Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Smash => Topic started by: Rublyow on November 11, 2015, 05:07:18 AM
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I just recently completed this build and, oh man, is something not right. With the volume all the way down, there is a loud hum, a bit of buzz, and crazy microphonics -- to the point that it starts to feedback through the speakers and get louder and louder. Also when I turn the volume knob, there is a loud scratchy sound. But when playing music, things generally sound ok (despite the noise in the background), however, even when the volume knob is totally maxed out the listening volume never gets above moderate levels.
I've already looked over all of the wiring and everything visually looks ok, but I'm going to go back and re-solder anything that looks even mildly suspicious. All of the measurements checked out fine, though most of the terminals marked with an asterisk gave a resistance of "1" on my ohmmeter (but not a zero reading) with no fluctuations. All of the voltage readings were within spec, though the readings at terminals 4, 9, 20 were a bit high (160 vdc rather than 150). Is there anything else I should be looking for while re-soldering?
I've read elsewhere that hum/microphonics can be an issue with high gain and high sensitivity speakers. My speakers are DIY, about 98 db sensitivity, and I'm running though the power amplifier stage of an old integrated Onkyo A10 amp, at 85 watts at 8 ohms, 50 kohms input impedance. Any thoughts?
Thanks!
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Does the 0D3 give you a nice, purple glow?
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It does -- the glow is quite striking compared to most tubes. In general, all of the tubes and LEDs light up.
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Are you running the amp/speakers in your signature? If you had something like 105dB sensitive horns on a 1500 Watt amplifier, there's a strategy to dealing with that.
If not, I would be suspicious of flaky solder joints in the preamp.
-PB
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I'm just running a cd line-level source through the Smash, then into the Onkyo A10 power amp stage, and out of Tempest Fusion speakers. It's a lot less than 105 db horns and 1500 watt.
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Your speakers and the amplifier section of the A10 being driven by the Smash should get to uncomfortable listening levels quite easily.
Based on that piece of information, along with having hum in the circuit, I would be looking for poor ground connections.
-PB
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The spec I found online was 0.15v input sensitivity for the Onkyo A10 amp. That's very sensitive, considering you are putting 2.00 volts into the preamp. Unfortunately it did not specify if that was for full output or some other measure.
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So I went back and touched up all of the joints. It's a bit of an improvement -- the microphonics are down to normal levels, but the hum is still quite loud (my guess would be 120 hz). When I hook up the Eros phono the volume is great (it gets to uncomfortable levels at about 3 out of 10), though with the line-level CD player it's still is pretty weak even at max volumes, but maybe this is just an issue with the gain of the cd player. I've checked and rechecked for any mis-wiring and can't find anything.
One thing I did notic on recheck, the A4 and C4 terminals are supposed to measure 1.57 volts but are actually measuring 3.68 volts each. Is this within normal range, or does this indicate an issue somewhere?
Paul -- I've attached the spec sheet for the Onkyo A-10 amp. It has the input voltage as 1.5 volts. Is this the same thing as input sensitivity?
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Your CD player is not putting out a line level signal. Does it have its own volume control? Have you used it before alongside the Eros with another preamp? Were the levels comparable?
Is the hum a problem when you use the Eros, or is it just on the CD?
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The hum problem exists regardless of what's plugged in, and doesn't change when I adjust the volume, or even if I have a source plugged into the Smash at all. As long as the Smash is on and connected to the amp, there's a hum. I tried switching wall outlets, but no change.
The CD player is just a crappy old discman, but I just tried plugging in my ipod and it's the same general volume, though maybe a tad louder. I use this setup 99% of the time for vinyl, so I don't really have any other sources to plug in....
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...Paul -- I've attached the spec sheet for the Onkyo A-10 amp. It has the input voltage as 1.5 volts. Is this the same thing as input sensitivity?
Thanks, that's better information. The source I found listed only the preamp sensitivity, not the power amp section sensitivity.
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One thing I did notic on recheck, the A4 and C4 terminals are supposed to measure 1.57 volts but are actually measuring 3.68 volts each. Is this within normal range, or does this indicate an issue somewhere?
That's very, very abnormal. This still tends to point to a very flaky ground connection, or a missing ground connection all together. I also believe that your noise could be a loose 220uF/250V power supply capacitor. With the amplifier off, they should feel nice and tight if you wiggle them.
The 3.68V measurement is enough to say that the preamp has not passed its voltage checks and should not be used. Can you recheck all your other voltages to see what you have? I am guessing there is at least one other voltage for each channel that is way off.
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Alright, went back again, rechecked all of the wirings step-by-step, and touched all of the joints up again. I rechecked the voltages, and I made a mistake reporting the previous voltages -- the 3.68 vdc is actually on the A7 and C7 terminals. The A4, C4, A8, C8 all read about 1.6. When I turn up the precision on the ohmmeter, terminals A6 and C6 read -0.22 and -0.1 rather than exactly zero, but that was the largest deviation I could find from the specified voltages. The full readings are below.
I just fired it up again, and no change at all to the hum. One thing I noticed, with no source plugged in, the hum is loudest when the volume is all the way down, and drops to almost bearable levels when the volume pot is at about 50% volume, then rises again when the volume pot is at 100%. Not sure if this helps diagnose the issue at all.
Also, I double and triple checked the 220uf/250V capacitors and they are definitely stuck on solid.
So how else can I trouble shoot this problem? Is there any way to verify it's not an issue with one of the tube, or is there any other way to diagnose where/if there is a flaky ground connection other than continually re-solding the joints?
Not that I image it's terribly helpful, but I've also attached a couple of images in case there are any glaring errors that I keep missing.
Thanks!
terminal VDC
1 0
2 0
3 0
4 152
6 0
7 0
9 153
10 0
11 0
12 0
13 0
15 49
16 49
17 214
18 0
19 0
20 153
21 49
22 49
24 0
25 0
28 0
29 214
30 0
31 0
34 0
35 230
36 0
A2 46
A3 49
A4 1.61
A5 0
A6 -0.22
A8 1.61
C2 46
C3 49
C4 1.5
C5 0
C6 -0.1
C8 1.6
B2 0
B3 214
B5 153
B7 213
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The fact that the hum level is affected by the volume control suggests that it is being picked up by the input wiring. Since you're not shorting the inputs, it's not as conclusive, but it's pointing in a direction.
Is it the same on all inputs?
Have you tried it in another room with different equipment?
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Yeah, the hum is the same on all inputs, regardless of if I have anything plugged into them or not. I unfortunately don't have another setup to try. I did plug in my Quickie just for comparison and that is pretty much dead quiet. And for a point of reference on what the hum sounds like, if I touch a tube on the Quickie while on (or even put my hand really close to a tube) it causes basically the identical hum I get out of the Smash. I did try some -12 db attenuators and that definitely made the hum quieter, but didn't really fix the issue.
Is it worth just rewiring the inputs and the volume/balance/selector switch, or is there a better way to trouble shoot the issue?
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These observations suggest the source may be radio waves from some external device, which is picked up by the input wiring and/or the tube itself. The most common culprits for steady hum/buzz of that sort are lighting ballasts or dimmers, and computer power supplies. But many, many other sources have been identified over the years. They can be very frustrating to identify if that's the case.
The possibility of ground noise remains viable though. Here are a few random things to check if you have not done so already: Does the Onkyo power amp have a 2-wire or 3-wire power cord? Is it plugged into the same outlet or power strip as the preamp? Were the screws at terminal 6 (the signal/chassis ground connection) properly tightened? Have you re-flowed the solder on all the ground buss terminals?
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Hi Paul, thanks for the reply. I've double checked that the screws are all on tight, and I will go back and touch-up the the ground buss wiring tomorrow, maybe even just redo the wiring altogether to be safe.
The Onkyo is a 2-prong plug -- maybe this is the issue? I've tried plugging the Smash into a separate outlet, but this didn't change anything. I've also tried turning off and unplugging every light fixture and piece of electronics I can find, but this didn't help. We do have dimmer switches in the house, but they are 2 rooms away, and have no effect whether they are off or on. I also tried RCA shorting plugs on all the inputs, to no effect.
One more observation, if I leave the Smash on for a while, I notice a faint crackle every minute or so. My guess would be just a little bit of tube noise, but figured i'd mention it on the off chance it helps track things down.
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The 2-prong plug isn't going to help anything. You can run a wire from the chassis of the Smash to the chassis of the Onkyo to see if that calms things down a bit.
-PB
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I tried running a ground wire to the Only with no success. I also borrowed a power amplifier with a grounded plug from a friend. The hum is still there, but this amp has a balance pot, and if I turn down the gain on the power amp to about 10% then the hum is almost totally gone (though not completely) and I still have plenty of volume with the Smash volume control. Is this just masking the issue? Or is it indicative of what's going on?
I totally redid the ground buss wiring, and otherwise I am thinking of doing a total redo of the volume and balance pots, and maybe the 220uf capacitors and all associated connections in case something is still lose. But I guess at this point I'm wondering if this is just a fool's errand, and the hum is most likely external in nature?
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I tried running a ground wire to the Only with no success. I also borrowed a power amplifier with a grounded plug from a friend. The hum is still there, but this amp has a balance pot, and if I turn down the gain on the power amp to about 10% then the hum is almost totally gone (though not completely) and I still have plenty of volume with the Smash volume control. Is this just masking the issue? Or is it indicative of what's going on?
This is fairly indicative of a system with a lot of sensitivity. I would buy two pairs of -12dB inline attenuators to use at the input of your power amplifier, as this will give you roughly the same results on any amp you wish to use.
-PB
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Alright I'll give that a try. When eventually transitioning to an amp with much lower output (e.g., the Seductor or Stereomour) will this present a problem? That is, will I still need the attenuators to reduce the hum, and will this in turn affect the ability of the amp to drive the speakers at adequate volume?
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When eventually transitioning to an amp with much lower output (e.g., the Seductor or Stereomour) will this present a problem? That is, will I still need the attenuators to reduce the hum
What you're really after is an amplifier with lower gain. To get hundreds of Watts out of an amplifier with only a volt or so of signal at the input requires much less gain than getting four Watts out of the same volt at the input.
, and will this in turn affect the ability of the amp to drive the speakers at adequate volume?
This is a function of mostly the sensitivity of the loudspeakers in conjunction with the available power from your amplifier.
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Just an update -- I borrowed a Quicksand from a friend to see if there way any benefit of having the power amplifier be totally out of the ground loop situations. It didn't really help things, there's still a hum (though sounds a bit more buzzy through the Quicksand). Interestingly, when I listen through the headphones with the speaker wires plugged in, there is no hum in the heapdhones, but when I unplug the speaker wires from the Quicksand, the hum becomes very loud through the headphones. Maybe it's just an issue of the heaphones and speakers "splitting" some of the signal, but still a bit weird.
Anyway, still working on tracking things down. Are there any good ways to isolate if it's an issue with the Smash wiring versus an issue with the wiring in my house (i.e., a light dimmer). Any other thoughts would be appreciated. I'm starting to run out of ideas. Maybe it's time for a full rewire....
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Try a pair of these and let us know what happens:
http://www.parts-express.com/harrison-labs-12-db-rca-line-level-audio-attenuator-pair--266-244 (http://www.parts-express.com/harrison-labs-12-db-rca-line-level-audio-attenuator-pair--266-244)
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Ok, some progress. I was able to try the Quicksand with the -12 db attenuators and that basically fixed the problem. There is a tiny bit of hum when you're right next to the speakers, but nothing beyond what I'd expect from a DHT amp.
But using the attenuators did drop the max volume from the speaker by a huge mount, basically such that even at max volume it's well below "comfortably loud". I know the Quicksand isn't really meant for large speakers, even if they are efficient (98 db sensitivity), but how might I go about avoiding this issue in the future? (Sorry if this is a really uniformed question!) Should I be cautious about adding more gain to the preamp section of my rig? For example, would doing the Smash-up upgrade be ill-advised since it's going to increase gain even more? Or are there ways around this, like adding a passive attenuator between the Smash and my amp? Or, lastly, is it just an issue with what power amp I use? I was thinking about trying the Seductor next, but now I'm a bit concerned that this isn't a good match since the Seductor has no gain control.
Thanks!
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They make a 6db version, you could try that...John
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That makes sense, but still seems like a temporary patch to me. If I add more gain to the preamp stage, again, for example, with the Smash-up upgrade, or by upgrading my phono stage with more gain, then I would expect the issue to get even worse, correct? Is there a better way to deal with this issue besides jerry-rigging in-line attenuators? And what would I look for in a power amp to to determine if its likely to exacerbate the problem or make it better?
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Smash-up may improve your situation.
Try clip leading A6 and C6 to ground, then listen for noise. (Grounds are terminal 6 and 7)
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Hi Paul, I finally had a chance to try clipping A6 and C6 to ground, but no change at all to the hum. One odd thing that was a bit noticeable before but is fairly striking now, when I turn the volume knob all the way up (with no source connected) the hum totally disappears, and is loudest when the volume know is all the way down. Anyway, just one last ditch effort to diagnose things. I'm going to give it one more solder touch-up job this weekend as well.
I should also so that overall things sounds great when music is playing. It's light years ahead of my old solid state... just a bit annoying during the quiet passages and between tracks.
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Just an update. I installed the Smash-up upgrade and that definitely helped things. The hum is probably about 30% of what it was before, and is totally gone with -12 db attenuators. The extra gain also gives me a bit more headroom, so even with the attenuate I can reach decent volumes with low-level input sources. Definitely an improvement.
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Those attenuators sure are handy under the right circumstances. I'm glad you're all up and running!
-PP