Tape delay FDBK
« on: April 26, 2019, 06:26:18 PM »
Thinking of a little Feedback mixer box for use with a tape recorder. Something I can use to creat e the feedback loop for repeats. I quickly through together this ve schematic but am I over complicating this?  I haven't worked out resistor or cap values. Also thinking about a wet/dry mix knob .

Thanks for any feedback  :D

« Last Edit: May 02, 2019, 05:44:38 PM by RSRecords »


Brian Roth

Re: Tape delay FDBK
« Reply #1 on: April 26, 2019, 09:21:34 PM »
There is no way that your diff amp inputs can ever work. 

Bri

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squarewave

Re: Tape delay FDBK
« Reply #2 on: April 26, 2019, 09:37:13 PM »
Your input amps are invalid. Also, you don't really need balanced IO for this. An unbalanced input and a simple quasi-balanced output would be fine. Also, do you not want to mix in dry?

Re: Tape delay FDBK
« Reply #3 on: April 26, 2019, 11:04:31 PM »
Your input amps are invalid. Also, you don't really need balanced IO for this. An unbalanced input and a simple quasi-balanced output would be fine. Also, do you not want to mix in dry?

Yea the input amps were just thrown in as place holders.   I'd probably use a THAT 12XX chip if I use balanced inputs.  I'll redraw so it makes sense.

I'd like to add a dry mix pot as well but I'm unsure how to structure that.

Re: Tape delay FDBK
« Reply #4 on: April 27, 2019, 10:04:07 AM »
OK so actually thought about this a little more.  New scheme attached, hopefully makes more sense.
Channel in I would like balanced input probably just borrow a design from Self using a 5532. Send to tape and from tape are fine unbalanced but it would still need to be buffered right? Any benefit to being unbalanced if it still needs an amp?

 Effect out will probably be balanced  with a that corp as well just to play nice with the patchbay...

Having some input level control would be nice to as well as a mix control and feedback control.  In a perfect world hi/lo filters would be cool but they'd need to be variable to be really useful in my mind.

Would a Dry mix be as easy as a linear pot with the wiper to ground? Sort of a reverse pan pot?





mjrippe

Re: Tape delay FDBK
« Reply #5 on: April 27, 2019, 11:40:27 AM »
You might google the Teac AX-10.  It was a box that enabled sound-on-sound or tape delay with your reel to reel.

squarewave

Re: Tape delay FDBK
« Reply #6 on: April 27, 2019, 12:59:23 PM »
That circuit is much better.

Note that the "proper" way that this would be done in a studio would be to use an AUX send on your console or digi-interface+daw and then recover the wet-only echo on another channel and mix it into whatever track(s) to taste. That method is superior because 1) you can adjust levels to account for the fact that consumer and even pro-sumer Reel-To-Reel machines cannot handle high level signals used by pro consoles or digi-interface+daw setups, 2) you don't run the dry part of the track through another gizmo and most important 3) you can mix in just enough of wet to taste and not add the full noise of the source output.

But I could also see how you would not want to go through all of the setup for that in which case a little gizmo like you're making is nice. Although little mixers are pretty cheap.

If you want something that integrates with the patchbay, you could do something like this:



I have one these on each of my two 1U patchbays. They are completely passive and quite effective. Output impdance is ~1K or less which is fine if you're just running into something else that will buffer.

Basically the way this "mult-mix" works is the 4 inputs mix balanced signals into a 4 output "mult" above the inputs. In this version, you have two inputs that mix equally, one that attenuates -3dB (relative to a 0dB input) and another input that attenuates -9dB (relative to a 0dB input). You can also get one -6dB mix by plugging into -3dB and -9dB (because the load is the other inputs, the -3dB input is actually not attenuated at all so you get 0dB and -6dB).

So to implement tape echo with feedback like you're doing, you would use one mult-mix (which I will call MM1) to split the dry to tape and to a second mult-mix (MM2) and then mix the tape output into MM2 (at 0, -3, -6 or -9dB). So the "input" of the whole thing is MM1's input and the "output" is MM2's output. You can also get feedback by taking the MM2 output and feeding it back to MM1 (at 0, -3, -6 or -9dB). And, as a cool side-effect, if you yank the input from MM1, it removes the load so that the output of MM2 doubles back through MM1 and suddenly you get a 100% feedback loop which causes that characteristic spaceship saturation sound. To stop the loop and go back to the mix, plug the bantam jack back in.

Re: Tape delay FDBK
« Reply #7 on: April 27, 2019, 02:02:20 PM »
Cool thanks, I’ll check out that teac box!

That circuit is much better.

Note that the "proper" way that this would be done in a studio would be to use an AUX send on your console or digi-interface+daw and then recover the wet-only echo on another channel and mix it into whatever track(s) to taste. That method is superior because 1) you can adjust levels to account for the fact that consumer and even pro-sumer Reel-To-Reel machines cannot handle high level signals used by pro consoles or digi-interface+daw setups, 2) you don't run the dry part of the track through another gizmo and most important 3) you can mix in just enough of wet to taste and not add the full noise of the source output.

But I could also see how you would not want to go through all of the setup for that in which case a little gizmo like you're making is nice. Although little mixers are pretty cheap

Totally get that the proper way Is with six and and return. No true console here but I kind of like the idea of a little purpose built gizmo. I’ll start bread boarding tonight and see what happens. I think the mix would be full wet 90% of the time. Could be a handy feature though. Maybe a switch to engage the mix knob?

What do you think of the “mix” circuit? Im not sure if it’s going to work as drawn.

squarewave

Re: Tape delay FDBK
« Reply #8 on: April 27, 2019, 03:31:29 PM »
What do you think of the “mix” circuit? Im not sure if it’s going to work as drawn.
I think it's fine. It should work. But I'm wondering if you swap the input pins 2 and 3 on Rtn from Tape XLR, you could drop the extra op amp. But again, you don't need balanced to / from tape. Even if you have balanced IO on the tape machine, it's usually better to skip it if you can using the monitor connection if there is one. If it's a consumer machine with RCA you could just have RCAs with the "Rtn from Tape" signal conductor connected to an unbalanced inverting input. Then the two mixed signals should be in phase and you don't have an odd number of op amps.



Re: Tape delay FDBK
« Reply #10 on: May 01, 2019, 11:52:21 AM »
well damn. That looks pretty great. I think I might try that with some quasi bal i/o.

Re: Tape delay FDBK
« Reply #11 on: May 02, 2019, 05:56:39 PM »
So decided to move forward with my own conjuring... I think this is pretty close. I have a few of the THAT corp chips around so I decided to use those. Left a lot of resistor/cap values blank atm to experiment. Also added a resistor (R16) to lower the feedback level so I can make it start oscillating at the extreme end of the feedback pot. I know that entirely dependent on the external levels but I wanted the option and I think that will work as drawn. Anybody care to destroy this thing? Anything I'm just wrong about haha? Rip it up! I'm trying to get better.


squarewave

Re: Tape delay FDBK
« Reply #12 on: May 02, 2019, 07:36:28 PM »
You swapped HIRTN and LORTN but you didn't remove the extra inverting stage. So your IC5B output is not really positive. Get rid of IC2A and just send tape inverted signal.

Then, because you will have an odd number of op amps you might as well use a regular op amp instead of INA137. The difference in performance is not worth another IC.

You can make a quasi-balanced output by putting a resistor the same size as R1 between LOSND and AGND. Use 100 ohms. That will isolate the two grounds to some extent and potentially reduce ground related noise.

You should put a coupling cap on the output of IC5B because any offset is going to put DC through all of the pot wipers.

If you want the 1646 output not to be inverted, you will need to also swap HIOUT and LOOUT.

Note that since the circuit does not have a lot of gain, using 5532 isn't really necessary. A FET op amp like TL071 or OPA2132 would be good and you won't have to worry about offset into the 1646 (which would make for a whacky offset on the output).

Re: Tape delay FDBK
« Reply #13 on: May 03, 2019, 08:28:35 PM »
You swapped HIRTN and LORTN but you didn't remove the extra inverting stage. So your IC5B output is not really positive. Get rid of IC2A and just send tape inverted signal.

Then, because you will have an odd number of op amps you might as well use a regular op amp instead of INA137. The difference in performance is not worth another IC.

You can make a quasi-balanced output by putting a resistor the same size as R1 between LOSND and AGND. Use 100 ohms. That will isolate the two grounds to some extent and potentially reduce ground related noise.

You should put a coupling cap on the output of IC5B because any offset is going to put DC through all of the pot wipers.

If you want the 1646 output not to be inverted, you will need to also swap HIOUT and LOOUT.

Note that since the circuit does not have a lot of gain, using 5532 isn't really necessary. A FET op amp like TL071 or OPA2132 would be good and you won't have to worry about offset into the 1646 (which would make for a whacky offset on the output).

Thanks squarewave. I can't tell you how much I appreciate the help.

So I ditched the INa137 for a standard opamp bal input. (TL074 for everything less the output).
If I get rid of  the inverting amp the send will be coming straight off of the  V.E. summing amp (IC5A). I guess my hang up is I always see the inverting amp after a summing amp but I guess an inverting amp is just that...an inverting amp.
This seems much more slick with a TL074.  Schematic attached! I also renumber everything so it won't line up with previous schemes.
« Last Edit: May 04, 2019, 05:04:44 PM by RSRecords »

squarewave

Re: Tape delay FDBK
« Reply #14 on: May 04, 2019, 03:53:55 PM »
I'm not able to download that attachment.

Re: Tape delay FDBK
« Reply #15 on: May 04, 2019, 05:03:56 PM »
I'm not able to download that attachment.

Weird try this?

squarewave

Re: Tape delay FDBK
« Reply #16 on: May 04, 2019, 06:34:24 PM »
Why did you un-swap RTN+ and RTN-? Now you're out of phase again. Leave RTN+ and RTN- swapped like you had before. Then I think the circuit would function as desired.

But you don't need a lot of those caps. As long as AGND is connected to the chassis and you're not using a virtual ground for use with a signal supply, you can drop C1, C2 and, with a low offset op amp like TL07x, you don't need C17, C9 or C10.  You only need C7 and C8 so that if something connected to an input has an offset, the DC won't get into the pots.

R1 and R2 are doing nothing.

Also, you don't need any of the 47p. You only need a cap like that in circuits that have significant gain or with certain older op amps. But TL07x doesn't need bypass caps like that.

I'm not sure I would put in R12. It's perfectly fine to feedback 100% so that you can get Space-Echo overdrive sounds.

Re: Tape delay FDBK
« Reply #17 on: May 04, 2019, 09:08:55 PM »

good call. Swapping the rtn was  an unintentional mistake when I redrew it.

I was unsure of the blocking caps, I figured it’s easier to jump them than  to squeeze them in later. But I’m ignorant of different opamp characteristics so I’ll yake your word on it.

R12 is meant to adjust the feedback so that the distortion and  oscillating starts later in the pot travel. I dunno makes sense in my mind. Can always jump it

I appreciate the help.
I’ll make these changes and try it out.


Re: Tape delay FDBK
« Reply #18 on: May 17, 2019, 09:20:54 AM »
finally put this together. Sounds pretty great and the feedback works perfectly. The mix circuit isn't quite right however. I'm not getting full wet or full dry and the middle sounds very phasey. I think I'll need to implement a polarity switch not the return. Any other obvious reason that the mix circuit isn't functioning like I thought it would?


squarewave

Re: Tape delay FDBK
« Reply #19 on: May 17, 2019, 09:54:44 AM »
Whoops. Is dry signal doubling back trough the feedback pot? Does the issue occur when the feedback is all the way off?
« Last Edit: May 17, 2019, 09:58:08 AM by squarewave »


 

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