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 Post subject: Re: 4S preamp project
PostPosted: 22 Jan 2019, 11:01 
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Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
The volume pot is on the input, as in the schematic above. The signal goes from the input connector to the input selector switch, to the volume pot and through the 10k resistor to the grid of the first stage.

I would like to solve the 120hz switching noise first. When it’ll be working right, I’ll be able to improve my circuit and lower the noise level. I don’t want to do modifications before I sort this issue.

One thing I don’t understand is that I am not able to see the 120hz noise with my scope. It may be because of the way I set it up when on the table. Should I "load" the output of the preamp to try to emulate the power amp and reproduce the actual conditions the preamp sees when it’s used ? I know that resistors are used on the output to test power amps, but what about preamps ?

I think that the grounding issue could be present only when connected to the power amp, but not when nothing is connected to the preamp’s output. my power amp is connected to mains by an old style cord without the third pin (never know how to call it) and my preamp has a standard IEC socket with the third (center) pin connected directly to the aluminum chassis. When the ground lift separates the signal ground and mains ground, the preamp has to take its ground through the only path available : input and output RCAs.

Could it be possible that my power amp has the left channel connected to mains ground, thus creating this noise? I guess that the enclosure must be grounded in some way, even if it does not have the third pin on its mains outlet ? If so, both grounds meet in my electrical pannel and create a big ground loop in the left channel ?


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 Post subject: Re: 4S preamp project
PostPosted: 22 Jan 2019, 19:18 
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Joined: 23 Feb 2017, 02:02
Posts: 455
Without the third pin means your power amp dosen't have any ground, generally means you can get away without humm even with poor preamp grounding.
No 3rd pin means the amp is fully lifted from ground, zero grounding to mains.

Scope to check noise is generally hard, you have to use ears to check. no resistor required for testing pre amp noise.

You cannot connect pre amp ground to case ground. It has to be a direct seperate wire that goes directly to IEC socket ground otherwise it will humm.
The case still is grounded to mains directly with one wire, then second wire goes directly from IEC socket ground to your pre amp ground.
Thats the second time I've said that.


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 Post subject: Re: 4S preamp project
PostPosted: 23 Jan 2019, 09:07 
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Joined: 19 Oct 2018, 15:30
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Location: Montréal, Québec
I know you’ve said that before, but I don’t understand why because adding a ground directly to the IEC socket would defeat the whole purpose of having a ground lift switch ? Or should I just wire my switch directly to the IEC socket ?


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 Post subject: Re: 4S preamp project
PostPosted: 23 Jan 2019, 12:15 
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Joined: 06 Apr 2009, 10:08
Posts: 1584
Location: US Pacific Northwest
Gab wrote:
I know you’ve said that before, but I don’t understand why because adding a ground directly to the IEC socket would defeat the whole purpose of having a ground lift switch ? Or should I just wire my switch directly to the IEC socket ?
Read this post and see if it makes sense. http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628&start=71

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 Post subject: Re: 4S preamp project
PostPosted: 23 Jan 2019, 14:36 
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Joined: 23 Feb 2017, 02:02
Posts: 455
Gab wrote:
I know you’ve said that before, but I don’t understand why because adding a ground directly to the IEC socket would defeat the whole purpose of having a ground lift switch ? Or should I just wire my switch directly to the IEC socket ?

Keep your case grounded for saftey reasons, then wire your amp ground directly from IEC to switch then amp ground.


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 Post subject: Re: 4S preamp project
PostPosted: 24 Jan 2019, 09:18 
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Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
Suncalc wrote:
Gab wrote:
I know you’ve said that before, but I don’t understand why because adding a ground directly to the IEC socket would defeat the whole purpose of having a ground lift switch ? Or should I just wire my switch directly to the IEC socket ?
Read this post and see if it makes sense. http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628&start=71


It does makes sense. Now I know that my ground lift switch should be closed


Last edited by Gab on 24 Jan 2019, 12:57, edited 1 time in total.

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 Post subject: Re: 4S preamp project
PostPosted: 24 Jan 2019, 12:33 
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Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
I will do modifications to my switch and will keep you updated.

Btw, the preamp is silent with no sources and volume to 0. A small noise is audible when turning the volume up, but something that I would consider normal


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 Post subject: Re: 4S preamp project
PostPosted: 24 Jan 2019, 14:58 
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Joined: 23 Feb 2017, 02:02
Posts: 455
Reason to never ground your amp using your case is because case tends to always sheild and have ground currents flowing, good examplse is all equipment has transfomers and all of the radation is being shunted to ground via grounded case and you connect your amp there your gonna pick up noise.

Grounding your amp to a grounded case means your grounding your amp to a non zero voltage point.

My understanding of groundloop noise is that ground point voltages in diffrent system are not always excatly the same, the use of groundnding to mains helps you maintain a zero voltage level accross all cricitcal ground points eliminating noise.

When only one equipment is grounded and others is not then you can get away easily with poor grounding.
But when more than one is grounded you need good grounding or you'll humm baddly.

With only one system grounded there tends to be little ground current flowwing between devices.
As the non grounded system can be grounded to mains via the only single grounded system and the key is no large ground currents flowwing because the voltage diffrence from connecting diffrent devices together is zero.
The non grounded systems is non refrenced meaning you can connect to a grounded system and theres no votlage diffrence betwen gnd and non gnd device.

But more than one system grounded one point in the system is zero volts depnding on ground point then there is power supply current flowwing in ground which bias up the other ground points to non zero.
If you didn't ground properly then the input ground is not excalty zero and you connect to next device theres lare currents flowwing and noise.
If you keep rca grounded drirectly then the amplifier ground may not be excatly zero anymore and you have noise.

Gab wrote:
I will do modifications to my switch and will keep you updated.

Btw, the preamp is silent with no sources and volume to 0. A small noise is audible when turning the volume up, but something that I would consider normal

I think your there at normal noise levels already.


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 Post subject: Re: 4S preamp project
PostPosted: 31 Jan 2019, 16:58 
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Joined: 19 Oct 2018, 15:30
Posts: 117
Location: Montréal, Québec
I hope it’s not normal, because even with the modifications you suggested, I can still hear a hum at normal listening levels. It is loud enough I can see it trigger the led vu meters on my power amp.

I think it got better after the ground lift switch re-wiring. I wired the switch between the iec socket’s ground and the bus bar. Both channels sound the same and the hum serms to be equal on both sides. I’m getting closer ! I’ll try to modify the parts that are likely to produce hum in hope of finding the problem.

I do know if it makes sense, but i’m curious about removing the ground connections of the output RCAs. It would separate this part from the preamp’s gound, and make shure I am not creating a ground loop between the RCA sockets of the 4s outputs and the power amp inputs. Does that makes sense ?

Again, thank you for all the support !


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 Post subject: Re: 4S preamp project
PostPosted: 31 Jan 2019, 21:09 
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Joined: 23 Feb 2017, 02:02
Posts: 455
As the amp is designed to have pot on input, that means the amp will always pick up some noise. (The power amp input is not grounded when volume is zero)
Amps without feedback like 4s have to have extremely clean power supply to be able to keep output low noise.

If you want to put pot on input to keep output imedance low then your making sacraficing to have more noise.
A solution could be using 10k pot on output and have a proper preamp that can drive a 10k load.

With both channels having same noise then thats generally a good sign that theres probally not any plorbems.

So your thinking of removing the ground connections of output rcas to mains ground?
You could try that but I doubt it will make much or any diffrence, it will probally worsen it.

The best grouning scheme I've found is to have the ground of input and output rca grounded to gether with large copper wire, then somewhere between input and output rca have a wire that goes to mains ground. Then the ciritcall ground return points in the amp also have wires that hook straight up to ground.


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