Revox C36 output
« on: March 25, 2020, 05:45:23 PM »
Hi guys,
I've spent the last few days restoring my old Revox C36 tupe tape recorder. What a lovely machine!! The only caveat is it only has a speaker output... Do you think it would be possible to add a line output to this machine?
Thanks!


Re: Revox C36 output
« Reply #1 on: March 25, 2020, 06:26:57 PM »
You could tap the signal at the volume pot. You need a high impedance input for that or you can add a 12AU7 as cathode follower to realize a line out.
« Last Edit: March 25, 2020, 06:45:24 PM by rock soderstrom »

Re: Revox C36 output
« Reply #2 on: March 26, 2020, 05:57:00 AM »
You could tap the signal at the volume pot. You need a high impedance input for that or you can add a 12AU7 as cathode follower to realize a line out.

Hi and thanks!
By needing a high impedance input, do you mean I could just take the signal off (before?) the volume pot and feed it into anything that has a high impedance input? Or should I use a transformer as an interface?
I was thinking about the cathode follower thing, which is how they implemented the line out in later tube Revox machines, but I have absolutely no idea how to "design" one...

Re: Revox C36 output
« Reply #3 on: March 26, 2020, 11:49:20 AM »
Do you think it would be possible to add a line output to this machine?
18 volt regulator fed from the 23.6 VDC heater supply,
TL071 configured for single supply operation,
signal from level control.
« Last Edit: March 27, 2020, 01:03:17 AM by gridcurrent »

fazer

Re: Revox C36 output
« Reply #4 on: March 26, 2020, 11:22:31 PM »
You could also use DI’s from volume pot with pad and go into mic pres.   

Gridcurrents is good for permanent  outputs For a RCA based hifi playback .

squarewave

Re: Revox C36 output
« Reply #5 on: March 27, 2020, 02:03:07 AM »
It looks like there is a normalled loud speaker output jack? Just use that. The level is probably just about right. It's low impedance. What's the problem? Then you're not hacking on it. It is a nice machine.

You would need a dummy load though. But because of the feedback on the secondary you might not need to use such a low value high wattage resistor. If the speaker was 16 ohms 4W for example and you used like 100R, now you're only dissipating ~0.5W. You could probably put that inside the jack if it's 1/4" (four 470R 0.25W bundled in parallel). It would probably work better / clearer / less distortion with a lighter load anyway.

But I'm not a tube guy. For all I know it might just as easily start to oscillate and add a whine to the recording.

Re: Revox C36 output
« Reply #6 on: March 27, 2020, 06:37:22 AM »
It looks like there is a normalled loud speaker output jack? Just use that. The level is probably just about right. It's low impedance. What's the problem? Then you're not hacking on it. It is a nice machine.

Dang.... so easy... hahaha! I just did that, took the speaker out both to the left speaker of my stereo and to a line input on my receiver playing through the right speaker and the level and sound are indeed just right! Now I just need to experiment with different dummy loads! If anyone wants to add something, feel free! Thank you so much, Squarewave!
« Last Edit: March 27, 2020, 10:42:25 AM by remsouille »

RuudNL

Re: Revox C36 output
« Reply #7 on: March 27, 2020, 10:39:28 AM »
I think the 'gridcurrent' version is the most elegant solution, because you can use both outputs independent from each other.
(And no need for a dummyload.)
There is a solution for every problem!

http://www.vansteenisaudio.nl

Re: Revox C36 output
« Reply #8 on: March 27, 2020, 10:45:06 AM »
I think the 'gridcurrent' version is the most elegant solution, because you can use both outputs independent from each other.
(And no need for a dummyload.)
Well, the thing is, I absolutely don't need a speaker output! And just adding a resistor across the output seems less intrusive than modifying the machine, does it not?

RuudNL

Re: Revox C36 output
« Reply #9 on: March 27, 2020, 11:13:08 AM »
Opinions differ, but of course it is up to you what to do.
(Personally I don't think a speaker amplifier will improve the quality of the signal...)
There is a solution for every problem!

http://www.vansteenisaudio.nl


Re: Revox C36 output
« Reply #10 on: March 27, 2020, 11:31:48 AM »
Cert
Opinions differ, but of course it is up to you what to do.
(Personally I don't think a speaker amplifier will improve the quality of the signal...)
Certainly! I was just looking for the less intrusive way to have a line out! The power amp probably won't improve the sound in a theoric way, but it certainly does have an impact on the sound of the output. What I've been hearing coming out of the speaker since this morning is nothing short of astonishing.  A very colourful HIFI sound when playing back pre-recorded music, and also a superb (pre)amp. I can't believe how good my 60's Hofner verithin bass sounds through this thing (and I own a 60's B15N). It truly is an amazing little machine, and a very colorful alternative to the Studer A80s I use at the studio!
Some time, I'll record the output of the CR36 and the A80 playing  the same tape so we can hear how much the C36 colours the sound!

squarewave

Re: Revox C36 output
« Reply #11 on: March 27, 2020, 12:41:21 PM »
Opinions differ, but of course it is up to you what to do.
(Personally I don't think a speaker amplifier will improve the quality of the signal...)
That "speaker amplifier" has probably upwards of 60 dB of open loop gain and what looks like multi-feedback topo (cathode of pentode grounded through secondary of OT). So as tube amps go, it's actually a pretty good circuit. But distortion will depend somewhat on load. They probably designed it for 1% at nominal output level. Something like that. So you if back-off the load a little (not too much or the frequency response could start to get wonky), you could bring the distortion down a little. So yes, it could very well improve the quality of the signal. If it did not measurably improve the quality of the signal, I would wager that that would only be because the signal quality was already good enough that lighter load just doesn't make that much of a difference.

But I personally would not care about a little distortion. In fact, I would probably crank the thing into 1%+. Isn't that half the reason for using a machine like this anyway? If it were not for the fact that the capstan is probably spinning when you turn the machine on, I would probably run signal through it monitoring input just for the sweet tone of a band-passed single ended pentode output.

Re: Revox C36 output
« Reply #12 on: March 27, 2020, 01:02:47 PM »
That "speaker amplifier" has probably upwards of 60 dB of open loop gain and what looks like multi-feedback topo (cathode of pentode grounded through secondary of OT). So as tube amps go, it's actually a pretty good circuit. But distortion will depend somewhat on load. They probably designed it for 1% at nominal output level. Something like that. So you if back-off the load a little (not too much or the frequency response could start to get wonky), you could bring the distortion down a little. So yes, it could very well improve the quality of the signal. If it did not measurably improve the quality of the signal, I would wager that that would only be because the signal quality was already good enough that lighter load just doesn't make that much of a difference.

But I personally would not care about a little distortion. In fact, I would probably crank the thing into 1%+. Isn't that half the reason for using a machine like this anyway? If it were not for the fact that the capstan is probably spinning when you turn the machine on, I would probably run signal through it monitoring input just for the sweet tone of a band-passed single ended pentode output.

W!at he said.

Re: Revox C36 output
« Reply #13 on: March 28, 2020, 08:04:02 PM »
A bag of 5W 16 ohm resistors should do the trick to experiment with parallel resistors.


 

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