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4S preamp project
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=6392
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Author:  Gab [ 11 Nov 2018, 19:22 ]
Post subject:  4S preamp project

I began to build the 4S preamp with a cathode follower output as in this tread :
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5186

Author:  Gab [ 12 Nov 2018, 08:25 ]
Post subject:  Re: 4S preamp project

Here are more details about this build. I first want to thank Matt for creating this circuit and to all others who answered my questions in the forum.

I got into audio (mostly vintage) 1.5 years ago. I'm 28. A friend of mine showed me an ad on Facebook and 6 hours and 500km later I was back home with a nice pair of Cerwin Vega AT15 and a Yamaha receiver. From this point I started to buy, trade and sell audio gear. Fast forward to this summer. I replied to an add on kijiji and ended up being given a lot of tube gear. Mains and output trans, a box full of tubes, Hammond chassis, resistors of all kinds (mostly Vishay RN65D) and all kind of parts. The man was building guitar amplifiers in the past, but had to part with this stuff due to space issue.

As an ex car mechanic, I had some electrical knowledge, but nothing close to what is needed to build an amplifier. I started to read (a LOT) but I wanted to build something simple with the parts I had in the meantime. Since I already had the power transformer for this preamp laying around, and almost all 12A-7 tube possible in different brands, I decided to give it a try.

I ordered the parts I didn't have in ordre to build the 4S, but in the meantime I learned about something called output and input impedance. Then I knew that the 4S couldn't drive my Yamaha M-50 solid state amp. This is where the cathode-follower version rapidly answered most of my questions. A second part order was made last friday.

As you can see in the pictures, there is a bit more holes in my chassis than what is actually needed for the «original» circuit.

Here are my plans :
- 3 selectable inputs
- Rotary switch input selector at the front
- Ground lift switch at the back
- High/Low impedance switch at the back
- stepped attenuator or regular audio taper pot ?
- separate the PSU from the rest in the enclosure

Features :
- All vishay metal film resistor (RN65D or CMF55 series)
- Cornell Dubilier caps for the preamp part
- Hammond 1444-16 aluminium chassis (10x6x2 inches)

Author:  Gab [ 18 Nov 2018, 20:44 ]
Post subject:  Re: 4S preamp project

Update : I cut, drilled and tapped a aluminum plate to separate the power supply from the signal parts of the amp. The heater wiring is almost done. Metal work is done too. Now it’s time to build the electrical circuit !

Author:  Suncalc [ 18 Nov 2018, 21:12 ]
Post subject:  Re: 4S preamp project

So I have to ask... What is the purpose of the little hole next to the rectifier tube?

Author:  Gab [ 19 Nov 2018, 08:10 ]
Post subject:  Re: 4S preamp project

Suncalc wrote:
So I have to ask... What is the purpose of the little hole next to the rectifier tube?


I was waiting for this one ! I drilled the 7/8" hole for the socket at the wrong place. The socket was supposed to be centered with the transformer, but instead I drilled the hole that was intended for the socket’s mounting screw. I thought about making a plate to center everything, but decided to leave it like that and use the hole for something else.

Since you are the one who designed this circuit, what do you think of my layout ? Do you have anything to suggest ?

Thanks for your answers, it’s always appreciated

Author:  Suncalc [ 19 Nov 2018, 11:21 ]
Post subject:  Re: 4S preamp project

Gab wrote:
I drilled the 7/8" hole for the socket at the wrong place.
I would like to be able to say I've never had that happen but I think we've all done that from time to time. Using it as an anchor point for a terminal strip seems like a good fit.

The layout looks solid. I assume that the socket closets to the divider is for the cathode followers?

Author:  Gab [ 19 Nov 2018, 13:07 ]
Post subject:  Re: 4S preamp project

Suncalc wrote:
Gab wrote:
I drilled the 7/8" hole for the socket at the wrong place.
I would like to be able to say I've never had that happen but I think we've all done that from time to time. Using it as an anchor point for a terminal strip seems like a good fit.

The layout looks solid. I assume that the socket closets to the divider is for the cathode followers?


I'm kind of a freak when it comes to symmetry and things lining up. I wanted to have the fewest bolts through the enclosure's top so that's why I bolted the choke down to the divider. On the other hand, not having the retifier centered will leave some space that I can use for something else (maybe).

I'm still not shure wich tube will go where. I see advantages to both ways. It would be easier to swap tubes if the cathode followers are near the divider, but I think that the wiring would be simpler the other way. What do you think ?

Also, I'm still not decided on how to power the LEDs. I could rectify/filter the heater supply or use the B+, or many other things. The input selector will switch the L+R channels as well as the LED supply at the same time. My fear is inducing hum or noise by sending unfiltered (or not filtered enough) dc near my input signal before it gets amplified. Do you have any suggestions ?

Author:  Suncalc [ 19 Nov 2018, 22:40 ]
Post subject:  Re: 4S preamp project

I'd put the cathode follower by the divider just to keep the signal stage as far as possible from power section. But then again, I'm kind of paranoid concerning such things.

I'd strongly recommend you power the LED from the B+. Just give the LED a couple of milliamps and let it be. It's simple (only one resistor), all DC so there are no switching transients to worry about, and it serves as a good indicator of not just "power" but B+ voltage. Just my opinion of course.

Author:  Gab [ 20 Nov 2018, 08:37 ]
Post subject:  Re: 4S preamp project

Suncalc wrote:
I'd put the cathode follower by the divider just to keep the signal stage as far as possible from power section. But then again, I'm kind of paranoid concerning such things.

I'd strongly recommend you power the LED from the B+. Just give the LED a couple of milliamps and let it be. It's simple (only one resistor), all DC so there are no switching transients to worry about, and it serves as a good indicator of not just "power" but B+ voltage. Just my opinion of course.


I agree. The divider isn’t blocking everything and ease of use when tube rolling isn’t that much of an issue. For the leds, i’ll try to calculate how much resistance I need to drop the voltage to an acceptable level.

I worked on the psu last night. I’ll be able to move on to the signal side very soon... if I finally get my order from mouser wich is already 4 days late. Canada’s postal services strikes and lockouts won’t help my problem neither...

Author:  Suncalc [ 20 Nov 2018, 10:39 ]
Post subject:  Re: 4S preamp project

Gab wrote:
For the leds, i’ll try to calculate how much resistance I need to drop the voltage to an acceptable level.
Use current not voltage. The LED Vf is so small in comparison with your B+ as to be insignificant. Just decide how may milliamps you want through the LED, divide the B+ voltage by that current, and use the indicated resistor. And contrary to popular opinion, most LEDs work just fine with a couple of milliamps of forward current.

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