hum on stand-by

[Leevi]

I noticed a low level hum coming from the speaker even when the stand-by is on.

Are there any good way to prevent that?
/Leevi

[terminalgs]

what kind of amp?  Some amps disconnect HT when switching stand-by, others are "mute" functions and simply ground or cancel signal post-preamp somewhere.  The latter leaves the power section functional, so any hum that would appear there normally, would appear there in a "mute" stand-by.

[Leevi]

It's my own project, a stereo amp based on Vox AC50. power amps.

Only the power amps and power supply has been wired. The stand-by disconnects B+ after rectifier.


/Leevi

[kagliostro]

Ahem, if you disconnect B+ how is possible you have a signal on the speaker ?

Have you put a resistor in parallel with the standby switch (as to avoid pops) ?

Or are you referring about humm coming directly from the PT ? (I've an old RCF amp that has this kind of problem, a vibration on PT laminations)

K

[HotBluePlates]

I noticed a low level hum coming from the speaker even when the stand-by is on.

Only the power amps and power supply has been wired.

Power transformer or choke radiating hum directly into output transformer(s)?

Physical vibration of transformers?

[Leevi]

Have you put a resistor in parallel with the standby switch (as to avoid pops) ?Or are you referring about humm coming directly from the PT ? (I've an old RCF amp that has this kind of problem, a vibration on PT laminations)

There is no resistors over the switch only a cap that I have removed.
The hum is not mechanical hum.

Power transformer or choke radiating hum directly into output transformer(s)?

That might be the explanation since one of the OTs and PT are placed close each other.
I have to investigate more.

Thanks
Leevi

[kagliostro]

First you can try to put a sheet of iron between the OT and PT


if this didn't give result try to unbolt the screw that fix the transformers and move it listening if something changes


K

[Leevi]

The root cause is too short distance between PT and OT.
Furthermore the PT is huge since it's meant for high power amp.


I tried to place a metal sheet between the transformers without success.
Do you think the metal sheet should be properly bolted to the chassis
in order to get some effect?


My biggest problem now is that there is not that much room to place the OT somewhere
else on the chassis. Any good ideas?


Leevi

[sluckey]

Show us a hi rez pic of the top side of the chassis.

[eleventeen]

The metal sheet really should be grounded, and very well grounded. On an experimental basis, you can use clip-leads, or you can drill a hole in the shield and run a nice fat SHORT wire to ground. If clip-leads, they should be heavy-duty. Did you use steel or aluminum for your shield? Also, are the laminations of the two trannies rotated 90 degrees relative to each other? Maybe try TWO shields between the trannies if you have room. Sometimes these things can not be fixed, but that doesn't mean you should not try. Look at a top view of a Dyna MkIII power amp. Those trannies are VERY close to each other, but rotated 90 degrees. That can make a big difference.

[Leevi]

Here the picture:


http://i218.photobucket.com/albums/cc183/kz14100/StereoAmp2_zps04d402b8.jpg


/Leevi

[Leevi]

Did you use steel or aluminum for your shield?

The is chassis is aluminium. It thought to use steel plate for isolation.


/Leevi

[eleventeen]

it's not your OT, that looks fine. it's your (unshielded) choke blasting into your preamp tubes. With an unshielded choke right next to your preamp tubes, color me pessimistic as to your ability to fix this with a shield. That's gonna be tough. Power transformer placement is also tough, BUT, even if my idea for moving your choke works, you may STILL need a shield between the PT & the preamp tubes. (and, it may not work!)


Another thing to try, very easy, disconnect one side of the choke (right where it is---this will prove that the choke is the villain--or not) and substitute a power resistor in its place. If that makes a BIG difference, that would suggest your problem is the choke placement and its radiated field. I bet you could get the choke under the chassis (a method used by MANY manufacturers of "small footprint" compact amps in the 50's-60's) mounted horizontally, roughly under the cutout near your AC entry.

[Leevi]

it's not your OT, that looks fine. it's your (unshielded) choke blasting into your preamp tubes. With an unshielded choke right next to your preamp tubes, color me pessimistic as to your ability to fix this with a shield. That's gonna be tough. Power transformer placement is also tough, BUT, even if my idea for moving your choke works, you may STILL need a shield between the PT & the preamp tubes. (and, it may not work!)

I cannot say anything about preamp since it has not been wired yet.
I have just got the both power amps (stereo) wired and thought to test them first. In the picture the problem OT has been taken out.
Furthermore the hum comes when stand-by is on i.e. the amp is not operating and no current is going through the choke.


/Leevi

[kagliostro]

If you want you can try with something like this Transformer Shield Box

http://www.enovaz.it/categorie-1572/Scatole-metalliche-per-trasformatori.aspx

K

[Leevi]

I tried one steel plate covering over the OT without success.


I'm almost sure that the problem cannot be solved with the current layout so
I'm planning to move the OT to the opposite corner.


/Leevi

[eleventeen]

I am assuming the little tube between the power tubes is the PI?


Pull that one & check.

[kagliostro]

I was considering that also big brands like Leak had PT very close to the OTs




K

[DummyLoad]

rotate the PT 90deg. it may help.


--pete

[shooter]

Is it both L&R channels or just the channel closest to the PT?

[HotBluePlates]

rotate the PT 90deg. it may help.

I had the same thought; leave the OT laying flat as it was, but try rotating 90 degrees and see if the hum improves.

[Leevi]

rotate the PT 90deg. it may help

I had the same thought; leave the OT laying flat as it was, but try rotating 90 degrees and see if the hum improves.

I thought that as well but there is not that much space since the OT is not dimensionally symmetric. I'll move the OT and I also move
the choke to the current place of the OT. Then it is not too close to the preamp tubes.

The amps like Leak and others were the reasons why I ended up to this kind of layout which didn't work in my case.


/Leevi

[DummyLoad]

I noticed a low level hum coming from the speaker even when the stand-by is on.

Are there any good way to prevent that?
/Leevi

both speakers? or just the one closest to the PT?


--pete

[Leevi]

both speakers? or just the one closest to the PT?

Only the closest, another is quiet.


/Leevi

[DummyLoad]

what is the function of the 9pin tube closest to the PT?


--pete

[Leevi]

what is the function of the 9pin tube closest to the PT?

PI


Leevi

[kagliostro]

You tried to shield the OT, did you tried to shield the choke ?


can you give a try to this method

...taken from the ESP (Elliott Sound Products) website.

"In order to reduce the radiated flux from an E-I transformer core, you will sometimes see a copper or brass band* wrapped around the winding and the outside of the core, as shown in Figure 13.4. This acts as a shorted turn to the leakage flux only, and greatly reduces magnetic interference to adjacent equipment. The band must be soldered where it overlaps to ensure a very low resistance"fig. 13.4

Source: http://sound.westhost.com/xfmr2.htm


K

[DummyLoad]

try this:

remove the PT and remove the OT use barrier strips to extend the secondaries/primaries back into the chassis

there should be no hum. if there is, it's the choke interacting with the PI or one of the other tubes adjacent to the choke. this problem is unlikely the choke, but don't rule it out.

install the PT, if there's hum then it's the PT interacting with the PI. if there's no hum, then assume the PT is interacting with the OT. relocate the PT.

--pete

[kagliostro]

Ciao Pete

Leevi report that the problem is with standby ON, can the iteraction between PT and/or choke with PI or other tubes be udible during standby ?

Franco

[DummyLoad]

Only the power amps and power supply has been wired. The stand-by disconnects B+ after rectifier.

franco - i just re-read the thread. it's seems that it is PT OT mutual inductance.

--pete

[kagliostro]

Thanks Pete

Franco

[Leevi]

Done and solved! No more hum, not even with full master volume.
I'll continue with preamp.



See the new layout:


http://i218.photobucket.com/albums/cc183/kz14100/StereoAmp3_zps7a42b830.jpg


/Leevi

[kagliostro]

Well and now that all is fine  , can you share some info about the beauty ?


Franco

[Leevi]

It's very much based on VOX AC50 (1976) normal channel. Since it is stereo there are
two amps on the same chassis. Effect loop and master added. Power amp in can be used for
line signal from external music source. Preamp can be used for guitar and mic.


I have thought to replace the original tone stack with Baxandall.


It can also be run with 6L6s by selecting higher speaker impedance.


http://www.korguk.com/voxcircuits/circuits/ac50_2.jpg


Reason why I selected VOX AC50 was its high headroom.


/Leevi

[DummyLoad]

It can also be run with 6L6s by selecting higher speaker impedance.

with a bias change?

--pete

[Leevi]

with a bias change?

Yes, the bias control has a wider range than in the original AC50.


/Leevi