sonolink

Tube Preamp pedal + DI Output question
« on: July 22, 2020, 03:39:41 PM »
Hi again everybody,

I've posted a few questions regarding this tube preamp pedal I've been working on for a while now, and I am almost finishing this project now (at last!!). It's coming out great thanks to all of the kind help I have had in this forum, and sounds huge.
I'd like to do a bass version, but without cabsim. My doubts are about the DI output. It should be like a parallel out that is "always on" for a soundcard or a mixer and you should be able to use either independently or both together.

1-Would the DI out section of the attached schem be correct?
2-I think I need to add protection caps in case someone leaves phantom power on at the mixer. 100uF/63v electrolytics  would be ok?
3-Is it worth considering an output pot for the DI circuit? Is that "usual"?
4-Is 100k a correct value for R10 ?

Thanks a lot for your help

Cheers
Sono
« Last Edit: July 24, 2020, 04:00:55 AM by sonolink »
Why sleep when you can mix?
M.P.Stavrou


sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #1 on: July 22, 2020, 03:40:28 PM »
Same schem with different resolution...

https://ibb.co/P4gwmKb
Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #2 on: July 24, 2020, 04:00:31 AM »
pretty please....
Why sleep when you can mix?
M.P.Stavrou

Heikki

Re: Tube Preamp pedal + DI Output question
« Reply #3 on: July 24, 2020, 04:43:23 AM »
Are you using the op amp with single supply configuration like drawn on the schematic?

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #4 on: July 24, 2020, 04:52:38 AM »
Thanks for your reply Heikki :)

Are you using the op amp with single supply configuration like drawn on the schematic?

Mmm, I'm not sure what "single supply configuration means". I'm feeding 9VDC to  the opamp pin 8...
Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #5 on: July 25, 2020, 05:25:14 AM »
Bump
Why sleep when you can mix?
M.P.Stavrou

gyraf

Re: Tube Preamp pedal + DI Output question
« Reply #6 on: July 25, 2020, 06:36:02 AM »
What tube, what B+?

R9 is too big, or your "MASTER" is too low-ohmic at 10KA

the two positions of "diode clip" are the same unless different diodes are used

/Jakob E.
..note to self: don't let Harman run your company..

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #7 on: July 25, 2020, 07:17:15 AM »
Thanks for chiming in Jakob,

What tube, what B+?
Tube is ECC83. B+ is 200VDC

R9 is too big, or your "MASTER" is too low-ohmic at 10KA
Originally I had a 1M pot as Master. Then PRR sugested this:
https://groupdiy.com/index.php?topic=74593.msg944826#msg944826

the two positions of "diode clip" are the same unless different diodes are used
Yes, the idea is to use LEDs and 4001s to have different flavors

Originally this project had a cabsim between the preamp out and the DI section input. What I really need to know is if it's ok to just cut out the cabsim section since I don't need it to use it as a bass pedal.

In the original DI circuit by Elliott Sound R10 is 10k. It was suggested by Abbey to use 100k because of the Cabsim output. Should I go back to the original 10k or stick to 100k since the cabsim is no more?

Can someone please answer these questions? I try my best to learn but I need some assistance here please :)

1-Would the DI out section of the attached schem be correct?
2-I think I need to add protection caps in case someone leaves phantom power on at the mixer. 100uF/63v electrolytics  would be ok? Maybe 10uF?
3-Is it worth considering an output pot for the DI circuit? Is that "usual"?
4-Is 100k a correct value for R10 ?

Thanks again for your time and help
Sono
Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #8 on: July 26, 2020, 12:05:30 AM »
Bumpety Bump...
Why sleep when you can mix?
M.P.Stavrou

volker

Re: Tube Preamp pedal + DI Output question
« Reply #9 on: July 26, 2020, 02:45:55 AM »
1. No, it's not. Since you run the opamp of a unipolar voltage, you need to bias it accordingly. See here for "Voltage Divider Biasing". Basically R10 needs to connect to half your supply voltage = 4,5V instead of ground.
2. Yes you can do that if that is a concern, 10µF/63V will be fine.
3. You already have the master volume. Unless you want to have different levels at the same time for your regular output and the DI another volume won't be of any use. If you want that though, you need another opamp stage to buffer the potentiometer. Or you could put another 10k in parallel to the master.
4. No, make it 1M. You also need an input capacitor of 10n with the correct biasing scheme.

edit: I didn't fully think this through before re your second question. When you change the bias to what I suggested the output of the opamp will sit at 4,5V, a output capacitor of course is mandatory and you need to put the positive side of the capacitor towards the opamp output. When phantom power gets switched on, the other side will go higher (48V vs. 4,5V) and the capacitor will be the wrong way. Two solutions for this problem: use a 10µF film capacitor (available from Wima for example at 63V or 100V, since it is only guitar 4,7µF would be big enough as well). Or you reference the 9V supply negatively to ground, the opamp output would sit at -4,5V with capacitor minus towards the opamp and all will be fine when phantom is switched on.
« Last Edit: July 26, 2020, 04:05:50 AM by volker »


sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #10 on: July 28, 2020, 12:53:39 PM »
Thanks for your reply Volker :)
Sorry for my late reply.

I've read your link, and found it very enlightening. There is something I don't understand though: when I originally built this circuit there was a cabsim at the output of the preamp and I asked here for advice on how to add the following circuit by Elliott Sound as a DI Output:
https://sound-au.com/project87.htm#cheat

Abbey was kind enough to draw me how to do it on the cabsim schem, and indicate that "You may have to increase R1 [the resistor between the opamp +input (pin 3) and GND] to about 100k, in consideration for the 4.7k resistance at the output of the cab simulator."



What I don't understand is, why wouldn't the DI circuit opamp work as described by Abbey placed directly at the output of the preamp?

Thanks again for your time and help :)

Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #11 on: August 02, 2020, 05:37:58 AM »
Pretty Bump....
Why sleep when you can mix?
M.P.Stavrou

ruffrecords

Re: Tube Preamp pedal + DI Output question
« Reply #12 on: August 02, 2020, 07:33:30 AM »
Because the input impedance of the (bipolar) op amp is not high enough. However, if you used a FET input op amp then you could use it as the DI and delete the preceding parts.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #13 on: August 02, 2020, 03:25:18 PM »
Thanks a lot for chiming in Ian :)

Would this work?



Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

ruffrecords

Re: Tube Preamp pedal + DI Output question
« Reply #14 on: August 02, 2020, 04:35:15 PM »
Thanks a lot for chiming in Ian :)

Would this work?
Cheers
Sono

Probably not. The output impedance of the single FET stage is likely to be too high which is why most simple DI's follow the FET with a transistor. If you took the op amp circuit of the prvious post and used a FET input op amp like a TL072 you good use that the replace the single FET circuit.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #15 on: August 02, 2020, 04:43:45 PM »
You mean this?



but with a TL072 instead of the NE5532?

Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

ruffrecords

Re: Tube Preamp pedal + DI Output question
« Reply #16 on: August 03, 2020, 04:49:06 AM »
That is basically right. There are a couple of practical issues you need to take care of. The TL072 +input needs to be biased to about half the 9V supply rail, you need a dc blocking capacitor between the same input and the pot, and blocking capacitors at the output as in Rod elliots original impedance balanced output

Here are the schematics \I recommend people use for gain make up with my passive EQ designs:



The bottom one is the one you need to base your circuit on. You do not need R6, R9, C7 and C8 because you want unity gain so just connect the - input direct to the output. The bias circuit is R11, R12, C6 and R7. The blocking capacitor is C10.

This circuit has an unbalanced output but you can simply substitute Rod Elliot's impedance balanced out out circuit.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #17 on: August 03, 2020, 05:19:26 AM »
Thanks a lot for your help Ian! :)

Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #18 on: August 03, 2020, 01:20:25 PM »
Sorry if this is a dumb question, but in your circuit, R8 is a pot right? What does it do, is it the Make Up Gain control?

Just trying to learn and understand here :)

Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: Tube Preamp pedal + DI Output question
« Reply #19 on: August 03, 2020, 02:12:50 PM »
Would this be correct?



I changed the input blocking cap to 100nF (it was 330nF on your circuit) since all the blocking caps on this circuit seem to be 100nF, and added blocking 10uF film caps (following Volker's suggestion) at the XLR output.

I have a question: isn't C12 (input blocking cap) redundant since C9 was already there before the Master pot?

I hope I got it correctly this time  :)

Thanks a lot for your guidance :)

Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou


 

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