Anyone have the same issue? Any takers? I built a Tubes4Hifi SP14 preamp with the Khozmo digital controller about a year ago. I paid in my opinion a lot of money for this thing including extras for upgraded parts. Since I built it I’ve been battling a slight hum and buzz. So fricking annoyed at this point. Over the past 11 months, I’d work on it, get frustrated and come back to it a couple months later and here we are today. I’ve worked on and off with Roy from T4HF quite a bit in the beginning and have tried just about everything. I’ve changed tubes which made the noise more tolerable but it’s not right. The SP14 feeds a pair of M-125 monoblocks from VTA Bob Latino that I also built. I don’t feel the issue is with the monoblocks because they are pretty much totally silent when connected to my Bluesound Node 2 streamer. There is something isolated to this preamp that doesn’t play well with the monoblocks. Over the weekend I brought the SP14 down and tried it on another system which uses a Jolida JD1000 integrated amp which could be used as a straight amp in bypass mode. The SP14 preamp was pretty quiet. A quick description of the noise sounds like a guitar amp which the gain turn up. Would it be best to send the SP14 back to Roy and pay him to figure it out but I’m afraid he may not see the issue on his end and now I’d be throwing even more money at it. Any thoughts on where I can be picking up this noise and what can be done about it. Thank you in advance
Are the amp's chassies properly interconnected ?
Should be if all are using 3-wire mains and interconnected in a grounded
outlet. If not, try to connect them all with a groundwire connected to a
suitable screw on all 3 amps ( and all other stuff connected together)
Another thing, is there a cable-tv and is this cable or set connected to
this cable in turn connected to the amp ?
with the amps ?
Are the power amplifiers quiet when no input cables (or shorting plugs) are connected?
Is there noise from the power amplifiers when you connect a single mono RCA cable
between the two power amplifier inputs? It's doubtful that the preamp is the only problem here.
Is there normally any kind of connection between the power amplifiers
other than the RCA cables from the preamp (headphone jack, etc.)?
Try plugging the amplifiers' AC cords into the same physical outlet or power strip.
Is there noise from the power amplifiers when you connect a single mono RCA cable
between the two power amplifier inputs? It's doubtful that the preamp is the only problem here.
Is there normally any kind of connection between the power amplifiers
other than the RCA cables from the preamp (headphone jack, etc.)?
Try plugging the amplifiers' AC cords into the same physical outlet or power strip.
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@petertub, I have tried connecting a ground wire amongst the 3 pieces with no success. All of the pieces are connected with high quality 3 prong connectors. The wall outlet is a Hubbell audiophile grade 20 outlet.
@rayma, when I pulled the rca’s out of the preamp, the amps hum but when I short the connectors (center pin to ground) the amps are totally silent. I don’t believe that signals trouble with the amps. Also, when the amps are connected directly to the streamer, they are both silent.
@rayma, when I pulled the rca’s out of the preamp, the amps hum but when I short the connectors (center pin to ground) the amps are totally silent. I don’t believe that signals trouble with the amps. Also, when the amps are connected directly to the streamer, they are both silent.
Did a licensed electrician install the Hubbell outlet? It could be wired wrong.
The new AC plugs could be wired wrong. Or both. Is the preamp's power cord
plugged into the same outlet as the amplifiers' power cords are?
If the preamp is connected to the amps, but with no inputs plugged into the preamp, is it quiet then?
The fact that the preamp was ok in a different system says a lot. It's a system problem.
Just because the noise started when a new preamp was connected, does not prove the preamp is at fault.
The new AC plugs could be wired wrong. Or both. Is the preamp's power cord
plugged into the same outlet as the amplifiers' power cords are?
If the preamp is connected to the amps, but with no inputs plugged into the preamp, is it quiet then?
The fact that the preamp was ok in a different system says a lot. It's a system problem.
Just because the noise started when a new preamp was connected, does not prove the preamp is at fault.
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No, I installed the outlet. I’ve been in the trades for over 30 years. I’m more than qualified to change the outlet.
Yes the preamp is plugged into the same outlet as the amps. There are other outlets in the room but they are all the same circuit. I’d have no way to pull a dedicated circuit up to that space.
If the preamp is connected to the amps by itself, (nothing plugged into it, no other source inputs) there is the noise issue.
Yes the preamp is plugged into the same outlet as the amps. There are other outlets in the room but they are all the same circuit. I’d have no way to pull a dedicated circuit up to that space.
If the preamp is connected to the amps by itself, (nothing plugged into it, no other source inputs) there is the noise issue.
Is the Jolida JD1000 integrated amp's preamp output available to test-connect to your monoblocks? Could be something else to try for comparison purposes.
When you say 'guitar amp turned up', do you mean the hiss or is there hum, too?
What provides the power for the Bluesound Node 2 streamer? Battery by chance?
When you say 'guitar amp turned up', do you mean the hiss or is there hum, too?
What provides the power for the Bluesound Node 2 streamer? Battery by chance?
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Are you sure that the house wiring is correct as well? Are you certain that all the AC wiring
involved with the system is correct? I've seen entire houses wired wrong. Have you tried using
cheater plugs (only do this for testing) and reversing the plugs systematically to minimize the hum?
Have you tried using a DVM between each chassis and utility earth (with no audio cables attached)
to check for AC voltage on each component's chassis?
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I have a similar problem with an SP14
I built my SP14 about 2 years ago.
If use the SP14 with a medium high gain power amp(Erhard audio ST70), and play thru high sensitivity speakers (~97db) there is an annoying background hum - sounds like 120hz.
If I use the SP14 with a low gain amp (single ended 45, 2A3, or 300B) the hum is inaudible unless my ear is in the speaker cone.
If hook an oscilloscope to either SP14 output jack I can see a very small (.8mv) waveform with no signal running thru. The wave resembles the top half of a truncated sine wave. When I hook my multimeter to the speaker terminals I get a reading between 4 and 5 mv with the same wave shape which is audible and annoying.
I spent some time reviewing and attempting to debug the issue with Roy Mottram. I concluded that the SP14 was bad match with the high efficiency speakers and the gain of the ST70.
I really like the sound of the SP14 when I use the low gain amps, and that's the way I use it.
When I choose to use the ST70, I use another preamp which produces a tolerable level of hum thru the speakers.
If you have access to an oscilloscope, I'd be interested in seeing what you see at the output of your SP14. I found it necessary to use the scope to be able to see and measure very small (.8mv) signal at the output.
I built my SP14 about 2 years ago.
If use the SP14 with a medium high gain power amp(Erhard audio ST70), and play thru high sensitivity speakers (~97db) there is an annoying background hum - sounds like 120hz.
If I use the SP14 with a low gain amp (single ended 45, 2A3, or 300B) the hum is inaudible unless my ear is in the speaker cone.
If hook an oscilloscope to either SP14 output jack I can see a very small (.8mv) waveform with no signal running thru. The wave resembles the top half of a truncated sine wave. When I hook my multimeter to the speaker terminals I get a reading between 4 and 5 mv with the same wave shape which is audible and annoying.
I spent some time reviewing and attempting to debug the issue with Roy Mottram. I concluded that the SP14 was bad match with the high efficiency speakers and the gain of the ST70.
I really like the sound of the SP14 when I use the low gain amps, and that's the way I use it.
When I choose to use the ST70, I use another preamp which produces a tolerable level of hum thru the speakers.
If you have access to an oscilloscope, I'd be interested in seeing what you see at the output of your SP14. I found it necessary to use the scope to be able to see and measure very small (.8mv) signal at the output.
@Rick PA Stadel- unfortunately I can go into bypass mode for the power amp but can’t be used as a pre only. It’s a thought though, I have a McIntosh MX-130 I can bring up and try that on the monoblocks. Good thought, I’ll try that tomorrow night. As for sounding like Guitar amp, no more hiss than normal which is quite low. It’s almost like buzzing sound out of the tweeters, buzz & hum out the midrange and hum out of the woofers.
@rayma- The cheater plug you mention, are you referring to the 3 to 2 plug ground adaptors? If so, yes I tried that earlier tonight with no benefit but I didn’t flip the plug over. I will also try that tomorrow night.
@rayma- The cheater plug you mention, are you referring to the 3 to 2 plug ground adaptors? If so, yes I tried that earlier tonight with no benefit but I didn’t flip the plug over. I will also try that tomorrow night.
The only reason to ever use one is for testing by reversing the plug.
The safety grounds must always remain connected. Never float the chassis,
that can be deadly.
See page 40 here:
https://web.mit.edu/jhawk/tmp/p/EST016_Ground_Loops_handout.pdf
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Good observation!
Do you have noise with volume turned down ? Does it change by switching
input ? Does it change by changing volume ?
Will the noise change if you disconnect the protective ground ( cheater plug ) ?
And what is the noise anyway? Do you have access to a scope to show the noise ?
The volume control has no effect. This thing buzzes the same amount at zero as half way turned up. The input selector has NO effect. The same input 1-4. I tried the cheater plug for a minute. When I saw it made no difference, I removed it and plugged back in without it. I don’t know what kind of noise it is. I don’t have a scope. I suppose I could possibly look for a cheap used one and purchase it. But then I’d have to learn how to use it as well. That may be a stretch.
Thanx.
It seems to be the B+ ( or at least the outermost stage) that generates noise.
At lest grounding issues are eliminated. It would be interesting to measure
internals. Do you got schematics with the unit ?
The issue likely the same as I had with VTA built M-125s.
See teaching the VTA M-125 to sing
If you use a Y cord with a shorting plug or just plug 1 RCA cable into both amps, they will likely hum due to the fact that the M-125 has a 9 point ground tied to Earth Safety.
I quietened my M-125s by nearly 40db. I now have to look at the tubes to see if the amps are on.
See teaching the VTA M-125 to sing
If you use a Y cord with a shorting plug or just plug 1 RCA cable into both amps, they will likely hum due to the fact that the M-125 has a 9 point ground tied to Earth Safety.
I quietened my M-125s by nearly 40db. I now have to look at the tubes to see if the amps are on.
Sound Character
More information of how the system is connected and what components could help
Buzzes are usually from an external device like a lamp dimmer or possibly Switch Mode Power Supply [SMPS].
Plug in amp only with input shorting plug.
If not quiet, problem is not SP14.
Try a different circuit. Kill lamp dimmers, computers, etc.
If quiet, connect pre amp only with line input w shorting plug
If quiet, add devices until noise appears.
More information of how the system is connected and what components could help
Buzzes are usually from an external device like a lamp dimmer or possibly Switch Mode Power Supply [SMPS].
Plug in amp
If not quiet, problem is not SP14.
Try a different circuit. Kill lamp dimmers, computers, etc.
If quiet, connect pre amp only with line input w shorting plug
If quiet, add devices until noise appears.
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