levels. For line level signals such as those at a preamp output, the logic can be as you say, NO, NC or even both. NO reed relays are often a bit cheaper as a permanent magnet is often added to obtain the NC function. The dual contact version of reed relays are the most commonly used, but in fact a reed relay can be built with as many contacts as desired as the contacts are all independent, anyway.

Quote
In switch mode power supplies (and this is NOT one of those) the small value (<470 uF) lytics are the most common failure. I would not 'shotgun' all the caps as they appear to be decent grade and since it's not a switching power supply, the ESR (equivalent series resistance) of the caps is not critical to the performance of the unit.

I don't agree with this. IME electrolytic failure to minimal or negligible capacitance is an equal-opportunity failure affecting just about any kind of circuit. Perhaps switchmode power supply caps fail more, possibly because they tend to run above ambient temperatures due to high peak currents. In my life I've replaced mostly signal-carrying electrolytics because there are often more of them than any other function.

Your caution about replacing just the failed parts is good, but effective in-circuit checking is not always easy, and if you replace one, this increases the probability that more have failed, while unsoldering one end of the cap to check it gets you over half way to simply unsoldering the whole thing. 

Reasonably robust temperature controlled soldering irons and big blue plastic solder suckers that you cock like a BB gun  are getting to be under $10 items on eBay, and so you just get one of each  and learn how to use them. Picking up a spare set of fine soldering iron tips and keeping a fresh one in the iron is very economical as well.

Probably the right thing is to proceed checking individual caps by means of  circuit function checks as opposed to slavishly replacing everything in sight. If the turn on timer is not changing the state of the output relay, based on voltage measurements, then going further with the timing cap is logical. If the device still has good bass response measurements, then going further with the signal-carrying caps may be safely skipped.  It is not that hard to break something by trying to fix it in ways that it does not need.

I should add that the ca. $15 universal electronic parts checkers AKA capacitor ESR testers sold on eBay are far too cheap and effective to not have, and IME work unbelievably well. 

Ditto for the $13.95 true RMS 6000 count DVMs recently mentioned here.

Quote
It does not seem to be 'service friendly' as getting at the bottom of the PC board looks like a chore but worth the trouble. If it was mine I'd repair it even without the manual though I always prefer to have a manual.

Yeah, they went  cheap with the case but audio  devices that I have that are packaged this way are legion.  That's why I have power screw drivers and magnetic parts dishes... ;-)

Re: Diagnosing Preamp Turn-on Thump

Reply #12
Thanks to Google... The manual can be downloaded from David's Audio

No schematic or circuit diagram. Maybe google deeper...

The 15 seconds is correct. Maybe trying to emulate the valve (everything was) equipment of my youth: you could turn stuff on and forget about before it started making noises.  ;D
The most important audio cables are the ones in the brain

Re: Diagnosing Preamp Turn-on Thump

Reply #13
OP here.  Thanks for the continued support.  I have opened up the Athena preamp and it the same version as ABK's photos.  Per comments, I will replace the small 100uf, 25 volt cap (C1) that is next to the mute relay.  While I'm in there, I might as well replace the power supply filter caps.  I thought I'd use some Panasonic FM's or Nichicon PW or HE's.  That's probably overkill but they're all pretty inexpensive.  I can't imagine the choice being terribly important on this preamp.

My question is this:  should I do a "blanket" replacement of all the electrolytics while I'm in there?  The unit came with almost entirely Rubycon (electrolytic) caps. I see no signs of leakage anywhere, but these are 18 years old - I bought the preamp new along with the companion Delilah in 1989.  Lastly, would there be any benefit from changing out any of the other non-electrolytic caps?

Re: Diagnosing Preamp Turn-on Thump

Reply #14
I've replaced many hundreds (a few thousand) caps in old Sony broadcast gear but the caps generally  aren't really bad until quite old.  At 18 years you can certainly justify the replacements particularly when the 'chore' factor is added in. Take lots of pictures as you might forget lead placement and such.

I buy thousands of caps each year for work. Both Panasonic FM, FC and Nichicon PW are on the preferred list. My criteria are highest ripple current followed by highest hours. Price comes last. On a real busy day I might use $15 worth of caps. "Cheap" ones might drop that to $12. Who cares? Organic polymers are getting cheap enough to consider. Some common value are only double the price of 'lytics. On surface mounts I'm changing to polymers. BTW, surface mount is easier than through hole and much faster.

If I find 1 bad cap of a specific value and there are more of the same, ALL get changed. I don't recommend shotgun changing of caps  because of potential board damage but if you're good, it is the best way performance wise.

I use a Metcal soldering iron - not cheap but truly first rate. Everything else is a 'soldering tusk'.


Re: Diagnosing Preamp Turn-on Thump

Reply #15
I use a Metcal soldering iron - not cheap but truly first rate. Everything else is a 'soldering tusk'.

Do you mean to imply that without a ca. $1K soldering iron, it is risky or unwise to try to fix through-plated boards?

Re: Diagnosing Preamp Turn-on Thump

Reply #16

Do you mean to imply that without a ca. $1K soldering iron, it is risky or unwise to try to fix through-plated boards?


No but it certainly improves the odds of success. I had a 23 year old monitor that the pads on the single sided board fell off the board with my normal 700 degree tip but a 600 degree tip was OK. They also have 500 degree tips.

In a Metcal the heating element IS the thermostat and will not overshoot the temperature setting like most others will, You can dunk a hot Metcal tip in a glass of water and it will be back at temp in 6 seconds.

 My Metcals ( I have 2) were bought used on eBay and can be had in the low $100s for a Thermaltronics wand and Metcal power unit. For best price buy the power unit and wand separate;y. Tips should only be bought new whether on eBay or Mouser.

If you can't solder properly with a Metcal, get a new hobby. My daughter in 6th grade had no problems at all.


Re: Diagnosing Preamp Turn-on Thump

Reply #17
OP here.  First off, the Athena is 28 years old, not 18.  Oops.  Maybe that strengthens the argument for recapping all the electrolytics.  Second, I finally got to see the backside of the PCB and everything is clean, except for the mute relay.  There's some kind of gunk on the underside of the board directly under the relay, although the relay looks fine from the top.  I didn't think the relay was filled with anything, but there's no where else the (not resin) gunk could have come from.  Anyway, the relays are still available so that'll get replaced, too.

I use a Weller WESD51.  I like it fine.

Re: Diagnosing Preamp Turn-on Thump

Reply #18
OP here.  As I am searching for replacement caps, I'm finding that many of them are only rated for 1k or 2k hours of service.  I'm talking mostly about Panasonic FC or Fm and Nichicon PW and HE series.  (And I can't even find a lot of the ones I need in those "audio" series.)  Not wanting to worry about making this modification again for a long time, the short lifetime is a concern.  Any thoughts on this?  I'm working through Digikey so far; am I missing another source with better options?

Re: Diagnosing Preamp Turn-on Thump

Reply #19
OP here.  As I am searching for replacement caps, I'm finding that many of them are only rated for 1k or 2k hours of service.  I'm talking mostly about Panasonic FC or Fm and Nichicon PW and HE series.  (And I can't even find a lot of the ones I need in those "audio" series.)  Not wanting to worry about making this modification again for a long time, the short lifetime is a concern.  Any thoughts on this?  I'm working through Digikey so far; am I missing another source with better options?

Generally, the life spec is for the specified maximum temperature which eyba between 85 and 105 degrees centigrade. This is much higher than the 45 C actual likely temperature I'd expect.  Cap life goes down exponentially with temperature rise, so much longer actual life is to be expected.

https://www.advanced-energy.com/application_notes/ap22.pdf

" The expected lifespan will double for every ten-degree drop in temperature." 

So a 85 degree rated cap's life will double 4 times at 45 degrees or 16 times longer.  A 105 degree cap can be expected to last two more doublings or 64 times longer.

Re: Diagnosing Preamp Turn-on Thump

Reply #20
That solves that concern.  Thank you.

Re: Diagnosing Preamp Turn-on Thump

Reply #21
To put the finishing touches on this thread, I finished recapping the entire preamp last night and now everything works fine, including the muting circuit - the turn on/off thump is now gone.  In the muting circuit, I replaced the relay (a booger to find) because there was some corrosion underneath it, along with two electrolytics also in the mute circuit, one of which has a slight amount of leakage.  Overall, I'd say that about 70 percent of the electrolytics had leaked at least a little.  However, they all measured within spec (at least for capacitance).  Nevertheless, I'm glad I decided to replace them all.

I used primarily Panasonic FM's for the polarized caps and Nichicon ES's for the non-polars.  The originals were all Rubycons and despite a small amount of leakage from most of them (only 28 years old!), I consider them to have held up pretty well.

Thanks for all the help and advice.

Re: Diagnosing Preamp Turn-on Thump

Reply #22
Glad to hear that it all worked out well :)
The most important audio cables are the ones in the brain