Tube preamp huge DC thump!

[Andypairo]

Hello,
I'm trying to fix a friend's tube preamp, a AMC model CVT1030a.
The problem is that, at turn on and off, if outputs a huge DC thump, you can see the woofer going full run on both sides (in and out).
I learnt how dangerous can it be testing it with a test speaker.. the woofer got fried after 2 times I switched it....
Since I'm new to tubes I have to ask your help...
I was able to find a schematic of the CVT1030, that matches quite well.
I suspect the capacitor C209, labelled as NP 22uF / 64V, that in this amp has a different value (220 uF / 16V) and is polarized.

Can anybody help me?


Cheers

Andrea

 

[adamamp]

Just guessing but is it possible that the regulated SS power supply induces this effect? I have added B+ regulated power supplies to some classic HiFi, HH Scott, Trio and Eico. In each case there was a need for a slow charge(adding low ohm resistance to AC primary, more on that later if required) to avoid disturbing turn on interaction. I love the sonic effect of regulating a tube amp but using tubes and SS in conjuction can have its problems mostly related to power up. There could be another prob with the amp that you have though. Simple fix, turn power amp on after preamp and turn it off before preamp, a good practice anyway. IMO, user results may vary.

 

[pedroskova]

you can avoid this just by turn on/off sequence:

turn on: source -> preamp -> amp

turn off: amp -> preamp -> source

 

[djmiddelkoop]

well, besides that I don't see why the designer should use a 220uF cap here, or even a 22uF, where a 2.2uF would do very well, I suspect he saw the problem and tried to fix it with the 2 zener diodes used as a clamp.

First I would suggest is the switch on/off cycle described by Pedroskova, than toggle the switch from N to M or D, and replace the mentioned cap by a 2.2uF cap, preferable a film type rather then electrolytic. This last action will at least reduce the amount of the thump.

Dick.

 

[fdegrove]

Hi,

well, besides that I don't see why the designer should use a 220uF cap here, or even a 22uF, where a 2.2uF would do very well,

Seems to me they were offered every possible cap that had two 2s in its name at a bargain price...

2.2µF into 1M? They're nuts and it's bad design practice to boot.

Cheers,

 

[analog_sa]

Not sure if i understand. There seems to be a muting function in order to prevent thumps. Is it still working? Is it manual or automatic?
The 16v rating appears to be incredibly low, even the original cap seems underrated. Is it possible the output caps have developed a leak? I'd immediately replace them with 2.2-4.7uF/250v films.

And i just noticed there is bipolar supply. Not sure what happens if one of the polarities ramps up first. A delayed mute seems a must.

 

[Andypairo]

Hi,



Seems to me they were offered every possible cap that had two 2s in its name at a bargain price...

2.2µF into 1M? They're nuts and it's bad design practice to boot.

Cheers,

Can you elaborate a bit.. not sure I understand

Not sure if i understand. There seems to be a muting function in order to prevent thumps. Is it still working? Is it manual or automatic?
The 16v rating appears to be incredibly low, even the original cap seems underrated. Is it possible the output caps have developed a leak? I'd immediately replace them with 2.2-4.7uF/250v films.

And i just noticed there is bipolar supply. Not sure what happens if one of the polarities ramps up first. A delayed mute seems a must.

The only "muting" function available is to switch manually to the Phono function (the other 2 positions of this switch are "direct" and "normal", that enables control tones).. but I can't believe they can sell a preamp that can damage the speakers so easily.
The fact that this problem is getting worse with time seems to me a project's fault that slowly damages a component (maybe this cap?)

Cheers Andrea

PS I put on my net disk (www button) a scan of the service manual. Being in Tif format you need to download it to view.
If resolution is not enough let me know and I'll send you a higher-res pic.

 

[Burnedfingers]

At the risk of sounding harsh why not just turn the darn power amp off first? No more thump and no more lost sleep.

Joe

 

[Burnedfingers]

Whoops!

Sorry pedroskova didn't see your post. You are 100% correct.

Why is it that I suspect the usage of a solid state amp here?

Why not install a switch to short the output to ground until the amplifier is up and running? Maybe if you want to get fancier a timed relay circuit to mute the output for an acceptable time period upon turn on and an immediate drop out on stut down.

Joe

 

[Andypairo]

Whoops!

Sorry pedroskova didn't see your post. You are 100% correct.

Why is it that I suspect the usage of a solid state amp here?

Why not install a switch to short the output to ground until the amplifier is up and running? Maybe if you want to get fancier a timed relay circuit to mute the output for an acceptable time period upon turn on and an immediate drop out on stut down.

Joe

The switch is already present, if I set it to "phono" the thump doesn't occour.
But since this problem is getting worse (and at the beginning it didn't even happen) I suspect something's going to fail soon, so it's not only a switching sequence problem but something different.

Cheers

Andrea

 

[Andypairo]

Yesterday I tried replacing the "NP" caps with film types, 10 uF MKT, but things didn't change.

What else could cause this?

Remember that this problem hasn't always been there so there must be something wrong!

On the suspect list I had the el-cheapo volume pot, really small, but, apart from a bad tracking at low volumes, it doesn't show other problems...

Please help me...

Cheers

Andrea

 

[Cassiel]

Hi, I have this same problem. What's the cause of this? Can anybody guess? Six seconds after turn on it appears this DC 'surge' (around 1.5VDC) on my preamp outputs. It suddenly goes up and then disappears and it also happens when I turn it off. It's an Aikido preamp.

I did find out when I connected it to a chip amp with no input caps and now my speakers have departed (oh I'm not sure) to a better world. I also have a F5, can this surge kill speakers with that amp too? I'm running out of speakers.

 

[cjkpkg]

Hi, I have this same problem. What's the cause of this? Can anybody guess? Six seconds after turn on it appears this DC 'surge' (around 1.5VDC) on my preamp outputs. It suddenly goes up and then disappears and it also happens when I turn it off. It's an Aikido preamp.

I did find out when I connected it to a chip amp with no input caps and now my speakers have departed (oh I'm not sure) to a better world. I also have a F5, can this surge kill speakers with that amp too? I'm running out of speakers.

Yes it will...

My 2 aikido amps do the same thing. Well at least one of them do...the reason I say that is that I recently changed the psu to one of them to a tube regulated circuit. I have not tested the behavior since. I have gotten in the habit of always turning on the aikido a few seconds before the power amp.

But I think a lot of it had to do with the b+ being solid state so the plates were getting hit with a good dose of DC before the heaters were fully doing their job. Even with good quality coupling caps some DC was making it to the power amp and eventually to the speakers.

 

[Cassiel]

Thanks for letting me know.

I have gotten in the habit of always turning on the aikido a few seconds before the power amp.

Yeah, that solves the problem but it's still a big risk. One mistake and goodbye speakers. My dead speakers were mini monitors, computer speakers, they sounded pretty good and I'm kinda sad but when I think it could have happened to some other speakers (7.000 euros out of the window ) I feel kinda happy about my bad/good luck.

I don't know, a relay circuit maybe, I really don't want to do that though. My power supply is SS and I don't want to change that either. I can't believe this is normal behavior in the Aikido preamp.

 

[cjkpkg]

I have relay protection on a SS amp I use and the relay opens with more than 5vdc IIRC...the aikido will trip the relay on a cold start. I am running 1uF bypassed by 0.33uF Russian pio.

I will have to measure the dc sometime.

 

[Cassiel]

Mine is a 2uF cap. I have just checked again and with nothing connected, no inputs and outputs, it goes up to 15VDC. Whoa. Could I be getting false readings? I have used this preamp with headphones, and they're still alive. It is always after six seconds, always on time. Swiss precision.

 

[DF96]

All circuits have switch-on and switch-off transients. The modern obsession with LF response down to earthquake-like frequencies makes things worse as the large coupling caps take a while to charge up. Many modern circuits have a lower LF point than the PSU which feeds them so they amplify any DC shifts.

Two solutions:
1. design the circuit to have a sensible LF rolloff i.e. smaller coupling caps.
2. have a reliable mute circuit.

 

[Burnedfingers]

This thread has been around for years and people have been having the same problem when trying to use a tube linestage or preamp with a SS amplifier. When you turn on the tube line stage or preamp it has to have time to become stable and once it has THEN turn on your SS amplifier and there will be no thump. Before the tubes warm up it is normal to get some type of voltage ramp in my experience.

You have your choice...either wait before turning on your SS power amp or re-work the delay time on your SS amplifier relay circuit or the possibility of adding caps in the circuit to block the DC which is probably present upon turn on from your tube preamp/linestage.

 

[cjkpkg]

I am using these on my SS amp with the Aikido and they work great - even when powered with DC.

 

[wicked1]

I recently hooked my aikidi pre up to a powerful amp, and notice something similar.. when I get a spike on my mains (heater turning on somewhere in the house, etc), the output of the aikido gets a large slowly alternating voltage that moves my speaker cones all the way in and out. Never noticed it on less powerful amps, but it could definitely damage my speakers w/ this amp!

I haven't started looking in to it yet.. My first guess is the ps noise injecting cap.. I'll make it a lot smaller. I'll start a new thread when I dig in, but I'm glad to hear I'm not the only one w/ odd aikido issues.