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Old 3rd August 2008, 03:58 AM   #1
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Default Practice Amplifier

Hi everyone;

I was just working on this, and wanted to get some inputs prior to fabbing PCBs and buying parts.

This is a practice amp with the following features
- 4 x AA NiMh batteyr pack
- DC power jack on rear
- 50mA constant current charge using LM317L
- Charge and Power LEDs
- 1/4 mono input
- adjustable from clean to dirty, using simple diode clipper
- modified tone stack with bass and treble adjustment +/- 8dB
- Volume goes to 11
- headphone jack 3.5mm which disables the speaker
- LM4861 boomer driving 4" 8 ohm speaker, about 1.2W

The below pic is one layer, the oppostie is 100% poured copper ground
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File Type: jpg pcb bottom.jpg (95.4 KB, 393 views)
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Old 3rd August 2008, 04:04 AM   #2
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Default PCB edge view

see below, I am using to work on the case and front panel design
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Old 3rd August 2008, 04:08 AM   #3
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And here is the schematic...

Your thoughts?
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File Type: pdf schematic.pdf (24.1 KB, 135 views)
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Old 5th August 2008, 02:19 AM   #4
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"These amps go to 11....." nice touch......
IIRC Crate did that for a while....

one practice i picked up many years ago that you might want to consider. put the dropping resistor for the power LED on the high side of the diode. that way if for some reason the LED leads touch the grounded case (such as might happen during testing the unit while partially disassembled), the LED won't get burnt out.
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Old 5th August 2008, 02:40 AM   #5
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Nice circuit. I'm a bit nervous about that BYP divider point not being buffered. It's well bypassed, so should be ok for ac, but will there be any problem with the feedback ratio on the distortion amp? I'd also consider the charger circuit- I'm too impatient to use a safe value of constant current charge, but at higher currents you really need a cutoff so it goes into float mode. If the input voltage is just right, it would work ok, but maybe consider one of the cheap battery charger chips. That way you could use NiMH cells to maximum advantage (they don't really like trickle charging).
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Old 5th August 2008, 05:16 AM   #6
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Uncle Jed - excellent suggestion. Not a big issue on this design, as I am using the dual diode in one black block thingie. I usually do it the way you suggest, but never for any particular reason.

Conrad - good call. Every single battery charger IC I have ever seen was SMT. It's not that I am afraid (see www.minthesizer.com), but the board is 100% through hole at this point.

I thought I could easily add a TL431 as a shunt, but then read more on NiMH, not a good idea. MAX713 is the closest to easy, but even that is a 50% increase in the total circuit complexity.

So I have an easy solution: NiCd batteries. Not very sexy, but they should do the job.

On the unbuffered ref, the worst case current is about 2V on 10k, or 200uA. Into 10uF, this would be about 20mV ripple at 100Hz. Also, the BTL amplifier output will reject this ripple.

Thanks again for your inputs. Your reponses show you really spent some time looking at this. I'll post here when I get results later.
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Old 5th August 2008, 12:27 PM   #7
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you could use a MOSFET to cut off the charging current. or you could use Lead-acid gell cells and use the circuit you have. a small gel cell (like the batteries that were used in the carry packs for motorola brick phones) will power the amp for about 10 hours per charge, and don't weigh very much.
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Old 5th August 2008, 01:17 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Careful selection of the speaker will make a big difference.
As it will be small a sealed cabinet should be used.

Possible candidates :

http://www.partsexpress.com/pe/showd...number=290-010
http://www.mcmelectronics.com/product/54-310
http://www.mcmelectronics.com/product/55-1595
http://www.mcmelectronics.com/product/55-1205
(published response of this one is "guitar speaker" like)

If you do not go for any of the above I'd suggest finding
a high efficiency paper TV type speaker, something like :
http://cpc.farnell.com/LS00464/audio...questid=542775

/sreten.
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Old 5th August 2008, 01:29 PM   #9
RDV is offline RDV  United States
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http://www.mcmelectronics.com/product/55-1290

RDV
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Old 5th August 2008, 02:39 PM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by RDV
http://www.mcmelectronics.com/product/55-1290

RDV

Hi,

That driver has a dip where you ideally would like a peak for guitar.
The dip is down to 80dB/W, in reality the driver is ~ 85dB/W.
This is why I did not include it. Tested by Zaph at Zaphaudio.com.

/sreten.
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