minor hum problem

[ubza1234]

I recently installed a prefab valve preamp in my car. sounds amazing, however there is a little hum. it's not that big a deal because it's so quiet you can't hear it once the engine has started, but in a perfect world, it wouldn't be there.

I've tried what I see a the obvious stuff. the power supply transformer is in a steel box, with the steel box earthed to the car chassis. if I disconnect the earth wire, the hum is loud so that seems to be working well.

I'm also using a pure sine wave inverter.

I tried earthing various spots to the chassis e.g the ground wire of the pre amp itself, the chassis of the inverter, earthing them separately, together.. most things just made it worse.

without knowing the full ins and outs of the amp (because I different design it, it was prefab) are there any other things I can check or try?

I may also add, when I first turn on the unit, as it heats up, this loud woosh and hum sound plays through the speakers and dies back to barely anything. it may or may not be useful in solutions.

thanks allot guys!

 

[gootee]

If the wire pairs to or from the transformer have any gap between the wires, they will transmit hum fields into the air. Same with the wires to and from the rectifier(s). Then if the signal input/ground pairs have any gaps between their signal and ground wires, they will receive the hum like an antenna. So you should make sure that all of those wire pairs (and all other wire pairs) are tightly twisted, all the way to each end.

 

[ubza1234]

that's easy enough. just to clarify, my transformer has a centre tapped winding (3 wires) and a second winding with 2 wires. do I have to twist each winding separately or can I twist them into one big cable? because the later would be neater, but I'm not sure if that would cause problems?
cheers

 

[loudnoise]

Before doing any modification, why dont you try powering the amp with the home electricity? Just run a long cable from your garage to your car and test if the hum is still there or not?
On other hand, most of tube amp, especialy single-ended amp, alway have very ligh hum but can not be heard far from more than 1 meter. When using in car, we sit right next to speakers so....

 

[ubza1234]

I have twisted the power wires. the audio signal in and out is feed through shielded audio cable shop I can't twist those but they should be fine. twisting the power leads didn't do any difference to hum.

I can't really power it from mains power as i have wired it directly to the internals of the inverter,and I cannot easily remove the inverter. when I was testing it, I used mains power and there was still hum.

it sounds like perhaps it is just the design of this amp to produce a little hum and hiss.

 

[trobbins]

Have you shorted the signal inputs to their socket grounds (close direct short so no external wiring from the amp chassis, and no connection to any other ground)?

Does the amp have any volume or tone controls? If so, then were they an influence?

 

[ubza1234]

I haven't shorted the signal ground to the circuit ground. I have tried shorting the chassis ground to various spots including both input and then output (separately). no difference.

the volume control is connected to leads, which are shielded to ground. could this be a problem? even though the shielding is grounded?

also, the board is suspended above a copper board. would this influence anything? I'll upload a picture in a sec..

 

[ubza1234]

You can see in this pic what I did. The board is suspended above the copper plate using silicon. That is a large flat sheet of copper that covers the entire under side of the board and then has springs for shock absorption soldered from it to the lower chassis. The silicon holds the circuit board roughly 5mm from copper plate so there is no risk of shorts.

I tried grounding this copper plate to the car chassis but that made things much worse.

 

[trobbins]

Good backyard effort at the suspension - did you take it to Peddars to get a free test!

If possible, you should try some testing that shorts the signal ground at the pcb. Eg, remove input signal cabling and short the connection points at the pcb. If appropriate (check schematic), the external volume pot wiper could also be disconnected and shorted at the pcb. The aim is to reduce any possible influence of external grounding, and then to compare with your setup which may have external signal grounding issues.

 

[ubza1234]

the entire car is one of my proudest creations thus far! I think pedars would be impressed, it barely continues to bounce, after going over a bump. so pretty to watch it while the car is driving.. glowing.. bouncing.. eargasm giving..

I'm a little confused what you mean by short. so you mean disconnect various ground points like input output volume etc one at a time and short that to the pcb ground instead of the original ground?

 

[trobbins]

Sorry for ambiguity. No, short means to connect the signal input line to the ground line at the terminals/pads on the pcb that are used to go to say a remote rca socket via shielded cable. Similarly, disconnect the wires going to the vol pot (after checking circuit to see if that is ok), and short the wiper signal terminal to the ground terminal (it's likely that the pot resistor doesn't need to actually be in circuit, as it would be an ac coupled loading on the prior stage, and this test is all about checking if hum is in the following stages heading towards the speaker).

Ciao, Tim

 

[ubza1234]

ah! I see! genius. I'll try that when I get home from work.

it's not going to damage the amp shorting the output?

the volume pot uses the ground, its connected to one side of the pot contacts. being a ganged pot, I assume it is actually just varying between the audio signal and ground. nothing fancy with ac. but I could be wrong because I can't get my hands on the schematics

 

[trobbins]

The aim is to short to 0V the signal input of a stage (high impedance part of circuit - most likely the grid of valve which would only have say a 1Meg resistor to 0V in the circuitry).

Shorting an output is not the aim!

 

[ubza1234]

OK i tried shorting all the input wires and there was absolutely no difference. i connected a test speaker up to the output and input of the amp. the sound output was quieter then the input, i assume due to impedance matching or something. i feel like the noise is somewhere inside the actual circuit..

i also connected the speaker up to various spots on the volume control. ground and wiper produced sound at about the same volume as the input. also, with the volume turned up all the way up, if i touch the wiper tab with a screw driver, and then touch the metal of the screw driver, there is a whole lot of noise through one speaker, if i touch the other wiper contact the other speaker makes noise. with the volume turned up all the way up, if i touch the signal wire of the input the same noise comes through. it sounds a little like noise mixed with a super distorted 50Hz hum..

dang..

 

[gootee]

Going back to your first post, it seems clear that if you use a larger-diameter wire to "earth" the steel transformer box, then it should diminish the hum, at least by some amount.

What about the wires going from the transformer to the amp? How are they configured?

 

[ubza1234]

currently, the transformer is in its steel box sitting about 6inchs underneath the amp. 8inch wires run from the various windings that are now twisted to in groups (heater winding is twisted together and the main center tapped power wire is twisted together.) is that what you asked?

i will try adding another earth wire and see if that makes any difference later today.

could the wooshing and hiss be caused by the interference or is this something else?

 

[trobbins]

We're all flying a bit blind here, because there is no schematic diagram of all the connections and external or pcb parts.

There is also some doubt about your testing approach - especially when you describe that you are connecting a speaker to various parts of the circuitry - the speaker is connected only to the output terminals of the pcb (?!?) normally. A schematic works really well to understand where you are connecting test items.

 

[ubza1234]

sorry, i do not have access to the schematics. i understand there that isnt much good but theres not much i can do. i have emailed another company that sells the same PCB to see if they can supply the diagram.

adding the second earth cable has basically fixed the hum! thanks alot gootee!! there is still a little hiss, but i before i ask to help about fixing that, but i wont ask about that untill i get my hands on the schematics.

thanks again guys!

 

[trobbins]

A 'schematic' of amp pcb 'as a black box with terminals' is very important as it identifies all the electrical connections you are making to the pcb. Hiss and hum will most likely be a result of grounding loops involving your chassis connection and other external parts (signal source such as cd player, and speaker connection if not floating, and pots that may inadvertently connect to chassis), or unscreened signal loops.

A sketch is all that is really needed, but it must accurately show the wiring.

 

[ubza1234]

I tried emailing them about how to source the schematics but they said they don't have it.

this may not be of any use to you, but this is the board. N4-X4-Maratz-7-Tube-Preamplifier-Board

I suppose photos are of no use? you guys have been more then helpful, and until I can get my hands on the schematics I don't want to waste your time, as I understand how futile answers to my questions can be without them.

thanks again