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 Forum index » Instruments and Equipment » Strings and things
Peizo+Condenser preamp
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Uncle Krunkus
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PostPosted: Tue Apr 24, 2012 5:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Inventor wrote:
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!


"You should come on down to the lab!
And see what's on the slab!"

Some of these specimens would make your eyes bleed! Laughing

BTW,... It's 10pm here. Maybe we got major lag?

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Uncle Krunkus
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PostPosted: Wed Apr 25, 2012 4:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey Les!
The 6u8 that I settled on to go across R9 increases AC gain in the piezo pre-amp section.
I need to reduce the levels coming out of the condenser pre-amp. Shocked

Increasing R1 (DC supply for the ECM) would have to reduce the signal hitting the base of T1 wouldn't it? (But this would raise the noise floor yeah?)
So should I try more negative feedback first?

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PostPosted: Sat Apr 28, 2012 1:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Now you've got to tweak the circuit, Uncle, and I can't really advise except to say that what I would adjust first is R4, also C2 and C3 are candidates as they do the same job as C7 on the other circuit.

Les

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Uncle Krunkus
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PostPosted: Sat Apr 28, 2012 6:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey Les,
Yeah, I thought C2&C3 were likely suspects.
What I've done is reduce R4 to 7K5 which gives a bit more negative feedback to drop the overall gain. I seem to remember that negative feedback is also good for controlling distortion. Is that right?
The condenser was still too big across the blend pot though, so I've put a 51K in series off the condenser side.
Sounding pretty good now, so I'm giving it back to Mark to finish stringing it.

Now I'm starting a new, thinner design for the first harp he finished. It's got a different position for the preamp, so I have to change the layout a bit to fit.

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PostPosted: Sat Apr 28, 2012 7:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, I'm only really familiar with feedback in opamp circuits, but yeah that's the deal. For an inverting opamp configuration the gain is -Rf/Ri so lowering Rf lowers gain. Should be the same for this circuit as you have discovered empirically. Good work, looks like we have a working circuit for now.

Les

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Uncle Krunkus
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PostPosted: Sat Apr 28, 2012 6:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, I've always found that building stuff like this and ironing out issues as they come up is the best way to learn things that stick in your head for a long time. I know there is heaps of theory associated with this simple circuit which, for now, I have forgotten, but like I said, I don't retain equations very easily. Most of my design process is doing things like this which feed my storage of instinctive decisions for later projects. Does that make sense?
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PostPosted: Sun Apr 29, 2012 8:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, most of us learn by doing better than by studying. I find a mix is best for me.

Les

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Uncle Krunkus
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PostPosted: Fri May 04, 2012 1:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Well,
Version 1 is out the door. Cool
I'm now working on a new version which is a lot thinner, and longer, (the battery is flipped around onto the same side of the board as the rest of the components) which is great, as I now have two separate stripboards layed out, depending on if I need it to be thin, or short. Wink
The thin version is going into the second harp, which is a dark red timber, with a self-contained sounding box across the bottom of the strings. Surprised
The next stage is to test a completed unit. The problem at the moment is that the installed version 1, is unstrung, and the strung harp is yet to have version 2 installed. Rolling Eyes

Back later, gotta wash the pups. Very Happy

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PostPosted: Thu May 10, 2012 6:15 am    Post subject: Reply with quote  Mark this post and the followings unread

It's going well, but,...
As I expected, I have a failure.
This second board doesn't put out anything. Just a bit of hum.
I've checked connections, cuts, soldering, and traced it out, so I'm wondering which is the component most likely to fail, through excessive heat, etc.? The FET? The ECM?

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PostPosted: Thu May 10, 2012 3:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

trace the signal with your scope from the sensor through the amp, perhaps?

Les

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PostPosted: Fri May 11, 2012 4:19 am    Post subject: Reply with quote  Mark this post and the followings unread

The scope wasn't showing anything across either the piezo or the (powered) ECM insert. Then I tried across the main output, and at about line levels, I can see the radio on the scope. So it was working! then I turned the blend completely mic, and nothing, back to piezo, and the scope's showing nothing again!?!?
Maybe the scope lead is dodgy?
I've gotta try and find my stash, I've got five scope leads around here somewhere. I s'pose you don't have the problem of losing stuff at the moment hey Les? When you're restricted down to a specific set of tools, it's harder to lose things. I think this could even be a universal truth! Wow! There are good things have come of adversity. All over the world. All the time.

And the same will be so for this pre-amp!
I built it for fuck's sake!
The fault can run, but it can't hide! Laughing

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Uncle Krunkus
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PostPosted: Fri May 11, 2012 6:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Problem fixed!!
So listen up all you DIY peeps!

I'm using those leaf style 6.5mm sockets. Fonik uses them a lot on his setup, but he's always bought the better quality ones with the chrome front.
I just wanted to try them out for this project, so I got the cheaper version. And I got 'em from Jaycar, which, sometimes are not so great, quality wise.

Well,
The third metal finger which contacts the tip of the 6.5 was just not quite bent down enough to make contact with the plug!! I just gave it a bit of a tweak with some pointy-nose pliers, and now everything is working fine!!

So,.... even when you know that the whole circuit is fine. (this is the second copy) things can still make you track a fault. As DIY people, we need to accept this as part of the territory I s'pose.

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PostPosted: Fri May 11, 2012 3:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

yep, true that. I always get machined sockets - pricey but always work, never fail!

Les

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PostPosted: Fri May 11, 2012 3:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks again for your consistently optimistic and logical advice Les! Very Happy
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PostPosted: Sun May 13, 2012 3:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Okay,
So now to tackle the level/distortion problem.
I've attached 6 fly leads which I can check on the scope with the board installed. So I can play any string, and see what it looks like on the CRO.
The output of the ECM insert is very soft, but smooth. No distortion. As soon as you look at the junction of Q1 & Q2, there's a 100 fold gain, and the bottom of the wave regularly clips with harder playing. I thought I could re-bias Q1, but it's already too loud at this stage, so maybe I should just wind back the gain. The signal will then drop into a well biased section on Q1 as well. Does that make sense?
I just had a look, and R4 (feedback) is the only DC connection to Q1s Base. When I reduced it from 10K to 7K5 I probably inadvertantly dragged the bias down, thus creating the clipping on the negative excursions.
So, I'm thinking of taking it back to 10K, and finding another way of controlling how much signal goes through this first transistor.

Any thoughts? Shocked

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PostPosted: Sun May 13, 2012 4:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Or,......?

What if I put a series cap in the feedback loop so it would only effect the AC gain?
No,... that would remove the DC from the base of Q1 altogether.
Maybe a sidechain?
5K and series cap in parallel with the 10K feedback resistor

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PostPosted: Sun May 13, 2012 5:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Could I reduce the level into Q1 by simply putting a resistor in series with the input?
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PostPosted: Mon May 14, 2012 8:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've replaced R4 (feedback) with a 10K as originally designed. I also put a 4K7 and a 10uF in series, around the 10K. (to hopefully reduce the AC gain)

There's a definite improvement. The level is similar, but it seems to be pulled back from the negative clipping levels a bit.
It's still clipping though, so I'm thinking about the series resistor in to Q1 to just force the level down.

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PostPosted: Tue May 15, 2012 9:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Try removing the emitter degeneration capacitor (wow that sounds so.. so scifi!).

Les

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PostPosted: Wed May 16, 2012 2:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Do I just remove it completely? or change to a smaller value?
Can I do the series resistor in? or will that raise the noise floor?

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PostPosted: Thu May 17, 2012 2:27 am    Post subject: Reply with quote  Mark this post and the followings unread

I'd try removing it completely and see what happens, should be lower gain, then if it's too low, increase the feedback resistor.

Les

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PostPosted: Fri May 18, 2012 1:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Inventor wrote:
Try removing the emitter degeneration capacitor (wow that sounds so.. so scifi!).


Capt'n!
I've disconnected the flux capacitor from the emitter in the first stage of the ECM transducer circuitry, and the power density levels are no longer in Red Zone proximity!!

Charge phasers and fire at will!!

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Uncle Krunkus
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PostPosted: Sat May 19, 2012 6:03 am    Post subject: Reply with quote  Mark this post and the followings unread

You still there Les?
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PostPosted: Sat May 19, 2012 6:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
You still there Les?


yes, IV in left hand, cannot type easily. congrats, mission accomplished!

Les

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PostPosted: Sun May 20, 2012 7:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah dude,
Much thanks to you. The output right across the blend is great, clean mic sound, and add as much piezo as you like.
It's a great feeling when something you're working on goes from a 4/10 to an 8/10. And so far, I haven't found any phase problems at all. I did a spectrum analysis on soundforge, which shows no predominant troughs or peaks, and a similar fundamental amplitude right across the two octaves of the harp. Cool

NTS - I have to update the schems and layouts! Surprised Laughing

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