Revox C36 output

[remsouille]

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!

 

[rock soderstrom]

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.

 

[remsouille]

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...

 

[gridcurrent]

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.

 

[fazer]

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 .

 

[Bo Deadly]

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.

 

[remsouille]

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!

 

[RuudNL]

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.)

 

[remsouille]

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]

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...)

 

[remsouille]

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!

 

[Bo Deadly]

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.

 

[remsouille]

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.

 

[Piedwagtail]

A bag of 5W 16 ohm resistors should do the trick to experiment with parallel resistors.

 

[remsouille]

Does anyone know what L1 on the schematic could be? It should be some sort of variable something, but I can't find anything like that in the machine..
Also, do you think it would be safe to have a 5w 8ohm resistor strapped across the speaker output all the time? This way I wouldn't have to worry about using a "loaded" output cord. That would drop the load to something like 4ohm... can the output transformer handle that?
Thanks!

 

[rock soderstrom]

Does anyone know what L1 on the schematic could be? It should be some sort of variable something, but I can't find anything like that in the machine..

L1 is a variable inductance. (coil)

 

[remsouille]

L1 is a variable inductance. (coil)

That's what I figured! What function would it serve?

 

[rock soderstrom]

That's what I figured! What function would it serve?

I guess it is a LC Filter for equalization of the playback amp.

 

[Bo Deadly]

Does anyone know what L1 on the schematic could be? It should be some sort of variable something, but I can't find anything like that in the machine..

It's a "trap". With the cap it makes a tuned notch filter. It's supposed to filter out any bias signal that bleeds into the recovery amp.

Also, do you think it would be safe to have a 5w 8ohm resistor strapped across the speaker output all the time? This way I wouldn't have to worry about using a "loaded" output cord. That would drop the load to something like 4ohm... can the output transformer handle that?
Thanks!

The service manual says that a load doesn't make much of a difference at all. But I would probably not load it with 4 ohms. You might stress the tube. Also, I'm not sure how you came up with 4 ohms. Maybe you're thinking the speaker is 16 so 8 makes it look like 4 to the tube. But IIRC the service manual indicates the speaker is actually 8 ohms in which case a 5W 8 ohm load is just about right.

 

[remsouille]

[quote author=squarewave
Also, I'm not sure how you came up with 4 ohms.
[/quote]
What I meant was to have an 8ohm load wired to the the speaker output all the time, so with the speaker in the machine I would have two 8 ohm loads in parallel, 4ohms total. And you're right, L1 is a bias trap!!