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Peizo+Condenser preamp
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Uncle Krunkus
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PostPosted: Fri Apr 06, 2012 5:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Managed to stuff that up!
(And I used to do it all the time! Laughing )
Is there any way that could be made easier Mosc, or Jan?

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Uncle Krunkus
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PostPosted: Fri Apr 06, 2012 5:47 am    Post subject: Reply with quote  Mark this post and the followings unread

T1 & T2 are BC549C
T3 is an MPF102

Sorry about the terrible quality, I haven't uploaded anything in a looong time! Laughing

I'll have better quality shots real soon. (hopefully! Rolling Eyes )

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Oskar



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PostPosted: Fri Apr 06, 2012 5:52 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm actually finding this very exciting! I don't circuits from Adam, but I DO know that amplifying stringed instruments is a challenge, so hats off to you lot for even trying. If I had ANY knowledge of electronics . beyond changing batteries - I'd be whining and moaning to be in on this.
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Uncle Krunkus
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PostPosted: Fri Apr 06, 2012 6:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Really appreciate all your input dudes,
this is a great conflagration of minds!
In heaps of ways (especially on theory) I only half understand, and then it takes a while for it all to sink in, (the Klee was a classic example of this). But laying the parts out into a real world solution needs to be done ASAP, as there's lots to be learned about it's physicality.
I think Mosc said that he would often design electronics from the front panel back. As in, get the ergonomics comfy, feel the space taken up by all the needs, wants, and likes of the circuit. Then design the circuit itself to meet those needs exactly.
Sorry, I'm just rambling on here.
Therefore,...........

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Uncle Krunkus
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PostPosted: Sun Apr 08, 2012 6:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey everyone,
I'm just having a go at posting a schem.
Which I just re-learned how to draw.
It's only the condenser pre-amp section.


schem test.JPG
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schem test.JPG



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Uncle Krunkus
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PostPosted: Sun Apr 08, 2012 6:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Okay, I should have cropped the .jpg Embarassed
But that's cool, I'll get there! Laughing

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PostPosted: Sun Apr 08, 2012 6:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Keep 'em coming, it's very interesting to follow the process from this side of the Globe!
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Uncle Krunkus
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PostPosted: Mon Apr 09, 2012 4:56 am    Post subject: Reply with quote  Mark this post and the followings unread

Here's the schem of what's on the stripboard built and tested.
The condenser side was dead before because of an unsoldered emitter on T1!
Now both pickups are working quite well. I need to start testing how well it would fit into the actual harp, and what it sounds like in there, but I'm also wondering if there are any improvements to the circuit that anyone can think of.
Any advice, as usual, would be most welcome.


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PostPosted: Mon Apr 09, 2012 7:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle, looks like that piezo disc symbol is shorted out - delete the wire across it. I would consider adding an opamp circuit for phase adjust, though I do not know of one off the top of my head. You'll want to tap the circuits prior to their dc blocking capacitors for that one (directly at the transistor terminals).

For phase adjust, google us up something called an "All Pass Filter" and post links to references and I'll advise from that. An all pass filter is a special filter that passes all the signal with equal amplitude but adds a phase delay. We want one in the audio range that is adjustable by potentiometer.

Les

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Uncle Krunkus
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PostPosted: Tue Apr 10, 2012 2:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Before we go down that path,.....
Look at the two R10s(!?)
See the way one comes after the cap and goes to ground with the output from the junction between the two, the other one (like you advised) is just a series resistor off the cap. Shouldn't they both be the same? I'm thinking one would tend to shunt the blend pot to ground, whereas the other will behave differently.

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Uncle Krunkus
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PostPosted: Tue Apr 10, 2012 2:34 am    Post subject: Reply with quote  Mark this post and the followings unread

BTW, yeah, you're right Les, there were actually 2! shorts across that piezo. Doesn't really matter for drawing, but would muck up the netlist if this got PCBed.
I read somewhere that you can improve the piezo circuit by putting a cap around the 560R resistor. Not sure why, but would that be a good thing or not?

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PostPosted: Tue Apr 10, 2012 2:46 am    Post subject: Reply with quote  Mark this post and the followings unread

OK, uncle krunkus, let's back up a minute. When you said the piezo circuit had an output impedance of 220k was that based upon the fact that you saw a 220k resistor at the output? Because that is not the output impedance. To me, that circuit appears to have an output impedance of less than 1k, as determined by the pullup resistor on the transistor primarily.

The output impedance of a circuit is not the resistance that you see shunted at the output. The purpose of that shunt 220k resistor is to center the output signal about ground since it appears after the dc blocking cap.

The output impedance of a circuit is determined by first measuring the open circuit voltage (or simulating or calculating or estimating it), and then determining the short circuit current. Then Rth = Voc / Isc.

However often you can "look into" the circuit and determine it's output impedance from experience or with simple techniques that are good approximations. In this case I have done this by seeing that a 1k resustor pulls the signal up and a transistor pulls it down (which will be of similar strength to the 1k resistor due to biasing) for an output impedance of about 500 Ohms (1k||1k).

Now that I think about it, why on earth would a circuit have such a high output impedance of 220k anyway? I failed to do a reality check.

My advice therefore, in this circuit, is to REMOVE THE 200K RESISTOR and probably the 220k resistor as well.

The circuit should work fine then. I apologize for not catching this the first time I saw the schematic yesterday (or thenabouts). Good question krunky uncle, you identified a major problem that was cause by your mistake and my blind acceptance of it. Hang in there, we'll get this harp a'workin'!

Les

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PostPosted: Tue Apr 10, 2012 2:51 am    Post subject: Reply with quote  Mark this post and the followings unread

About putting a cap across the 560 ohm resistor, that resistor is there to set the gain of the transistor by biasing it, using a technique called source degeneration, if I recall my grad circuits class correctly. I never really got into transistor circuit design all that much (well, some), so I don't know about a cap across it but I can tell you what a cap across it would do.

It would short out the source degeneration resistor (say that ten times fast, lol) at higher frequencies, increasing the AC transistor gain while still keeping the DC biasing correct. I would think this would be an improvement to the circuit and would offer higher gain of the signal that you want to amplify. So yeah, I'd say you heard right.

Les

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Uncle Krunkus
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PostPosted: Tue Apr 10, 2012 6:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Okay, now we're talking!
This is good stuff Les.
Lose both the 200K and 220K and take the caps straight to the blend pot.
Annndddd,..... I get to experiment with caps across the 560R. My gut feeling is that 10nF would be a good starting point. Whatcha reckon?

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PostPosted: Tue Apr 10, 2012 6:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, a little more theory for you to chew on uncle. The cap and resistor will be impedances in parallel, so the formula for parallel impedance applies. You will recall it to be product over sum, or (R * jXc) / (R + jXc). At DC Xc is infinite so you just get R. At large AC frequency values Xc will short and you'll get maximum gain from the transistor.

What we want to do is choose a capacitor value that will have Xc being small compared to 500 Ohms or whatever it is, at ALL of the frequencies of your harp. That way you won't get phasing and varying amplitude response across your usable spectrum. This may require a fairly large cap there, not sure, but here's the math.

We want Xc to be small. well, Xc = 1/(omega * C), where omega is 2 * Pi * f. So Xc = 1 / (2 * Pi * f * C) < 500 Ohms, or C > 1 / (500 Ohms * 2 * Pi * 80 Hz). Note that the < changes to a > when we take the reciprocal of both sides. Plugging that expression in the calculator we get C > 4u F.

So I would start with a 10 microfarad cap for C and "see" how that works. There will be a quiz on this material Thursday, so study up Mr. Krunkus!

Les

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PostPosted: Tue Apr 10, 2012 6:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh and a caveat to the above: I have never worked with this particular calculation before so we will have to see what happens a la trial and error. In case of problems i'd try smaller caps. Also it might be advisable to put a 0.1uF ceramic cap in parallel with the 10uF electrolytic cap to lower the series resistance at higher frequencies.

Les

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Uncle Krunkus
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PostPosted: Tue Apr 10, 2012 2:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'll hopefully start these mods later today Les.
Thanks heaps for all this related theory. This is the best way for me to re-learn this stuff. By actually applying it immediately, and feeling out the results.
I was dux of maths for my first 4 years of high school. I took on 3 unit maths in yrs 11&12, but unfortunately my depression started when I was about 15yo, and it almost immediately started to effect my ability to remember equations, and how to apply them. This wasn't so bad in physics, because that was just so intuitive for me that I would derive the equations from first principles if and when I needed them. But trig function equations, in maths class, would just leave me gazing into space.

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PostPosted: Thu Apr 12, 2012 3:32 am    Post subject: Reply with quote  Mark this post and the followings unread

I've modded the design to this schematic, which loses the 200K and 220K resistors, and adds a 6u8 around the 560R resistor.
It is sounding heaps louder, now I have to check it in the instrument.


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PostPosted: Fri Apr 13, 2012 4:36 am    Post subject: Reply with quote  Mark this post and the followings unread

I tested the prototype this arvo, in and around the actual harp, while Mark was playing it. My gut reaction is that the condenser is picking up too much,as it often will distort even just next to the sounding hole, I may have to crank it's gain back somehow.
The other thing I realised is that the 50Klin I used for the blend is now way too big. As the output impedances of the two pre-amps have dropped to about 2K, there is a section in the middle of the 50Klin pot which is quite dead. Each end features the two pickups quite well, so I think I should bring it down to say 5K or 10K depending on what I've got.
Otherwise, we were quite impressed with the "live" sound coming out of the reference speakers.
Mark left me one of the harps, and the prototype is getting mounted in there ASAP. This will be heaps better as a test bed, and will give me the chance to record some playing and analyse what the pickups are actually picking up.

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PostPosted: Tue Apr 17, 2012 6:31 am    Post subject: Reply with quote  Mark this post and the followings unread

I replaced the blend pot with a 25KLin, and that has improved the overall sound heaps, both in volume and quality.
It seems like the condenser is quite loud by comparison now, but I need to check that in the harp. The vibrations into the piezo could be a lot more than I get from tapping the desk. Anyway, we'll see.
I jammed the piezo into the back of the bridge on my bass, and had a great time. Nice, wooden sound.

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PostPosted: Tue Apr 17, 2012 11:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Great news! I'm happy to learn that things are going well with your project, Uncle!

Les

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PostPosted: Mon Apr 23, 2012 3:34 am    Post subject: Reply with quote  Mark this post and the followings unread

No, the condenser mic is still way too loud. I'm wondering if the ECM itself just can't handle the levels in there. Even with the gain turned right down on the mixer, I can't get rid of the distortion coming through, and if I do get close, the piezo can't be heard at all. Sad
So,...
Could I reduce the levels coming out of the ECM by upping the resistor that supplies it's DC? Kinda starving the little amp inside the ECM? Or should I try lowering the negative feedback resistor? Or do I just pad it with a series resistor to the blend pot? (I think this will just produce a smaller, yet still distorted, signal.)
Still, like before, properly adjusted, the piezo comes through quite nice! Very Happy

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PostPosted: Mon Apr 23, 2012 7:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus, remember that we added that big cap across R9 to increase the AC gain. If I understand you correctly, now the gain is too large, so just reduce the cap. Maybe a lot.

Les

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PostPosted: Tue Apr 24, 2012 2:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah! Les!
Truly wise in the ways of logic you are! Embarassed Laughing

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PostPosted: Tue Apr 24, 2012 4:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Ty. As you decrease the cap, remember that it will affect the phase shift which will greatly change the time domain waveform of the notes, however I recall that the Human ear cannot distinguish the phase of harmonics, so all is good the waves will sound the same.

The only difference would be a change in sound if, say, you were to put a distortion box on the harp or something. You're not insane enough to put a distortion box on a harp are you? Of course you are!

Hahahhahahahhaha!!!

Les

p.s. good morning Uncle Krunkus!

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