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Tin Taxi Busking Amp Project


Vox DA5We have been using a VOX DA5 for busking.  It's not bad. The sound is good, but it's not really ideal for an acoustic duo. 
There is a guitar input with tone control and lots of special effects which we don't use , a single mic input with a volume control and an aux input with no control at all.  We have been using a 3:1 passive mixer to get more inputs

I decided to build something more aligned to what we do. The things we needed were

  1. 4 inputs which can be either mics or instruments
  2. Better battery life compared to the VOX without loss of power
  3. More volume from the mics than the VOX
  4. Clipping indication
  5. Simple operation
  6. Inexpensive

What follows is the final iteration, I learned a lot in the process.

The Power Amp
To get the power, I bought a Class D Amp.  It is capable of providing 30 Watts (15+15) into 8 ohms with a 24V supply. The module is sold as PAM8620 although that is actually the ID of the chip. It is sold by lots of Chinese suppliers
Although it is a stereo amp I only use one channel, and a 10 or 12V supply. The amp can drive to within a volt of PAM 8620the rails or which gives about 5 or 7.5 Watts (depending on power supply) into an 8 ohm speaker. It has a volume control and I found it could reach max power with only 0.7Vp-p at the input with the volume turned up full giving it a max gain of 40 (or 32dB)

Class D outputThe output is pulse width modulated and a brute to observe on an oscilloscope.
Load with filterI used an 8 ohm load instead of the speaker to save my eardrums and with a filter across it I could see the waveform.
Then I could see a normal sinewave, and how the amp clipped. Nothing like a standard class A or B amplifier which would clip with a flat top

Clean Sine WaveClipped Sine wave
On the left a "clean sine wave and on the right the waveform as the amp saturates and distorts

The Mixer/Pre-amp

MX400To get the 4 channel input I use a Behringer MX400 mixer which I had bought some years ago.  It takes a 12V DC supply.  Despite the fact it sold as a line mixer and not a microphone mixer I found it worked fine with mics and instruments with both active and passive pickups.  It has a max gain of 100 (or 40dB).

It's not the quietest of amps, but there is a lot of background noise anyway while busking so a little hiss will go unnoticed.  In fact the hiss was slightly less than the DA5 original amp.

Clipping indication
As I mentioned above we need more power for the mic inputs then the standard VOX DA5. I added an extra preamp to the VOX and it worked but after one session a friend told us that we had been distorting.  It's very hard to tell from behind the mic. So a clipping detector was needed.
Clip Detector SchematicI agonized over this.  I finally came up with the simplest circuit I have ever seen using only three components. A 100ohm resistor is in series with and AC light emitting diode and a 10uF capacitor. That's it. It is placed across the output of the mixer and the led comes on when the voltage exceeds +/-1.7V.
The power amp volume is then set so that with 3.4Vp-p signal  is just not saturating.  We note the position of the pot. This is important.

(I set the knob on the volume control
so that the index line points at the clipping led at this point)
With practice we can find the mixer settings which will just not cause the led to light, then turn up the power amp to suit the location as long as we don't go above the predetermined pot position.

If we want to use a single mic that we cannot get really close to, then additional gain would be needed.  Probably around 10.  The simplest method is to add a low noise battery driven amp into the mic lead.
The neatest method would be to replace the mixer amp with a single channel higher gain amp.  Maybe less versatile though as we could not add other inputs.

The class D amp is much more efficient than the VOX and also take much less quiescent current so we can run for longer on the same batteries as the VOX.  In addition I can power it from a bank of 8 rechargeables, or 8 C cells. I have the following options

Battery PacksTwos bank of 8 rechargeable AAs  providing 9.6Volts. and 2000mAH each

One bank off 8 AA Alkaline batteries providing 12V and 2500mAH

One bank of 8 C cell alkaline batteries providing 12V and 8000mAH.

The Cost
To keep the cost down, I used the VOX cabinet.  It's a nice size with a good speaker capable of taking 10 Watts and there is plenty room inside for batteries.
Now I went through quite a few iterations of the amp so I now have quite a few spare parts in my junk drawer.  But if I were to do this again I'd seek out an old broken practice amp and pay a few pounds for it, Fit the power amp under £10, a MX400 for around £20 and I would expect all of the other bits to come in at £10-£15. So around £50-£60

Tone Control
I finally decided against a tone control in the amp. The only place it could go would be between the mixer and the power amp which would mean that the clipping detector would be unreliable. 

Current set up is that, at clipping, the mixer output is at +/1.7V.  It is powered by 12V so should have a bit to spare. According tho the data sheet, it should manage +/- 16dBu which is 4.9V rms or 14V p-p (from a 12V supply!!! Yeah, right). but it can probably really manage about 10Vp-p or 5V peak.
But the mic amp is working flat out to give +/-1.7V.from the mics

SO any extra gain has to come from the power amp.
It is currently set up to saturate at 3.4Vp-p input, but with the gain turned up full it can do this at 0.7V p-p.  This would cover the insertion loss of the tone control.
I would just have to find the setting where it saturates and try not to go above ths.

Worth a try.

The output does sound a bit lacking in HF - Maybe a simple LF drop off would do the job.

Also the same tone correction would be applied to all 4 channels.
Now it is a busking amp so the same voices are using the same mics so maybe a passive fixed tone adjustment circuit could be fitted in the mic leads - a future project??


Top PanelsI removed the DA5 as a complete unit so it could be put back or used elsewhere. This left me with a box with some holes in it and a speaker

I cut and bent a piece of aluminium to fit the top and rear aperture copying the DA5 amp measurements and hole spacing.

The top panel (top) is pretty simple, especially compared to th DA5 (bottom). The green led is the power indication, the other led is the clipping indicator, and the knob is the volume.

Spire nutsI then drilled the aluminium plate to take spire nuts as used by the DA5.

After fitting to the box and doing a quick test I found the aluminium buzzed like billy-O so i stuck some hardboard to the back using double side sponge tape. 

I drilled holes in the hardboard to fit some Neodymium ring magnets to hold the mixer in place during performances if we want it there. (But it is fitted to long cables so we can use the mixer around a metre from the amp)

The aluminium did not look "cool" so I covered it in black Gorilla tape.

Inside the amp
Inside the amp. The Class D amp is the PCB on the left. The black tape holds in the magnets to keep the mixer in place. The clipping detector and power indicator circuitry is mointed on the terminal strip.  The hardboard provides damping.

Amp - front viewAmp rear view
Front and rear views

Amp with mixer seperatedThe Mixer can be used up to four feet from the amp with the cables used.

Amp-storage compartmentThe mixer, shown in the storage area for transport. It attaches magnetically quite firmly to the speaker.

The battery pack in use is on the left, and the reserve one on the right.

Overall the amp is very simple.  The commercial mixer feeds the commercial class D amp which feeds the speaker. The battery pack feeds them both through a power switch. A simple peak detector is connected onto the link between the mixer and the class D amp.  

Weight: 4.5Kg with two sets of batteries

Power: Peaks at less than 1 Volt from rail. So 22Vp-p or 17.2Vp-p  depending on battery which corresponds to 7.5W or 4.6W.
(Note: The original DA5 would not manage more then 3Watts without clipping on batteries)

Quiescent Current: 40mA including mixer


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