Braun LV 720 Speaker Thread

[Bratwurst7s]

Braun made 2 different active speakers in the early 1970's, first the LV 1020 in 1971-'76, and the LV 720 from 1973-'76.

The LV 1020 has shown up in a few threads here but as far as I can tell using the search function the LV 720 has no Audiokarma presence. Until today.

For background info here are 2 threads that I know of concerning the LV 1020:
https://audiokarma.org/forums/index.php?threads/braun-lv-1020.426737/
https://audiokarma.org/forums/index.php?threads/braun-lv1020-replacement-woofers.788492/

And since the special LF700 "Kangaroo" stands will come up later here is a link to a thread about them started by sx-70:
https://audiokarma.org/forums/index...-stands-mounting-hardware-information.792224/

Photos, info and the start of a resto of a pair of LV 720's to follow...

Cheers,
James

edit 08 Sept 2019:
Since this thread has gotten pretty long and has a few twists and turns I decided to add a sort of an index. The following subject headers point to post numbers that pertain to the subject. Some posts are listed in more than one header. As I/we add to the thread I'll edit the index to keep it up to date.

General information: 1, 2, 3, 4, 151, 177
Documents, spreadsheets and zips of such: 3, 10, 11, 20, 76, 79, 80, 88, 161
Speaker cabinets: 3, 18, 19, 21, 22, 23, 31, 33, 37, 42, 47, 52, 55, 56, 59, 64, 70, 87, 92, 93, 97, 99, 100, 139
Speaker stands: 1, 20, 62, 63, 65, 67
Speaker Drivers, general info: 5, 92, 93, 97, 98, 99
Tweeter: 9, 25, 36, 41, 76, 78, 140
Midrange: 13, 15, 26, 71, 76, 77
Woofer: 14, 58, 97
Amplifiers, general info: 6, 7, 16, 81, 82, 83, 84, 137, 142, 169, 171, 173
Amp power supplies: 80, 81, 82, 110, 112, 113, 114, 162, 170
Crossover boards: 84, 138, 162, 169, 187, 188
Tweeter amp boards: 84, 137, 153, 162, 163, 165, 165, 168
Midrange amp boards: 84, 129, 153, 162, 163, 164, 165, 166, 167
Woofer amp boards: 84, 115, 116, 117, 118, 120, 123, 125, 128, 153, 162, 163, 164, 165
Resistors: 88, 101, 116
Capacitors: 81, 82, 83, 84, 85, 101, 105, 106, 107, 108, 109, 129, 162, 170
Transistors: 79, 153, 154, 155, 156, 157, 159, 160, 161, 163
Trimmer potentiometers: 101
Bridge rectifiers: 80, 101, 110, 111
RCA plugs: 130, 131

And, since the spreadsheets that I made are LibreOffice files here is a link to download LibreOffice. It's free and opensource:
https://www.libreoffice.org/download/download/

 

[Bratwurst7s]

Reading about the LV 1020 got me pretty interested in these speakers but they are pretty rare even here in Germany and also pretty expensive. It looks like there were a lot more of the 720's sold because they come up fairly often but they still usually command a fairly hefty price.

Once that I had finally gotten my longstanding Yamaha CX-600 pre-amp project to the point where it actually works I started thinking about what to do with it. Because I really have no need for it any longer now that I have a CX-1+MX-1. Then a few weeks ago I saw a pair of pretty sad looking LV 720's on ebay for a buy-it-now or make offer. They were/are missing the grills and painted black. But looking at the photos I noticed one of the back side that showed that there is some walnut veneer under that paint.

So I decided what the heck, made an offer which was accepted and so I now have a pair of these things for the grand sum of €100 including shipping. Today I finally had the time and space on my workbench to start to take a close look at them and take a few photos.


One of the cabinets has taken a hit up front on one side. And one of the woofers has a partly pushed in dust cap. I won't know the shape of the rest of the cabinets and veneer until I can get the paint off.

Cheers,
James

 

[Bratwurst7s]

So the LV 720 is basically a powered version of the L 710. Except of course that while all of the drivers are the same sizes they are actually all different drivers and unique to these speakers. Exactly what the differences are I don't know yet.

edit/correction: The tweeters in the LV 720 are the same as the L710, LV 1020 and a bunch of others (LC 2/4). The mids and woofers are unique.

HiFi Engine has the service manual, in German:
https://www.hifiengine.com/manual_library/braun/lv-720.shtml

Here are a few fotos of the plate-amps:


After pulling an amp I see that there are brass inserts for all of the mounting screws. Nice.

Cheers,
James

 

[Bratwurst7s]

hifimuseum has a detailed write-up on these speakers, only in German sadly. In fact pretty much all of the info that I can find on the LV 720 in in German, it seems that they had a fairly small footprint in the USA. Or were they available at all in the US?

Mine are only 9 numbers apart on the serial numbers.

http://www.hifimuseum.de/braun-lv720.html
https://www.aktives-hoeren.de/viewtopic.php?f=16&t=1015



Cheers,
James

 

[Bratwurst7s]

The drivers all appear to be in good condition optically, aside from some dust on the tweeter & mid on one box and that single woofer dust cap. The woofers all have rubber surrounds.



Because the height and width of the cabinets are the same as the L 710 the front grills are the same. Which leaves me hope that with patience I can find some.

Cheers,
James

 

[Bratwurst7s]

Here are some initial photos of one of the plate-amps. I'll be going into detail later when I look them over, this is just a starter.

The amp is mounted to a hefty aluminum casting for cooling, with said casting doubling as the rear face of the amp, with the cooling fins an integral part of the casting.

They have a substantial transformer, which has 2 separate windings for the woofer and high/mid amp sections, each of which has it's own bridge rectifier and smoothing/filter caps.



Cheers,
James

 

[Bratwurst7s]

There is a crossover board that feeds each amp section, which also includes an overall level control and a separate level control for the tweeter and woofer amps. This is a true tri-amp. There is a separate amp board for tweeter (20w), midrange (20w) and woofer (60w). Interesting to note that the older LV 1020 only has 15w for the tweeter and 40w for the woofer.



Thatz it for now, it's time to go out front and listen to some music.

Cheers,
James

 

[sx-70]

Like and subscribed.

 

[Bratwurst7s]

So I've step by step removed the drivers and looked them over now.

The tweeters. Now everything that I've read so far states that the tweeters should be model number LC-2/4, part number 2422 741. So when I grabbed the first cabinet near to hand and pulled a tweeter I was surprised to see an older LC-2/3, P/N 2422 731. But the date inked on it passed with the other drivers when I removed them, 11 Jan 1974. And the Braun cross reference sheets that I have are from 27 Aug 1975 so I just figured that the early models had the 2/3 tweeter.

Btw, the Braun model numbers of can be deciphered so: In German a driver is known as a "chassis", and LC can be translated as LautsprecherChassis (LoudspeakerDriver) = LC. The 2/4 or 5/3 or 18/8 of the drivers works out to diameter/series. So: LoudspeakerDriver-(dia)/(series).

In spite of the above the LC-2/4 (and 2/3) tweeters are actually 25mm.

The tweeter measured 4.8ohms DCR and has a mounting hole spacing of 60 x 80mm. It is installed with M4 x 20mm machine screws and has press-nuts installed in the cabs, as do all of the other mounting screws.

The wires are: yellow/green connected to plus+, and black(5) connected to minus-. The black wires are consistent with what I've become accustomed to here in that they have an underlined number printed on them in white every 2.5cm. LOL, it was different in the '70s but nowadays a yellow/green wire is always used for ground. So don't get confused.

So when I got to the second cab and pulled the tweeter I didn't really pay much attention to the way it looked until I looked at the sticker on the magnet, which had P/N 2461 701 printed on it. Huh. This is the 19mm tweeter used in the L-520, 630, 730 and 830 speakers. And it measured "open" when I checked it for ohms. So, dead. In retrospect, if one looks at the 1st 2 photos in post#2 above one can clearly see that the tweeter in the left speaker is smaller than the one in the right speaker.

So it looks like at least one tweeter had been killed in the past and replaced with a 19mm tweeter. Lucky for me I've been looking in ebay for LC-2/4 tweeters and found a pair for €15 each, so I just bought them tonight.



Cheers,
James

 

[sx-70]

Was going to ask if you did a “before” listen but they wouldn’t have sounded right with a tweeter out. Such a great price on those tweeters! Nice score there.

Braun driver docs:

 

[Bratwurst7s]

I made a conscious decision not to hook up the amps until after at least removing them and checking them out. After pulling the 2nd one yesterday I'm glad. I'll post photos later but the trim pots on each amp board are very badly tarnished and the fuse/fuse-holder on the woofer amp is badly corroded. I worry about runaway idle current blowing out the amps so those trim-pots will be replaced with Bourns multi-turn before I ever put power to the amps.

Thanks for the tables. That first tweeter one I don't have. The 2nd & 3rd I do. Also, here are a couple of cross reference charts:



Cheers,
James

 

[sx-70]

Fun (really fun) 10 year old link I found provided by @E-Stat in another forum:

https://www.aktives-hoeren.de/viewtopic.php?f=16&t=1015

edit: oops, just saw you had it already above. I'm going to keep this here though because Braun fans should click on it!

Think I could live with this set up.




This is my favorite thread ever.

 

[Bratwurst7s]

Fun (really fun) 10 year old link I found provided by @E-Stat in another forum:

https://www.aktives-hoeren.de/viewtopic.php?f=16&t=1015

edit: oops, just saw you had it already above. I'm going to keep this here though because Braun fans should click on it!

Think I could live with this set up.


View attachment 1523108

This is my favorite thread ever.

Well, that thread is pretty good so it can't hurt for it to be repeated.

That photo shows the CES-1020 "pre-ciever". They go for good money even now. There is also a quad pre-amp, the CSQ-1020. All of these things connect with Din-7 plugs/cables and one thing that I have to do is buy or make some interconnects. It's my understanding so far that pins 6&7 of those connections are for the standby function that would be provided by the CES-1020 to turn on the amps. If that function isn't needed then one can use Din-5 plugs, they will apparently fit in the Din-7 socket.

So the midrange drivers are the LC 5/3, P/N: 2415 931. Compared to the 5cm domes in the L-630's that I had these things have a much larger magnet. I'll post some comparison photos later, today I just have a few of the backside of the mids. The LV 1020 used a LC 5/2 mid, I don't have any idea yet what the differences are. The 1020 service manual shows the mids and tweeters as being 8 ohm in the block diagram. The LV 720 SM doesn't specificly show the driver impedance that I can find. Well the driver spec page shows Nennscheinwiederstand (nominal impedance) for the woofers and mids as 1 ohm, which I'm thinking must be a typo. The mids in my speakers measured 5.8 and 5.9 ohms DCR.

The mids are connected with black wires, #3 = plus+ and #4 = minus-. The mounting screws are M5 x 25mm and have a 120 x 120mm hole spacing. Both mids have a date stamp of 02 Jan 1974.

But we have a problem with one of the mids. Just like you had with one of your woofers in your L 710's Jon, the plus lead lug has fallen off of one of them. So I'm going to have to repair that.



Cheers,
James

 

[Bratwurst7s]

The woofers are LC 18/8, P/N: 2443 911. They are dated 14 Jan 1974 and all 4 of them measured 6.5ohms DCR. They have runner surrounds.
Wires to the upper woofer are Black(1) on plus+ and Black(2) on minus-. The lower woofer is connected in parallel to the upper woofer with jumper wires, Yellow to plus and Black to minus.
The mounting screws are M4 x 20mm.



Cheers,
James

 

[sx-70]

When I said this is my favorite thread I meant this LV-720 one!

Here’s a LC-5 mid for comparison.




Wonder what that extra bit on the back of the LC-5/3 is?

 

[Bratwurst7s]

Here are a few photos of the second amp, note the corroded fuse & holder on the middle (woofer) board and the badly tarnished prim-pots and speaker wire connection lugs.



Cheers,
James

 

[Bratwurst7s]

When I said this is my favorite thread I meant this LV-720 one!

Here’s an LC-5 mid for comparison.
View attachment 1524804

Wonder what that extra bit on the back of the LC-5/3 is?

Well, I'll try to keep the thread interesting.

I don't know about that flat dome cap on the back of the magnet. Do you suppose that it could be a cover for an extended pole piece? The mids in the L 630 have them also. I'll take some photos of them together later for comparison.

Cheers,
James

 

[Bratwurst7s]

Well, at least whoever raped these cabinets with that black paint didn't spray the bases. At least they spared me having to sand it off there.



Cheers,
James

 

[Bratwurst7s]

Earlier in the week I worked on repairing that bashed in front edge on the one cab. I figured that the black paint was actually helping hold together the splintered parts of wood so I wanted to try to repair it before doing any sanding.

I took some wood glue and squeezed it into all of the cracks and splits in the wood and then wiped off the excess. Wife contributed some wax-paper from the Command Center of out apartment (the kitchen) and I had already cut a pair of hardwood strips to clamp things together. 4 c-clamps worked like a charm to squeeze the section back into place.

After letting it dry overnight I did the same thing front to rear with a long clamp.



Cheers,
James

 

[sx-70]

Hopefully that black paint comes off easily. This is going to be another of your great line of restorations.

I had those little felt discs on the bottom of my 710’s. I didn’t realize they were oem Braun parts: