Instrument to "Line Level"
« on: December 28, 2020, 11:27:19 AM »
Trying to add an instrument input to a line level device. I'm thinking this should work. I'm unsure of r27/28 values. 10k/1k=10= 20dB
20dB enough?

edit: shows opa2134 but will probably be a TL072


JohnRoberts

Re: Instrument to "Line Level"
« Reply #1 on: December 28, 2020, 12:10:12 PM »
you need a dc path (resistor to ground) at + input of second op amp.

JR
It's nice to be nice....

Re: Instrument to "Line Level"
« Reply #2 on: December 28, 2020, 12:25:48 PM »
you need a dc path (resistor to ground) at + input of second op amp.

JR

Oh good point. I always omit that for some reason.

if I stick with 22uF, resistor should be 360ohm for 20Hz. 750 would give about 10Hz. That seem right?

squarewave

Re: Instrument to "Line Level"
« Reply #3 on: December 28, 2020, 04:37:47 PM »
Just skip the first op amp entirely. 10M input Z resistor is too high. Make it 1M or 2.7M max. And your pot is backwards.
« Last Edit: December 29, 2020, 12:04:28 AM by squarewave »

abbey road d enfer

Re: Instrument to "Line Level"
« Reply #4 on: December 28, 2020, 11:29:38 PM »
Just skip the first op amp entirely. 10M input Z resistor is too high. Make it 1M or 2.7M max.
+1

Quote
And your pot is backwards.
Seems correct to me.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

squarewave

Re: Instrument to "Line Level"
« Reply #5 on: December 29, 2020, 12:04:54 AM »
Seems correct to me.
Indeed. I had my glasses on upside down.

Re: Instrument to "Line Level"
« Reply #6 on: December 29, 2020, 09:15:33 AM »
Indeed. I had my glasses on upside down.

All good. I think it’ll just be an internal trim.
Thanks!

Whoops

Re: Instrument to "Line Level"
« Reply #7 on: December 29, 2020, 10:20:45 PM »
Trying to add an instrument input to a line level device.

Seems a good idea for a project.

Which "Line Level" do you want at the output? Balanced +4 dBu or unbalanced −10 dBV?

I guess it would be a pretty useful box if the output is +4 dBu Balanced.

I would use a FET front end, before the opamp and would make the input impedance 10Mega, that makes it useful also for all kinds of piezo pickups (acoustic guitar, Doublebass, Cello, etc)



Re: Instrument to "Line Level"
« Reply #8 on: December 29, 2020, 10:35:45 PM »
Seems a good idea for a project.

Which "Line Level" do you want at the output? Balanced +4 dBu or unbalanced −10 dBV?

I guess it would be a pretty useful box if the output is +4 dBu Balanced.

I would use a FET front end, before the opamp and would make the input impedance 10Mega, that makes it useful also for all kinds of piezo pickups (acoustic guitar, Doublebass, Cello, etc)

The idea is to interface with another mixer sort of unit I'm working out. Eventually the output will be +4 balanced.

abbey road d enfer

Re: Instrument to "Line Level"
« Reply #9 on: December 31, 2020, 01:43:56 AM »
Seems a good idea for a project.

Which "Line Level" do you want at the output? Balanced +4 dBu or unbalanced −10 dBV?

I guess it would be a pretty useful box if the output is +4 dBu Balanced.

I would use a FET front end, before the opamp and would make the input impedance 10Mega, that makes it useful also for all kinds of piezo pickups (acoustic guitar, Doublebass, Cello, etc)
I have always found that having that kind of input impedance resulted in too much boominess.
This article seems to confirm that:
https://www.gitec-forum-eng.de/wp-content/uploads/2019/03/poteg-6-piezo-pickups.pdf
K&K preamps have an input Z of 1Meg.
Typical bridge-mounted piezo pick-ups have a quite low capacitance (about 1nF), which results in a LF cut-off frequency of about 140 Hz, but most other piezo pick-ups have a much larger capacitance. I have fitted my  Yamaha C7 grand with two disc p/u's, that I measured at 2.5nF capacitance, feeding DI's with 1 Meg input Z, which results in 63Hz LF cut-off. No one complains.
Of course, I don't use them for recording, just for rehearsals.
« Last Edit: December 31, 2020, 01:53:04 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


Whoops

Re: Instrument to "Line Level"
« Reply #10 on: December 31, 2020, 11:13:06 AM »
Of course, I don't use them for recording, just for rehearsals.

I use piezo pickups on Doublebass, Cello, acoustic Guitar and Piano both for recording and Live sound.
Connecting the Piezo to a DI with 10 Mega input impedance sound much better than a typical 1 Mega input DI, that always sounded harsh and thin to me.
I found that 5M to 10M is my favourite load for Piezos, many people seem to have found the same also.
Like Radial says on their website "The ultra-high 10 meg-ohm impedance is used to compensate for the harsh, squawky tone that usually plagues piezo-equipped instruments.", I totally agree with their statement.

abbey road d enfer

Re: Instrument to "Line Level"
« Reply #11 on: December 31, 2020, 01:11:47 PM »
I use piezo pickups on Doublebass, Cello, acoustic Guitar and Piano both for recording and Live sound.
Connecting the Piezo to a DI with 10 Mega input impedance sound much better than a typical 1 Mega input DI, that always sounded harsh and thin to me.
I found that 5M to 10M is my favourite load for Piezos, many people seem to have found the same also.
Like Radial says on their website "The ultra-high 10 meg-ohm impedance is used to compensate for the harsh, squawky tone that usually plagues piezo-equipped instruments.", I totally agree with their statement.
This is not what I found, and K&K and Ovation seem to agree with me...Each one his own, I guess.
And there is actually such a variety of piezo pick-ups that what is right for one maybe marginal for another.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Instrument to "Line Level"
« Reply #12 on: December 31, 2020, 01:12:02 PM »
I use piezo pickups on Doublebass, Cello, acoustic Guitar and Piano both for recording and Live sound.
Connecting the Piezo to a DI with 10 Mega input impedance sound much better than a typical 1 Mega input DI, that always sounded harsh and thin to me.
I found that 5M to 10M is my favourite load for Piezos, many people seem to have found the same also.
Like Radial says on their website "The ultra-high 10 meg-ohm impedance is used to compensate for the harsh, squawky tone that usually plagues piezo-equipped instruments.", I totally agree with their statement.

Would be simple enough to have a dip switch or something to change the resistor value. This will be used as part of a tape delay so lack of low end isn't a problem. I think it would actually be a good thing to roll off some lows...Who puts delay on bass?!?

Whoops

Re: Instrument to "Line Level"
« Reply #13 on: December 31, 2020, 02:24:23 PM »
This is not what I found, and K&K and Ovation seem to agree with me...Each one his own, I guess.
And there is actually such a variety of piezo pick-ups that what is right for one maybe marginal for another.

Connecting a Piezo pickup to a 10 Mega input DI or a 10 Mega input FET Buffer is pretty common practice in the Industry and the benefits are pretty well documented.
I was impressed the first time I listened to an acoustic guitar piezo and a Doublebass piezo connected to a 10mega input DI, instead of the traditional 1mega input, the plastic and mid-rangy peaky tone that I associated with Piezo pickups was gone.
I could attest then that there was a reason why Piezo pickup buffers and preamps have a 10 Mega input impedance and why it was common practice to do so.
Unfortunately most standard DI's, even active ones had only 1mega input impedance.

That's the reason why a some of piezo pickup manufacturers were recommending users the Countryman Type 85 DI when a piezo preamp was not used.   
Thats the case with the excellent Carlos Juan and Yamahiko pickups and Shadow also.













Whoops

Re: Instrument to "Line Level"
« Reply #14 on: December 31, 2020, 02:28:30 PM »
Would be simple enough to have a dip switch or something to change the resistor value. This will be used as part of a tape delay so lack of low end isn't a problem. I think it would actually be a good thing to roll off some lows...Who puts delay on bass?!?

Well if you are using your circuit with a Tape Delay output then you don't need an instrument input (Hi-Z), the output of the Tape Delay is already Low-Z Line Level.

Re: Instrument to "Line Level"
« Reply #15 on: December 31, 2020, 02:36:37 PM »
Well if you are using your circuit with a Tape Delay output then you don't need an instrument input (Hi-Z), the output of the Tape Delay is already Low-Z Line Level.

No sorry, I wasn't being clear. It's a tape delay interface-- a send and return with feedback and mix control. I just posted the latest schematic here. https://groupdiy.com/index.php?topic=72366.0 The DI is just another input.

dbelousov

Re: Instrument to "Line Level"
« Reply #16 on: January 01, 2021, 03:11:19 AM »
An "instrument level" is a very vague term. I have a guitar with the Bare Knuckle Nailbombs (passive) and a bass with Bartolini guts, both of them easily overload professional ADCs with an unity gain buffer, so they have to be attenuated, not boosted.
« Last Edit: January 02, 2021, 10:34:29 AM by dbelousov »
Dmitry

Re: Instrument to "Line Level"
« Reply #17 on: January 01, 2021, 11:44:03 AM »
An "instrument level" is a very vague term. I have a guitar with the Bare Knuckle Nailbombs (passive) and a bass with Bartolini guts, both of them easily overload professional ADCs with a rail-to-rail buffer, so they have to be attenuated, not boosted.

Yea, this won't work with that.

Whoops

Re: Instrument to "Line Level"
« Reply #18 on: January 01, 2021, 04:24:42 PM »
An "instrument level" is a very vague term. I have a guitar with the Bare Knuckle Nailbombs (passive) and a bass with Bartolini guts, both of them easily overload professional ADCs with a rail-to-rail buffer, so they have to be attenuated, not boosted.

I find it really hard to believe your pickups will have more voltage than +4 dBu Line Level, that's 1,228VAC, even if your instruments output that voltage there's around 16 Db's headroom before the AD converter reaches 0DbFS and clips.
I don't know any passive or active guitar/bass pickups that will overload a +4 dBu Line Level input.

RSRecords is talking about a Line Input not a Mic input

dbelousov

Re: Instrument to "Line Level"
« Reply #19 on: January 02, 2021, 06:09:51 AM »
Some metal guitar players report about whopping 4-5 volts p-t-p from the hottest passive pickups on the market.

My Modulus bass overloads ADCs. Because of its insane signal level this is the way how I record it when I need a "clean" signal, it's just an active buffer, without preamps, etc. It works fine for fingers, can't handle the slap, though, without attenuation. Yes, instruments like this are rare, but they exist. I call this bass "the metal hate machine" because of its output and very aggressive sound.

BTW. RSRecords, may be you find some ideas here. It was implemented with the ADA4627 and a pair of the LM4562. The primary "floating" output is +6 dB, the "zero impedance" outputs can be easily configured to gain. The schematic can be implemented with an only one quad low noise FET op-amp at the cost of the one "ZI" output.
« Last Edit: January 02, 2021, 06:13:13 AM by dbelousov »
Dmitry


 

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