REVOX A77 Baby

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Soeren_DK

Well-known member
Joined
Jun 7, 2004
Messages
526
Location
Denmark
Hallo in the hut...

Yes i am now the lucky owner of a old REVOX A77 1/4 2 tracks.. I bought this unit from an very nice guy her in denmark. He works for the firm Greengroove so if you need a very good basamp you should try this one. It is little expensive but it is the money worth.. IT SOUND SUPER...

Okay a little side leap... :cool:

I have this tape recorder i will use for mastering but it's not with balanced in and out.. I thought i will get deep in this machine and build a balanced in and out unit.. But now I am only a electrician so this is a little big for me. Can someone help me with that? :wink:
 
I've seen Danmarks Radio-versions of the A77, mounted with Beyer transformers on the inputs and Neutrik transformers on the outputs. Sounding very, very good indeed.

I have schematics for the A77, but not for the transformer balancing (which should probably not be too difficult)

Jakob E.
 
Okay... Yes its a Danmark Radio version... hehe...
Will you mail the original schem for the A77 to me? :wink:

How difficult is it?
 
You can download the manual from here:
http://fileshare.eshop.bg/equipment_mfg/REVOX_5.html
Also for B77Mk2.

chrissugar
 
Throwing in my 2 cents worth:
It would be a fine DIY exercise to convert some unbalanced gear to balanced, however you might want to try your revox AS IS with the gear you'd most often be using it with, following the proper guidelines if connecting with balanced gear (searchable on this forum i'd imagine) just to see if it isn't already fine - level wise it could be ok.

My main reason for saying so is that as far as I know balanced is mostly for longer runs of cable, and there's a slight tradeoff in fidelity getting there - unless you are going for a "transformer"sound. There are a few pieces of gear out there that are balanced all the way through the circuit, but most are unbalanced on the inside. To make yours balanced you'll be adding components into the signal path, which in this case may only serve to degrade your signal quality.

That being said I have a revox G36 that I'm over-DIYing to death - new power supply caps, all DC heater supply... and a few things removed... I'll be finding out the hard way if I really needed the internal speaker amp.

Here's where I found my service manual in pdf format:
http://www.beersoft.de/

It cost $$ but has been indispensible.

Best of luck! :guinness:

Franklinh
 
If you drive the A77 from a +4dBu source, balanced or unbalanced, you're going to clip the input amplifier, and the clipping sounds utterly horrible. The thing seems to burst into oscillation when it clips.

So whether you try to drive it in its unbalanced state, or fit a balancing circuit or transformer to it, pad the signal down at least 14dB.

Peace,
Paul (who learned the hard way)
 
I can se the prospect in rebuild the Revox... I will keep it in old style and put it on the shelf and use it for the first time today...

:oops: Thanks everybody for your time... :wink:
 
Don't forget that the A77's have a two-pin power inlet, and the device is double-insulated, so the chassis is not connected to mains earth. I've wired A77's and B77's into balanced studio setups with no hum problems. As mentioned above, you need to pad the input by -12dB or so, and boost the ouput accordingly. But the pad can be two resistors in the phono shell, and the output makeup can be the line trim on a console input (or a simple op-amp based buffer/booster/unbal-to-bal converter, with or without transformers.

Just thought I'd mention the double-insulation thing, because breaking ground-loops is the main reason for using transformer in/outs in this situation...unless you want transformer sound- but the A77's sound pretty nice anyway with no extra colouration. I've got 1/2 and 1/4 track A77's, used every day!

:thumb:

Mark
 
[quote author="Mark Burnley"] As mentioned above, you need to pad the input by -12dB or so, and boost the ouput accordingly. But the pad can be two resistors in the phono shell, and the output makeup can be the line trim on a console input (or a simple op-amp based buffer/booster/unbal-to-bal converter, with or without transformers.[/quote]

I did mine on the RCA input jacks; 8.06k series, 2k shunt. As long as you don't mind the input impedance of ~10k (and nearly everything else unbalanced is that, so why should you?) it works fine. And the input has enough gain that if I want to feed it with a -10dBV input it still works fine.

For a while I got to know Revoces quite well, and came to admire most of the design decisions they made. (Although I still don't understand why the input amp breaks into oscillation when it's overloaded. They fixed that on the B77.)

Peace,
Paul
 
Hi Soeren_DK

Here is the set of schematics of my regreted A77
Maybe these will help you.

http://site.voila.fr/flyingfader/audio/revox/A77.part1.rar
http://site.voila.fr/flyingfader/audio/revox/A77.part2.rar

regards

Fly
 
Don't forget that the A77's have a two-pin power inlet, and the device is double-insulated, so the chassis is not connected to mains earth.

Hey Mark,

My revox G36 has the same mains power input, just hot and neutral, but the chassis IS ground for everything. Is that kosher for hooking this up to more modernized gear, unbalanced, with the shield being part of that ground? The reason I ask is I'm having some slight buzzing issues, sort of a general "powered on" noise that I want to reduce, I'd be wondering if this power ararngement has any influence here. Any ideas?
 
Well,

It depends when you hear the buzzing. The chassis ground is the signal ground as you point out, but it has no direct connection to mains ground in the lead to the machine. But a ground loop can still occur if you connect two pieces of unabalanced gear to the machine- the "floating" chassis then connects two possibly mains-grounded pieces of gear.

A few things to try:

Disconnect everything except the mains cord from the machine. Connect a pair of headphones to the headphone socket (or use a floating battery-powered headphone amp or battery-powered monitor amp fed from the output sockets), and set to "Input Monitor". Turn up the monitor and input levels for each input and listen for hum. Then connect your usual Input connectors- listen carefully for any hum. If none is present at this stage, try recording a short passage of music to tape and monitor the off-tape sound. Any hum? This just rules out any hum present and/or induced in the input circuitry, and rules out the tape record/replay paths.

Then, connect your output leads as usual and listen through the normal path. If you then hear hum, try removing the Input leads from the Aux input sockets. Does the hum disappear?

Once you've ascertained that it's the external connections causing the hum, you can do something about it.

Let us know how that test goes, and then let us know what input and output devices you've got it connected to.

Mark
 

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