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It is currently 20 May 2019, 22:36

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 Post subject: Re: 4S preamp project
PostPosted: 01 Feb 2019, 13:34 
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Joined: 19 Oct 2018, 15:30
Posts: 101
Location: Montréal, Québec
This is how my preamp is grounded. Everything is grounded to this copper wire. It goes all the way to the power supply.

I’m also curious about trying better quality potentiometers, but i’m not shure if it’s what i’m hearing. And the noise doesn’t go away when I raise the volume. It stays the same whatever I do. It’s just more difficult to hear as the music gets louder...


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 Post subject: Re: 4S preamp project
PostPosted: 21 Mar 2019, 06:03 
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Joined: 19 Oct 2018, 15:30
Posts: 101
Location: Montréal, Québec
I have found why I had a large buzz only on one channel. Some of the resistors of the buffer were soldered to the central lug of the terminal strip, wich is also the mounting bracket to the chassis. That was a stupid one ! After checking the circuit at least 5 times one part after the other I haven’t found anything wrong. But one day, I was looking at my ground layout and saw it. I can tell for shure that I will never ever do this mistake again :D

Now I can listen to it and I really appreciate how it sounds. Still have a small hum on the input side, so it is not perfect, but it’s way better than it was. While trying different tubes, I was getting a louder hum with tubes with a higer gain, and less hum with the 12au7, so I guess that the hum comes from something before or in the amplification stage. In the second picture you can see how it looks at this point.

Next things on my list are :
-install a true X2 capacitor to replace the blue ceramic disc that I used to be able to test the amp
-better potentiometer of a higher value (can’t turn the volume more than an eight before it gets too loud)
-redo the input selector switch part with sheilded cables and maybe a different switch system
-install a new lift switch (fried the last one while soldering)
-knobs
-bottom cover and feet

Feedback is welcomed ! I’d like to have your opinion on the way I grounded the amp. I can take better pictures if needed.

Thanks to everyone who helped me with this project. I learned a lot and it is mostly because of you guys !


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 Post subject: Re: 4S preamp project
PostPosted: 21 Mar 2019, 14:12 
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Joined: 23 Feb 2017, 02:02
Posts: 403
HI FI potnetial meters do make a diffrence, I find bourns 91 to be very good is conductive plastic low price and only avlaible in 10k options, another disadvantage is limited volume adjustment range and is slightly unaccuarte at very low volumes. But works very well if you don't have too much gain in the system.

Bourns 91 is massive upgrade compared to alps rk27 , but limited volume adjustment range (hard to stay in very low volumes as poor matching at very low volumes). and only avlible in 10k options.

I've also noticed your using just wires not even twisted for input and heat shrinked together, thats not good.
You want to use coax cables since it will reduce noise and improve sound quality, twisted pairs is recomended for fully diffrential amps.


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 Post subject: Re: 4S preamp project
PostPosted: 21 Mar 2019, 17:52 
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Joined: 19 Oct 2018, 15:30
Posts: 101
Location: Montréal, Québec
ILoveHiFi wrote:
HI FI potnetial meters do make a diffrence, I find bourns 91 to be very good is conductive plastic low price and only avlaible in 10k options, another disadvantage is limited volume adjustment range and is slightly unaccuarte at very low volumes. But works very well if you don't have too much gain in the system.

Bourns 91 is massive upgrade compared to alps rk27 , but limited volume adjustment range (hard to stay in very low volumes as poor matching at very low volumes). and only avlible in 10k options.

I've also noticed your using just wires not even twisted for input and heat shrinked together, thats not good.
You want to use coax cables since it will reduce noise and improve sound quality, twisted pairs is recomended for fully diffrential amps.


I've got something to deal with the input wires. My local parts store had a complete coil of vintage Belden twisted pair 22awg. I also bought a couple feet of three conductors, double sheilded cable in case the twisted pairs aren't enough. It was really cheap and I think it's practical to have one cable for all the left or right channels.

I also bought a switch to replace my ground lift swtich and wire everything correctly. Still a work in progress LOL


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 Post subject: Re: 4S preamp project
PostPosted: 23 Mar 2019, 02:56 
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Joined: 23 Feb 2017, 02:02
Posts: 403
For wiering inside the amp or short wires I like to use smaller coax cables, something like the size of rg316, for longer wiering or for amp to amp for rca connection I like to use thicker rg58 size coax cable.

The larger wire has less resistance and is more immune to noise, slightly better in sound.

Twisted pair works bad on any ampflier other than fully diffrential amplifiers.
Because the ground of input is gounded to mains or power supply, this means the noise voltage will not appear on the negative wire which is ground. that means theres no canceling effect and the noise goes straight on the postive input wire.

Twisted pair only reduces noise when noise voltage appears on both + and - wire and the apmlifier has a diffrential input.
Same noise voltage appearing at + (non inverting) and - (inverting) temrnial means the noise is cancled out and is zero.

Practicall testing with about 60minutes on just one song and repetive listening on both a fancy twisted pair and very cheap coax cable, showed the cheap coax calbe to have less noise and less distrtotion.
The fancy twisted pair was a gift from a hifi mate that owns very expensive equipment 30k+ amps. It is distorting the signal and has noise.


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 Post subject: Re: 4S preamp project
PostPosted: 23 Mar 2019, 22:31 
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Joined: 23 Feb 2017, 02:02
Posts: 403
I like to use smaller wires for short or inside amp wiering.
Thick ones like rg58, things get very messy and is very hard to solder, is massive time consuming for very small or no sound quality gains.


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 Post subject: Re: 4S preamp project
PostPosted: 25 Mar 2019, 19:09 
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Joined: 06 Apr 2009, 10:08
Posts: 1578
Location: US Pacific Northwest
Be careful of shielded coax cable as it can exhibit significant capacitance per foot. This adds to the Miller capacitance and must be taken into account when calculating the high frequency rolloff. RG58 is about 31µµf/ft and RG316 is about 29.5µµf/ft. Not a lot, but it can add up if you're not careful.

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 Post subject: Re: 4S preamp project
PostPosted: 25 Mar 2019, 20:51 
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Joined: 23 Feb 2017, 02:02
Posts: 403
Not too much diffrence from the audiophile twisted pair compared to the cheapo coax cable.
The cheapo ones have arround 200-300pF/m which is abbout doubble or more than rg316.
With a total capacitance of about 420pF it still perfomed better than the fancy twisted pair which was shorter in length.

Audio source was a psp output which can drive haeadphones I imagine its quite capacitance tollerative.
The twisted pair may come closer if the source is less tollerative to capaictance however the small noise pick up will still exist which is poor.


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 Post subject: Re: 4S preamp project
PostPosted: 28 Mar 2019, 22:24 
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Joined: 19 Oct 2018, 15:30
Posts: 101
Location: Montréal, Québec
I don’t get why you consider twisted pairs as something fancy...

I tried both twisted pairs and two conductor sheilded cables, but neither had any kind of impact on the hum. (Only grounded one end of the shield to the chassis)


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 Post subject: Re: 4S preamp project
PostPosted: 29 Mar 2019, 06:05 
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Joined: 23 Feb 2017, 02:02
Posts: 403
In context it ment that the twisted pair listened to for comparison was a very expensive one, therefore its fancy.

You should be able to hear some small noise pick up when you have longer runs on twisted pairs.
Inside the amp the amp case is already ground, acting as some sheilding from external signals already.


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