"Personal monitoring system" design

Stompboxes circuits published in magazines, books or on DIY electronics websites.
lukatosh
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Post by lukatosh »

Hello!
i'm Lucas... Writing from chile!

Well... I play the acoustic guitar and everytime that i play with the band i can't hear myself through the mix or even the monitors.

So... I would like to make a simple unit that allows me to split my original guitar signal in two equal signals. One will go directly to the mixer(OUT2) and the other(A) will go to a MXR headphone amplifier circuit(B) so i can hear myself through personal headphones...

The idea is to have this unit on all the time by a 9v dc adapter... probably the 2nd out of the splitter would be connected to a 3PDT to switch in between a tuner or the direct out.

So... i ask for help :)

The way i present the design... is correct? do you have any suggestions?

since... point A and B would be connected all the time, should delete R3 - 22M?

The values of the 2 resistors in the Vref net... are ok? 10k since now i'm feeding two op amps?

I would like to finish this before start working on a pcb :)

Thanks so much!

Best Regards From chile!

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jymaze
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Post by jymaze »

Hey,

It looks good,except R3 and R7 that are way too high for the purpose: I would limit both of them to 2.2Mohms (or even less actually, like 470k if you intend to never plug a passive source in B). And make R2 and R4 2.2Mohms too by the same occasion to have a nice 1M as input impedance for the guitar.

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Post by deltafred »

jymaze wrote:Hey,

It looks good,except R3 and R7 that are way too high for the purpose:
+1
R7 should be about 100k for correct opamp operation, R3 (I would guess at) 1M.

Your output stage will probably be susceptible to crossover distortion, there are far better ways of biasing the output transistors than with 2 diodes. With headphones crossover distortion will soon get very tiring.
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Post by jymaze »

Alternatively, to simplify the design a the headphone amp, you could use a LM386. It is more than enough for the purpose and sounds decent.

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Post by deltafred »

jymaze wrote:Alternatively, to simplify the design a the headphone amp, you could use a LM386. It is more than enough for the purpose and sounds decent.
+1

My thoughts exactly but last time I suggested using a 386 instead of a crudely biased push pull output stage I got shot down and told they were crap and a diode biasing would sound better. You gotta love the internet!
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Post by jymaze »

Actually, if I had to use discrete transistors as a buffer after an opamp, I would use a diamond follower (resistor biased). It uses 4 transistors instead of 2, but it is a perfect design.

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Post by lukatosh »

Dear friends, thanks so much for your suggestions, i really want to make this the best way we can do!

since my english is not so good, i would like to clarify some points...

1) Input impedance, 1M would be fine to be used on acoustic guitar with onboard preamp? (takamine EG530C)

2) Changed the amplifier circuit to an LM386 based circuit... Fixed gain... it is on a MONO configuration, but thats ok...

i've used this circuit:
Image

3) what are your suggestions to this new desing?

4) Can I connect L and R (Tip and Ring of the out Jack) channels to the out after 470uF capcitor?

Image

Thanks so much!!

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Post by jymaze »

1M of input impedance will be fine, and yes, your should connect both L and R to the output.

DO NOT use R5 and C6, unless you want to have distortion and/or blow up your ears! It will be probably pretty loud without it already.

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Post by lukatosh »

Thanks for your help!

The minimum gain of the lm386 is 20 db... is there a way to reduce that even more? or just set the gain at minimum and then make a voltage divider at the output?

which would be appropiate values for this?

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Post by Jarno »

I think your initial choice of components wrt the headphone amp was spot on, I myself dislike LM386 with a passion for anything other than a Clari(not) build.
Instead of the 2n3904/2n3906 I have used BD139/BD140, and it sounds great. If you do not need that amount of power (no 32ohm headphones) BC327/BC337 is an option.
I have a schematic and PCB layout somewhere, let me know if you want to change your design and I'll dig them out.
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Post by smackoj »

Jarno; hi. I just bought a couple BD139/140 chips to use in a design and found out the design is badly flawed. So, I'm scrapping that idea and looking for a proven design to use
the new BD chips in. I would certainly appreciate you posting the design you mentioned above.

thanks, smacko jack

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Post by deltafred »

smackoj wrote: I just bought a couple BD139/140 chips to use in a design and found out the design is badly flawed.
Yea, fixed diode biasing is crap for a multitude of reasons.

It can sort of work if you have the biasing diodes in very good thermal contact with the output transistors but is still a crap shoot and very dependant upon the vbe of the transistors and the forward volts drop of the diodes. These can vary considerably in the production process.

There are far better ways of doing it that minimise crossover distortion and power consumption. Low power amplifier chips being by far the easiest. Your PC, phone, TV, car radio, and mp3 player don't have discrete amplifiers in them and they sound fine.
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Post by deltafred »

jymaze wrote:Actually, if I had to use discrete transistors as a buffer after an opamp, I would use a diamond follower (resistor biased). It uses 4 transistors instead of 2, but it is a perfect design.
Do you happen to have a working example (that could be used as a headphone amp), looks interesting and could be a great little project.
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Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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Post by rocklander »

deltafred wrote:
jymaze wrote:Actually, if I had to use discrete transistors as a buffer after an opamp, I would use a diamond follower (resistor biased). It uses 4 transistors instead of 2, but it is a perfect design.
Do you happen to have a working example (that could be used as a headphone amp), looks interesting and could be a great little project.
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Post by deltafred »

rocklander wrote:
deltafred wrote:
jymaze wrote:Actually, if I had to use discrete transistors as a buffer after an opamp, I would use a diamond follower (resistor biased). It uses 4 transistors instead of 2, but it is a perfect design.
Do you happen to have a working example (that could be used as a headphone amp), looks interesting and could be a great little project.
[smilie=bump.gif]
IIRC there was a parallel thread to this (Could be wrong of course) where a diamond buffer schematic was given I will scour my hard drive for a schematic and also look for the thread.
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Post by deltafred »

Schematic below, I never tried it because I bought a faulty Samson S-Phone multi headphone mixer-amp off the bay and fixed it.
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Post by rocklander »

thank you sir. so what voltage are we looking at?
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Post by deltafred »

rocklander wrote:thank you sir. so what voltage are we looking at?
A bit more digging and I find that I was PMed a link by jymaze but it was not for the schematic above, no idea where I got that from. Hope it ok to post it here

http://phonoclone.com/diy-bb.html

Looking at the power supply on the link it will be +/- 12v but I would try it at a lower voltage and also try single ended and decouple the headphones with a large value electrolytic.
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Post by rocklander »

hmm.. thanks.. it just went past my level of understanding :oops:

I may just use an lm386 amp like MicTesters.

in a similar chase of 'free-lunch' ... I'm hoping to use a couple of rechargable AAA batteries. that'd be a grand total of 2.4V..
I have googled "max1044 calculator" (and 7660 calculator) and not come up with much, so I'll ask here and look even stoopider than normal.
if I whack 2.4V into a standard voltage tripler circuit (I find a plethora of examples getting +9v <-> -9v and also see things like this which make a voltage tripler
Image) can I get >6VDC, or are there more resistor/cap/diode changes I'll have to add? :scratch:
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Post by deltafred »

You might be able to use one of the cheap step up converters, I have just started messing with a 1 GBP board from Ebay, China. Try searching for "XL6009" with the worldwide option set.

It does say 3v minimum so you may need to use 3 batteries.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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