LM3875 + Broskie Cathode Follower. - diyAudio
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Old 13th March 2013, 09:16 PM   #1
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Default LM3875 + Broskie Cathode Follower. (WITH PICS)

Hi folks.

I havn't posted anything for a while but have still been active with the soldering iron.

Having built several LM3875 amps either using the excellent Peter Daniels kits or P2P and following what National recomend i like the amp. My system at home is based around a pair of 1200's and a analogue dj type mixer. I also use an analogue Phonic studio mixer and a pc running Traktor scratch pro for the decks, utillising time code vinyl.

Ive always been aware of balanced audio inputs / outputs and understand the principles of the differential amplifier / buffer. As both my mixers have xlr balanced outputs i decided to upgrade my system to balanced.

As well as this ive been tinkering with valves and finally decided to build something using them. I went through the usual hrs of looking at buffers and pre-amps using triodes etc. Nearly all i found coupled to an LM3875 were in the inverted setup. I was keen to use Audio sector boards for the output amps and they are non inverting. I then looked at the Broskie Cathode Follower. It seemed perfect for all my requirements.

LM3875 non inverted PD kit.
Balanced to un-balanced buffer.
Good starter for valve project.

The project is almost finished. I tested it and every thing was better than i expected except i roughed the valve buffer out with salvaged components first time round and had few mis matches so ive re built that section.

Over next few post il put up photos and diary etc
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Last edited by tom wesley; 13th March 2013 at 10:37 PM.
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Old 13th March 2013, 09:29 PM   #2
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The project is housed in a 19 inch rack mount 3u case which i liberated from a cheap chinese amp. (chassis was surprisingly good).

I used two traffos. The B+ comes from a traffo that powered a small valve tape recorder that came from a skip. It powered a dual triode using a voltage doubler so thats what i did. The traffo for the LM3875's came from a kenwood stereo. It has multiple secondaries. 23-0-23 is powering the chips and 12v rectified for the valve heaters. This is due to me using 6H1N valves and the valve traffo's heater winding wasn't great.
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Old 13th March 2013, 09:53 PM   #3
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Default Some build pics

Photo0070.jpg

This is the traffo for the LM3875's and valve heaters.

Photo0067.jpg

This is my P2P voltage doubler. Im get a nice 175vdc from this an valve traffo. (not in pics)
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Old 13th March 2013, 10:16 PM   #4
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The can type rectifier in the first pic was used to test the heater circuit. it has since been removed and replaced with a board containing MUR860 diodes and some electrolytics. This cicuit running from the red traffo is 12v dc and draws a constant 520mA. This is with the heaters in series. The datasheets suggest 12.6vdc @ 600mA +/- 50mA. Im confident im close enough, if anyone can advise me further on this please do. Ive decoupled the heater pins on each valve with poly cap.
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Old 13th March 2013, 10:28 PM   #5
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The thick red cable is for the mains inlet, switch and traffo's. I scrounged it off the guys who came to fit fire detectors in my block of flats. It is mains rated twin plus earth solid core with a metal fire shield and bloody brilliant for any mains connections.

The big blue caps are the main supply caps for the 3875's (10,000uf 43v) with a small poly underneath the board and 2.2k resistors to bleed the caps. The "underneath" of this board also holds 4 mur860's arranged for a center tapped secondary.
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Old 13th March 2013, 10:40 PM   #6
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I will be posting a full collection of pics to go with this build, and schematics etc as ive adapted certain parts to suit.
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Old 14th March 2013, 06:49 PM   #7
Fenris is offline Fenris  United States
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Since a good cathode follower should ideally do nothing but provide a lower output impedance than input impedance, what function does this perform? The LM3875 already has a very high input impedance.

It might have been more interesting to try Broskie's Super Triode:
SuperTriodes & Inverted ES Speakers
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Old 15th March 2013, 09:02 PM   #8
Ciu is offline Ciu  France
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Quote:
Originally Posted by Fenris View Post
It might have been more interesting to try Broskie's Super Triode:
SuperTriodes & Inverted ES Speakers
Hello Fenris

JR Broskie wrote " we must also increase the 520-ohm feedback resistor to ensure that the solid-state amplifierís gain exceeds the triodeís mu. In the example shown above, the feedback resistorís value has been increased to 100k, which results in a gain of 100 for the solid-state amplifier "

The triode 6dj8 has mu value of 33, do you know the reason why JRB has choosen a very hight gain of 100, and not ~40 - 50 ?
Thanks
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Old 15th March 2013, 09:12 PM   #9
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by Fenris View Post
Since a good cathode follower should ideally do nothing but provide a lower output impedance than input impedance, what function does this perform? The LM3875 already has a very high input impedance.
It lets the builder say there's a tube in there!
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Old 15th March 2013, 11:56 PM   #10
Ciu is offline Ciu  France
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Quote:
Originally Posted by Fenris View Post
It might have been more interesting to try Broskie's Super Triode:
SuperTriodes & Inverted ES Speakers
Or this one ?
http://www.tubecad.com/2008/04/27/ai...design%201.png
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